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Basics for Stress Analysis of Underground Piping using Caesar II

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

What is a Buried Pipe/Pipeline?

Underground or buried piping (or Pipeline) is all piping that runs below grade. In every process industry, there will be some lines (Sewer or drainage system, Sanitary, and Storm Water lines, Firewater or drinking water lines, etc), part of which normally runs underground. However the term buried piping or underground piping, in a true sense, appears for the pipeline industry as miles of long pipe run carrying fluids will be there.

Analysis of Underground Piping System

Analyzing an underground pipeline is quite different from analyzing plant piping. Special problems are involved because of the unique characteristics of a pipeline, code requirements, and techniques required in the analysis. Elements of analysis include pipe movements, anchorage force, soil friction, lateral soil force, and soil pipe interaction.

Typical Buried Piping
Fig. 0 Typical Buried Piping

Why Pipeline Stress Analysis is different from Piping Stress Analysis?

To appreciate pipe code requirements and visualize problems involved in pipeline stress analysis, it is necessary to first distinguish a pipeline from plant piping. Unique characteristics of a pipeline include:

High allowable stress of Pipeline

A pipeline has a rather simple shape. It is circular and very often runs several miles before making a turn. Therefore, the stresses calculated are all based on simple static equilibrium formulas which are very reliable. Since stresses produced are predictable, the allowable stress used is considerably higher than that used in plant piping.

High-yield strength pipe

To raise the allowable, the first obstacle is yield strength. Although a pipeline operating beyond yield strength may not create structural integrity problems, it may cause undesirable excessive deformation and the possibility of strain follow-up. Therefore, a high test line with a very high yield to the ultimate strength ratio is normally used in pipeline construction. The yield strength in some pipes can be as high as 80 percent of ultimate strength. All allowable stresses are based only on yield strength.

High-pressure elongation (Bourdon Effect):

The movement of the pipeline is normally due to the expansion of a very long line at the low-temperature difference. Pressure elongation, negligible in the plant piping, contributes much of the total movement and must be included in the analysis.

Soil-pipe interaction

The main portion of a pipeline is buried underground. Any pipe movement has to overcome soil force, which can be divided into two categories: Friction force created from sliding and pressure force resulting from pushing. The major task of pipeline analysis is to investigate soil-pipe interaction which has never been a subject in plant piping analysis.

Low Design Temperature

Normally these lines do not have high design temperatures (of the order of 60 to 82 degrees centigrade) and only thermal stress checking is sufficient for the underground part. Common materials used for underground piping are Carbon Steel, Ductile iron, cast iron, Stainless Steel, and FRP/GRP.

In this article, I will try to explain the steps followed while analyzing such systems using Caesar II. However, this article does not cover the basic theory for analysis.

Inputs Required for U/G Pipe Stress Analysis

Before proceeding with the analysis of buried piping using Caesar II collect the following information from the related department

  1. Isometric drawings or GA drawings of the pipeline from the Piping layout Department.
  2. Line parameters (Temperature, Pressure, Material, Fluid Density, etc) from the Process Department.
  3. Soil Properties from the Civil Department.

Caesar II for Underground Piping Analysis

The CAESAR II underground pipe modeler is designed to simplify user input of buried pipe data. To achieve this objective the “Modeler” performs the following functions for analysts:

Allows the direct input of soil properties. The “Modeler” contains the equations for buried pipe stiffnesses that are outlined later in this report. These equations are used to calculate first the stiffnesses on a per length of pipe basis, and then generate the restraints that simulate the discrete buried pipe restraint.

Breaks down straight and curved lengths of pipe to locate soil restraints. CAESAR II uses a zone concept to break down straight and curved sections. Where transverse bearing is a concern (near bends, tees, and entry/exit points), soil restraints are located in close proximity, and where axial load dominates, soil restraints are spaced far apart.

Allows the direct input of user-defined soil stiffnesses on a per-length-of-pipe basis. Input parameters include axial, transverse, upward, and downward stiffnesses, as well as ultimate loads. Users can specify user-defined stiffnesses separately, or in conjunction with CAESAR II’s automatically generated soil stiffnesses.

Modeling steps followed in Caesar II

The modeling of buried piping is very easy if you have all the data at your hand. The following steps are followed for modeling:

From the isometric model, the line is in the same way as you follow in the case of an above-ground pipe model i.e, enter line properties in Caesar Spreadsheet, enter lengths by breaking the line into several nodes, or select an existing job for converting it into an underground model.

The analyst can start the Buried Pipe Modeler by selecting an existing job and then choosing Input-Underground from the CAESAR II Main Menu. The Modeler is designed to read a standard CAESAR II input data file that describes the basic layout of the piping system as if it was not buried. From this basic input, CAESAR II creates a second input data file that contains the buried pipe model. This second input file typically contains a much larger number of elements and restraints than the first job. The first job that serves as the “pattern” is termed the original job. The second file that contains the element mesh refinement and the buried pipe restraints is termed the buried job. CAESAR II names the buried job by appending a “B” to the name of the original job.

When the Buried Pipe Modeler is initially started up, the following screen appears:

Buried Piping
Fig. 1: Sample Caesar II Spreadsheet for Buried Piping

This spreadsheet is used to enter the buried element descriptions for the job. The buried element description spreadsheet serves several functions:

  • It allows the analyst to define which part of the piping system is buried. 
  • Allows the analyst to define mesh spacing at specific element ends. 
  • Allows the input of user-defined soil stiffnesses.

From/ To node

Any element of pipe in CAESAR II can be defined by two elements first is a start point and the second is an endpoint. In the buried pipe model, before conversion, the From/ To nodes remain the same as in the unburied model.

Soil model No

This column is used to define which of the elements in the model are buried. A nonzero entry in this column implies that the associated element is buried. A 1 in this column implies that the analyst wishes to enter user-defined stiffnesses, on a per-length-of-pipe basis, at this point in the model. These stiffnesses must follow in columns numbers 6 through 13. Any number greater than 1 in the soil model no. column points to a CAESAR II soil restraint model generated using the equations outlined later under Soil Models from analyst-entered soil data.

From/ To mesh type

A critical part of the modeling of an underground piping system is the proper definition of Zone 1 bearing regions. These regions primarily occur:

  • On either side of a change in direction
  • For all pipes framing into an intersection
  • At points where the pipe enters or leaves the soil
  • CAESAR II automatically puts a Zone 1 mesh gradient at each side of the pipe framing into an elbow. Note it is the analyst’s responsibility to tell CAESAR II where the other Zone 1 areas are located in the piping system.

User-defined stiffness & ultimate load

There are 13 columns in the spreadsheet. Columns 6 to 13 carry the user-defined soil stiffnesses and ultimate loads if the analyst defines soil model 1. The analyst has to enter lateral, axial, upward, and downward stiffnesses & loads.

Procedure

Select the original job and enter the buried pipe modeler. The original job must already exist and will serve as the basis for the new buried pipe model. The original model should only contain the basic geometry of the piping system to be buried. The modeler will remove any existing restraints (in the buried portion). Add any underground restraints to the buried model. Rename the buried job if the CAESAR II default name is not appropriate.

Enter the soil data using Soil Models.

Describe the sections of the piping system that are buried, and define any required fine mesh areas using the buried element data spreadsheet.

Convert the original model into the buried model by the activation of option Convert Input. This step produces a detailed description of the conversion.

Exit the Buried Pipe Modeler and return to the CAESAR II Main Menu. From here the analyst may perform the analysis of the buried pipe job.

The steps to create a soil model and other important considerations are covered in the following article: Underground Piping Stress Analysis Procedure using Caesar II. Click here to read that article.

Online Video Course on Buried/Underground Piping/Pipeline System Stress Analysis

If you need to join an online buried pipe stress analysis course using Caesar II to learn the above using a practical case study then click here to join the same.

Few more Resources for You..

Video Tutorial of Buried Piping Stress Analysis using Start-Prof
Basics for Stress Analysis of Underground Piping using Caesar II
What if Piping Continuation is Unknown? Part 2. Underground Piping
A short presentation on Various Analysis methods for Underground Piping Using Caesar II
Piping Stress Analysis Basics
Piping Design and Layout Basics

Process Design of Marine Systems

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

Refineries and petrochemical plants obtain feedstocks and dispatch products in a number of ways. The most common methods are by pipeline, road, rail, and sea.   A marine import/export facility is a very important item for a refinery or petrochemical plant.  Under-design of the marine facilities can result in process plant throughput being constrained.  Over-design can lead to a substantial increase in capital cost, which can have a significant effect on the overall economics of a Project.  

Whereas the majority of process unit operations are continuous processes, marine loading/unloading is always a batch operation.  It is therefore most important that all the requirements and activities associated with the start-up and shutdown of each type of loading/unloading operation are covered in the specification/design of marine facilities.  This article will assist process engineers with the design of facilities for importing feedstocks and exporting products by sea.  This document covers the key process design issues associated with the design/specification of marine systems, including jetties/berths, single-point moorings, and the associated loading systems.

Marine System design guidelines

The following guidelines can be followed during the design of a marine system

At an early stage of the design of a marine facility, an estimate of the throughput of each material to be handled and the capacities of tankers that will transport them must be agreed upon by all parties, as a starting point for the design work.  This information is fundamental to the design.

Initial Estimate of Number of Berths Required- When throughput/tanker size data has been agreed upon, it is recommended that an initial estimate is made of the number of berths required.

Types of Berth

Berths can be divided into the following categories:

  • Single Point Mooring (SPM): Feedstocks/products are pumped via sub-sea pipeline(s) from/to a tanker that has moored up to a floating buoy, located off-shore. 
  • Single Berth: One berth, either on a jetty or on the quayside, normally with multiple loading arms for one tanker to load/unload products/feedstocks
  • Multiple Berths, forming a harbor: A number of berths, normally on a jetty, enabling several tankers to load/unload products/feedstocks simultaneously
  • SPMs are favored when the maximum tanker size is such that dredging and/or a very long jetty structure would be required to provide sufficient draught i.e. when large tankers have to be accommodated.  Single berths are seen most often in deep water locations and in river estuaries.

Location of Shipping Tanks / Pumps

In some cases, a Client may not have finalized the location of the product (or feedstock) tank farm before the design work starts.  In such cases, it should be noted that pumping to/from tankers is generally at much higher flow rates than the rates at which feedstocks/products are consumed/produced by a process plant.  Hence the pumps/pipelines etc. serving marine systems are generally much larger than the pumps/pipelines connecting process plants to storage tanks.  For this reason, in cases where there is a significant distance between a process plant and the supporting marine facilities, it is more economic to locate the shipping tanks/pumps close to the marine facilities, rather than close to the process plant. 

Segregation of Feedstocks / Products

It is important to ensure that marine loading facilities are designed to segregate products/feedstocks. Segregation is best achieved by designing dedicated marine facilities for each material.  But this is often impractical or uneconomic.  At a multiproduct berth, it is normal for loading arms to be used for several similar products.  If economics dictate that pipelines must be multi-product (e.g. when the marine loading facility is a long way from the export tanks), flushing or pigging facilities (or a system to detect and slop the interface material) are required.

Types of Tankers/ Tanker Types in a Marine System

Broadly speaking tankers can be classified as either conventional, pressurized, refrigerated, or semi-refrigerated. For all tanker types, onshore pumps are used for loading, and onboard pumps are used for unloading.  Key features of these types of tanker are as follows:

A conventional tanker carries fluids with true vapor pressures of less than one atmosphere minus a suitable safety margin.  This margin tends to vary slightly between clients and carriers, but usually, the maximum vapor pressure is approximately 0.75 atm.  The tanker comprises a number of compartments that can be filled or emptied independently. Most tankers also have slop tanks, separate from the product tanks.  Tankers carrying highly flammable or oxidizable materials often have inerting facilities, by which exhaust gases from the tanker’s engines are passed into the vapor space in the tanks.  

Pressurized liquid gas tankers usually have fewer storage compartments than conventional tankers.  The smaller one might comprise a single-pressure container.  It is usual for pressurized liquid gas tankers to use a vapor connection for the transfer of displaced vapor at both the import and export locations.

Refrigerated gas tankers are usually bigger than pressurized tankers. They generally have more compartments and can withstand little pressure.  The refrigeration plant on board is usually designed to counter heat in leak only, hence only liquids that are fully cooled can be loaded. 

Semi-refrigerated tankers are sometimes used, and these combine the attributes of both pressurized and refrigerated tankers.

It should be noted that the maximum operating pressure of pressurized and semi-refrigerated tankers varies from vessel to vessel.  The metallurgy of refrigerated and semi-refrigerated tankers governs the minimum temperature allowed and hence the products that can be handled.

Marine System Tanker Capacities/ Capacities of tankers

It should be noted that when designing a terminal, the number and volume of feed/product tanks are required to depend to a great extent on the range of tanker capacities that have to be accommodated. The capacity of a tanker that carries material at ambient conditions is generally given in Dead Weight Tonnes (DWT). The capacity of a tanker that carries refrigerated or pressurized material is generally given as a volume. Typical ranges of tanker capacities are as follows:

Tanker Types vs Capacities
Fig. 1: Tanker Types vs Capacities

Tanker Loading and Unloading Rates:

 Typical loading/unloading rates for tankers are as follows:

Tanker Size vs Design flowrate
Fig. 2: Tanker Size vs Design flowrate

Even if the design loading/unloading rates for the specified tankers are appreciably less than the figures given above, it is recommended that these design loading/unloading rates are used (for the purposes of hydraulic design, but not for the purposes of calculating the berth occupancy) to give the marine terminal flexibility and provide a robust design.

Berth Turnaround Time for Marine System design

The Berth Turnaround Time is a term often used to describe the total elapsed time from the moment a tanker starts its approach to a berth, to the moment that the tanker is clear of the port area, thereby allowing another tanker to begin its approach. Realistic time intervals for a tanker using a berth or jetty are as follows: 

Berth Turnaround Time
Fig. 3: Berth Turnaround Time

Demurrage/ Berth Occupancy

Demurrage is a charge paid by the Refinery/Petrochemical plant operator to the ship operator/owner in the event of a tanker being delayed.  Examples of circumstances in which demurrage fees are payable include:

  • A tanker arrives at a marine facility on schedule, but there is no berth available for the tanker.
  • A tanker starts loading as scheduled, but for reasons outside the tanker captain’s control, the loading operation takes longer than the pre-agreed maximum time.

Demurrage fees are charged on a daily basis and can become a very significant operating cost. During the design of a marine facility, berth occupancies should be calculated for various scenarios, in order to reduce/minimize demurrage charges. In practice, it is not possible to avoid incurring some demurrage charges, without over-designing the marine facility. As a very rough guide, the following demurrage costs can be used, but the actual costs should be provided by the Client or a marine consultant.

Tanker Size vs Demurrage Charges
Fig. 4: Tanker Size vs Demurrage Charges

Single Point Mooring (SPM) Systems

SPMs provide a useful method of loading/unloading tankers from/to onshore facilities when it is not practicable to build a jetty.  Jetties tend to be impracticable in shallow water (e.g. in estuaries or on continental shelves) when loading/unloading deep-drafted vessels.  SPMs are also appropriate in deepwater locations.

The concept of the SPM is simple.  The tanker approaches the mooring from the down current.  The bow is made fast to the buoy.  With the main engines stopped, the tanker now hangs from the buoy with the bow into the current as required.  If there is a change of current (or wind) direction, the tanker swings freely around the buoy (known as weather vaning).

Floating hoses from the buoy are hoisted to the tanker’s manifolds and connected, thus permitting loading/unloading.  The buoy is connected by the subsea pipeline to the storage facilities.  The hoses have to be connected to the pipeline by a swivel so full 360º movement of the tanker can be accommodated without interfering with hose operation.

Jetty Topside Facilities

Jetty topside facilities generally include some/all of the following equipment items/systems:

  • Loading arms and associated hydraulic facilities
  • Surge alleviation systems 
  • MOVs
  • ESDs 
  • stripping pumps 
  • slops/drainage collection systems 
  • vapor return facilities 
  • VOC handling systems 
  • fire detection/protection equipment
  • operator shelter/control room

Marine Loading Arms

Loading arms are made up of several sections of pipe, connected by swivel joints.  The section on the shore side of the ‘apex’ of the loading arm is known as the inboard arm and the section on the tanker side of the ‘apex’ is known as the outboard section.  See the following Figure for a diagram of a loading arm.

marine Loading Arm
Fig. 5: Marine Loading Arm

Metering Systems

When loading/unloading tankers, accurate metering is normally a requirement, for both custodial and regulatory reasons.  Metering with sufficient accuracy can be achieved using either tank gauging or certain types of flowmeter.

Deballasting Facilities

Most tankers have to be partially filled with ballast water when they are not carrying cargo, in order to make the tankers seaworthy i.e. stable in rough weather.  Seawater is generally used as ballast water.

Surge

Marine loading/unloading systems are often designed to pump at high flow rates over large distances.  Hence surge pressures (eg due to the closure of a MOV or a pump trip) can be large.  It is most important that during detailed design, a surge analysis is carried out on each marine loading pipeline.  Peak surge pressures should be calculated for all likely scenarios, to check that the design pressure of pipework/equipment (plus an allowance for short-term over-pressure) is not exceeded, even in the worst case.

VOC Recovery/Disposal

When loading products that are stored at ambient conditions, it is common for the displaced vapor to be simply vented into the atmosphere, thereby emitting volatile organic compounds (VOCs) into the environment.  For very low vapor pressure products, the number of VOCs emitted by a tanker when loading is small if not negligible.  But for a high vapor pressure product, such as motor gasoline and naphtha, a significant amount of product can be lost to the atmosphere, such as VOCs.

Drainage at Births

It is common to have a curbed area at each berth, in order to catch any material which may leak from, for example, the couplings on loading arms.  The curbed area drains into a sump tank located below the berth on the jetty structure.  The sump tank includes pumps that return the drainings to a slop oil tank or a ballast water tank or alternative disposal onshore.

Fire Detection and Protection

Remote-controlled monitors and hydrants are commonly provided at each berth, to fight a fire and to protect equipment from the effects of the fire.  The fire may be at the berth/on the jetty or on the tanker at the berth.  It is normal practice for a jetty/berth to have water sprays/water curtains to protect the structure/equipment and to provide means of escape for personnel in case of fire. It is common to provide valved connections on a fire main in the vicinity of a jetty, to enable a fire tug to connect up and pump water into the system, as a standby.

Selection of Valves: Valve Selection Procedure

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

Valve selection is the process of choosing the appropriate valve for a specific application based on various factors such as the type of fluid or gas being handled, the operating conditions, and the required performance characteristics.

Valves are mechanical devices used to regulate the flow of fluids or gases in a piping system, and proper valve selection is crucial to ensure the safe and efficient operation of the system. The selection of a valve should take into consideration several factors, including the flow rate, pressure, temperature, and chemical compatibility of the fluid or gas being handled, as well as the required level of control or regulation.

The type of valve selected can also affect the performance of the system, as different valves have different characteristics, such as flow capacity, pressure drop, and shut-off capability. Common valve types include ball valves, globe valves, gate valves, butterfly valves, and check valves, each with its unique features and benefits.

In addition to considering the fluid or gas properties and operating conditions, valve selection may also involve evaluating factors such as cost, reliability, ease of maintenance, and compatibility with other system components. Working with a qualified valve supplier or engineer can help ensure that the appropriate valve is selected for the specific application and that it meets all necessary safety and performance standards.

Valves are important mechanical (sometimes electro-mechanical) devices that help in controlling the fluid flow through pipes or tubes. Selecting the right kind of economic valve for a specified fluid service requires many considerations. This article provides guidelines for the selection of normal-type valves. (Special-type valves are not covered in this article).

It should be recognized that Valve Selection is a very extensive and complex subject and it is not possible to select only one type based on this article. The valve selected to work under the same conditions will differ from one client to another. It is necessary to compare the cost and come to an agreement with the client.

Valves in Operating Plants
Fig. 1: Valves in Operating Plants

Criteria for Valve Selection

The selection of valve type requires an understanding of the following.

  • Valve Function
  • Service Characteristics
  • Cost of Valve

Valve Selection Based on Valve Function

There are two basic principles by which valves are constructed.

(1) The first principle is developed in three ways:

  • Moving the stopper by direct thrust onto the orifice seating.  This obturating movement is the basis of globe valves.
  • Rotating the stopper. This is the basis for plug valves or ball valves.
  • Sliding the stopper across the face of the orifice seating.  This is the basis of gate valves.

(2) The second principle, squeezing action, is the basis of all diaphragm valves.

These are shown in the following Figure (Fig. 2)

Squeezing Action for Diaphragm Type Valves
Fig. 2: Squeezing Action for Diaphragm Type Valves

 (3) Each of the four-valve motivations has its own advantages and disadvantages, as listed in the following table (Fig. 3).

Table comparing the valve features
Fig. 3: Table comparing the valve features

To summarize, there are four basic valve types, each type having its own individual characteristics and offering its own particular advantages or disadvantages.

For this reason, many modifications and variations of the basic type are made by valve manufacturers. An appraisal of the chief characteristics of the above four basic valve types is given in the following paragraphs

Globe Valve Selection Criteria

The globe valve is used for flow regulation or as a block valve where resistance to flow is not critical and positive closing action is required. They have a tortuous configuration which is typical of the closing method and results in higher resistance to flow compared with other valves.

Gate Valve Selection

Gate valves are used for on/off operation on hydrocarbon, general process, and utility services for all temperature ranges.

They have straight-through configurations which are typical of the sliding method, therefore, minimum resistance to flow.

The disadvantages are that they have the slowest action of all valves because the gate has to be moved a distance greater than the bore of the valve, which also means that the valve is of necessity bulky in height.

Selection of Plug valve

Plug valves have a quarter-turn operation, plugs are tapered or parallel plugs, and are suitable for most on-off clean processes and utility services, including non-abrasive slurries.

They have straight-through configurations typical of the sliding method.

The disadvantages of the basic plug cock are two-fold.

Firstly, its quick action can lead to a water hammer on hydraulic installation when it is closed too rapidly, and secondly, it is difficult to combine tight shut-off with ease of operation.

Basic Valve Types
Fig. 4: Typical Basic Valve Types

Ball Valve Selection Guidelines

A low torque quarter-turn valve, with a low resistance to flow, is suitable for many on-off utility and process services.

It has a straight-through configuration typical of the sliding method of closure.

Butterfly Valve Selection Procedure

A low torque, quarter-turn, rotary action valve with a straight-through flow configuration in which the disc is turned through 90 degrees, close to open position, in axial turning bearings and is used as a control or block valve. It is of compact design and may be obtained with or without flanges and linings. The seating arrangement may be soft (use of body lining, trapped ‘O’ ring, etc.) or metal to metal.

When to Select the Diaphragm Valve

Diaphragm valves have straight-through flow or weir configurations employing the flexing method and may be used for low-pressure chemical plant processes on-off or regulating the operation of most gases and liquids, e.g. slurries, viscous fluids, and fluids that are chemically aggressive.

The inherent disadvantage is that the diaphragm has to be made from an elastomeric material, usually either a natural or synthetic rubber, and this limits the application of the valve with respect to both temperature and pressure. Also, because of the stresses induced in the diaphragm, the valve has a shorter working life than other valve types. 

Typical Valve Types

Typical valve types for various operating functions are given in the following table.

Valve Selection Based on Function
Fig. 5: Valve Selection Based on Function

Valve Selection based on Service Characteristics

Valve selection is dependent on the characteristics of the services as follows.

  • Fluid type
  • Fluid characteristics
  • Pressure and temperature limitations
  • Operation and maintenance requirements

1. Fluid Type:

The fluid being handled should be classified as follows,

  • Liquid (including Two-Phase)
  • Gas
  • Steam
  • Slurry
  • Solids

2. Fluid Characteristics:

The characteristics and condition of fluids and slurries require careful identification since these are often the most significant factors in selecting the correct type of valve.

Clean fluids generally permit a wide choice of valve types, for dirty fluids the choice is often restricted and may require a specific type of valve.

The fluid is divided into the following characteristics.

(1) Clean Service

(2) Dirty Service

  • Dirty service involves fluids with suspended solids that may seriously impair the performance of the valve unless the correct type of design is selected. This type of service is often of major significance since many valves are very sensitive to the presence of solids. Dirty service is further classified as abrasive or sandy.
  • Abrasive Service is a term used to identify the presence of abrasive particulate found in piping systems and includes the presence of pipe rust, scale, welding slag, sand and grit which are damaging to many valves. These materials can damage seating surfaces and clog working clearances in valves often resulting in excessive force required to operate valves, sticking, jamming, and leakage through the valves.
  • Sandy service is a term identifying severe abrasive and erosive conditions and is used in oilfield production to identify the production of formation sand with reservoir crude oil or gas.

(3) Slurry Service

  • Slurry service involves liquids with substantial solids in suspension.
  • Slurries vary widely in nature and concentration of solids, and hard abrasive solids of high concentration can cause severe abrasion, erosion, and clogging of components.
  • Soft, non-abrasive solids can cause clogging of components.
  • In certain chemical processes, polymerization may block the cavities, preventing valve operation.

(4) Solids

  • There are many other conditions where solids may be present in the form of hard granules, crystals, soft fibers, or powders.
  • The transporting media may be liquid or gas. Air or fluidized bed systems may be used for some particulate. Specialized valves are available for many of these services but development work may sometimes be necessary.

(5) Hazardous & Flammable Service

  • Hazardous and flammable material is normally specified in Fire Service Law and/or the Client’s specifications.
  • Toxic substances such as chlorine, hydrofluoric acid, hydrogen sulfide, CO, phenol, etc.
  • Highly corrosive fluids such as acids and caustic alkalis.

(6) Corrosive Service

  • Corrosive service is a term generally used to identify clean or dirty fluids containing corrosive constituents that, depending on concentration, pressure, and temperature, may cause corrosion of metallic components. Corrosive fluids include sulfuric acid, acetic acid, hydrofluoric acid (HFA), wet acid gas (wet CO2), sour gas (wet H2S), and chlorides. Many chemicals are highly corrosive.
  • The choice of suitable corrosion-resistant materials for valve pressure retaining components (boy bonnet) and trim is necessary to avoid corrosion that can impair the integrity or performance of the valve.

(7) Viscous Service

  • Viscous service is a term that generally identifies a wide range of dirty or clean fluids with pronounced thickness and adhesive properties that, for the range of operating conditions(pressure, temperature, and flow) may require high operating torques and cause a sluggish response affecting seating. Fluids include high-viscosity oils (lube and heavy fuel oil) and non-newtonian fluids e.g. wax crude, gels, and pastes.
  • The choice of valves for viscous service can vary depending on fluid properties. Special attention should be given to the check valves where a sluggish response may cause operating difficulties and even hazardous conditions.

(8) Fouling Service

  • Fouling or scaling service involve liquids that form a deposit on surfaces. Such deposits may vary widely in nature, with varying hardness, the strength of adhesion, and rates of build-up. Valves for these services require careful selection, particularly where thick, hard, strongly adhesive coatings occur. The temperature of the fluid may be a vital factor and in some cases, valves may need tracing or be steam jacketed or of purged design.

(9) Solidifying Service

  • Solidifying service is a general term used to identify fluids that will change from liquid to solid unless maintained at the correct conditions of temperature, pressure, and flow. It is a term generally associated with fluids such as liquid sulfur and phthalic anhydride where valves of steam-jacketed design may be required or heavy fuel oil where valves often require tracing to maintain temperature and operability.

3. Pressure and Temperature Limitation:

  • Valves are normally allocated a rating according to the maximum operating pressure and temperature or the ratings of the piping system and flanges.
  • The temperature also limits the materials used in the valve construction, particularly the internals, trims, seals, rings, or lubricants.
  • These are normally defined in the “Piping Material Specification”.

4. Operation and Maintenance Requirements:

Operation and maintenance requirements can influence selection and design. Consideration should be given to:

  • Fire Resistance: There may be a requirement for valves having non-metallic (soft) seating to include ‘fire-safe’ features so that in the event of the soft seat and/or seals being damaged or destroyed by fire the valve will still be operational and any leakage will be within the acceptable limits laid down by a particular standard or specification.
  • Operability
  • Leak tightness (internal and external)
  • Maintainability
  • Storage and commissioning
  • Pipeline requirements (e.g. ability to pass cleaning pigs)

Valve Selection Considering the Cost of Valves

The costs of valve types cannot be simply compared because they vary depending on the countries where they are produced, vendors, and the balance of demand and supply.

In most cases, clients call for stress relieving, annealing, radiographic inspection, etc. of the valves, which affects the cost of the valves. Therefore, it is necessary to ask the Procurement Division to inquire about suitable vendors regarding these valves, based on the service conditions and the client’s requirements, after they are defined.

After a draft of the P&ID is issued, the personnel in charge of the preparation of the P&ID should cooperate closely with the piping design engineers in charge, during the in-house review period of the P&ID, and should determine the valve types and prepare piping material specifications in parallel.

  • Generally, gate valves are less expensive than globe valves of the same specification. However, the cost of 1-1/2″ or smaller-dia. malleable gate valves are higher than that globe valves.
  • Class 150 Gr.304SS TFE-seat 4” or smaller-dia. ball valves are less expensive than gate and globe valves.
  • Class 150 cast iron/malleable iron TFE-seat 2″-4″ ball valves are less expensive than gate and globe valves whose rating and material are the same.
  • In recent years, domestic vendors have developed and distributed light and inexpensive 2″ or smaller wafer-type ball valves, which should be positively adopted in chemical plants.
  • The cost of soft-seat butterfly valves, applied in the range of approximately 10 kg/cm and 80°C or lower, is the lowest compared with other types of valves.

Metal-seat butterfly valves could be used in the same applicable range as general gate valves if the same material is used for the metal seat. Also, as Class 600 and 900 butterfly valves have been developed and distributed, they should be positively applied to process lines. If the butterfly valves are throttled in use, care should be taken to avoid cavitation.

Few more Valve Related useful Resources for you…

A brief article on Valve Inspection & Testing
Ball Valve Design Features: A Literature
Details about control valves
An article on ROTARY SELECTOR VALVE (RSV) and MULTI PHASE FLOW METER (MPFM)
A short presentation on Control Valve Sizing
Routing Of Flare And Relief Valve Piping: An article
Nonstandard Valve Model for Pipe Stress Analysis: Angular, 3-way, 4-way, with Actuator
Modeling Relief Valve (Pressure Safety Valve) Thrust force

Valves are one of the important mechanical (sometimes electro-mechanical) device that helps in controlling the fluid flow through pipes or tubes. Selecting the right kind of economic valve for a specified fluid service requires many considerations. This article provides guidelines for the selection of normal type valves.

Guidelines for Sizing of Restriction Orifice for Single-phase Fluids

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

In our last article, we discussed restriction orifices in detail. The same article also lists the important factors on which the sizing of a restriction orifice depends. Click here to recapitulate those before proceeding with actual RO sizing steps.

Of many kinds of flow restriction devices, restriction orifices (RO) are frequently used, because they are simple and economical devices. RO is applied to regulate the flow rate or pressure. This article will provide the guidelines for the sizing of the Restriction Orifice. It should be noted that this standard practice is applicable to single-phase fluids only.

Inputs Required for Restriction Orifice Sizing

The following is a summary of input data to be prepared for the design of RO:

(1) Operating Conditions

  • Flow rate
  • Upstream & downstream pressure
  • Temperature (for vapor)
  • Line Size

(2) Physical properties

  • For liquid service: density, vapor pressure
  • For vapor service: molecular weight, Cp/Cv, Z-factor, viscosity

(3) Minimum allowable value of cavitation index for liquid service

Output from the Restriction Orifice Design

(1) Single orifice

  • Orifice diameter
  • The pressure at vena-contracta
  • Velocity at orifice
  • Calculated cavitation index (for liquid)
  • Critical or non-critical (for vapor)

(2) Multi-stage orifice

  • Required stage number
  • Orifice diameter of each orifice
  • Distance between adjacent orifice plates
  • Inlet and outlet pressures of each orifice
  • The pressure at vena-contracta of each orifice
  • Velocity at each orifice
  • Calculated cavitation index of each orifice

Principles of RO Sizing Calculation

Flow restriction orifice calculation for Gas Service

Critical Pressure Ratio:

The critical pressure ratio, rc can be obtained from the following equation.

Critical Pressure Ratio for RO Sizing

Orifice Diameter:

The equation for orifice diameter should be selected using equation (2), depending on whether the flow is critical or non-critical.

(A) Critical flow (sonic region)- When the ratio of downstream pressure to upstream pressure, r4, is smaller than or equal to the critical pressure ratio, rc, the following equation of orifice diameter for a critical flow should be used.

Orifice Diameter Equation for Critical Flow

(B) Non-critical flow (subsonic region)- The following equation of orifice diameter can be used for the non-critical flow region.

Orifice Diameter Equation for Non-Critical Flow

Restriction orifice calculation for Liquid Service:

Orifice Diameter:

Orifice Diameter Equation for Liquid Service

Cavitation Index

In order to avoid the cavitation problem, the minimum allowable value of the cavitation index, Kd, should be selected based on the following:

(1) Cavitation index Kd=0.37 shall be used for the usual case. At this critical cavitation condition, the noise is steady but still light. No erosion will occur. (Once the orifice chokes and supercavitation occurs, no damage by erosion will exist near the orifice. This is because the damage is caused by the collapse of the cavities and the collapse occurs far downstream during supercavitation)

(2) On some occasions such as in the following cases, use Kd=0.93 as an incipient cavitation condition in order to avoid severe economical risk.

  • Material is high grade such as stainless steel or higher and pipe size is larger than 12”.

Interval of Orifices

In the case of the multi-stage restriction orifice, the minimum distance, as shown below, should be provided between orifices, to avoid a reduction in RO performance.

  • For concentric orifice: L ≥ 5.4*D*(1-β)
  • For eccentric orifice: L ≥ D

Deflection Ratio of Eccentric Orifice

The deflection ratio of the eccentric orifice “e” is 0.75.

e = 2δ /(D − d0 ) = 0.75 (14)

Where,

  • δ: Pipe center to orifice center length (m)
  • d0: Orifice hole diameter (m)
  • D: Pipe inside diameter (m)
Image showing Deflection Ratio for Eccentric Orifice
Image showing Deflection Ratio for Eccentric Orifice

Restriction Orifice Plate Sizing Calculation Procedure

A. Single RO system-

① Decide the minimum allowable value of the cavitation index to meet a given situation.

  • Kd = 0.37: for the usual case
  • Kd = 0.93: for the conservative case

② Assume dO and calculate β by equation (8).

③ Incompressible coefficient of contraction CCi shall be calculated by equation (7).

④ Orifice diameter (dO) shall be calculated by equation (13).

⑤ Compare the assumed d0 with the calculated dO in step ④.

⑥ Calculated dO shall be replaced with the previous dO and repeat step ② to step ⑤ until the dO agrees with the calculated dO.

Calculate the cavitation index, Kd by equation (15), and compare it with the minimum allowable value.

If the cavitation index ≥ 0.37 (or 0.93), then the orifice diameter is acceptable.

If the cavitation index < 0.37 (or 0.93), then a single orifice is unable to accommodate the required pressure drop. In that case, a multi-stage orifice system should be applied.

B. Multistage Restriction Orifice Calculation

① Decide the minimum allowable value of the cavitation index to meet a given situation.

  • Kd = 0.37: for the usual case
  • Kd = 0.93: for the conservative case

First stage orifice: located at the outlet

② Assume upstream pressure of the first stage orifice.

③ Assume dO and calculate β by equation (8) for the first stage orifice.

④ Incompressible coefficient of contraction CCi shall be calculated by equation (7).

⑤ Orifice diameter (dO) shall be calculated by equation (13) using assumed upstream pressure.

⑥ Compare the assumed d0 with the calculated dO in step ⑤.

⑦ Calculated dO shall be replaced with the previous dO and repeat step ③ to step ⑥ until the dO agrees with the calculated dO.

⑧ Calculate cavitation index, Kd, using Equation (15), and compare with minimum allowable value.

If the cavitation index > 0.37 (or 0.93), increase the upstream pressure and repeat the steps from ③ to  ⑧.

If the cavitation index < 0.37 (or 0.93), decrease the upstream pressure and repeat the steps from ③ to ⑧.

If the cavitation index equals or slightly bigger than 0.37 (or 0.93), the design of the first stage RO is completed and go to step ⑨.

n-th stage orifice

⑨ Set the upstream pressure of (n-1)-th stage orifice for the downstream pressure of the n-th stage orifice.

⑩ Assume the upstream pressure of the n-th stage orifice.

⑪ Repeat the steps from ③ to ⑧, until the cavitation index is equivalent to the minimum allowable value.

Special Consideration of RO Design:

(1) The minimum hole diameter of RO-

To prevent the plugging problem with RO caused by debris, the hole diameter should be greater than the following values:

  • For the clean liquid service: 2mm
  • For the clean Gas service: 1mm

When a diameter smaller than the above values is required, the strainer or filter to remove debris should be provided upstream of RO.

(2) The necessity of minimum straight run length-

Basically, the objective of RO is rough control of the flow rate and should not be used for strict control of the flow rate. Therefore, it should not be necessary to take a straight run of piping both upstream and downstream of RO to keep performance. However, for erosional services such as slurry or flush services, countermeasures for erosion shall be considered.

(3) The calculated hole diameter of RO should be rounded to the conservative size for easy manufacturing.

Restriction Orifice Sizing Spreadsheet

A restriction orifice sizing spreadsheet is an electronic tool or software program that helps in calculating the size of a restriction orifice based on various parameters such as flow rate, fluid properties, pressure drop, and pipe dimensions. Often they may come in the form of restriction orifice calculation excel.

Restriction orifices are used in piping systems to control the flow rate of a fluid, and their proper sizing is crucial for the system’s safe and efficient operation. A restriction orifice sizing spreadsheet typically uses industry-standard equations and formulas to calculate the required orifice diameter and other necessary parameters.

The input parameters required for the calculation typically include fluid density, viscosity, pressure, temperature, pipe diameter, and the desired pressure drop across the restriction orifice. The spreadsheet then calculates the required orifice diameter and other related parameters such as Reynolds number, velocity, and flow rate.

The use of a restriction orifice sizing spreadsheet can simplify the orifice sizing process and reduce errors associated with manual calculations. However, it is essential to use reliable and accurate spreadsheet templates or software programs and to validate the results obtained from the spreadsheet against industry standards and guidelines.

Few More Handpicked Learning Resources for you…

The safe way to install restriction orifices
Level Gauges for Industrial Applications
Overview of Piping – Instrument Interface: An article
Basics of Piping Design and Layout
Piping Stress Analysis Basics
Piping Material Basics
Articles Related to Piping Interface Departments

Pipeline Virtual Anchor Length Calculation

Written by Anup Kumar Dey in Pipeline,Piping Interface,Piping Stress Analysis,Piping Stress Basics

What is Virtual Anchor Length?

A pipeline restrained by fixed anchors will experience a series of stresses including longitudinal, bending, and axial. Virtual anchor lengths are taken as the distance required for the frictional force provided by the soil surrounding the pipe to equal the forces applied by thermal/ pressure expansion and the soil’s resisting friction per unit length of pipe.

Anchors are placed strategically along the pipeline to help prevent movement. Unrestrained pipeline movement can cause damage to the connecting piping and equipment. The length of piping required to form the virtual anchor is known as the active length. It must be noted that the force required to fully restrain a pipe is not a function of its length. Factors that influence the required force to restrain the pipe include temperature, pressure, and percentage strain within the pipe.

Soil provides a constant frictional force along a buried steel pipeline. The magnitude of the frictional force depends on the burial depth, pipe weight, soil density, and coefficient of friction between the soil and steel. Therefore, starting from a free end, the total restraint exerted by the soil on the pipe gradually increases until it reaches the fully restrained load at the virtual anchor. At this point, the naturally occurring forces are balanced with a restraint point. Similarly, moving along the pipe away from the virtual anchor, the pipe expansion becomes gradually minimized until a point of zero expansion is reached indicating the pipe is fully held in place.

Axial expansion is calculated by taking the average of the full axial restraint at one end of the pipe and zero restraint at the opposite end of the pipe. When compared to above-ground piping, the total axial expansion at the free end of a buried pipeline is half of the calculated value for a similar scenario involving the above-ground pipe.

In reality, most pipes do not have a totally free end but have some resistance due to soil restraint as the pipe exits the ground and from the connection to above-ground piping. This acts to reduce the expansion at the ‘free’ end. Soils with lower friction resistance or pipes with less depth of cover have longer active lengths and thus have greater expansion at the free end.

Visual Representation of virtual Anchor Length
Fig. 1: Visual Representation of virtual Anchor Length

Virtual Anchor Length Calculation Sequence

Variables Required for Virtual Anchor Length Calculation

Pipe Properties

The following Pipe Parameters are required for Pipeline Virtual Anchor Length Calculation:

  • Do = Outside Diameter of Pipe
  • Di = Inside Diameter of Pipe
  • D = Mean Diameter of Pipe = Do – TNom
  • TNom = Nominal Wall Thickness of Pipe
  • TAc = Actual Thickness of Pipe =TNom -CA
  • v = Poisson’s Ratio
  • a = Coefficient of Thermal Expansion
  • ᵨP = Pipe Density
  • CA = Corrosion Allowance
  • E = Modulus of Elasticity of Pipe
  • ᵨF = Pipe Fluid Density
  • L = Length of Pipe
  • A = Cross Section Area of pipe
  • WP = Full Weight Pipe and its fluid.

Design & Other Conditions

Required Soil Parameters

  • µ = Coefficient of Friction between Pipe & Soil
  • ᵨS = Soil Density
  • H = Depth of Burial

Other Design Parameters

  • Sh = Hoop Stress
  • Sa = Axial Stress
  • e = Strain
  • FEx = Total Force Due to Expansion
  • FEx(T) = Expansion Force Due to Temperature Change
  • FEx(P) = Expansion Force Due to Pressure
  • FP = Force due to Pressure
  • FF = Frictional Force
  • La = Anchor Length

Virtual Anchor Length Calculation

Theoretically, there will be pipe movement from the entry point due to thermal expansion. Also, an expansion will be there due to the pressure. Opposing these two is the frictional force between pipe and soil. Let us find these factors first:

Expansion Force due to Temperature Change:

Expansion Force due to Change in Temperature will be:

FEx(T) = E × A×a × DT ………………………………..(1)

Expansion Force Due to Pressure
Friction Force at Soil-Pipe Interface

Few more Pipeline-related useful Resources for You.

Underground Piping Stress Analysis Procedure using Caesar II
Comparison between Piping and Pipeline Engineering
A Presentation on Pipelines – Material Selection in Oil & Gas Industry
Corrosion Protection for Offshore Pipelines
Start-up and Commissioning of the Pipeline: An Article
DESIGN OF CATHODIC PROTECTION FOR DUPLEX STAINLESS STEEL (DSS) PIPELINE
AN ARTICLE ON MICRO TUNNELING FOR PIPELINE INSTALLATION
A short presentation on OFFSHORE PIPELINE SYSTEMS: Part 1
Factors Affecting Line Sizing of Piping or Pipeline Systems

Pipe Stress Analysis from Water Hammer Loads

Written by Alex Matveev in Piping Stress Analysis,Piping Stress Basics,Process,Start-Prof

A water hammer is a hydraulic shock that occurs in a piping system when a fluid (usually water) is suddenly stopped or forced to change direction. This sudden change in the fluid’s momentum can create a pressure surge that travels through the piping system, causing a loud banging noise, and vibrations, and potentially damaging the system.

Water hammer occurs when a valve is closed too quickly, causing the fluid to stop abruptly, or when the flow direction of the fluid is suddenly changed. The pressure surge created by the water hammer can be many times greater than the normal operating pressure of the system, potentially causing pipes to burst or fittings to fail.

Water hammer can be prevented or minimized by designing the piping system with gradual changes in direction and using properly sized piping and fittings. Additionally, valves should be closed slowly to prevent sudden changes in the flow direction, and air vents or expansion tanks can be installed to absorb pressure surges.

Water hammer can also be controlled by using pressure relief valves or surge tanks to release excess pressure before it can cause damage to the system. It’s important to address the water hammer promptly to prevent damage to the piping system and ensure safe and efficient operation.

It is well-known that water hammer is caused in a piping system by sudden closure or opening of the valves or due to pump trips. During those events, the pressure increases to a huge extent and that causes an unbalanced load in the piping system which is known as the Water hammer load. All these loads are created due to water hammer acts in the changes in directions and can be of sufficient magnitude to cause the piping system failure. So, during the design phase, water hammer analysis must be performed to calculate the water hammer loads and design piping and support systems to avoid failures. In this article, we will discuss the pipe stress analysis methodology for water hammer loads.

In piping systems, sudden changes in flow cause pressure unbalanced forces between elbows, valves, tees, reducers, and other inline components. These forces damage the piping system when it is not properly designed to accommodate them.

The main advantage of using PASS/START-PROF + PASS/HYDRO SYSTEM is that the 3D piping model can be easily converted between these software packages both ways.

3D piping model from PASS/START-PROF can be converted into the 3D piping model in PASS/HYDRO SYSTEM, then generate 3D water hammer loads for all nodes at certain moments of time (not only just in one node!) and pass it directly into stress analysis software PASS/START-PROF automatically (not manually!).

PASS Suite START-PROF + HYDRO SYSTEM allows us to model it very easily, just in several clicks, and help to find a way to reduce water hammer damage to the piping system. Let’s study how to perform stress analysis of the piping system from water hammer loads.

The simple piping system created in PASS/START-PROF software is shown below screenshot:

Simple Stress System for Water hammer load explanation
Simple Stress System for Water hammer load explanation

After saving the PASS/START-PROF file, we can open it using the PASS/HYDRO SYSTEM software.

Importing the file in HYDROSYSTEM
Importing the file in HYDROSYSTEM

Set pressure 0.15 MPa at node 1, and 0.16 MPa at node 11

Inputting for Water hammer Analysis
Inputting for Water hammer Analysis

Choose water steam library

Choosing Object properties for Water Hammer Analysis
Choosing Object properties for Water Hammer Analysis

Define the period of surge analysis 2.5 sec, data output stem 0.01 sec, and dynamic load factor DLF=2.0.

Entering Dynamic parameters for Water hammer loads
Entering Dynamic parameters for Water hammer loads

Select Valve in node 12 and in the “Waterhammer” tab set that valve is immediately closed.

Setting the valve for Water Hammer Analysis
Setting the valve for Water Hammer Analysis

Run Isothermal Flow analysis

Calculating isothermal Flow
Calculating isothermal Flow

Run water hammer analysis

Running the Water Hammer Analysis

Running the Water Hammer Analysis

After that export unbalanced pressure forces into (.ctpf) file. Also, HYSROSYSTEM allows exporting data into (.frc) file that can be opened by CAESAR II.

Exporting Unbalanced pressure forces into FRC file
Exporting Unbalanced pressure forces into FRC file

(.ctpf) file contains forces multiplied by DLF factor for all nodes in piping models at several most dangerous moments of time. The most dangerous moments of time is found automatically by HYROSYSTEM software.

Force vs Time Plot for water hammer loads
Force vs Time Plot for water hammer loads

This is the (.ctpf) file structure

Structure of CPTF file
Structure of CPTF file

This means, that PASS/HYDROSYSTEM generates unbalanced pressure loads for all nodes in the piping system at a certain moment of time.

Water Hammers loads with respect to Time
Water Hammers loads with respect to Time

Now open PASS/START-PROF, open Operation Mode Editor and import waterhammer forces into first operating mode.

Importing water Hammer loads
Importing water Hammer loads

Eight additional force-based load cases are imported from the file

Creating additional load cases for water hammer analysis
Creating additional load cases for water hammer analysis

You can check the force values at any node properties

Water hammer loads at nodes
Water hammer loads at nodes

Now run piping stress analysis and see results

Deformed Shape at any moment of time

Deformed shapes due to water hammer loads
Deformed shapes due to water hammer loads

Stress table at any moment or time

Stress Table during water hammer analysis
Stress Table during water hammer analysis
Detailed stress value during water hammer analysis
Detailed stress value during water hammer analysis

And maximum stresses from all load cases (static and all moments of time during waterhammer)

Maximum stresses
Maximum stresses

The same table can be checked for displacements, support loads, flange leakage, expansion joint deformations, etc.

Output results
Output results

Watch the following video with detailed analysis process described above:

Video Tutorial for Pipe Stress Analysis from water hammer loads

Some more ready references for you…

Introduction to Pressure Surge Analysis
Water Hammer Basics in Pumps
Understanding Centrifugal Compressor Surge and Control
Stress Analysis using Start-Prof