Buried GRP/FRP pipe Laying and Installation Procedure

Written by Anup Kumar Dey in Pipeline,Piping Design Basics,Piping Interface

This Write-up/ Article guides to the proper installation of buried fiberglass piping systems. Designing a piping system to the latest engineering standards and techniques makes up for half of the job. The other half consists of the installation and the implementation of the design specifications. The requirements for installing buried fiberglass piping systems differ significantly from those of other conventional materials, for example from steel piping requirements.

GRP (glass-reinforced plastic) pipes, possibly built with mortar siliceous aggregates, are classified as “flexible” pipes, since they can work in a deflected condition, up to 5% of the diameter (long-term), fully in conformity with safety requirements.

Soil-Pipe System

The external loads (soil and traffic) above a GRP buried pipe cause a reduction of the vertical diameter and a consequent increase of the horizontal diameter (deflection).

This horizontal movement develops a passive soil resistance that enhances the pipe’s support by contrasting the deflection and increases its lift (fig.1). Please see the next picture.

**Fig. 1: Figure showing Soil Pipe Interaction**

Thanks to the flexibility of the pipe, all of the external loads, such as soil and traffic that are loaded on the pipe, are sustained by a combination of the pipe’s stiffness and the stiffness of the soil surrounding the pipe.

Deflection

The amount of deflection depends on the soil load, the live load, the native soil’s characteristics, the pipe’s backfill material, the trench width, the filling, and on the pipe’s stiffness.

Buried fiberglass pipes generally accommodate 4-5% of long-term deformation without structural damage. An appropriate selection of the pipe’s stiffness class and its corresponding installation method allows for maintaining the pipe deflection within acceptable values.

Termonologies

The figure (Fig. 2) below shows the meaning and the position of the elements that are used in this article, such as foundation, bed, primary backfilling, secondary backfilling, etc.

**Fig. 2: Figure Showing Terminologies used for the article**

Following are listed a few terms and concepts that are used for soil description:

fines = particles passing through the ASTM No. 200 sieve (with an opening of 0.075 mm), made of silt and clay
fine-grained soils = soils where fine-grained particles are >50%
coarse-grained soils = soils where fine-grained particles are <50%; made of sand and gravel
sand = soil retained by the ASTM No.200 sieve but passing the ASTM No. 4 sieve (opening 4.5mm)

General Recommendations for GRP Pipe Installation

The conditions of the different soils crossed by pipelines to be laid should be determined before installation.
If this information is missing, is not available, or is incomplete, an investigation of these soils will have to be carried out.
The result of this investigation not only will give the information that is necessary to define the suitable backfilling and compaction procedures but will also define possible areas of unsuitable materials, in order to minimize the use of selected material
Fine-grained soils with medium/high plasticity, such as highly plastic clay and silts, or organic soils, generally are unsuitable for the backfilling area.
The parameters that define the soil’s behavior have a determinant influence on the dimensioning formulae and on all of the verifications that are necessary for buried PRFV pipes.

In Situ Soils

It is important to determine the in situ soil conditions prior to the installation and even prior to pipeline design.

Data to be collected are:

soil composition: the ratio between coarse-grained particles and fine-grained particles
compaction degree (for soils with a predominance of coarse-grained particles) or cohesive strength (for fine-grained soils), that can be ascertained by means of penetration and shear tests
groundwater conditions Investigations are addressed to evaluate the modulus of soil reaction (E’n) of the native soil at the pipe elevation and how it can affect the global reaction of the embedment.

Native soils with very low characteristics may reduce remarkably the stiffness of the embedment.

Since in most projects, the embedment materials and the rate of compaction are required to develop a modulus of soil reaction in the range of 7-14 Mpa, any normally consolidated and undisturbed native soil, is able to produce a modulus of soil reaction of the same magnitude or higher.

Embedment Materials

The material used for the bedding and for the backfilling of the pipe is classified according to its composition and its compaction degree

Soil Stiffness Categories

The soil Classification and the Soil Stiffness Categories are summarised in the following table (Fig. 3)

For further details regarding soil classification, please see ASTM-D2487.

Most coarse-grained soils (SC1, SC2, and SC3) make acceptable beddings and pipe zone backfill materials.
Fine-grained soils with medium to high plasticities, such as CH and MH, and organic soils such as OL, OH, and PT generally are proven to be unsuitable for pipe zone backfill materials. High plasticity and organic soils request special design considerations.
The maximum grain size for backfill materials is 18 mm.
Pipe zone backfill material must be compatible with the native trench so it will not wash away nor migrate into the native soil. Likewise, one must prevent the migration of the native soil into the pipe zone backfill area. Either of these events would result in a loss of side support for the pipe and consequently cause an excessive deflection.
Migration can only occur if there is a movement of water in the pipe zone. When using incompatible materials, they will have to be separated with a filter cloth.

FRP Pipe Burial

Storage and Handling

When storing fiberglass pipes, prior to their assembly and burial, use at least three supports (wooden beams). Separate each row of pipes with supports. When a pipe is stored directly on the ground, ensure that the surface is smooth and rock free. Do not roll the pipe on the ground.

Prior to installation, inspect each section of the pipe on both the internal and external surfaces in order to locate possible damages. Inspect also all joints, surfaces, and edges. Do not use damaged pipes without having first consulted our site supervisors.
Only lift pipe sections with fabric straps.

Excavating the Trench

On most construction sites it is best to execute the trench excavation, the pipe installation, and the backfilling consecutively in order to minimize logistic problems and reduce supervision costs.
The bottom of the trench must be flat, continuous, smooth, and free from large rocks. The excavation of the trench bottom has to be deep enough to provide a minimum of 150 mm of bedding under the pipe.
Trench construction will vary according to the different types of soil encountered (stable or unstable, granular or soft). In any case, the bottom of the trench must be flat and continuous.

Stable Trench Walls or Rock Trench

Soils are considered stable only when it is possible to excavate the whole of the trench wall vertically, without the need for any supports or shores.

Unstable Trench Walls & Bottom

With unstable soils, the trench wall may be excavated vertically with the insertion of sheet piles, bulkheads, or shores (installation method 1).

Installation in unstable soil-Shoring should preferably be done using a small thickness and it will have to be removed in concomitance with the backfill. Every backfill layer should be placed and compacted as the sheeting is being pulled out of equivalent height, in order to avoid the occurrence of voids beneath the sheeting and maintain the requested compaction. Refer Fig. 4

The second installation method is done with the trench wall angle at the natural slope of the soil.

Granular Soil Trench

In this case, the trench walls will be at the natural slope (Fig. 4) of the native granular material.

Soft Soil Trench

When the native soil is composed of soils with high plasticity, very compressible, with a water content percentage on the dry soil weight exceeding 50%, as for soft clays, very melted mud, etc., the granular soil used for the laying bed and for the backfilling can be absorbed by the native soil. In this case, it is suitable to cover the bottom and the walls of the trench with a geotextile filter fabric, which has the function of separating the layers to prevent the granular materials composing the bed and the backfilling from being mixed or buried.

Trench Width

The trench width must be wide enough to guarantee a minimum distance between the pipe and the trench wall that can allow the backfilling compaction, according to the type of material used and the compacting method.

Furthermore, in case of excavation in soils that are not able to guarantee the side support requested by the project, the trench will have to be widened, according to the designer’s prescriptions, in order to stabilize the trench wall.

Suggested values for the trench width (L) are the following:

DN ≤ 400 mm L= DN + 400 mm
400< DN ≤ 1000 mm L= DN + 600 mm
DN > 1000 mm L= DN + 800 mm

The depth of the trench must be executed in such a way as to guarantee the dimensions of the bedding in accordance with the prescriptions of the following paragraphs. If the soil is not able to give the vertical support requested by the project, the trench will be deepened by 20cm or more, according to the prescriptions given by the designer, in order to obtain stabilization of the soil.

Moreover, should butt and strap joints be executed directly inside the trench, this must be widened and the trench bottom must be adequately lowered, allowing the lamination operations to be practical. These spaces will then be filled during the trench filling.

Multiple Pipes

If several parallel pipes are laid in the same trench, all of the pipes should have the same bottom level. The clear spacing between adjacent pipes must be at least 0.2 m, in any case not less than the diameter of the smaller pipe, and can be limited to 1 m.

The same spacing will be used for crossing pipes at different heights.

Trench Excavation Below Water Table

When an unstable soil condition is encountered that is caused by a water table, the bottom of the trench must be drained before laying the pipes. This can usually be accomplished by lowering the water table to approx. 30 cm below pipe level by means of pumps and stabilizing the bottom as previously described.

To minimize the soil dewatering, only a length of trench to place one or two sections of pipe should be opened and then be backfilled.

Foundation

The foundation shown in the above figures is required when the trench bottom is unstable, i.e. made of soils whose displacement, due to variation in stress or moisture content, is very high.

According to the different conditions of instability for the trench bottom, the installation contractor may require different types of foundations such as:

stabilization and reclamation of the bottom, by removing an ulterior layer of at least 200 mm of depth and replacing it with stabilized gravel or sand, into which the unstable soil will not penetrate (ground capacity from 0.7 to 0.9 kg/cm2), that is able to redistribute the vertical pressures more regularly.
pouring of lean concrete with a minimum depth of 150 mm (ground capacity from 0.5 to 0.7 kg/cm2);
Foundation made with piles capped by concrete (ground capacity lower than 0.5 kg/cm2).

The above indications must be more closely followed as the diameter of the pipe to install is larger.

Bedding

On the bottom of the trench, the bedding will be laid by using the materials that are described in the following sections. The bedding must have a thickness corresponding to 15% of the pipe’s diameter and in any case a minimum thickness of 150 mm that provides the pipe of uniform and continuous support over its entire length.
The bedding surface must be even and recesses have to be projected in correspondence to every pipe joint. These recesses have to be backfilled after pipe installation and joining.
The use of pea gravel, crushed stone, or sand as bedding material, with fine content not exceeding 12% is recommended. Fines are considered the materials, which pass through the ASTM 200 sieve. The dimensions for the bed materials’ grain diameter should not be greater than 20mm.
The bed must be compacted until reaching 70% of its maximum density, before the pipe installation
Different kinds of materials and compaction ratios can be authorized by the Designer.

Backfilling

Backfilling is divided into two areas

Primary backfilling, which extends vertically from the culvert of the pipe up to 70% of the diameter; should preferably be composed of the same materials used for the bedding (maximum fines content 12% and maximum grain diameter 20mm), symmetrically laid by alternating layers of 20-25cm, compacted one by one, until reaching 70% of the maximum density for the specific material (90% Proctor Standard). Be sure that the areas under the pipe are filled up and compacted with accuracy, in order to grant valid support over an arc of at least 60%.
Secondary backfilling, extending as far as 15 cm above the invert of the pipe; it can be made with the excavation resulting materials, even if it is preferable to use the same material as for the bed and the primary backfilling, symmetrically laid with alternating layers of 30-40 cm compacted one by one.
Backfilling up to the ground level has to be completed with native material.
During the backfilling one must avoid any pipe impact that may be caused by stones or by any other material that could damage it. If the backfilling is disposed of in a nonsymmetrical way or with non-uniform compaction, the alignment of the pipeline may be altered in such a way that it could influence the pipe’s seal.

Use of Different Materials

Materials that are different from the ones above described, but approved by the Designer, can be used for the composition of the laying bed and the backfilling, as long as the content of the granular materials (retained by the ASTM 200 sieve) is greater than 30% and the liquid limit is lower than 50%.

Please take note that by increasing the fine content of the material, the energy required for the tamping of the material itself will increase. Furthermore, it’s useful to intensify quality controls and check the right laying of the backfilling.

FRP Pipe Installation

To install the pipes, the following procedure can be used, as a function of the type of joint and of the pipe diameter:

a) for any type of joint and diameter: lay and align the pipe bars on the bed that has been previously prepared, and perform the junctions inside the trench.

b) for pipes with joints that guarantee axial continuity:

lay on the bed two or three bars, previously joined outside the trench, in order to reduce the number of junctions to be performed inside the trench.
align and join the pipe bars alongside the trench or above it, by using ties; lower the jointed pipeline into the trench by using several hoisting equipment being careful not to cause excessive deformations; this method can be used for small diameters only.

In the case of bell/spigot or socket joints with o-rings, please verify that the angles that have been given do not exceed those allowed. Refer Fig. 5

**Fig. 5: Deviation angles with respect to Diameter**

Typical Compaction Methods

Compaction can be made by using a pulse compactor or other suitable systems. In case the backfill is composed of sand, the compaction can be obtained by saturation. If this method is used, it is necessary to verify the draining capacities of both the bed of the trench and of the native soil; if necessary provide suitable drainage systems and use an adequate quantity of water, in order to prevent the pipe from floating.

The laying trench should be filled up as soon as possible, as far as ground level or for a height of 1.3 diameters, in order to prevent the pipe from floating, in case of laying in presence of a water table or in little draining soils.

Modulus of Soil Reaction

Pipes are generally checked in the actual working conditions, by using a certain Modulus of Soil Reaction, which becomes one of the design’s mandatory prescriptions, together with the:

material to be used for embedment
compaction degree (soil density) and
trench width.

If the installer is given the possibility to change one of the above parameters, the following table is shown the Modulus of Soil Reaction that can be achieved with different materials and soil densities.

Installation Control

Deflection checks must be carried out when the first installed pipes have been backfilled. Further periodical checks must be done throughout the entire project.

Where it is practical, measurement has to be taken of the density of the pipe zone material primarily compacted to ensure compliance with the design assumptions.

Concrete Block Connection

When connecting with concrete blocks and walls, it is necessary to follow some prescriptions, in order to avoid damage to the pipes, due to these two phenomena;

high differential settlements between manufactured product and pipe, due to the considerable weight of the concrete works in comparison with the relatively light PRFV pipes; this phenomenon is particularly clear if the concrete work, or a part of it, is made after having already laid and buried the pipeline, and it can cause high longitudinal shear and flexural stresses to the pipe;
Very sudden passage of the transversal section of the pipe from a deformed configuration due to the normal deflection, to an un-deformed circular configuration in the area, filled up with concrete; this phenomenon only occurs to a short section, at the end of the concrete block, due to a complex stress condition.

In both cases, the larger the pipe diameter, the more evident the phenomenon, and it is necessary to take care that the deflection is minimum near the manufactured product.

It is suggested to take the following precautions, according to the actual working conditions

in correspondence to the connections with the concrete, always wrap the pipe with a rubber sheet, (Fig. 6) 100/200 mm large and 10/30 mm thick, in order to reduce strain concentrations:
design and verify with accuracy the concrete works and thrust blocks, with particular care to the settlement check
improve the stiffness of the backfilling near the manufactured products, by widening and deepening the trench (Fig. 6) and filling it up with well-compacted granular materials; the variation of the trench section should take place gradually, over a length of about two diameters.

As an alternative, backfill the pipe with lean concrete (70 kg/mc) for a length of about one diameter (Fig. 6), near the connection without using form works, this allows the lean concrete to flow, following the natural angle of friction

**Fig. 6: Some precautions to follow during concrete block connection**

create a strong PRFV ribbing on the pipe close to the connection with the concrete manufactured product;
If you can’t keep the differential settlement within an acceptable limit, one should use a flexible joint (bell/spigot or socket joint), which allows rotation of 1-2° just out of the concrete manufacture (generally it is not necessary for diameters up to 300 mm).

Online Video Course of FRP/GRP/GRE Pipeline Stress Analysis using Caesar II

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FRP Piping/Pipeline Stress Analysis using Caesar II

What is HAZOP? Mechanism, Advantages, Applications of HAZOP Analysis

Written by Anup Kumar Dey in Process

What is HAZOP Analysis?

The full form of HAZOP is a Hazard and Operability Study. This is a comprehensive multi-disciplinary team exercise to critically review (Analyze) the piping design (Layout/ Routing/Placement of branches/inline instrumentation items/equipment, etc.) with respect to Hazardous and Operational considerations and requirements.

A Hazard and Operability (HAZOP) analysis is a structured and systematic examination of a planned or existing process or operation in order to identify and evaluate problems that may pose risks to personnel or equipment or prevent efficient operation. This is a well-documented study and forms part of the quantitative risk assessment.

The HAZOP analysis technique was initially developed to analyze chemical process systems but has later been extended to other types of systems and also to complex operations and to software systems.

A HAZOP analysis is a qualitative technique based on guidewords and is carried out by a multi-disciplinary team (HAZOP team) during a set of meetings. A few of the common HAZOP analysis guidewords are listed below:

No or not
Other than
More
Early
Less
Late
As well as
Before
Part of
After
Reverse (of intent)
Others can be crafted as needed

HAZOP Analysis Mechanism

HAZOP analysis is now a mandatory activity. It is a qualitative, experience-intensive exercise. It is in the form of deviation analysis. After the process design, the steady-state specifications of each stream in the flowsheet are known.

The HAZOP analysis team exhaustively asks itself questions as to what will happen if this specification deviates on the positive or negative side of the expected steady-state value. It debates the possible causes and consequences of each such eventuality. Anything that appears to them as likely to lead to hazardous situations is debated further, and possible means of avoiding the same or raising alarm if it happens so that remedial action can be taken, etc. are recommended. This may lead to the recommendation of additional instrumentation on lines and equipment, Hi-Lo alarms and trips, etc. may be required to be provided.

The idea of HAZOP analysis is to foresee a hazardous situation and take measures and abundant precautions to avoid them and increase process safety.

Hazop analysis is a structured analysis, conducted after the design review, to ensure the design is suitable for all the intended operating conditions and complies with the HSE requirements. This process also ensures that the fundamentals of the design are thoroughly explained, understood, and examined.

Basically, HAZOP analysis is performed in four phases as listed below:

Definition Phase
Preparation Phase
Examination and
Documentation and follow-up phase.

Important characteristics of all four HAZOP analysis phases are provided in the image below.

The examination phase is the most important HAZOP analysis phase among the four phases. A typical flowchart of the HAZOP Examination phase is provided below:

Advantages of HAZOP analysis

The benefit of the HAZOP is that early identification and assessment of critical hazards provide essential input to project development decisions. This leads to a safer and more cost-effective design with a minimum cost of change penalty. Other benefits of HAZOP are

It works as a qualitative risk assessment tool
During the brainstorming session based on past experiences, all lessons learned safety points are taken care of to prevent re-occurrence.
HAZOP is a very simple and effective method.
As HAZOP is a cross-functional team effort; experiences from all disciplines combine into the design for highly safe operation.
HAZOP is a powerful communication tool for obtaining early buy-in on the approach.

Key Documents required for HAZOP analysis

Important documents required during a HAZOP Study are:

PFD / PFSs (Process Flow Diagram/ Process Flow Schemes)
P&ID / PEFSs (Piping & Instrument Diagrams/ Process Engineering Flow Schemes)
Basis of Design
Operating, controlling, and safeguarding philosophy
Plot plans & Hazardous area classification drawings
Cause & Effect diagrams

HAZOP Analysis Team Composition

Normally a HAZOP review team consists of

Chairman (independent)
Lead engineers from Process, Instrumentation & Control (both from the Design team and Client/ End-user)
Operations engineer (from Client/ End-user)
Lead engineers from Mechanical/Piping, Pipeline, HSE, and Electrical as required. (Both from the Design team and Client/ End-user)

The following image shows a typical team composition for HAZOP Analysis.

HAZOP Analysis Timings

The HAZOP analysis study should preferably be carried out as early in the design phase as possible to have an influence on the design. On the other hand; to carry out a HAZOP we need a rather complete design. As a compromise, the HAZOP is usually carried out as a final check when the detailed design has been completed.

A HAZOP study may also be conducted on an existing facility to identify modifications that should be implemented to reduce risk and operability problems.

Typically HAZOP analysis in EPC design companies is performed 3 to 4 weeks after the design review meeting, once the design review points are incorporated in the key documents.

Application of HAZOP Analysis

In recent times, the HAZOP study has gained worldwide acceptance across various industries due to its inherent benefits. The main industries are:

Chemical,
Petro-chemical
Pharmaceutical
Refinery
Food
Oil and gas
Nuclear
Fertilizer
Power plants

Few more useful resources for you…

What is Engineering Process Safety?
Safety Rules during A Field Visit By A Design Engineer
An article on Crane safety during Construction
HAZOP (Hazard and Operability) Study: A brief introduction
An article on Excavation Hazards at Construction Sites
Hazardous Area- Theory, Classification and Equipment selection: A short presentation
An article on THE HAZARDS OF PRESSURE TESTING

Types of Atmospheric Storage Tanks: Fixed Roof vs Floating Roof Tanks

Written by Anup Kumar Dey in Mechanical

Atmospheric Storage Tanks are an integral part of any refinery or process plant as they are used to store fluids for various purposes for short or long duration. Feedstock tanks and product tanks provide buffer capacity between process plants, which are generally in continuous operation, and the supply/dispatch of feedstock/products, which are batch operations in general. Intermediate tankage is provided where there is a requirement to buffer the flow of an intermediate from one process unit to another. Component tanks are the intermediate tanks between the process units and the blenders. These tanks are necessary to de-couple blender operation from process unit operation, in order to give the blender sufficient degrees of freedom to blend the product optimally.

Swing Tanks

In determining the requirements for feedstock/product, intermediate and component tankage, it may become apparent that some of the tanks will only be in service for a relatively small proportion of the time. In these circumstances, there is potential for specifying/designing tank(s) for more than one duty. Reducing the number of tanks can have a significant impact on capital costs. Such multi-purpose tanks are often referred to as “swing tanks”.

Tanks are frequently used at the start and finish of a process and it is, therefore, necessary to know the quantity and quality of the contents. Most fixed roof tanks are calibrated after construction, and a dip hole is frequently fitted to the top of the tank so that the depth of the liquid within can be measured accurately with dip tape. This dip hole is often used to collect samples using a specialist container that can be opened once it has reached the required depth.

Modern-level instrumentation can be very accurate and has in many cases replaced the need for regular tank dipping. However, it is good practice to retain the dip pipe both as a means of checking the level instruments and as a means of getting samples. Storage tanks can be of the following types:

Roofless Tanks

This type of tank (Fig. 1) is normally used for water services including fire water, cooling water and service water. Potable water will normally be stored in a tank with a roof. Although open-top tanks are sometimes made of steel, it is common to construct them from concrete, or glass-reinforced plastic (GRP). The main advantage of a roofless tank is its low cost.

Fixed Roof Tanks

Fixed roof tanks (Fig. 1) usually make up the majority of tanks used in a typical tank farm. The tanks are generally used for low vapour pressure materials rather than higher vapour pressure materials, as these would tend to give high VOC emissions during tank breathing and could generate an explosive mixture in the vapour space.

Fixed roof tanks are used typically for gas oil, fuel oil, and sometimes for kerosene in cold locations.

Tanks are very good settling vessels and any water or similar liquid that is not completely removed by the processing unit is likely to settle in downstream tanks. This can affect the material selection for the tank, the provision of drains, or the provision of mixers if it is necessary to keep the two phases in suspension.

Floating Roof Tanks

Floating roof tanks are used for higher vapour pressure materials as they reduce both evaporative losses and the concentration of explosive mixtures. However, it is essential that the vapour pressure can never reach atmospheric pressure.

The design of the floating roof can be pan, pontoon, or double skin. However, regardless of type, all floating roofs need to have a seal. These are to minimise the losses from the tanks and to prevent the ingress of rainwater into the tank. Most modern tanks are fitted with double seals to reduce VOC emissions. A scraper seal is fitted if the tank contents are prone to adhering to the tank walls. Floating roof tanks are used typically for crude oil, naphtha, gasoline, and kerosene in tropical locations.

Various Types of storage Tanks — **Fig. 1: Various Types of Tanks**

Internal Floating Roof Tanks

Internal floating roof tanks (Fig. 1) are becoming of increased importance as a result of tighter environmental requirements. They are more expensive than conventional fixed or floating roof tanks, but they do offer a number of advantages.

They can be used in place of a conventional floating roof tank where there is a need to exclude the possibility of air contamination. In this case, an internal floating roof tank can be nitrogen blanketed.
They can be used for floating roof applications where there are heavy snowfalls. Snow can sink a conventional floating roof. Tanks built for this reason may be provided with ample ventilation apertures in the sidewalls just below the roof.
They can be used where the exclusion of water is very important. Rainwater has been known to find its way around roof seals and leak rainwater into the tank. This type of tank is used for Jet Fuel where water should be eliminated from the product.
They can be used to reduce VOC emissions since the vapour space above the internal roof can be vented via emission control equipment.

There are some disadvantages to IFRTs in addition to increased cost.

The vapour space above the internal roof, unless specially ventilated, will be prone to the production of explosive mixtures. This often has to be blanketed.
In practice, there are likely to be limitations in tank size (maximum diameter approximately 40 m ) resulting in an increase in the number of tanks.
In order to work in association with other facilities like vapour removal units, the tanks may need to be designed for slightly higher pressures

Dome Roof Tanks

Sometimes there is a need to store materials, such as very light naphthas, that have vapour pressures in excess of atmospheric pressure but are not sufficiently high to justify a sphere or a bullet.

API 620 allows for such instances by permitting designs up to a design pressure of 15 PSIG. Such vessels can have a variety of shapes, but the most commonly employed is the dome-roofed tank. A pressurised dome roof is always more costly to fabricate than a coned one. This is usually due to the major additional cost of the foundations. The additional pressure is likely to push out the flat floor, and a special foundation is usually required to stop the tank from deforming. Over/under pressure protection for such tanks is also more like those facilities provided for pressure vessels.

There are instances when tanks such as these are needed, but most installations avoid their use by ensuring that the light hydrocarbon streams involved are safely disposed of without storing them.

Design Standard for Atmospheric Storage Tanks

Atmospheric storage tanks are essential components in various industries, storing a wide range of liquids and gases under ambient pressure. API 650: Welded Tanks for Oil Storage is the most widely used design standard for Atmospheric storage tanks.

API 650 covers the design and construction of welded, cylindrical, atmospheric tanks. It provides guidelines on material selection, tank design, and testing to ensure the tanks can safely handle the intended contents. Key aspects include:

Material Specifications: Steel grades, thicknesses, and welding practices.
Design Requirements: Minimum requirements for tank strength, stability, and safety features.
Inspection and Testing: Procedures for ensuring tanks meet performance standards and are free of defects.

Differences Between Fixed Roof Tank and Floating Roof Tanks

The major differences between a fixed roof tank and a floating roof tank are tabulated below:

Feature	Fixed Roof Tanks	Floating Roof Tanks
Roof Type	Fixed, non-movable roof	Roof that moves up and down with the liquid level
Construction	Permanent, rigid structure	Floating structure that adjusts with liquid level
Applications	Used for a wide range of liquids including water, chemicals, and oil	Primarily used for volatile liquids like gasoline and crude oil
Ventilation	Usually requires a vent or pressure relief system	Floating roof reduces vapor space, often requires fewer vents
Vapor Loss	Higher potential for vapor loss and emissions	Reduced vapor loss and emissions due to reduced vapor space
Maintenance	Easier to inspect and maintain due to fixed nature	Requires careful inspection and maintenance of the floating mechanism
Construction Cost	Generally lower due to simpler design	Higher cost due to complexity of the floating roof system
Operational Cost	Potentially higher due to greater vapor losses	Lower operational costs due to reduced vapor emissions
Sealing Efficiency	Less efficient in sealing against vapors	Highly efficient in sealing against vapors due to the floating roof
Safety Considerations	More prone to atmospheric contamination and fire hazards	Better at minimizing fire hazards due to reduced vapor space
Flexibility in Size	Suitable for various sizes and volumes	Typically used for larger tanks with high volatility liquids
Temperature Control	May require additional measures for temperature control	Floating roof can help with temperature control by minimizing vapor space
Environmental Impact	Higher potential for environmental impact due to vapor loss	Lower environmental impact due to reduced vapor emissions
Leak Detection	Easier to detect leaks at the roof level	More complex due to the floating roof mechanism and need for specialized detection

Fixed Roof Tanks vs Floating Roof Tanks

Additional Notes:

Fixed Roof Tanks: These are straightforward in design and construction, making them less costly initially. However, they may incur higher long-term operational costs due to greater vapor losses and the need for more frequent ventilation and environmental controls.
Floating Roof Tanks: These are more complex and expensive to build and maintain but are advantageous for storing volatile liquids. The floating roof helps minimize vapor loss, reduces environmental impact, and often improves safety by limiting the amount of vapor space that could potentially ignite.

Design Pressure

Although atmospheric storage tanks are intended to operate at about atmospheric pressure, in practice they do have a design pressure, even if it is only a few millibar gauges.

Tank Breathing

When the gas enters or leaves a vapour space in a liquid storage tank, the tank is said to be breathing. Breathing can occur from a number of causes.

A rise or fall in the liquid level.
A rise or fall in the tank temperature (consider the effect of a rain shower on a tank that has been exposed to the sun all day).
The escape of gases coming out of solution, or even the ingress of gases being absorbed such as vapour breakthrough.
The escape of gases entering the tank from other sources.

The vents on the tank have to be large enough to pass all coincident vapour flow, both in-breathing and out. API RP 2000 forms the basis for establishing tank breathing rates.

The type of vent to be used depends on the pressure to be held in the tank. If the tank is to be held as close as possible to atmospheric pressure, and air ingress is acceptable, then a covered vent (to keep out birds etc.) will be used. The only pressure generated will be from the frictional resistance of the gas entering or leaving.

If the tank is to be blanketed, then it will probably need to operate at a slightly higher pressure so that sufficient margins can be allowed between in-breathing and out-breathing for the blanketing controls to work. It would be normal to expect a blanketed tank to be fitted with a pressure/vacuum valve (PVV) and have a controlled supply of blanket gas. Under normal operation, in-breathing by the tank will cause the blanket gas to be introduced. Should it fail, then the vacuum facility will permit air ingress on the grounds that it is better to take in the air than to collapse the tank. Out-breathing causes the pressure section of the PVV to operate. The control signal for the letdown control of the blanket gas must come directly from the tank, as the frictional pressure drop from the controller to the tank is likely to be significant in comparison with the set pressure.

When out-breathing is required to pass to some kind of treatment facility, then clearly higher pressures are required as a driving force. In these circumstances, it is normal to provide PVVs with flanged outlets.

Inerting

Fixed roof tanks and internal floating roof tanks normally have a vapour space that can accumulate volatile gases and/or contain air. If explosive mixtures can be produced in the vapour space, or if it is necessary to exclude air for reasons of contamination, then these tanks will require a gas blanket. In most cases, the most appropriate blanketing gas will be nitrogen. However, occasionally fuel gas may be used. When fuel gas is to be used, it is essential to ensure there are no inadvertent releases to the atmosphere, and that any PVVs discharge to an appropriate disposal system.

Types of Flow Meters and Their Applications

Written by Anup Kumar Dey in Instrumentation,Piping Design Basics,Piping Interface

A flow meter is a device that measures the flow rate of a fluid. Flow meters are flow-measuring instruments used to measure the linear, nonlinear, mass, or volumetric flow rate of a gas or a liquid. Flow meters are also known as Flow Gauges or flow measurement instruments. Accurate flow measurements of gases and liquids are required for better control and quality of industrial processes.

This article will provide the following information in brief:

Types of Flowmeters/ Flowmeter types
Operating Principle of Each type of Flowmeter
Design Features of each flowmeter.
Advantages and Disadvantages of each type of Flowmeters
Selection and Application of those Flowmeters

Types of Flowmeters | Flowmeter Types

Broadly two types of flow meters are widely used in industries:

Volumetric Flowmeters and
Mass Flowmeters

Volumetric Flowmeters

Volumetric flow meters got their name because these flow meters measure the fluid volume passing through a specific location in a set period of time. Volumetric flow meters provide an instantaneous analog, digital, or pulse output of the volumetric flow rate of the liquid or gas. Various types of Volumetric Flowmeters are available as listed below

Differential Head type
- Orifice plates
- Venturi meters
- Annubar
Differential Area type (Rotameters)
Electromagnetic flowmeters
Ultrasonic flowmeters
Turbine flowmeters
Vortex flowmeters
Positive Displacement Meters

Mass Flowmeters

Mass flow meters measure the fluid mass flow rate that travels through a tube per unit of time. There are two types of mass flowmeters as mentioned below

Coriolis Mass flowmeter and
Thermal Mass flowmeters

Differential Head Type Flowmeters

The difference in pressure exists between the upstream & downstream sides of a restriction in a confined fluid stream, which is related to the square of fluid velocity.
- Q α √ ▲P

Where Q = Volume flow rate and ▲P = Differential pressure between taps

**Fig. 1: Differential head type flowmeters**

Types of Orifice plates (Fig. 1)

Concentric orifice plate: Most commonly used
Segmental & Eccentric orifice plate: Used for fluids containing suspended solids.

Tappings for the Orifice plates:

Corner taps (< 1 inch)
D and D/2 taps ( 2 to 16 inches)
Flange taps (> 16 inches)

Features of Orifice Plates

Design Pressure: No limitation. Limited by DP transmitter
Design Temperature: No limitation. Limited by DP transmitter
Sizes: Maximum size is the pipe size
Flow range: limited only by pipe size.
Fluids/ Applications: Cryogenic/clean gases & liquids/ Steam (saturated/superheated)
MOC: No limitation (Steel/ Monel/nickel/ haste alloy)
Accuracy: It varies from ±0.25% to ±0. 5% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5

Advantages of Orifice Plates

Easily installed between flanges.
Fabrication is simple and inexpensive.
No limitations on the materials of construction, line size, and flow rate
Cost is relatively independent of pipe diameter since the cost of DPT is fixed.
No process interruption for the exchange of the DP transmitter.

Disadvantages of Orifice Plates

High permanent pressure loss & hence high energy consumption to overcome pressure loss.
Impractical for systems with low static pressure.
Measuring ranges from about 3:1 to 5:1.
Accuracies decrease with Beta ratios above approximately 0.7.
Subject to damage by water hammer and foreign objects.

Venturi Meters

A venturi tube (Fig. 2) measures flow rates by constricting fluids and measuring a differential pressure drop. In the upstream cone of the Venturi meter, velocity is increased, and the pressure is decreased. Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument. Further details of the venturi meter are provided here

**Fig. 2: Figure showing Venturi meter and Annubar Flowmeter**

Features of Venturimeters

Design Pressure: No limitation. Limited by DP transmitter/ pipe pressure ratings.
Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratings
Sizes: 25 mm to 3000 mm
Fluids/ Applications: Clean Liquids/ clean gases
Limited applications: Dirty /corrosive/viscous Liquids & Dirty gases
Flow range: limited only by pipe size and beta ratio.
MOC: No limitation (cast iron/ carbon steel/ SS/Monel, Titanium, Teflon, Hastelloy, Naval Bronze/haste alloy)
Accuracy: It varies from ±0.25% to ±0. 75% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5

Advantages of Venturimeters

Lower head losses than orifice plates reducing the capital expenditure on pumping eqpt. / save pump energy costs
No process interruption for the exchange of the DP transmitter.
Can be used for temperature extremes
Cryogenics or High Temperatures

Disadvantages of Venturimeters

Highly expensive
Larger and heavier to handle.

Annubar Flowmeter

The Annubar flowmeter is a device to measure the fluid flow (liquid, vapor, or gas) in a pipeline. The flow is measured by creating a differential pressure. As per Bernoulli’s theorem, this differential pressure is proportional to the square of the fluid velocity in the pipeline. The annubar flowmeter measures this differential pressure which is then converted to flow rate using a secondary device.

The probe is installed in the median line as a pressure sensor.
With the flow, the probe records both the static and the dynamic pressure via the probe openings.
In the minus chamber of the annubar, lying on the opposite side, only the static pressure has any effect
The differential pressure corresponds to the dynamic pressure in the pipeline & the flow can is calculated directly.

Features of Annubar Flowmeters

Design Pressure: Upto 97 bars (38 Deg. C) / 55 bars (370 Deg. C)
Design Temperature: Upto 400 deg.C
Sizes: 50 mm to 3000 mm
Fluids: Clean Liquids, gases, and steam
MOC: Brass/steel/ stainless steel/ Hastelloy
Accuracy: It varies from ±1% to ±2% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5

Advantages of Annubar flowmeters

The integral manifold head allows direct mounting of DP transmitters
Hot tapping: Insertion/ installation without system shutdown
Very low-pressure drop

Disadvantages of Annubar flow meters.

Not suitable for viscous and slurry applications
Can be used only for clean fluids.

Variable Area Flowmeters/ Rotameters

Variable area flowmeters or rotameters are one of the widely used common flowmeter types. It consists of a vertical glass or plastic tube. The size of the tube varies from top to bottom; because of this, it got its name as a variable area flowmeter. Inside the tube, there is a free-moving metering float.

A free-moving float is balanced inside a vertical tapered tube
As the fluid flows upward the float remains steady when the dynamic forces acting on it are zero.
The flow rate is indicated by the position of the float relative to a calibrated scale.

Design Features of Rotameters

Design Pressure: Upto 350 PSIG (GLASS TUBE) / 720 PSIG (METAL TUBE).
Design Temperature: Upto 400 deg.C (GLASS TUBE) / 538 Deg. C (METAL TUBE).
Sizes: up to 75 mm
Fluids/ Applications: Clean liquids, gases, and vapors
Flow range: upto 920 cubic m/hr for liquids & 2210 cub.m/hr for gases
MOC: Borosilicate glass/brass/steel/ stainless steel/ Hastelloy
Accuracy: It varies from ±1% to ±2% of actual flow.
Rangeability is 10:1
Upstream length/ Downstream straight length is 10 / 5

Advantages of Rotameters

Simple, robust, and linear output
It does not require external impulse or lead lines.
The pressure drop is minimal and fairly constant.

Disadvantages of Rotameters

Vertical installation only.
Glass tubes limit pressure & temperature and are subject to breakage from hydraulic & thermal shock
Glass tubes eroded by undissolved solids & unsuitable for metering alkaline solutions
Metal tube meters are more expensive.
Foreign particles can accumulate around the float & block the flow

Magnetic Flowmeters

Operate on Faraday’s Law of magnetic induction.
When a conductive fluid moves in a magnetic field, a voltage is generated between two electrodes at right angles to the fluid velocity and field orientation.
The flow tube has a fixed area & field intensity so the developed voltage is linearly proportional to the volumetric flow rate.

Design features of Magnetic Flowmeters

Design Pressure: 20 BARS to 172 BARS
Design Temperature: Upto 120 deg.C with Teflon liners / 180 Deg.C with ceramic liners
Sizes: 2.5 mm to 3000 mm
Fluids : Liquids (clean/ corrosive/dirty/viscous/ slurry)
Velocity range: 0.1 to 10 m/s
MOC: Liners: ceramic/ teflon/rubber : Electrodes: Platinum/ hastelloy/SS
Accuracy: It varies from ±0.5% to ±1% of actual flow.
Rangeability is 10:1
Upstream length/ Downstream straight length is 10 / 5

Advantages of magnetic Flowmeters

Flow rate unaffected by fluid density, consistency, viscosity, turbulence, or piping configuration.
Highly accurate due to the absence of moving parts/ external sensing lines
Corrosion-resistant using Teflon liner and platinum electrodes
Wide flow measuring ranges & no pressure drop

Disadvantages of Magnetic Flowmeters

Costly, relative to other flowmeter types.
The temperature of the fluids being metered is limited by the liner material rating.
Cannot be used for gas flow measurements

Vortex Flowmeters

An obstruction is placed across the pipe bore at a right angle to fluid flow.
As fluid flows, vortices are shed from alternating sides of the body & this shedding frequency is directly proportional to fluid velocity.
Detection of the vortices by means of pressure changes in the vortex stream.
The rate of creation of vortices is directly proportional to the flow rate.

Design Features of Vortex Flowmeters

Design Pressure: 138 bars
Design Temperature: -200 Deg. C to 400 Deg.C
Sizes: 15 mm to 300 mm
Fluids: Gases (clean/ dirty) and clean liquids
Velocity range: 0.3 to 10 m/s (liquids) and 6 to 80 m/s (gases)
MOC: mostly in stainless steel, some in plastic
Accuracy: It varies from ±0.5% to ±1% of actual flow for liquids and ±1% to ±1.5% for gases
Rangeability is 20: 1
Upstream length/ Downstream straight length is 20 / 5

Advantages of Vortex Flowmeters

Minimal maintenance, no moving parts.
Calibration using fluid flow is not required & unaffected by viscosity, density, pressure, and temperature within operating specifications.
Digital or analog output.

Disadvantages of Vortex Flowmeters

At low flows, pulses are not generated and the flowmeter can read low or even zero.
Reynolds number should be greater than 10000.
Vibration can cause errors and inaccuracy.
Correct installation is critical as protruding gasket or weld beads can cause vortices to form, leading to inaccuracy.
Long, clear lengths of upstream pipework must be provided, for orifice plate flowmeters.

Figure showing Vortex Flowmeter, Ultrasonic flowmeter and Turbine Flowmeter — **Fig. 5: Figure showing Vortex Flowmeter, Ultrasonic flowmeter, and Turbine Flowmeter**

Ultrasonic Flowmeters

A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on the pipe wall, one (set) on the upstream and the other (set) on the downstream.
The time for acoustic waves to travel from the upstream transducer to the downstream transducer (t_d) is shorter than the time it requires for the same waves to travel from the downstream to the upstream (t_u).
The larger the difference, the higher the flow velocity.

Design Features of Ultrasonic Flowmeters

Design Pressure: 207 bars (insertion type)/ unlimited (clamp on type)
Design Temperature: -180 Deg. C to 260 Deg.C
Sizes: 3 mm to 3000 mm
Fluids : clean gases, clean/corrosive liquids (with little/no solids/ bubbles)
Velocity range: 0.3 to 15 m/s
MOC: mostly in stainless steel/ alloys
Accuracy is: +0.5% of flowrate for insertion type/+1% to +3% of flowrate for clamp on type
Range ability is 10: 1 to 300: 1
Upstream length/ Downstream straight length is 10 / 5
Bidirectional flow measurement
For insertion type, hot tapping in pressurized pipelines is possible

Advantages of Ultrasonic Flowmeters

No obstruction/ moving parts in the flow path
No pressure drop
Low maintenance cost
Multi-path models have higher accuracy for wider ranges of Reynolds number
Can be used in corrosive fluid flow
Portable models available for field analysis and diagnosis

Disadvantages of Ultrasonic Flowmeters

Only clean liquids and gases can be measured
Higher initial setup cost

Turbine Flowmeters

Consists of a multi-bladed rotor mounted at right angles to the flow & suspended in the fluid stream on a free-running bearing.
The diameter of the rotor is slightly less than the inside diameter of the flow metering chamber.
Speed of rotation of rotor proportional to the volumetric flow rate.

Features of Turbine Flowmeters

Design Pressure: 1500 PSIG
Design Temperature: 150 Deg. C
Sizes: 5 mm to 600 mm (Full bore type)/ > 75 mm for insertion type
Fluids : Clean liquids/ gases and vapours
Velocity range: 0.3 to 15 m/s
MOC: mostly in stainless steel/ hastelloy
Accuracy is: +0.25% to + 0.5% of flowrate for full bore type/+1% to +3% of flowrate for insertion type
Range ability is 10: 1
Upstream length/ Downstream straight length is 15/ 5
Bidirectional flow measurement
For insertion type, hot tapping in pressurized pipelines is possible

Advantages of Turbine Flowmeter

Very accurate. Commonly used to prove other meters.
The digital output provides for direct totalizing, batching, or digital blending without reducing accuracy.
There is less tendency to read high in pulsating flow than in head or variable-area type meters.

Disadvantages of Turbine Flowmeters

Not usable in dirty streams or with corrosive materials.
Subject to fouling by foreign materials -fibers, tars, etc.
Bearings are subject to wear or damage. Shift in calibration if bearings replaced
It can be damaged by overspeeding (over 150 percent) or by hydraulic shock.
Pressure loss at rated flow varies & can be high.

Positive Displacement Flowmeters

This meter repeatedly entraps the fluid into a known quantity and then passes it out.
The quantity of the fluid that has passed is based on the number of entrapments.
The volume flow rate can be calculated from the revolution rate of the mechanical device.

Features of Positive Displacement (PD) Flowmeters

Design Pressure: 1500 PSIG (liquids)/: 100 PSIG (gases)
Design Temperature: 293 Deg. C (liquids)/: -34 to 60 Deg. C (gases)
Sizes: 6 mm to 400 mm
Fluids: Clean Liquids/ gases
Flow range: 0 – 20000 GPM (liquids)/ : 0 – 3000 cub.m/hr (gases)
MOC: mostly in aluminum, stainless steel, plastics, Hastelloy
Accuracy is + 0.5% to + 1% of flowrate
Range ability is 15:1

**Fig. 6: Positive Displacement Flowmeters and Thermal mass flowmeters**

Advantages of PD Flowmeters

Good accuracy and high rangeability
Can be used in viscous liquid flow
Low to medium initial set-up cost
Require no power supply and is available in a wide variety of readout devices

Disadvantages of PD Flowmeters

Maintenance is required at frequent intervals because of the `moving parts.
High-pressure drop due to obstruction
Not suitable for low flow rate
Not suitable for fluids with suspended solids
Gas (bubbles) in liquid could significantly decrease the accuracy

Thermal Mass Flowmeter

Operates by monitoring the cooling effect of a gas stream as it passes over a heated transducer.
Gas flow passes over two PT100 RTD transducers.
The temperature transducer monitors the actual gas process temperature, whilst the self-heated transducer is maintained at a constant differential temperature by varying the current through it.
The greater the mass flow passing over the heated transducer, the greater the current required to keep a constant differential temperature.
The measured heater current is, therefore, a measure of the gas mass flow rate.

Design Features of Thermal Mass Flowmeters

Design Pressure: 1200 PSIG
Design Temperature: 176 Deg. C
Sizes: 15 mm to 1000 mm
Fluids: Clean gases
Flow range: 0 – 2500 SCFM
MOC: mostly in stainless steel/ glass, Teflon, monel
Accuracy is +1% to + 2% of flowrate
Range ability is 10: 1 to 100:1
Upstream length/ Downstream straight length is 5/ 3

Advantages Of Thermal Mass Flowmeter

No temperature or pressure compensation required
Linear output (as temperature differential is proportional to mass flow)
Can be used on corrosive process streams if proper materials are specified
DC voltage or 4 to 20 mA dc outputs are available

Disadvantages of Thermal Mass Flowmeters

Practical for gas flow only
Subject to blockage by foreign particles or precipitated deposits due to small openings in the flowmeter
Power requirements are excessive in larger pipe sizes
Has to be taken out of the process line for servicing
Accurate field calibration is difficult

Coriolis Mass Flowmeter

When a moving mass is subjected to an oscillation perpendicular to its direction of movement, Coriolis forces occur depending on the mass flow.
When the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube.
On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist.
This twisting movement is sensed by a pick-up and is directly related to the mass flow rate

Coriolis Mass Flowmeter Characteristics

Design Pressure: 345 bar
Design Temperature: 200 to 426 Deg. C
Sizes: 1.5 mm to 150 mm
Fluids/ Applications: Liquids (clean/ dirty/viscous/ slurries) clean /liquified gases
Flow range: 0 – 25000 lb/m
MOC: mostly in stainless steel, Hastelloy/titanium
Accuracy is + 0.15% to + 0.5% of flowrate
Range ability is 20: 1
Bidirectional flow measurement

Advantages of Coriolis Mass Flowmeters

Capable of measuring difficult handling fluids
Independent of density changes, flow profile and flow turbulence. Hence straight lengths are not required.
No routine maintenance is required since no moving parts
High accuracy

Disadvantages of Coriolis Mass Flowmeters

Not available for large pipes (up to 150 mm only)
High flow velocities are required for detection resulting in a high-pressure drop
Expensive compared to other flowmeters
Difficulty in measuring low-pressure gases.

Application of Flowmeters / Selection of Flowmeters

Clean liquids/gases

Orifices
Venturi
Annubar
Variable Area
Magnetic (only liquids)
Ultrasonic
Vortex
Coriolis Mass Flowmeters
Thermal mass flowmeter (only gases)
PD meters

Dirty Liquids

Most suited: Magnetic/Coriolis Mass Flowmeters
Limited applications: Venturi meters

Dirty Gases

Most suited: Vortex meters
Limited applications: Venturi meters/Thermal mass flowmeter/Variable area flowmeter

CORROSIVE LIQUIDS: Magneticflowmeters/Ultrasonic flowmeters

NON-NEWTONIAN LIQUIDS: Coriolis Mass Flowmeters

VISCUOUS LIQUIDS: Coriolis/Magnetic/Positive Displacement Meters

ABRASIVE SLURRIES: Magnetic flowmeters/Coriolis Mass Flowmeters

FIBROUS SLURRIES: Magnetic flowmeters/Coriolis Mass Flowmeters (limited applications)

Saturated Steam

Most suited: Orifice DP meters/ Vortex flowmeters
Limited applications: Venturi meters/Variable area meters

Super-heated Steam

Most suited: Orifice DP meters
Limited applications: Venturi meters

Cryogenic Applications: Venturi-meters/Orifice plates

Parameters affecting Flowmeter Selection

There are various factors that influence the flowmeter selection for a specific industrial process. Some of those factors are:

The fluid phase and characteristics; for example, gas, liquid, steam
Flow conditions and flow range; for example clean, dirty, abrasive, or viscous fluid.
Process design parameters; for example, pressure, temperature ranges, density, viscosity, etc.
Pipe size, material, flange pressure rating, etc.
Accuracy desired.
Material of Construction (corrosive or non-corrosive fluid).
Maximum and Minimum measurement range and turn-down rates.
Repeatability and cost-effectiveness.
Environmental considerations, if any.
Regulatory and certification requirements.
Location of the flowmeter in the piping system.
Measurement in mass or volumetric unit.

Installation of Flowmeters

Flow measurement using flowmeters is an essential activity for any industry. So, it must provide reliable and accurate data. The accuracy and repeatability of measured data, to a large extent, depend on the correct installation of the flowmeter. Some of the critical steps that must be followed during flowmeter installation are listed below:

It must be installed in the proper location.
It should not be installed where there are vibrations or magnetic fields.
The flow direction must be known before installation.
Flowmeters should be installed on a straight pipe.
Some flowmeters may need straight lengths upstream and downstream of the flowmeter. It should be maintained for accurate results.
For liquid flow applications, the downward flow should be avoided.
Ensure that the flowmeter is completely filled with fluid.
Vapor or air in liquid lines and liquid droplets in gas lines should be avoided.
It’s preferable to install a filter upstream of the flowmeter to remove solids.
If repair is required, a bypass line should be provided.

Types of Pipe Materials and Their Selection / Metallic Pipe

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Design Basics

Piping Materials are a very important part of Piping Engineering. The success of a project and the cost to a great extent depends on piping materials. Hence, the proper selection of pipe materials plays an important role in the project economy, and it is always preferred to choose piping materials that meet all technical requirements but are cheaper. There is a wide variety of piping materials present in the market. Choosing the best and most economical pipe material for a specific service is really challenging and here comes the role of piping material engineers. They, in consultation with process engineers, decide the right kind of pipe materials. In this article, we will explore the types of piping materials and their selection philosophy.

Piping material is a broad term and is not limited to only the material of the pipe. It signifies the material of all piping components; pipes, fittings, valves, and other items. So, in a sentence, we can say that piping material means the materials of all the components that are present in a specific pipe class.

Parameters for Pipe Material Selection

As pipes and pipelines carry various liquids and gases having different process conditions, the proper selection of pipe material is very important. The main process parameters that govern the piping material selection process are:

Types of Fluid to be handled (Flammable, Explosive, Corrosive, Reactive, Volatile, or Hazardous to human skin).
Design and operating temperatures. (Whether the temperature of the fluid is low temperature, high temperature, or cryogenic)
Design pressure of the fluid.
Flow medium
Service Life of the Fluid Handling System
Material characteristics
Fracture tendencies at the temperature extremes
Corrosion and erosion
The environment where the pipe will be working

In addition to the above parameters, some non-process parameters that are considered for pipe material selection are:

Cost of the pipe material
Availability
Manufacturability, Weldability, etc.

Various piping codes and standards provide a long list of acceptable piping materials under their jurisdiction. These codes provide various details about those piping materials like design rules, allowable stresses at different temperatures, etc.

Pipe Material Selection Basics

Selection of basic Piping Metallurgy and Material (viz. CS, LTCS, AS, SS, etc.) for piping specification lies with the Process/Metallurgy Engineer. The main selection is based on process, licensor, and/or intrinsic metallurgy requirements to suit process medium, like corrosion, high temperature, pressure, etc.

This basic selection shall also address special considerations for PWHT, special valve trim for NACE, corrosive services like acids, amines, etc., and hazardous services like Hydrogen, Chlorine, Phosgene, Oxygen, etc.

Piping Material Basics

Metals are rarely used in their purest form as they have low mechanical strength. Alloying helps increase its properties like strength and ductility. (Easiest eg. is adding Carbon to Iron to produce ferritic Carbon Steel). Addition of alloying elements in proper proportions along with appropriate metal processing and heat treatment results in the optimization and improvement of its mechanical properties. Alloying also helps in improving corrosion & oxidation characteristics, machinability, weldability, etc.

Complex alloyed material is also being engineered for use in aerospace programs and applications. Metallic glasses and crystalline alloys have also been developed and metal alloys are sometimes even bonded with graphite, ceramic, and organic materials as composites for wider and more complex applications

Mechanical Properties of Pipe Materials

Certain mechanical properties are also considered while selecting pipe material for a specific service. They are

Modulus of Elasticity (Young’s Modulus) – the ratio of stress to strain and measured using tension tests
Elastic range: Material returns to its original shape after the load is released
Plastic range: Material is permanently deformed even after the load is released
Yield Strength – It defines the transition from elastic to plastic phase and it establishes the limiting value at which this transition occurs
Ultimate Tensile Strength – This defines the limit to which any further addition of load under constant strain would arrest the specimen elongation or thinning and would result in its failure.
Ductility – expressed in the elongation of a specimen and its reduction in the cross-sectional area before its failure. Established by measuring specimen length before elongation and minimum diameter before failure.
Hardness – Ability of a material to resist deformation. Hardness is tested by Brinell or Rockwell Hardness tests, both of which are indentation-type tests
Toughness – Ability of a material to resist sudden and brittle fracture due to the rapid application of loads. Measured using the Charpy V-Notch test.
Fatigue Resistance – Ability of a material to resist failure or crack initiation and its further propagation under repeated cyclic loading conditions

Material Terms and Definition

Before diving into the actual piping material selection process, let’s learn some of the important terms relevant to pipe materials.

Creep Strength – Ability of a metal to withstand constant weight or force at elevated temperatures without yielding
Brittle fracture – Sudden & rapid failure of a metal due to the application of energy with hardly any deformation
Stabilization – Addition of alloying elements to prevent carbon-chromium precipitation and formation of carbides, which reduces corrosion at higher temps
Intergranular Corrosion (IGC)– Corrosion occurring at grain boundaries of metals due to the depletion of chromium by the formation of the Cr Carbide layer, after reacting with carbon, which protects from further corrosive environments. (Min 12% Cr in SS). IGC is caused by reducing acids, oxidizing acids, and organic acids
Reducing acids – In Chemistry reduction means loss of oxygen and gain of Hydrogen – examples are Hydrochloric acid, Hydrofluoric acid, and hydrobromic acid
Oxidizing acids – Oxidation (a chemical reaction between metal & Oxygen) means a gain of Oxygen and loss of Hydrogen – examples are Sulfuric acid, Nitric acid, and Chromic acid
Organic acids – are of the carboxyl (COOH) group containing hydroxide (OH) – examples are Acetic acid, Formic acid, Citric acid
Stress Corrosion Cracking (SCC) – Failure of metal through a combined action of tensile stress and chemical corrosion. SCC also depends on service temp, solution environment, exposure duration, and metal properties
High-Temperature Hydrogen Attack – Results in the degradation of Carbon and Low Alloy Steel due to depletion (decarburization) of carbon (strengthening agent) in steel due to reaction with Hydrogen at high temps, thus causing loss of strength in metal.
Hydrogen Blistering – A low-temperature phenomenon where atomic hydrogen diffuses into steel and is trapped as non-metallic inclusion, which builds up pressure and eventually bulges and blisters steel.
Hydrogen-Induced Cracking (HIC) – A phenomenon similar to hydrogen blistering but HIC occurs in pipelines operating in sour services. Hydrogen blistering and HIC can be controlled by restricting sulfur content in steel to 0.005% or 0.010% max.
Oxidation – The chemical reaction of metal and alloys with oxygen in the metal in the air, to form oxides is called oxidation. This process results in scaling.

Effects of Alloying Elements on Metallic Piping Materials

The main alloying elements that influence the properties of pipe materials and hence their selection are

Carbon (C) – More carbon means more strength and hardness but less ductility and toughness.
Phosphorus (P) – High content decreases shock resistance & ductility, making the material brittle.
Silicon (Si) – Increases high-temperature properties, making the metal more stable by increasing tensile strength without increasing brittleness when under 2%. It also resists oxidization & is used as a deoxidizing agent
Manganese (Mn) – It improves hot working characteristics by increasing hardening when combined with sulfur
Nickel (Ni) – It improves hardenability by increasing the strength and toughness of steel. Combined with Chromium it improves impact and fatigue resistance. Improves low-temperature properties. Higher nickel content improves resistance to chloride cracking
Chromium(Cr) – It is a hardening element & improves material strength at a higher temp. Improves high temp oxidation & corrosion resistance of steel
Molybdenum (Mo) – It makes the steel harder and more stable by increasing its creep resistance at a higher temp. 2% Mo in steel also reduces high temp oxidation rate
Columbium/Titanium (Cb/Ti) – Commonly used stabilizing elements to improve sustained high operating temp properties of steel by reducing carbide precipitation. SS Type 321 and 347

Typical Pipe Material Selection Criteria

Now selecting a proper pipe material is a difficult task as there is a wide variety of pipe materials available in the market. Piping materials can broadly be of three types

Metallic Piping Materials
Non-metallic Piping Materials, and
Composite Piping Materials

Metallic pipe materials can again be subdivided into two groups; Ferrous piping materials and non-ferrous piping materials.
Composite pipe materials consist of both metals and non-metals.
Types of pipe materials can easily be graphically represented as shown below:

Cast Iron (CI) ASTM A126, A436 Pipe

The usual features of Metallic Cast Iron pipe materials are given below:

Metallic Cast Iron/Ductile Iron/Malleable Iron Pipes are brittle, low-strength materials used for low-temperature applications and basic utilities like air, water, drains, etc. Low-cost material.
CI shall not be used on severe cyclic condition services, excessive heat, thermal shock
DI & MI cannot be used at a temperature below -29° C & above 343°C (ASTM A47, A536)
Austenitic DI (ASTM A 571) may be used at temp up to -196°C max but not lower

Carbon Steel (CS) ASTM A53-B/A106-B/API 5L-B Pipe Selection

The usual features of Carbon Steel Metallic Piping materials are

Better than CI and has higher strength.
Used for higher temperatures (up to 800° F or 427° C).
Most process services include steam piping.

Low Temp Carbon Steel (LTCS) ASTM A333-Gr 1, 3, 4, 6, 8, 11, etc

Used for low-temperature services like chilled brine, chlorine liquid/gas, propylene, etc. (Bet -45°C to 485°C)
Has more Carbon and no alloying elements like Cr and Mo and contains Nickel which improves low-temperature properties
Impact properties/values at low temp are better than in CS (Charpy N Notch test)
Refer to ASTM 01.01 for impact test requirements for low-temperature/cryogenic services

Galvanized Carbon Steel Pipe

Use limited to about 200° F or 93° C for basic utilities like water, air, nitrogen
Normally piping connections are screwed to avoid damage to galvanizing due to welding

Lined Piping

Normally metallic, glass, non-metallic, cement-lined piping
Used for highly corrosive services like acids, caustic, process limited services, etc.
CS Cement lined pipe normally used in seawater applications

Alloy Steel (AS) – Also known as Cr-Mo Steel Pipe Selection

Used for high-temperature applications in CS base like process services, superheated steam, reformer gases, etc. above 400° C design temp (ASTM A335 Gr P1, 5, 11, 22, etc.)
C-1/2Mo steel can be used bet -29°C up to 454°C design temp
Cr-1/2Mo steels can be used bet -29°C and up to 550 to 600°C
PWHT or stress relieving is a must after welding

Metallic Stainless Steel (SS) Pipe – Austenitic Grade Cr-Ni-Mo

The main characteristic features of metallic stainless steel piping are

Used for high-temperature and process-critical services and for cryogenic applications
Selection governed by the process for specific service needs
ASTM A312 Gr TP 304 and 316 are normally used SS grades for pipes
The presence of 2% Mo in SS316 gives better overall corrosion resistance properties than SS304
SS316 has a higher resistance to pitting and crevice corrosion in chloride environments
Grade L series has lower C (0.035%) which improves its use for higher temp up to 1100°F (600°C), has a higher resistance to IGC and better weldability, Better mechanical strength at elevated temps & good high temp oxidation resistance up to 925°C.
Grade H – Controlled C between .04 to 0.1% & lower Ni provides improved high-temperature strength above 815° C.
Common applications of SS304 are food, steel utensils, beverages, the dairy industry, etc.
Common applications of SS316 are food, pharma, marine, medical implant steel, etc.
Grade 317 – use dictated by licensor/process
Grades 321 and 347 are metallurgically very stable in high-temperature applications because of the addition of Columbium and/or Tungsten
Impact testing is not required if C < 0.1%
Refer to ASTM 01.01 for impact test requirement for low-temperature/cryogenic services

Exotic Grades – Not commonly used – High cost!!!

Duplex SS: Grade 2205/2207 (UNS No. S31803/32760)

Cr-Ni-Mo steel
Has excellent strength & corrosion resistance, improved resistance to acids and chlorides, and good weldability (ASTM A928)

Nickel 200/201: (UNS No. N02200/N02201)

Pure Nickel-very good mechanical properties
Excellent resistance to corrosive media
Good mechanical strength at a high temperature
Good ductility at low temp, good weldability
Nickel 201 has a little more Carbon than 200 which makes its use ideal for highly corrosive caustic soda (Sodium Hydroxide) applications up to 300°C (ASTM B163).

Monel 400/500: (UNS No. 04400/05500)

Ni-Cu Alloy – ASTM B165
Good resistance to saline and acidic conditions
Ideal for high-velocity seawater/brackish water applications
High resistance to cavitation and corrosion
Monel 400 has oxidation resistance up to 550°C
Monel 500 has higher tensile strength and hardness and resists oxidation up to 650°C
Ideal for use with H₂SO₄and other acids
Ideal material for valve trims

Inconel Pipe Selection

Inconel 600/601: (UNS No. N06600/06601)

Ni-Cr-Fe Alloy – ASTM B167
Excellent oxidation and scale resistance properties up to 1200°C
Excellent resistance to corrosive media
Good weldability and is resistant to chloride stress corrosion cracking
Inconel 600 is used extensively in power plants Higher Cr content in Inconel 601 offers better oxidation and carburization resistance to sulfur applications

Inconel 625: (UNS No. N06625)

Ni-Cr-Mo-Fe Alloy – ASTM B705
Excellent strength and ductility between 700°C to 1100°C
The presence of Mo further increases corrosion resistance at higher temperatures
Ideal for phosphorus acids, organic acids, seawater, boiler tubing, etc.
Good weldability

Inconel 800: (UNS No. N08800)

Ni-Cr-Fe Alloy – ASTM B407

Inconel 800H: (UNS No. N08810)

Ni-Cr-Fe Alloy – This is a solution heat-treated high carbon version of Inconel 800 with improved elevated temp properties and strength

Inconel 825: (UNS No. N08825)

Ni-Cr-Mo-Fe Alloy – ASTM B423
High resistance to sulfuric acid, phosphoric acid, solvents, reducing acids, and seawater
Cupro Nickel: C70600, ASME B466/B467, B111Is an alloy of Cu & Ni, Cu 70-95% & Ni 30-95%
Has very good corrosion resistance, especially to seawater applications
Good mechanical strength & weldability
B466 seamless and B467 welded pipes are used in marine applications like seawater piping and fittings
B111 used in exchanger/condenser tubes
Also used extensively in the automobile industry, especially for brake tubing

Hastelloy C276: (UNS No. N10276)

Ni-Cr-Mo-W Alloy – ASTM B619/622
Best alloy for extremely corrosive conditions
Good for reducing and oxidizing applications
The presence of Tungsten (W-Wolfram) imparts excellent resistance to strong oxidizing services, hot contaminated acids, solvents, chlorides, etc.
Ideal for strong acids, formate acids, acetic hydride solutions, seawater, and saline solutions.
Corrosion-resistant to wet HCL, hydrochloride solutions, etc.

Alloy 20: (UNS No. N08020)

Ni-Cr-Mo-Cu Alloy – ASTM B464
Excellent mechanical properties, strength, and machinability
Excellent stress corrosion resistance to boiling 20-40% Sulfuric acid
Widely used in 98% Sulfuric acid service for valve trims
Ideal in food & pharma applications where product purity has to be guaranteed

Aluminum and Al Alloy Pipe (ASTM B345)

The main features of Aluminum and Aluminum alloy metallic piping materials are:

Excellent mechanical properties like strength, weldability, and formability
Good surface finish
High corrosion resistance
Used in Gas & Oil transmission and distribution piping systems
Also used in aircraft applications as it is light but strong

Titanium: ASTM B861 – Seamless Pipe/ ASTM B862 – Welded Pipe

Metallic Titanium piping materials have the following features:

These are practically pure grades of Titanium with about 6% Nitrogen
Has high strength, impact toughness, fabricability, formability, and weldability
Exceptional corrosion and erosion resistance which allows
Zero corrosion allowance
Used a lot in aerospace applications and engine components besides chemical, marine, refinery, chlorine, food processing, and pharma applications

Non-Metallic Piping Materials

The most common non-metallic piping materials are PVC, UPVC, CPVC, PP, LDPE, HDPE, PVDF, HALAR, PTFE, PFA, FRP, RTRP, etc.

Polyvinyls as Pipe Materials

(PVC, UPVC, CPVC) ASTM D1784/1785
Polyvinyl Chloride (PVC) is the most common material for plastic pipe
Unplasticized (U-PVC) is the same as PVC but has resins added as additives to make it harder
Service temp up to 60°C (140°F)
Joining methods are solvent welding, threading, or flanging
Chemically inert, good corrosion and weather resistance, high strength, good electric and thermal insulator
Used in chemical processing, chilled water distribution, chemical drains, etc.
Not suitable for oxidizing agents like concentrated sulfuric acid, nitric acid, esters, and amines
Polyvinyls will burn but do not support combustion because of their high chlorine content and will extinguish immediately upon flame removal
Chlorinated PVC (CPVC) has added chlorine which makes it suitable for use at higher temps up to 100°C (210°F) and offers better corrosion resistance to liquids
Used for hot process piping, corrosive liquids, hot and cold water lines, etc.
Joining methods are solvent welding, threading, or flanging

Polyolefins (PP, CPP, PE) as Pipe Materials

Polypropylene (PP) is the lightest thermoplastic piping material. Good strength and chemical resistance, resistant to sulfur-bearing compounds. May be used up to 80°C (180°F) applications. Excellent material for industrial drainage, petroleum industry, salt water disposal, chilled water, and demineralized water lines.

Copolymer Polypropylene (CPP) is a copolymer of propylene and polybutylene. Has excellent dielectric and insulating properties, high chemical resistance, toughness, and strength between freezing to 93°C (200°F) operating temps. Excellent abrasion resistance and good elasticity. Joining by socket fusion or butt welding.

Non-metallic Polyethylene (PE) Piping Material has four classifications:

Low-density polyethylene (LDPE) has more branching and less compact molecular structure, Lower mechanical strength than polyethylene; Ideal for food handling services, brine tanks, etc. Good for temps up to 60°C (140°F). Joining by hot gas welding. All polyethylenes have excellent chemical resistance to a wide range of common chemicals.
Medium Density Polyethylene (MDPE) is a thermoplastic that has a lesser density than HDPE. It has good shock and drop resistance and better stress cracking resistance than HDPE. It has a lower hardness and rigidity when compared to HDPE. Used a lot in gas piping and fitting and in packaging films. Joining by hot gas welding.
High-Density Polyethylene (HDPE) has minimal branching and a more compact molecular structure. More rigid and less permeable than LDPE. Good for temps up to 71°C (160°F). Used for abrasion-resistant piping, caustic storage tanks, control tubing, etc. Joining by hot gas welding.
Cross-Linked High-Density Polyethylene (XLPE) is a 3-dimensional polymer of extremely high molecular weight and close molecular structure. Superior resistance to environmental stress cracking and very high impact strength. Good for temps up to 71°C (160°F). Ideal material for large storage tanks for outdoor service

Fluoroplastics (PVDF, HALAR, PTFE, FEP, PFA) Pipe Materials

Polyvinylidene Flouride (PVDF) is a strong, tough, abrasion-resistant fluoroplastic material. Resists distortion and retains strength up to 135°C (275°F). Ideal for handling wet and dry chlorine, bromine, and other halogen services. It also withstands most acids, bases, and organic solvents. PVDF is not recommended for strong caustics. The best material for high-purity piping such as deionized water. Joining methods are thermal butt, socket, or electrofusion.

Halar (ECTFE) Ethylene Chlorotrifluoro Ethylene is a very durable copolymer of ethylene and chlorotrifluoroethylene. Excellent resistance to a wide variety of strong acids, chlorine, solvents, and aqueous caustics. Excellent abrasion resistance, electric properties, low permeability, and temperature capabilities from cryogenic to 170°C (340°F). Resistant to radiation. Halar has an excellent application for high-purity hydrogen peroxide application. Joining by thermal butt fusion.

Polytetrafluoroethylene (PTFE) PTFE offers the most unique and useful characteristics of all plastic materials. PTFE can handle liquids or gases up to 232°C (450°F). PTFE flows and is used as an excellent sealant material. Normally an opaque white material.

Fluorinated Ethylene Propylene (FEP) This fluoroplastic was invented by DuPont. It can be melt-extruded and fabricated by conventional methods which allow more flexibility in manufacturing. Excellent dielectric and chemical resistance properties similar to PTFE. Use limited to temp between -54°C (-65°F) to a maximum of 150°C (300°F). It has a glossy surface and is transparent when in a thin section. FEP is widely used for its high ultraviolet light-transmitting ability.

Perfluoroalkoxy (PFA) is Similar to PTFE and FEP. Better properties than PTFE permits conventional thermoplastic molding and extrusion. Good flexibility for tubing purposes. Higher mech strength up to 260°C temperature.

Acrylonitrile Butadiene Styrene (ABS) is A family of engineered thermoplastics with a range of performance characteristics. Acrylonitrile imparts chemical resistance and rigidity to this thermoplastic. Butadiene endows it with impact strength and toughness. Styrene contributes to ease of processing. Good for hostile environments like esters, ketones, alcohols, and hydrocarbons up to 93°C (200°F).

Sulfone Polymers These are clear thermoplastics used in corrosive environments. Temp range up to 150°C (300°F). High resistance to acids, alkali, and salt solution. Not suited for ketones, chlorinated hydrocarbons, and aromatic hydrocarbons. Used a lot in flow meters and sight gauges

Forms of Sulfone Polymers

VITON – A fluoroelastomer compatible with many chemicals at varied temp ranges. Used for sealing valves, pumps, and instruments. Excellent for mineral acids, salt solutions, chlorinated hydrocarbons, and petroleum oils. The maximum temp limit is 120°C (250°F).

EPDM – Is a terpolymer elastomer made from the ethylene-propylene-diene monomer. Good abrasion and tear resistance. Excellent chemical resistance to a variety of weak acids and alkalies. Not recommended for applications involving petroleum oils, strong acids, or alkalies. The maximum temp limit is 100°C (212°F).

Nitrile BUNA-N- Nitrile rubber is a copolymer of butadiene and acrylonitrile. It has excellent elastomeric properties. It has excellent resistance to aliphatic hydrocarbons and aromatic solvents. Excellent material for valve seating. The maximum temp limit is 100°C (212°F).

HYPALON – This is a DuPont registered name for its elastomer of chlorosulfonated polyethylene. Used widely for valve seats and seals. The maximum temp limit is 100°C (212°F).

NEOPRENE – is a chlorinated synthetic rubber used primarily as a seating and sealing surface for valves. The maximum temp limit is 100°C (212°F).

NATURAL RUBBER – IS polymer isoprene with the highest molecular weight. Derived from Hevea (Rubber) tree. Used as the diaphragm and sealing material because of its elastomeric properties and resistance to abrasion. The maximum temperature limit is 100°C (212°F).

Thermoset Plastics as Piping Materials

Fiberglass Reinforced Plastic (FRP) is a highly valuable engineering material for piping and vessels (epoxy glass fiber). Very vast industrial use because of low initial cost & low maintenance. The broad range of chemical resistance. High strength-to-weight ratio. Ease of fabrication and flexibility in design. Good electrical insulation properties. It can be used up to a temperature of 150°C (300°F). Such epoxy piping is commonly used in oil, mining, and chemical industries. Sometimes used for steam condensate systems. Used in industrial cooling towers. Also used in Chlorine gas, chlorine water, and brine services.

Reinforced Thermosetting Resin Pipe (RTRP) Vinylester resin epoxy-based thermosetting resin material cured by free polymerization. Better tensile strength, elongation, and fatigue resistance. Material has excellent alkali resistance of the epoxy and acid and oxidation chemical resistance of the polymer. Used a lot in large bore seawater piping. Non-metallic piping material use is generally restricted to about 120°C (250°F). Check with vendors for specific applications for services and corrosion, sizes, fitting dimensions, jointing procedures, etc.

Typical Pipe Material Service checks

Caustic :

Always check NaOH or KOH concentration before material selection
Use Baume’s scale (Caustic Cracking Curve RP0403) for reference
The typical material is CS and PWHT is required
The typical valve trim is Monel
If PWHT is not mandatory, do not allow steam out

Amine:

Services include MEA (Mono Ethanolamine), DEA (Di Ethanolamine), DIPA (Di iso propanolamine), DGA (Di Glycolamine), etc.
The typical material is CS and PWHT is required
Fresh amines do not cause SCC; PWHT is not required, but if exposed to lean and rich amines, they require PWHT
Restrict fluid velocity to < 2 m/sec for CS
The preferred valve trim is SS 316
Use SS for higher velocity and temps

Wet H₂S:

If H₂S concentration in water is > wt% 50 to 75 ppm, it is susceptible to SCC
PWHT required and material hardness to be restricted to 200 BHN
NACE MR-0103 valves are not required but acceptable. Many use NACE valves as a norm
CS with 300 series SS trim is typical for valves
Always consult client/process for specific metallurgy requirements

Hydrogen (API 910/941):

Hydrogen service is defined as a combination of H₂ partial pressure and temp above the curve for CS per API 941
Use Nelson’s Curve for material selection limits
CS can be used up to 232°C
232° to 330°C use 1.25Cr or 2.25Cr/Mo steel or Duplex steel
330° to 400°C use SS 321
400°C use SS 347 or Alloy 800
For higher pressure ratings (Class 600 & higher) valve casting inspection by radiography is a must and may need to go for non-standard valves – long lead item!!!
Check for PWHT requirement for piping & NACE requirement for valve trim based on pp of Hydrogen and wet H2S concentration

Hydrofluoric Acid (HF) – Concentration > 1ppm:

Very critical and hazardous service
The typical material is CS
PWHT is normally required, restrict hardness to 200 BHN
Use special precaution during the purchase of piping components, shielding, and color coding
Strictly follow piping specifications

Chlorine (Dry and Liquid):

Follow Chlorine Institute guidelines for material selection and design. Leakage is hazardous
Strictly follow pipe specification requirements
Ball valves with soft seats are typically used
CS is normally used in liquid chlorine services
PWHT is normally not required
Exercise high precaution in purchase spec for piping components w.r.t. cleanliness, packing, and shipping requirements

Ammonia (Aqueous and Gaseous application):

In aqueous (Ammonia with water), CS is a commonly used material
Strictly follow pipe specification requirements
In gaseous service, low-temperature and cryogenic materials and codes shall be used
Do not use CS if the temp is < -29°C

Oxygen (For concentration > 31%):

CGA (Compressed Gas Association) standard shall be followed
Strictly follow pipe specification requirements, special notes for design, and material selection
Commonly used piping materials are CS, SS, and Monel
Velocity in a pipe is a critical factor for material selection
PTFE is the preferred material as gasket filler and for valve stem packing
Avoid threaded piping
Cleanliness requirement is very critical during material handling/shipping after fabrication

Common Metallic Piping Materials

The widely used Metallic Piping Materials are as follows:

The most widely used seamless carbon steel pipe for high-temperature service up to 427⁰C is A106-B.
The most widely used seamless low-temperature carbon steel pipe for a temperature up to -46⁰C is A333-6.
Common metallic pipeline materials for liquid and gas transport are API 5L, Gr B, X42, X52, and X60.

Cathodic Protection: Definition, Working Principles, Types, Design, Advantages, Applications

Written by Anup Kumar Dey in Engineering Materials,Maintenance,Mechanical,Pipeline

Cathodic Protection is an industrial technique for controlling metallic corrosion. Cathodic protection is commonly used on buried and submerged metallic structures like pipelines, underground storage tanks, locks, subsea equipment, offshore floaters, harbors, and ship hulls. Corrosion is an electrochemical process that operates as an electrical circuit. Corrosion occurs in the anode by oxidation and metal is lost; whereas in the cathode, protection occurs through the reduction reaction. In the cathodic protection technique, the concerned metal is converted into the cathode of the electrochemical corrosion cell to reduce corrosion. In this article, we will explore the basics of cathodic protection principles and industry practices.

Basics of Corrosion

What is Corrosion?

Corrosion can be defined as the degradation of a metal by its electrochemical reaction with a non-metallic matter such as oxygen, sulfur, etc. This occurs by forming an electrical circuit through the exchange of electrons. A corrosion cell consisting of an anode, cathode, electrolyte, and metallic connection between the anode and cathode is formed. So, for corrosion to occur, all the above four parts must be present which creates a closed circuit condition. At the surface of the metal that is exposed to the electrolyte, the electrochemical reaction occurs. The anode corrodes due to an oxidation reaction.

What is an Anode?

This is the most important part of the corrosion cell where corrosion occurs. The anode is defined as the point where the electricity is passed from the metal surface to the electrolyte by chemical means. This chemical reaction is characterized by the metal losing an electron and combining with another element, usually, oxygen is known as the oxidation reaction. In the case of steel, the resulting material is iron oxide popularly known as rust.

What is a Cathode?

The cathode is the second necessary part of the corrosion cell where protection occurs. The cathode can be defined as the point where electricity is passed from the electrolyte to the metal surface by chemical means. This is a reduction reaction characterized by the metal passing electrons to the electrolyte.

Depending upon the electrical potential compared to the other electrode, An electrode can behave either as an anode or as a cathode. This electrical potential difference is the driving electromotive force of the cell and is the voltage difference between the anode and the cathode. The electrode which is more electrically active, or more negative in voltage, undergoes the corrosion, so by definition is the anode. On the other hand, the electrode that is more noble (less negative in potential) passes electrons to the electrolyte (reduction reactions) and by definition is the cathode and does not undergo corrosion.

What is an Electrolyte?

The electrolyte, the third part of the corrosion cell, is the location where ions flow and can be any material in contact with both the anode and the cathode. The electrolyte allows oxidation and reduction reactions to occur and includes the source of atoms required for ion transfer to and from the anode and cathode.

Metallic Path: The final part of the corrosion cell is the metallic path that completes the circuit and allows the electrons to flow. It can be any metal in contact with both the anode and the cathode. For example, In the case of a tank or pipeline, the tank or pipe itself can act as the metallic path.

Examples of Corrosion cell — **Fig. 1: Examples of a Corrosion cell**

Effect of Corrosion

One Ampere of current can cause a loss of 9 Kg of Steel in One Year.

Even a small amount of Current discharge, 1 mA can result in 7 holes of ¼” diameter in a 2” steel pipe of standard thickness in 1 year time.

Corrosion can be mitigated by….

Cathodic Protection
Selection of Materials
Coatings

What is Cathodic Protection (CP)?

Cathodic protection is the use of DC Current from an External Source to oppose the discharge of corrosion current from anodic areas of the structure. It minimizes the potential difference between anode and cathode and in turn reduces corrosion. On the application of enough current, the whole pipeline or structure will be at one potential; thus, avoiding the creation of anode and cathode sites.

Principle of Cathodic Protection

The principle of the Cathodic Protection system is determining the anode in a large corrosion cell for making the intended material as cathode overcoming smaller corrosion cells. This can be achieved by any of the following two methods:

Galvanic Cathodic Protection:

This method first selects a more active metal from the galvanic series. Then the metal is installed in the electrolyte and a metallic path is provided. This method of cathodic protection is called sacrificial cathodic protection, or galvanic cathodic protection. In this method, a galvanically more active metal is installed to act as the anode which sacrifices itself and protects the pipe/structure working as a cathode.

Galvanic cathodic protection systems are fundamentally very simple. The anodes in sacrificial anode cathodic protection systems need to be inspected periodically and must be replaced when consumed. The simplest systems consist of the selection of an anode fabricated from an active metal (normally, zinc, aluminum, or magnesium). Then that sacrificial anode is directly connected to the structure exposing it to the same environment as the structure/ pipeline to be protected.

However, for buried pipelines, the anodes are not directly attached to the structure. They are evenly distributed at a shorter distance from the pipeline. Then the pipeline is connected to the anode using a wire normally through a test station.

Cathodic Protection with Galvanic Anodes:

Magnesium Anodes
Zinc Anodes
Aluminum Anodes

Advantages of Galvanic Anode Cathodic Protection System

Simple in Installation.
No External Power Source.
Very few operation or maintenance requirements.
No Power Bills.
Easy to Design.
No expensive accessories like cables etc.
Economical for small structures.

Limitations of Galvanic Anode Cathodic Protection System

Low Driving Voltage.
Poor performance due to passivation.
Limited Current. An extremely small current is available in higher-resistivity electrolytes.
Low life.

Typical Application of Galvanic anode System

Small Pipelines with good Coating.
Harbor Facilities, Steel piles, Jetties, etc.
Vessels, Tanks, etc.
Plant facilities and Equipment, Seawater intakes, Screens, Condensers, Heat Exchangers, etc.

Impressed current cathodic protection:

In the case of impressed current cathodic protection, a source of DC current is installed in the system which provides additional energy to force the current flow from an installed anode to the pipe or structural material making it a cathode. DC source can be a solar cell, rectifier, generator, battery, or some other DC power.

The anode material is selected considering the cost and weight loss per ampere year of current. Graphite, high silicon cast iron (HSCI), platinum, or mixed metal oxide are used as anodes for impressed current cathodic protection systems as they are slowly consumed. The anodes should be periodically inspected and replaced if consumed.

**Fig. 4: Impressed current cathodic protection system**

Cathodic Protection with Impressed current Anodes:

High silicon chromium cast iron Anodes
Mixed Metal Oxide Anodes
Graphite Anodes

Advantages of Impressed Current Cathodic Protection System

Current and Voltage can be varied.
Can be used in almost any resistivity Environment.
Can be designed for remote monitoring and control.
Can be designed for the measurement of Instant OFF / ON.
No limitation on driving Voltage.
Economically feasible to replace the anode system when required.
The system is extremely flexible.

Limitations of Impressed Current Cathodic Protection Systems

Regular monitoring and maintenance required
Requires Main supply or another source of electric Power
Interference Problems must be considered.
Relatively large chance of premature failure or breakdown.

Theory of Cathodic Protection

Steel in soil/water Anodic
Earth / Sea water Cathodic

As a result:- Steel loses electrons and hence corrosion

To reverse the above:

Make steel Cathodic
Seawater / Earth Anodic

And this is the theory of cathodic protection where the intended material is converted into a cathode.

How does Cathodic Protection Work?

Direct Current is forced to flow from an external source to the Structure. When the flow of this current is so adjusted to overpower corrosion current discharging from all anodic areas thereby providing complete Protection.

Criteria for Cathodic Protection

Steel in Soil: Pipe to Soil potential must be between –0.85 V to -1.2 V with respect to Cu/CuSo4 Reference Electrode.

Steel in Water: Pipe to Electrolyte potential must be between –0.8V to –1.10V with respect to Ag/AgCl Reference Electrode.

BASIS FOR CURRENT DENSITY….

SOIL RESISTIVITY

>1000 ohm-cm 10 mA/m2
100-1000 ohm-cm 20 mA/m2
<100 ohm-cm 35 mA/m2

WATER RESISTIVITY

>150 ohm-cm 50 mA/m2
50-150 ohm-cm 75 mA/m2
<50 ohm-cm 110 mA/m2

**Fig. 5: Typical Anode and Monitoring**

Cathodic Protection System Design

Prior to deciding the type of cathodic protection system and its design, certain preliminary data must be collected.

Data required for deciding a Cathodic Protection System

The following data are required for deciding on a cathodic protection system

Details of Structural Dimensions and drawings
Surface Coating Scheme
Details of Soil Strata / Terrain
Presence of Foreign Metallic Structures.
Details of cased crossings
History of corrosive areas
Stray current conditions
Operating Temperature
Availability of AC Power
Corrosion history of structures in the area
Electrolyte resistivity and pH survey report to understand corrosion rate

Planning a Cathodic Protection System

Factors that govern the Cathodic Protection System Design:

Choice of Cathodic Protection system
Amount of Total Current to achieve Cathodic Protection
No of Cathodic Protection Installations
- Spacing between them &
- Current Output of each Installation
Type of anodes and ground bed configuration
Any special conditions at certain locations needing modification of the general CP Plan
Location of CP Test station.

Selection of a Cathodic Protection System

The size of the structure, soil resistivity, and past corrosion history dictate the selection of a cathodic protection system. A current requirement test to protect the structure is performed before the selection. In general, when the soil resistivity is low (< 5000 ohm-centimeters) and the current density requirement is low, a sacrificial anode cathodic protection system is selected. However, for large structures with larger current requirements, a properly maintained impressed current system is used.

Cathodic Protection System Design Sequence

The following flowchart provides the design sequence for the cathodic protection system design sequences.

The below-mentioned steps are usually followed while designing a sacrificial anode cathodic protection system design:

Review soil resistivity
Select anode: For extremely corrosive soil (<2000 ohm-centimeters), Zinc is used. Otherwise, magnesium is used.
Calculating the net driving potential for anodes.
Finding the number of anodes needed to meet ground-bed resistance limitations.
Estimating the number of anodes for the system’s life expectancy.
Selecting the number of anodes to be used
Selecting the ground-bed layout
Calculation of the life-cycle cost for a proposed design.
Preparing plans and specifications

Similarly, the below-mentioned steps are followed for designing an impressive current cathodic protection system design:

Review soil resistivity
Review current requirement test
Select anode: Anode selection is based on cost consideration.
Estimating the number of anodes needed to satisfy the manufacturer’s current density limitations
Calculate the number of anodes needed to meet the design life requirements.
Calculate the number of anodes needed to meet maximum anode ground-bed resistance requirements.
A select number of anodes are to be used.
Select the area for placement of the anode bed.
Determine total circuit resistance.
Calculate anode ground-bed resistance.
Calculate ground-bed header cable resistance.
Calculate structure-to-electrolyte resistance.
Calculate total circuit resistance.
Calculate rectifier voltage.
Select a rectifier.
Calculate system cost.
Prepare plans and specifications.

Why do we need Cathodic Protection in Plants

Initial Investment for Petrochemical complexes, Fertilizer Plants, and Refineries are very high.
The corrosion problems are not detected until some leak appears.
Leakages can be extremely disastrous causing fatal accidents and great financial loss.
Increases maintenance cost of repairs of leakages.
The corrosion problem can also cause plants to shut down thereby losing production.
Cathodic Protection can be installed at the time of the erection of the plant at a very low cost.
The CP cost could be 5-6 times if it is installed after completion of the project as it involves a lot of excavation and restoration of structures.

Complexities of Plant Cathodic Protection

Scattered Underground Pipelines
- -Pipes in Parallel
- -Bifurcations
- -Closely grouped network
Different pipes could be of different materials and coatings.
The area of influence shall vary from pipe to pipe.
Heavy underground civil structure reinforcements
Earthing network.
Tank Bottoms

Corrosion Control for Above Grade Storage Tanks using CP system

The Tank bottom in contact with soil undergoes corrosion. This could lead to leakage, loss of product, and cause environmental hazards.
It is much simpler and economical to install a CP system during the construction stage.

Cathodic Protection for Pipelines

The main data required for deciding a Cathodic Protection System for pipelines are

Details of Structure Dimensions
Surface Coating Scheme
Details of Soil Strata / Terrain
Presence of Foreign Metallic Structures.
Details of cased crossings
History of corrosive areas
Stray current conditions
Operating Temperature
Availability of AC Power

Equipment used for Cathodic Protection System

Anodes

MMO Anodes
Silicon Iron Anodes
Graphite Anodes

The anodes are installed in deep well borehole ground beds.

TYPICAL DEEPWELL INSTALLATION WITH MMO ANODES — **Fig. 7: Typical Deepwell installation with MMO Anodes**

Solar Generators

In corrosion Protection Stations where the input power is not available, Solar generators are used to power the CP system.

Anode Junction Box (AJB)

The anode junction boxes are used to distribute the DC power from the T/R unit to the anodes at each CP station. The anode tail cables and the positive header (from T/R) are terminated inside the AJB. One circuit with suitably rated shunts for monitoring the current output of the anode string.

Negative Distribution Box (NDB)

The negative header cable (from T/R) and negative drain cable from the pipeline are terminated inside the NDB at each CP station. Each circuit shall have a variable resistor to control the current and suitably rated shunt to monitor the current.

Solid State Polarization Cell

Pipeline to be earthed at all overhead powerline crossings where the voltage is greater than 100 kV. Connected across the pipeline and the system earthing. Prevents the CP current drain from the structure to the system earthing. Shunts all fault currents and induced currents to the earth.

Surge Diverters

Surge Diverters are used across Isolating Joints Facilities. Provides surge protection to Isolating Joints in case of a lightning strike or other faults.

Monitoring of the Corrosion Protection System

For routine maintenance and monitoring of the effectiveness of the CP system, the following test facilities are provided along the pipeline.

POTENTIAL MEASUREMENT FACILITY: For measurement of the pipe-to-soil potential with respect to a portable reference cell. These facilities are provided at a regular interval of one facility every two kilometers
DRAIN POINT TEST FACILITY: At each CP station a drain point test facility is provided. Two permanent reference cells, two polarization coupons, and one corrosometer probe are used to measure the pipe-to-soil potential with respect to a permanent buried Cu/CuSO4 reference cell and the instant OFF potential from a buried coupon. Corrosometer allows monitoring of the corrosion rate and thereby the performance of the system. Permanent Reference cells are used to feed data for the SCADA system.
FOREIGN SERVICE CROSSING/PARALLEL FACILITY: To carry out interference testing at foreign pipeline crossing or foreign pipelines running parallel to SGP. To facilitate the mitigation of Interference effects

Cathodic Protection System Monitoring Frequency

Economic and safety considerations require close supervision and maintenance of all cathodic protection systems. Monitoring can be divided into three categories:

Monthly Monitoring
Quarterly Monitoring
Annual Monitoring

Monthly Monitoring:

Recording drain point potentials
Ground bed resistance
Anode current output
T/R output and T/R settings
Solar output settings

Quarterly Monitoring:

Recording Structure to Electrolyte Potential at the measurement location. (Only ON potentials are to be recorded)
Bonding Currents to be measured.
Isolation joints are to be tested using the Swing Test.
Reports to be generated.
Solar output settings

Close Interval Potential (CIP) and Direct Current Voltage Gradient (DCVG) Surveys

It is recommended that a Close Interval Potential Survey should be conducted within one year after commissioning the Corrosion Protection system. This should be repeated once every three to five years.
Based on the CIPS data, the DCVG survey should be conducted immediately at sites where under protection has been observed during the CIP survey. It is also recommended that a DCVG survey is conducted for the entire pipeline once every three to five years.

So, we can conclude that cathodic protection is a very useful and widely used method of steel protection. However, this method is costly and requires periodic maintenance and replacement. Click to know more about the Design of Cathodic Protection for Duplex Stainless Steel (DSS) Pipeline

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