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Pipeline Construction Stages

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

Pipeline Construction means laying the pipes to serve their intended purposes. There are two kinds of pipelines: Liquid and Gaseous. The construction of both pipelines is similar. The construction of large-scale cross-country pipelines involves a multitude of activities.

The construction of a pipeline can be compared with a moving assembly line. A large pipeline construction project is usually broken into manageable lengths called “spreads,”. Each spread is attacked at a time utilizing highly specialized and qualified workgroups. Each spread consists of various crews having their own responsibilities. As one crew finishes its work, the next crew takes its position to complete its piece of the pipeline construction process.

Pipeline construction steps usually take years to complete. Many surveys, studies, and plans are required to be completed before the construction of the pipeline starts. A comprehensive plan addressing the societal, developmental, environmental, and safety considerations are prepared to build the pipeline.

Pipeline Construction Steps

The construction of pipelines is a multi-step process. Many months prior to the actual pipeline construction phase, planning and surveys are conducted. Pipeline Construction steps can broadly be classified into the following three categories:

  • Pipeline Pre-Construction Activities
  • Pipeline Construction activities and
  • Pipeline Post-Construction Activities
Pipeline Construction Activities
Fig. 1: Pipeline Construction Activities

Pipeline Pre-Construction Activities

Pipeline pre-construction activities shall include the following:

  • Reconnaissance Survey
  • Detailed Engineering Survey
  • Permits and Clearance
  • Cadastral Survey
  • Acquisition of Right of Way (ROW)
  • Acquisition of Land for Repeater Stations and Block Valves
  • Approval of QA/QC procedure

Reconnaissance Survey

The main objectives of a Reconnaissance survey in pipeline construction are

  • To establish the pipeline route
  • To avoid populated areas, forest, and mining areas
  • To keep the number of crossings to a minimum
  • To ensure easy approachability to the ROW
  • The utilization of existing ROW if any.

The reconnaissance survey is carried out by the following activities:

Information from the reconnaissance survey include 

  • Route map in 1: 10,000,00 scale
  • Field data like altitude, terrain, cost of building materials
  • Type and number of important crossings

Detailed Engineering Survey for Pipeline Construction

Detailed engineering survey for pipeline construction involves the following activities:

  • Route Survey
  • Ground Profile Survey
  • Crossing Details
  • Collection of hydrological data
  • Collection of data on the type of terrain, soil, and crop pattern
  • Soil Resistivity Survey

Data available from detailed engineering survey include

  • Route maps in 1: 50,000 scale
  • Profile maps in 1: 25,000 scale
  • Crossing details in 1: 200 scale
  • Soil Resistivity Report

Permits and Clearances for Pipeline Construction

General Permits required for pipeline construction are:

  • Railways
  • Roads
  • Canals and drains
  • Panchayats
  • Other Agencies

Statutory Permits required for pipeline construction are:

  • Environment & Forest Clearance
  • State Pollution Control Board Clearance
  • Chief Controller of Explosives

Cadastral Survey for the pipeline construction route

A cadastral survey of the construction pipeline route involves the collection of the following:

  • Collection of village maps
  • Transfer of IPs/TPs and marking of 18 meters. width of ROW on the village maps
  • Identification of plots falling under the proposed 18 meters ROW marked for acquisition
  • Calculation of plot areas
  • Collection of land owner’s name and addresses
  • Collection of land rates, crop yield, and rates for different crops

Acquisition of ROW for pipeline construction

  • The positioning of land acquisition personnel and notification of Competent Authority
  • Preparation and Notification of land schedule
  • Serving Notice to landowners
  • Hearing objections from landowners
  • Preparation and notification of schedule 
  • Award and disbursement of compensation

Compensation

  • Land compensation @ of 10% of land cost
  • Crop compensation
    • Standing crops
    • Presumptive crops
  • Preparation of notification for termination operation in respect of the lands and notification of the same

Pipeline Construction Activities

Pipeline Construction Activities can be broadly classified as below:

Front End activities for pipeline construction:

The front-end activities for pipeline construction are listed below

  • Opening of ROW
  • Clearing and grading
  • Hauling and stringing
  • Trenching
  • Bending
  • Welding & Radiography
  • Joint coating
  • Lowering
  • Backfilling

Back End activities of pipeline construction:

The back-end pipeline construction activities are

  • Tie-ins
  • Crossings
  • Hydrotesting
  • Valves installation
  • Final cleanup and restoration
  • Installation of pipeline markers
  • Documentation 
  • Clearing and Grading Operation:
  • Stacking of ROW.
  • Marking of ROW boundaries.
  • Clearing of trees, bushes, farm crops, undergrowth, and routes, electrical and telephone poles falling within the 18 M width of ROW.
  • The grading of ROW is sufficient to be consistent with the maximum permitted pipe bending radius.
  • Providing ramps, diversion at road crossings, hume pipe culverts for maintaining water flow across the ROW.

Hauling and Stringing of pipes

  • All care shall be taken for transportation of the pipes from the coating yard to the ROW without damage to the coating and pipe.
  • Stringing shall be done in such a manner that pipes are easily accessible and shall not hinder the movement of equipment.
  • In rocky areas, pipe stringing shall be done after rock trenching.
  • Stringing shall not be done for more than 10 km ahead of trenching.
  • Pipes of special grades or wall thickness shall be strung at the required specific locations.

Trenching for laying pipelines

  • The pipeline shall be laid at a distance of 5 meters from one edge of the ROW.
  • Stacking of trench line.
  • The width of the trench shall be equal to the pipe diameter plus 400 mm.
  • The depth of the trench shall be equal to the diameter of the pipe plus 1 meter.
  • Extra width and depth shall be provided in rocky terrain.
  • Stripping of the topsoil up to 30 cm of the trench and storing separately.
  • Suitable crossings for the passage of men, equipment, cattle, etc. shall be provided.

Bending of pipes

  • The bending of the pipe is required to negotiate changes in the vertical and horizontal alignment of the pipeline.
  • The bending procedure has to be approved before bending pipes
  • Cold field bends shall only be used
  • The radius of bends shall be limited to 40 D for pipes up to 18” dia and to 60 D for pipes of 20” and above.
  • Welding seams are to be kept in the plane passing through the neutral axis of bending.
  • Tangents of a minimum of 2 M in length are to be left at both ends of the pipe.
  • Check for ovality, thinning, wrinkles, and buckles.

Welding of pipeline

  • Welding procedure specification has to be prepared for approval of the procedure and qualification of the welders as per API 1104.
  • Welding is done using a vertical down technique with cellulose-coated electrodes.
  • An internal line-up clamp shall be used for proper alignment of the joint.
  • Initial weld: Root/ stringer bead.
  • 2nd run: hot pass to reinforce root bead.
  • 3rd/ 4th run Fillers;
  • 5th: Capping 

Radiographic inspection of pipelines for pipeline construction

  • Radiographic inspection is carried out by using X-rays.
  • Visual inspection of all welds shall be carried out by a qualified welding inspector having minimum qualification of Level – II certification.
  • All joints at the following locations shall be radiographed.
    • Initial 1 km.
    • At cased road/rail, submerged crossings,
    • Tie-ins (including golden tie-ins)
    • Marshy areas.
    • Valves and insulating couplings
    • 20% of balance mainline joints(100% here)

Field Joint Coating

  • 250 mm on either side of the pipe is left uncoated in the coating yard to facilitate welding.
  • The width of the sleeve shall depend upon the cut-back length provided in the yard-coated pipe.
  • Heat-shrinkable sleeves are used for coating welded joints.

Joint Coating procedure

  • The pipe surface is sandblasted to SA-21/2 specification.
  • The sandblasted area is heated up to 600C and the epoxy primary is applied on the surface.
  • The sleeve is wrapped around and then shrunk on the joint using a propane/ LPG torch.
  • Air bubbles trapped are removed using hand rollers.
  • The integrity of the joint coating is tested by conducting a peel test.

Lowering of Pipeline in the trenches for Pipeline construction

  • The excavated trench should be free from excess earth, rock, hard clods, and other debris.
  • The coating of the pipe string shall be checked for damages by using a holiday detector.
  • Repair of coating damages.
  • Sand padding and rock shield are provided in rocky areas before lowering.

Backfilling of Pipeline

  • Backfilling shall be done immediately after lowering.
  • Backfilling shall be done with earth free of hard lumps, boulders, rock, etc.
  • Sand padding over the pipe shall be provided in rocky areas.
  • Slope breakers shall be provided in steep gradients to avoid the washout of the trench.

Pipeline Construction Tie-ins

  • Situations in the mainline such as rail/ road/ river crossings etc. may cause a break in the continuity of mainline laying operation and are normally bypassed by the mainline laying crew.
  • The process of connecting the unconnected sections of the pipeline is defined as a tie-in operation.

Crossings in Pipeline construction Route

Type of Crossings:

  • Open cut                             :Roads, cart tracks, and minor watercourses.
  • Cased                                    :Railways, National Highways, and State Highways
  • Submerged crossings     :Major rivers
  • HDD crossings                    :Perennial rivers and canals

HDD CROSSING:

HDD Crossing
Fig. 2: HDD Crossing

Hydrostatic Testing of Pipeline during Pipeline construction

Objectives for Hydrotesting       

  • To establish that the pipeline has the required strength for which it has been designed.
  • To demonstrate leak tightness of the pipeline.

Parameters for choosing test sections   

  • Availability of water
  • Suitable place for disposal
  • Ground profile
  • Logistics

Pipeline Hydro-Test procedure:

  • Air cleaning the pipeline to clear all debris and muck
  • Gauging
  • Water filling with corrosion inhibitor
  • Thermal stabilization
  • Pressurization
  • Evaluation and acceptance

Pipeline Valve Installation

  • Block valves are either Hand operated or Motor operated.
  • Mainline isolation valves are provided at an approximate interval of 25 to 35 km. depending upon the size of the line.
  • Isolation valves are provided on either side of major rivers.
  • Tapings for the pig signaler and pressure transmitters are provided at the valve locations for monitoring the pressure, temperature, and moment of the pig.

Final clean-up, Restoration, and  Installation of Markers

  • After construction, ROW is leveled and restored to the entire satisfaction of the landowners/ authorities.
  • All drains, utility lines, and water lines damaged during construction are restored to their original position.
  • Pipeline markers such as kilometer posts, turning points/ direction markers, warning signs, and boundary pillars are provided.
  • ROW is notified for closure.
  • Payment of crop compensation.

Documentation of Pipeline Construction Activities

All pipeline construction activities must be documented for future reference purposes. The following should be maintained

  • Daily logbook
  • Separate register for each activity
  • Pipe Book
  • Welding inspection report
  • Radiographic inspection report
  • Tie-in charts
  • Pipe damage register
  • Deviation/ NCR register
  • Equipment & manpower mobilization report
  • Hydrostatic testing register
  • Claims register
    • Due to deviations
    • Due to a change in the work plan
    • Damage to pipes etc.

Major Pipeline Construction Equipment

List of major equipment required for construction:                        

Major Pipeline Construction Equipment
Fig. 3: Major Pipeline Construction Equipment

Pipeline Post-Construction Activities

Post-construction Activities of pipelines construction shall include the following:

  • Caliper survey
  • Line preservation
  • Cathodic Protection
  • Commissioning 

Caliper survey:

  • A caliper survey establishes the geometric integrity of the pipeline constructed and helps to establish base data for future reference.
  • Helps to eliminate un-noticed pipe defects during construction. 

Preservation:

  • The constructed pipeline is preserved when there is a substantial lag between mechanical completion and actual commissioning.
  • Preservation is carried out by water filling the pipeline with suitable dosages of corrosion inhibitor.

Cathodic Protection:

Commissioning of Pipeline

After the mechanical completion of the pipeline, the line is commissioned after the completion of the following activities.

Understanding Centrifugal Compressor Surge and Control

Written by Vijay in Mechanical,Piping Interface,Process

Meaning of Centrifugal Compressor Surge

Centrifugal compressor surge is a characteristic behavior of the compressor that occurs in situations when inlet flow is reduced and the compressor head developed is so low that it can not overcome the pressure at the compressor discharge. During a centrifugal compressor surge situation, the compressor outlet pressure (and energy) reduce dramatically which causes a flow reversal within the compressor.

The surge in a centrifugal compressor is considered to be a very dangerous and detrimental phenomenon as it results in compressor vibration that results in the failure of the compressor parts. Compressor surge normally occurs in centrifugal and axial compressors. Compressor surge is a cyclic event and this results in high strain on compressor bearings, seals, and the impeller. The resulting severe vibration can lead to damage to the motor compressor coupling and the baseplate.

Ask a chemical or mechanical engineer, what a compressor surge does, and he would shudder merely thinking of the consequences. The centrifugal compressor is the heart of any oil & gas facility and for the last 100 years has been subjected to scrutiny as to what is the perfect control mechanism.

The Surge in a centrifugal compressor can be simply defined as a situation where a flow reversal from the discharge side back into the compressor casing occurs causing mechanical damage

What Causes Compressor Surge?

Various reasons could contribute to a centrifugal compressor surge. The reasons are multitude ranging from a

  • Driver failure,
  • Misdistribution of load in the compressor
  • Power failure,
  • Restrictions in the inlet and outlet of the system
  • Upset process conditions,
  • Inadvertent loss of speed
  • Startup & shutdown problems,
  • Failure of anti-surge mechanisms,
  • Check valve failure,
  • Very high rotation speed and insufficient flow
  • Mispositioning of rotor
  • Operator error, etc.

Consequences of Centrifugal Compressor Surge

The consequences of a compressor surge are more mechanical in nature whereby ball bearings, seals, thrust bearings, collar shafts, impellers, etc wear out and sometimes depending on how powerful are the compressor surge forces, cause fractures to the machinery parts due to excessive vibrations. Other bad consequences of the compressor surge are:

  • The flow reversal could cause process-related problems leading to plant shutdown.
  • As the hot compressed gas is returning at the inlet, it will result in an increase in the compressor inlet temperature.
  • As long as this surge will prevail, a large dynamic force will act on the compressor impeller and blade.

Here is an image, that shows the bearings being dislodged from their containment. The effects of the compressor surge are also contagious and due to excessive shaft vibrations, the gearbox connected between the compressor and the driver is also not spared at the bearings and gear teeth.

The power of a compressor surge is also proportional to the capacities (flow, power, pressure ratio) and even in the case of small turbo compressors, the gear teeth wear out when the impeller rotates in the opposite direction during a surge. 

The bottom line is: Always avoid a surge in compressors and other rotating equipment.

Compressor Surge Control

Compressor Surge Control using Anti-surge Valve (ASV)-Cold Gas Recycle

The chief protecting agent of a centrifugal compressor is the anti-surge line/valve that recycles cold gas from the discharge side cooler back to the suction scrubber to keep the operating point away from the surge line.

Compressor Surge Control: Hot Gas Recycle Valve

Although the anti-surge valve (ASV) is the chief protector, in brownfield projects, often the ASV becomes inadequate to deal with a compressor surge due to the addition of new compressors in parallel or series (e.g., booster compressors), change of plant piping or change of vapor composition. In these situations, a necessity arises to recycle more flow for which an additional ASV with quick opening characteristics is installed in parallel to the first ASV. When such solutions still fail to stop a compressor surge event from occurring, a hot gas recycles (a.k.a HGV) is used as a last resort. The second image below shows a gas compressor with hot gas recycles whose operating point moved away from the compressor surge line during an emergency shutdown.

In recent decades, with tools such as dynamic simulation, the quantity of hot gas to be recycled can be determined without recycling immoderate amounts of hot gas that can overheat the gas compressor with bearings and seals failing. Excessive hot gas recycling also shortens the efficacy of the lube oil that is used for lubrication purposes.

The Hot gas recycles valve is always to be used in tandem with the ASV and only during an emergency shutdown (ESD). A hot gas recycle/bypass system consists of piping with an On-Off Valve that is motor operated and can have a full opening time of < 1 sec (for valves between 4” to 16”). For larger On-Off Valves (above 16”), the time is taken to be < 2 sec. In the case of an electric motor-driven compressor, the power source for the motor-operated HGV must be independent lest, during a power failure, the motor-operated HGV becomes futile.

The hot gas piping should also be laid as short as possible between the discharge line and the suction line to have a fast response. During an ESD scenario (e.g., power loss), taking a conservative approach for design purposes, the control output signal from the compressor driver after a trip, takes ~300 milliseconds to reach the Distributed Control System (DCS) and another ~300 milliseconds from the DCS to reach the HGV to open. However, with advances in technology, these timings can be considered at ~100 milliseconds.

In simple terms, lower response time increases the chances of responding faster to a compressor surge.

Deviations from Design Criteria for Compressor surge control

As a thumb rule, the hot gas system is sized for 50% (max) during the FEED stage. However, this needs to be checked with a dynamic simulation study since over-sizing the Hot gas system can cause the compressor to overheat the bearings and seals. As per API 617 (7th Edition, 2002), Clause 2.7.1.3, it states,

As a design criteria, bearing metal temperatures shall not exceed 100°C (212°F) at specified operating conditions with a maximum inlet oil temperature of 50°C (120°F). Vendors shall provide bearing temperature alarm and shutdown limits on the datasheets.

However, clause No. 2.7.1.3.1 of the said document also says,

In the event that the above design criteria cannot be met, purchaser and vendor shall mutually agree on acceptable bearing metal temperatures.

In reality, the Author has seen cases, where this deviation was taken up to ~135 deg.C depending on the manufacturer and believes that this is due to a variety of operating conditions between string test conditions and actual conditions.

Nevertheless, compressor operating temperatures must never exceed the stipulated or mutually agreed values in order to protect the compressor’s internals.

Compressor Surge Control Systems

In today’s world, no piece of machinery can be said to be protected by modern methods without implementing a control system. A compressor surge can occur in a matter of seconds or sometimes even milliseconds giving almost no time for operators to intervene. Hence a control system becomes a part and parcel of the centrifugal compressor package.

Although the good old Proportional-Integral-Derivative (PID) control was enough to avoid a compressor surge by minimizing the compressor recycle flow, it did not aid much in reducing/optimizing the power requirements. With a steady rise in oil consumption since the 1970s, the necessity of energy efficiency, safety, and environmental friendliness became a priority and demanded better control systems. To respond quickly to any process upsets, high computational speeds in controllers also became a necessity. This led to the rise of specialized control equipment known as ‘Black Boxes’ that was the alternative to panel-mounted instruments. Black boxes though addressed response times, suffered from frequent hardware and software revisions. Black box technology was proprietary with its own coding languages and often experienced compatibility issues when interfacing between different manufacturers’ models. This also meant having to sometimes shut down the machinery causing monetary implications and increased downtime if not made part of plant maintenance.

The Advent of a Programmable Logic Controller (PLC) for compressor surge control

With the limitations of using black box technology being recognized, industry honchos realized the necessity of standardizing and generalizing control systems and their respective programming languages. These standardization efforts led to documenting the IEC 61131 (International Electrotechnical Commission Standard for Programmable Controllers) in 1993 and subsequently revised in 2003.

Programmable Logic Controllers (PLCs) provided not only computational power but also were easily integrateable into the compressor controls. PLCs offered the advantage of scalability where new I/O could be added during any form of plant modification/expansion depending on the type of PLC used (e.g., modular or stacked). PLCs also offer Diagnostics capabilities, for example, to trace through the logs of controller output/data during fault analysis.

In earlier systems that depended on the black box principle, a primary PLC is supplemented with an auxiliary PLC that controlled systems like lube oil, seal oil / dry gas seals, startup sequencing, interlocks, etc. This also required interfacing them properly to allow operators to diagnose and do a root cause analysis in the event of, for example, a compressor trip. However, with integrated systems, that used a dedicated control PLC with a backup PLC and the necessary hard wiring, the cost of implementation also comes down while offering better efficiency, diagnostics, generic parts, and scalability.

Some more ready references for you

Introduction to Pressure Surge Analysis
Water Hammer Basics in Pumps
Pipe Stress Analysis from Water Hammer Loads

Ref: [1], [2], [3], [4], [5], [6]

Understand the Harmonic Analysis and Ensure the Pressure pulsation comply with API 674

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

Harmonic analysis is the dynamic analysis used to predict the steady-state dynamic response of the piping system subjected to sinusoidally varying loads. All kinds of externally applied loads like nodal, elemental, gravity, and thermal loads can be included as load input. Accordingly, load cases are required to prepare and included in the solution. Load components in each load case use the same factor and phase angle. Different load cases may have different factors and phase angles, but the frequency for all loads is the same. The points that will be covered in this article are:

  • Introduction
  • Reviewing the Static Model
  • Creating the Harmonic input
  • The Harmonic Analysis
  • Result Review
  • Acoustic vibration and resonance are caused by Positive displacement pumps

Introduction to Harmonic Analysis

The harmonic analysis considers the effect of a harmonic load being applied to the system. The load is usually applied as a sinusoidal function (Fig. 1), e.g. pressure pulsation from reciprocating equipment. Being of a cyclic nature, harmonic loading relates to the fatigue allowable of the design code and should be considered. Care should be taken when undertaking a harmonic analysis for the accuracy of input data. Information relating to existing field problems can be derived from the measurement of pressure pulsation, deflection, forces, etc.

Reviewing the Caesar II Static Model for Harmonic Analysis

Introduction to Harmonic Analysis
Fig.1: Introduction to Harmonic Analysis

Creating harmonic Input for Analysis in Caesar II

Follow the steps shown in Fig. 2

Steps for creating harmonic input
Fig. 2: Steps for creating harmonic input

The effect of damping on the response

Effect of Damping
Fig. 3: Effect of Damping

The Frequency-phase Dialog

The frequency phase dialog
Fig. 4: The frequency phase dialog

Harmonic Analysis Results Review

Results-Displacements:

Nodal Movements in Harmonic Analysis
Fig. 5: Nodal Movements in Harmonic Analysis

Results-Loads:

Loads in Harmonic Analysis
Fig. 6: Loads in Harmonic Analysis

Results-Stress:

Stress Window of Harmonic Analysis in Caesar II
Fig.7: Stress Window of Harmonic Analysis in Caesar II
Occasional Stress
Fig. 8: Occasional Stress
Fatigue Approach
Fig. 9: Fatigue Approach
Updated Stress Results
Fig. 10: Updated Stress Results
Caesar II Steps for adding Fatigue data
Fig. 11: Caesar II Steps for adding Fatigue data
Updated Caesar II output result with Fatigue Data
Fig. 12: Updated Caesar II output result with Fatigue Data

Author: This presentation is prepared by Mr. Deepak Sethia who is working in ImageGrafix Software FZCO, the Hexagon CAS Global Network Partner in the Middle East and Egypt. He has extensive experience in using Caesar II software and troubleshooting.

Steel Pipes Used in Process Industries

Written by Anup Kumar Dey in Engineering Materials

Steel pipes are widely used in the process-piping industries. Various grades of steel pipes are available in processing plants. In this article, we will discuss some of those in brief.

Carbon Steel Pipes

(Temperature Range -29 degree centigrade(C) to 427 degrees C) 

This is the most common and cheapest material used in process plants. Carbon steels are used in most general refinery applications. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. Carbon steels are extensively used in a temperature range of (-) 29 degrees centigrade to 4270 centigrade  

Low-Temperature Carbon steel (LTCS) pipes

(LTCS-Temp range -45.5 degrees C to 427 degrees C)

LTCS can be used at a low temperature of (- 45.5) degrees centigrade.   Killed Carbon Steel is defined as those which are thoroughly deoxidized during the melting process. Deoxidation is accomplished by the use of silicon, manganese, and aluminum additions to combine with dissolved gases, usually, oxygen, during steelmaking. This results in cleaner, better-quality steel which has fewer gas pockets and inclusions. Killed carbon steel is specified for major equipment in the following services to minimize the possibility or extent of hydrogen blistering and  hydrogen embrittlement:    

  • where hydrogen is a major component in the process stream.
  • where hydrogen sulfide H2S is present with an aqueous phase or where liquid water containing H2S  is present.
  • Process streams containing any amount of Hydrofluoric acid (HF), boron trifluoride (BF3), or  (BF) compounds; or
  • Monoethanolamine (MEA) and diethanolamine (DEA) in solutions of greater than 5 weight percent.

If by mistake carbon steel pipe is installed in place of LTCS, then click here to know what you can do to accept that.

Low Alloy Steel Pipe

(Temperature range -29 degrees C to 593 degrees C)

Low Alloy Steel pipes contain one or more alloying elements to improve the mechanical or corrosion-resisting properties of carbon steel. Nickel increases toughness and improves low-temperature properties & corrosion resistance. Chromium and silicon improve hardness, abrasion resistance, corrosion resistance, and resistance to oxidation. Molybdenum provides strength at elevated temperatures. Some of the low alloy steels are listed below.  

Carbon   1/2% Moly and Manganese   1/2% Moly:

These low alloy steels are used for higher temperature services and most frequently for intermediate temperatures for their resistance to hydrogen attack. They have the same maximum temperature limitation as killed steel (ASME Code   1000 deg. F) but the strength above 700 deg.F is substantially greater.

1% chrome   1/2% Moly and 1 1/4% Chrome   1/2% Moly:

These alloys are used for higher resistance to hydrogen attack and sulfur corrosion. They are also used for services where temperatures are above the rated temperature for C   1/2 Mo steel.

2 1/4 Chrome 1% Moly and 3% chrome   1% Moly: 

These alloys have the same uses as 1 1/4% Cr but have greater resistance to hydrogen attack and higher strength at elevated temperatures.

5% chrome   1/2% Moly:

This alloy is used most frequently for protection against combined sulfur attacks at temperatures above 550 deg.F. Its resistance to hydrogen attack is better than 2 1/4% Cr_ 1% Moly.

9% Chrome 1% Moly:

This alloy is generally limited to heater tubes. It has a higher resistance to high sulfur stocks at elevated temperatures. It also has a maximum allowable metal temperature in oxidizing atmospheres.

Stainless Steel Pipes

(Temperature range -257 degrees C to 538 degrees C)

They are heat & corrosion-resistant, noncontaminating, and easily fabricated into complex shapes. There are three groups of Stainless steel.

  1. Martensitic Stainless Steel
  2. Ferritic Stainless Steel and
  3. Austenitic Stainless Steel

Martensitic stainless steel pipes

Martensitic alloys contain 12-20 percent chromium with a controlled amount of carbon and other additives. Type 410 is a typical member of this group. These alloys can be hardened by heat treatment, which can increase tensile strength. Corrosion resistance is inferior to Austenitic Stainless steel and these are generally used in mildly corrosive environments.

Ferritic stainless steel pipes

Ferritic steels contain 15-30 percent chromium with low carbon content(0.1 percent). The higher chromium content improves its corrosion resistance. A typical member of this group is Type 430. The strength of these can be increased by cold working but not by heat treatment. Type 430 is widely used in nitric acid plants. In addition, it is very resistant to scaling and high temp oxidation up to 800-degree cent.

Austenitic stainless steel pipes

Austenitic steels are the most corrosion-resistant of the three groups. These steels contain 16-26 percent of chromium and 6-22 percent nickel. Carbon is kept low (0.08 percent max) to minimize carbide precipitation. Welding may cause chromium carbide precipitation, which depletes the alloy of some chromium and lowers its corrosion resistance in some specific environments, notably nitric acid. The carbide precipitation can be eliminated by heat treatment(solution annealing). To avoid precipitation special steels stabilized with titanium, niobium, or tantalum have been developed (Types 321,347 & 348). Another approach to the problem is the use of low-carbon stainless steel such as types 304L & 316L with .03 percent max carbon.

The addition of molybdenum to austenitic alloy (types 316, 316L) provides generally better corrosion resistance and improved resistance to pitting.   The chromium-nickel steels, particularly the 18-8 alloys, perform best under oxidizing conditions since the resistance depends on an oxide film on the surface of the alloy. Reducing conditions and chloride ions destroy this and bring on the rapid attack. Chloride ions tend to cause pitting and crevice corrosion. When combined with high tensile stresses they can cause stress-corrosion cracking.

Click here to learn about the differences between carbon steel and stainless steel

A detailed list of commonly used steels in hydrocarbon industries is given in the following  table:

ASTM Designated Common Ferrous Piping Materials
ASTM Designated Common Ferrous Piping Materials
Ferrous Piping Materials
Ferrous Piping Materials

Click here to know about Common Non-Ferrous Materials used in Process Piping Industry

Procedure to Restore the Corrupted Caesar II File

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

Those who are extensively using Caesar II software must have noted that some Caesar II files got corrupted due to some reason. And modeling the same file from isometric again is time consuming. At the same time, the man-hour used is lost without any fruitful result. The same happened to me yesterday. Unconsciously I deleted the required file and I was a bit worried as I had to redo the modeling again.

In such a situation, you can easily restore the complete Caesar model without much pain. This write-up will try to explain the method of restoring the Caesar II file which is corrupted or deleted by mistake. However, this will only work if you have performed the run function at least once. The load cases that you made will be lost and you have to make new load cases for the analysis. And I feel that’s better as making load cases does not take much time.

Whenever you prepare any Caesar file and then run the file for analysis a backup file of the stress system is automatically generated in the PC. Later that backup file can be used to restore the required Caesar file again. The steps are as follows:

1. Click the open button on Caesar II and you will get the following screen.

open file

2. Now click on the System button on the right side as shown in the above picture.

3. After that page up button on top and you will get the following screen.

restore

4. Now Click on the backup file as shown in the above figure and choose your file. You must remember the file name to restore the same. Choose the latest file as per the date and time to get the most updated information.

5. The Caesar file will be restored for you. Now save that file to the location where you want that to be and make the load cases as per your requirement for analysis. Hope this helps you to resolve a few of your problems and save man hours.

Pumps & Pumping Systems

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

What are Pumping Systems?

Pumping systems account for nearly 20% of the world’s electrical energy demand. Furthermore, they range between 25-50% of the energy usage in certain industrial plant operations. The use of pumping systems is widespread. They provide domestic, commercial, and agricultural services. In addition, they provide municipal water and wastewater services, and industrial services for food processing, chemical, petrochemical, pharmaceutical, and mechanical industries.

The function of a Pump

Pumps have two main purposes:

  • Transfer of liquid from one place to another place (e.g. water from an underground aquifer into a water storage tank)
  • Circulate liquid around a system (e.g. cooling water or lubricants through machines and equipment)

Components of the Pumping System

The main components of a pumping system are:

  • Pumps (different types of pumps are explained in section 2)
  • Prime movers: electric motors, diesel engines, or air system
  • The piping used to carry the fluid
  • Valves used to control the flow in the system
  • Other fittings, controls, and instrumentation
  • End-use equipment, which has different requirements (e.g. pressure, flow) and therefore determines the pumping system components and configuration. Examples include heat exchangers, tanks, and hydraulic machines

Pumping System Characteristics

The pressure is needed to pump the liquid through the system at a certain rate. This pressure has to be high enough to overcome the resistance of the system, which is also called the “head”. The total head is the sum of the static head and friction head.

Static head

Static head is the difference in height between the source and destination of the pumped liquid (see Fig. 1)

Static head is independent of the flow (see Fig. 1)

The static head consists of:

  • Static suction head (hS): resulting from lifting the liquid relative to the pump centerline. The hS is positive if the liquid level is above the pump centerline, and negative if the liquid level is below the pump centerline (also called “suction lift)
  • Static discharge head (hd): the vertical distance between the pump centerline and the surface of the liquid in the destination tank

The static head at a certain pressure depends on the weight of the liquid and can be calculated with this equation as shown in Fig. 1:

Pumping System and its Characteristics
Fig. 1: Pumping System and its Characteristics

Friction head

  • This is the loss needed to overcome that is caused by the resistance to flow in the pipe and fittings.
  • It is dependent on size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid.
  • The friction head is proportional to the square of the flow rate as shown in Fig. 2.
  • A closed-loop circulating system only exhibits a friction head (i.e. not a static head).

In most cases, the total head of a system is a combination of static head and friction head as shown in Fig. 2. The left figure is a system with a high static head (i.e. the destination reservoir is much higher than the source). The right figure is a system with a low static head (i.e. the destination reservoir is not much higher than the source).

Friction Head and Performance curve
Fig. 2: Friction Head and Performance curve

Pump performance curve

The head and flow rate determine the performance of a pump, which is graphically shown in Fig. 2 as the pump performance curve or pump characteristic curve.

Fig. 2 (Top Left) shows a typical curve of a centrifugal pump where the head gradually decreases with increasing flow.

As the resistance of a system increases, the head will also increase. This, in turn, causes the flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a short period without causing the pump to burn out.

Pump operating point

The rate of flow at a certain head is called the duty point. The pump performance curve is made up of many duty points.

The pump operating point is determined by the intersection of the system curve and the pump curve as shown in Fig. 3

The Best Efficiency Point (BEP) is the pumping capacity at maximum impeller diameter, in other words, at which the efficiency of the pump is highest. All points to the right or left of the BEP have a lower efficiency.

Pump Suction Performance

Pump Cavitation or vaporization is the formation of bubbles inside the pump. This may occur when the fluid’s local static pressure becomes lower than the liquid’s vapor pressure (at the actual temperature). A possible cause is when the fluid accelerates in a control valve or around a pump impeller.

Vaporization itself does not cause any damage. However, when the velocity is decreased and pressure increases, the vapor will evaporate and collapse. This has three undesirable effects:

  1. Erosion of vane surfaces, especially when pumping water-based liquids
  2. Increase of noise and vibration, resulting in shorter seal and bearing life
  3. Partially choking the impeller passages, which reduces the pump performance and can lead to loss of total head in extreme cases.

The Net Positive Suction Head Available (NPSHA) indicates how much the pump suction exceeds the liquid-vapor pressure, and is a characteristic of the system design.

The NPSH Required (NPSHR) is the pump suction needed to avoid cavitation and is a characteristic of the pump design.

Pump Operating point and Pump classification
Fig. 3: Pump Operating point and Pump classification

Type of pumps

  • Pumps come in a variety of sizes for a wide range of applications. They can be classified according to their basic operating principle as dynamic or positive displacement pumps
  • In principle, any liquid can be handled by any of the pump designs.
  • The centrifugal pump is generally the most economical but less efficient.
  • Positive displacement pumps are generally more efficient than centrifugal pumps but have higher maintenance costs. Click here to know the differences between a centrifugal pump and a positive displacement pump.

Positive displacement pumps are distinguished by the way they operate: liquid is taken from one end and positively discharged at the other end for every revolution.

In all positive displacement type pumps, a fixed quantity of liquid is pumped after each revolution. So if the delivery pipe is blocked, the pressure rises to a very high value, which can damage the pump.

Positive displacement pumps are widely used for pumping fluids other than water, mostly viscous fluids.

Positive displacement pumps are further classified based on the mode of displacement:

  • Reciprocating pump if the displacement is by reciprocation of a piston plunger. Reciprocating pumps are used only for pumping viscous liquids and oil wells.
  • Rotary pumps if the displacement is by rotary action of a gear, cam, or vanes in a chamber of the diaphragm in a fixed casing. Rotary pumps are further classified as internal gear, external gear, lobe, and slide vane, etc. These pumps are used for special services with particular conditions existing in industrial sites.

Dynamic pumps are also characterized by their mode of operation: a rotating impeller converts kinetic energy into pressure or velocity that is needed to pump the fluid.

There are two types of dynamic pumps:

  • Centrifugal pumps are the most common pumps used for pumping water in industrial applications. Typically, more than 75% of the pumps installed in an industry are centrifugal pumps.
  • Special effect pumps are particularly used for specialized conditions at an industrial site.      

Centrifugal Pumps

A centrifugal pump is one of the simplest pieces of equipment in any process plant. The figure (Fig. 4) shows how this type of pump operates:

The liquid is forced into an impeller either by atmospheric pressure or in the case of a jet pump by artificial pressure.

The vanes of the impeller pass kinetic energy to the liquid, thereby causing the liquid to rotate. The liquid leaves the impeller at high velocity.

The impeller is surrounded by a volute casing or in the case of a turbine pump a stationary diffuser ring. The volute or stationary diffuser ring converts the kinetic energy into pressure energy.

A centrifugal pump has two main components. First, a rotating component comprised of an impeller and a shaft. And secondly, a stationary component comprised of a casing, casing cover, and bearings.

Centrifugal Pumps
Fig. 4: Centrifugal Pumps

What is an Impeller?

An impeller is a circular metal disc with a built-in passage for the flow of fluid. Impellers are generally made of bronze, polycarbonate, cast iron, or stainless steel, but other materials are also used.

The number of impellers determines the number of stages of the pump. A single-stage pump has one impeller and is best suited for low head (= pressure)

Types of Impellers

Impellers can be classified on the basis of (which will determine their use):

  • The major direction of flow from the rotation axis
  • Suction type: single suction and double suction
  • Shape or mechanical construction: Closed impellers have vanes enclosed by shrouds; Open and semi-open impellers; Vortex pump impellers. Fig. 4 shows an open-type impeller and a closed-type impeller

Shaft

The shaft transfers the torque from the motor to the impeller during the startup and operation of the pump.

The function of the Pump Casings

Casings have two functions

  • The main function of the casing is to enclose the impeller at suction and delivery ends and thereby form a pressure vessel.
  • A second function of the casing is to provide a supporting and bearing medium for the shaft and impeller.

Types of Pump Casing

There are two types of casings

  • Volute casing (Fig. 5-A) has impellers that are fitted inside the casings. One of the main purposes is to help balance the hydraulic pressure on the shaft of the pump.
  • The circular casing has stationary diffusion vanes surrounding the impeller periphery that convert speed into pressure energy. These casings are mostly used for multi-stage pumps. The casings can be designed as solid casing (one fabricated piece) or split casing (two or more parts together)

Assessment of pumps

The work performed by a pump is a function of the total head and of the weight of the liquid pumped in a given time period. Pump shaft power (Ps) is the actual horsepower delivered to the pump shaft, and can be calculated as follows:

Pump shaft power Ps = Hydraulic power hp / Pump efficiency ηpump

or Pump efficiency ηpump  = Hydraulic power / Pump shaft power

Pump output, water horsepower or hydraulic horsepower (hp) is the liquid horsepower delivered by the pump, and can be calculated as follows:

Hydraulic power hp = Q (m3/s) x (hd – hs in m) x ρ (kg/m3) x g (m/s2) / 1000

Where:

  • Q = flow rate
  • hd  = discharge head
  • hs = suction head
  • ρ = density of the fluid
  • g = acceleration due to gravity

In practice, it is more difficult to assess pump performance. Some important reasons are:

Absence of pump specification data

Pump specification data (see Worksheet 1 in section 6) are required to assess the pump performance. Most companies do not keep the original equipment manufacturer (OEM) documents that provide these data. In these cases, the percentage of pump loading for a pump flow or head cannot be estimated satisfactorily.

Difficulty in flow measurement:

It is difficult to measure the actual flow. The methods are used to estimate the flow. In most cases, the flow rate is calculated based on the type of fluid, head and pipe size, etc, but the calculated figure may not be accurate. Another method is to divide the tank volume by the time it takes for the pump to fill the tank. This method can, however, only be applied if one pump is in operation and if the discharge valve of the tank is closed. The most sophisticated, accurate, and least time-consuming way to measure the pump flow is by measurement with an ultrasonic flow meter.

Improper calibration of pressure gauges and measuring instruments

Proper calibration of all pressure gauges at suction and discharge lines and other power measuring instruments is important to obtain accurate measurements. But calibration has not always been carried out.  Sometimes correction factors are used when gauges and instruments are not properly calibrated. Both will lead to incorrect performance assessment of pumps.

Energy efficiency opportunities

This section includes the factors affecting pump performance and areas of energy conservation. The main areas for energy conservation include:

  1. Selecting the right pump
  2. Controlling the flow rate by speed variation
  3. Pumps in parallel to meet varying demand
  4. Eliminating the flow control valve
  5. Eliminating by-pass control
  6. Start/stop control of the pump
  7. Impeller trimming
  8. Selecting the Right Pump

Selecting the Right Pump

Fig. 5-B shows typical vendor-supplied pump performance curves for a centrifugal pump where clear water is the pumping liquid.

In selecting the pump, suppliers try to match the system curve supplied by the user with a pump curve that satisfies these needs as closely as possible.

The operating point is where the system curve and pump performance curve intersect (as explained in the introduction)

The Best Efficiency Point (BEP) is the pumping capacity at maximum impeller diameter, in other words, at which the efficiency of the pump is highest. All points to the right or left of the BEP have a lower efficiency.

Pump Curves
Fig. 5: Pump Curves

The BEP is affected when the selected pump is oversized. The reason is that the flow of oversized pumps must be controlled with different methods, such as a throttle valve or a bypass line. These provide additional resistance by increasing friction. As a result, the system curve shifts to the left and intersects the pump curve at another point. The BEP is now also lower. In other words, the pump efficiency is reduced because the output flow is reduced but power consumption is not.

Inefficiencies of oversized pumps can be overcome by, for example, the installation of VSDs, two-speed drives, lower rpm, smaller impeller, or trimmed impeller

Controlling Flow: Speed variation

A centrifugal pump’s rotating impeller generates a head. The impeller’s peripheral velocity is directly related to shaft rotational speed. Therefore varying the rotational speed has a direct effect on the performance of the pump.

The pump performance parameters (flow rate, head, power) will change with varying rotating speeds. To safely control a pump at different speeds it is therefore important to understand the relationships between the two. The equations that explain these relationships are known as the “Pump Affinity Laws”:

  • Flow rate (Q) is proportional to the rotating speed (N)
  • Head (H) is proportional to the square of the rotating speed
  • Power (P) is proportional to the cube of the rotating speed

As can be seen from the above laws, doubling the rotating speed of the centrifugal pump will increase the power consumption by 8 times. Conversely, a small reduction in speed will result in a very large reduction in power consumption. This forms the basis for energy conservation in centrifugal pumps with varying flow requirements.

  • Controlling the pump speed is the most efficient way to control the flow because when the pump’s speed is reduced, the power consumption is also reduced.
  • The most commonly used method to reduce pump speed is Variable Speed Drive (VSD).
  • VSDs allow pump speed adjustments over a continuous range, avoiding the need to jump from speed to speed as with multiple-speed pumps. VSDs control pump speeds using two types of systems:
  • Mechanical VSDs include hydraulic clutches, fluid couplings, and adjustable belts and pulleys.
  • Electrical VSDs include eddy current clutches, wound-rotor motor controllers, and variable frequency drives (VFDs). VFDs are the most popular and adjust the electrical frequency of the power supplied to a motor to change the motor’s rotational speed.
  • The major advantages of VSD application in addition to energy-saving are:
  • Improved process control because VSDs can correct small variations in flow more quickly.
  • Improved system reliability because wear of pumps, bearings, and seals is reduced.
  • Reduction of capital & maintenance costs because control valves, by-pass lines, and conventional starters are no longer needed.
  • Soft starter capability: VSDs allow the motor to have a lower startup current.

Parallel Pumps for Varying Demand

Operating two or more pumps in parallel and turning some off when the demand is lower, can result in significant energy savings. Pumps providing different flow rates can be used.

Parallel pumps are an option when the static head is more than fifty percent of the total head.

The Fig. 5 shows

  • the pump curve for a single pump, two pumps operating in parallel, and three pumps operating in parallel.
  • that the system curve normally does not change by running pumps in parallel.
  • that flow rate is lower than the sum of the flow rates of the different pumps.

Eliminating Flow Control Valve

  • Another method to control the flow is by closing or opening the discharge valve (this is also known as “throttling” the valves).
  • While this method reduces the flow, its disadvantages are
    • It does not reduce the power consumed, as the total head (static head) increases. Fig. 6 shows how the system curve moves upwards and to the left when a discharge valve is half-closed.
    • increases vibration and corrosion and thereby increasing maintenance costs of pumps and potentially reducing their lifetimes
  • VSDs are a better solution from an energy efficiency perspective.

Eliminating By-pass Control

The flow can also be reduced by installing a bypass control system, in which the discharge of the pump is divided into two flows going into two separate pipelines. One of the pipelines delivers the fluid to the delivery point, while the second pipeline returns the fluid to the source. In other words, part of the fluid is pumped around for no reason, and thus is an energy wastage. This option should, therefore, be avoided.

Start / Stop Control of Pump

A simple and reasonable energy-efficient way to reduce the flow rate is by starting and stopping the pump, provided that this does not happen too frequently. An example where this option can be applied is when a pump is used to fill a storage tank from which the fluid flows to the process at a steady rate. In this system, controllers are installed at the minimum and maximum levels inside the tank to start and stop the pump. Some companies use this method also to avoid lowering the maximum demand (i.e. by pumping at non-peak hours).

Impeller Trimming

  • Changing the impeller diameter gives a proportional change in the impeller’s peripheral velocity
  • Changing the impeller diameter is an energy-efficient way to control the pump flow rate. However, for this option, the following should be considered:
  • This option cannot be used where varying flow patterns exist.
  • The impeller should not be trimmed more than 25% of the original impeller size, otherwise, it leads to vibration due to cavitation and therefore decrease the pump efficiency.
  • The balance of the pump has to be maintained, i.e. the impeller trimming should be the same on all sides.
  • Changing the impeller itself is a better option than trimming the impeller, but is also more expensive and sometimes the smaller impeller is too small.
  • Figure 6 illustrates the effect of impeller diameter reduction on centrifugal pump performance.
Centrifugal pump Performance
Fig. 6: Centrifugal pump Performance

Figure 6 illustrates the effect of impeller diameter reduction on centrifugal pump performance.

  • With the original impeller diameter, the flow is higher
  • With the trimmer impeller, the flow is lower
ParameterChange control valveTrim impellerVFD
Impeller diameter430 mm375 mm430 mm
Pump head71.7 m42 m34.5 m
Pump efficiency75.1%72.1%77%
Rate of flow80 m3/hr80 m3/hr80 m3/hr
Power consumed23.1 kW14 kW11.6 kW

The above table compares three options to improve energy efficiency in pumps:

  • changing the control valve, trim the impeller and variable frequency drive.
  • The VFD clearly reduces power most, but a disadvantage is the high costs of VFDs.
  • Changing the control valves should at all times be avoided because it reduces the flow but not the power consumption and may increase pump maintenance costs.

Few more Pump Related Resources for you..

Cause and Effect of Pump Cavitation
Major Factors Affecting the Pump Performance: An article
NPSH for Pumps: Explanation and Effect
Water Hammer Basics in Pumps for beginners
Mechanical Seals for Rotary Pumps