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What is Steam Blowing? Steam Blowing Procedure

Written by Anup Kumar Dey in Mechanical,Piping Design Basics,Piping Stress Analysis,Piping Stress Basics

Prior to commissioning, it is essential to thoroughly clean the internal surfaces of pipes. This cleaning process, known as steam blowing, is used to remove rust, dust, scales, and debris. If this procedure is not executed properly, it can lead to significant damage or a shortened lifespan of critical components such as steam traps and control valves.

Steam blowing is a critical process used primarily in the commissioning and maintenance of steam systems. It involves the use of high-pressure steam to clear out impurities and debris from the pipes of a steam generation system, ensuring the system operates efficiently and safely. In this blog post, we’ll delve into the fundamentals of steam blowing, its purpose, procedure, benefits, and key considerations.

What is Steam Blowing?

Steam blowing, also known as steam flushing, is a method employed to clean the internal surfaces of steam pipelines and equipment. It is most commonly used in power plants, petrochemical facilities, and other industrial applications where steam systems are prevalent. The primary goal of steam blowing is to remove any residual debris, welding slag, mill scale, or other contaminants that might obstruct the flow of steam or cause damage to the system.

Steam blowing is one of the initial cleaning operations before starting any power plant or steam lines. Steam blowing of MS lines, CRH, HRH, SH, RH, HP, & LP bypass pipelines of the turbine is carried out in order to remove welding slag, weld bead deposits, loose foreign materials, iron pieces, rust, etc. from the system, generated during manufacturing, transportation, & erection prior to turbine operation. The cleaning is accomplished by subjecting the piping systems to heating, blowing steam, and cooling cycles in sufficient number and duration until clean steam is obtained.

Why is Steam Blowing Necessary?

During the construction or maintenance of steam systems, particles, and debris can accumulate inside the pipelines. These contaminants can originate from welding processes, pipe manufacturing, and even the installation phase. If not removed, these particles can lead to:

  • Reduced Efficiency: Blockages or restrictions in the pipes can reduce the efficiency of steam flow, leading to higher energy consumption and operational costs.
  • Equipment Damage: Debris can cause erosion, corrosion, and damage to critical components such as turbines, valves, and heat exchangers.
  • Safety Risks: Inadequate cleaning can increase the risk of operational failures, which may lead to safety hazards, including potential explosions or leaks.

Working Principle of Steam Blowing

The working principle of steam blowing involves using high-pressure steam to clean and clear debris from pipelines and equipment. The process begins with the generation of steam at high pressure and temperature from a boiler or steam generator. This steam is then directed into the pipeline or system through specialized blow-off connections. The high-velocity steam creates dynamic pressure within the pipes, which effectively dislodges and carries away contaminants such as rust, dust, scales, and welding debris.

As the steam flows through the system, it sweeps out impurities by exerting significant force, thereby ensuring that the internal surfaces of the pipes are thoroughly cleaned. This process not only removes blockages that could impede the flow of steam but also helps prevent potential damage to critical components like valves and turbines. Proper monitoring and control of steam pressure and flow are essential to ensure the effectiveness of the cleaning and to maintain safety throughout the operation.

The effect of Steam Blowing depends on the following factors:

  • Thermal shock
  • Removal force of steam
  • Cleaning force of steam
Removal Force of Steam

Cleaning Force Ratio or CFR of Steam Blowing

The necessity to create in the system, a steam velocity greater than that is possible at MCR condition is obvious. These two velocities are expressed as a ratio “Cleaning factor” or “ Distribution factor” or “Cleaning Force Ratio” denoted by “X” or “K”.

CFR (Cleaning Force Ratio) is also known as Cleaning Factor or CF. CFR is an industry-accepted factor that determines the required dynamic pressure. CFR in steam blowing can be defined as the ratio of required dynamic pressure for cleaning to maximum dynamic pressure experienced during system operation.

Cleaning Force required (CFR) or distribution Factor

Cleaning Force Required

Preconditioning for Steam Blowing

  • Chemical cleaning should be completed.
  • SH’s primary and secondary de-superheater piping and RH’s emergency de-superheater piping ready for operation
  • All permanent piping & temporary piping insulated and supports/hangers are released with a cold setting
  • The silencer should be connected at a temporary pipe exit
  • Soot blowing for APH should be available
  • Makeup for the deaerator made ready
  • Motor-Driven BFP with all controls made ready
  • Hydraulic test of the following lines completed:
    • Feed Lines
    • MS, HRH, CRH Lines
    • MS to Aux. PRDS Line
    • All other auxiliary lines identified for steam-blowing
  • The sampling system made ready
  • Boiler auxiliaries proved serviceable and ready after a pilot operation like:
    • Fuel oil system
    • Compressors & Atomizing steam system
    • Start-up system ( for the continuous system)
    • Coal Mill system (for the continuous system)
    • CHP readiness
    • Economizer hopper and bottom ash hopper and its evacuation system (for the continuous system)
  • All safety valve discs installed after removing the hydro-static plugin drum(sub-critical), superheaters, and reheaters
  • Adequate communication between the control room, boiler, and TG are ensured.
  • Flow nozzle, control valves, and NRV flaps wherever applicable should be not erected before steam blowing and suitable spool pieces are erected. Strainers in the path should be removed.
  • Required number of Target Plates and holders made available
Steam Blowing

Chemical Cleaning Process

  • Boiler Front System Alkaline Flushing
  • Mass Flushing
  • Hot water Rinsing
  • Alkaline Flushing
  • Hot DM water Rinsing

Main Boiler System Acid Cleaning

  • Super Heater Filling
  • Mass Flushing
  • Alkaline Flushing
  • Hot DM water Rinsing
  • Acid Cleaning
  • Passivation
  • ACID CLEANING ( BY CITRIC ACID METHOD )will be done by the circulation method for the effectiveness of the cleaning process.
  • Acid cleaning will be followed by PASSIVATION so that the uniform protective coating of GAMMA FERRIC OXIDE is formed on the metal surface and corrosion/oxidation damage to the metal surface is prevented and continues during normal operation by dosing oxygen. The gamma ferric Oxide formed by using the chemical 1-2 % sodium Nitrite(NaNO2)

Steam Blowing Procedures Techniques

Normally, two methods are historically used for steam-blowing

  • PUFFING METHOD
  • PURGING METHOD / CONTINUOUS BLOW METHOD

PUFFING Method of Steam Blowing

To give a thermal shock to the contour being purged, to dislodge the scale, etc.

Procedure: Raise the boiler pressure to a pre-determined value (40-60 kg/cm2), shut off firing, and at the same time open the quick opening valve(EOTV), thus allowing the steam to escape to atm. with high velocity carrying with it the loose debris.

Precautions during the puffing method

The Pressure drop allowed in the drum is limited to the corresponding saturation temp. change of 40 OC.

Scheme of puffing method

Steam blowing done in stages

Stage-1(a):

  • SH, MSL, ESV, temporary lines from ESV to EOTV, EOTV to CRH line, CRH lines up to boiler end with the temporary exhaust pipe.
  • Tap-off lines from CRH to deaerator, auxiliary PRDS, HP heater 6a & 6b, gland sealing, etc. shall remain closed/isolated.
  • Stage 1a endpoint will be concluded by observing the indents on the target plate.

Stage- 1(b):

  • SH, MSL, HP bypass interconnection, hand-operated valve mounted in place of HP bypass valve, and CRH lines up to Boiler end with temporary exhaust piping.
  • In this stage, 6 to 8 blows will be given through HP bypass lines to ensure the cleanliness of the limb.
  • The boiler MS stop valve will be used for stage 1b. EOTV will be kept closed. Manually Operated Isolation Valves in HP bypass lines will be kept open fully.
Steam Blowing Scheme

Stage-2(a):

  • 1a plus reheater, HRH lines, Interceptor Valve, and temporary pipe.
  • CRH line along with the attemperator shall be welded with a reheater before the start of stage 2a. LP bypass lines shall be blanked during stage 2a. Stage 2a endpoint will be concluded by observing the indents on target plates

Stage- 2(b):

  • 2a up to IV + LP bypass lines with the temporary exhaust pipe.
  • Stage 2b blowing will be a parallel blow in paths 2a & 2b. LP Bypass blanks shall be removed for stage 2b. 4 blows will be given through the L.P. Bypass line to ensure the cleanliness of the limbs

Stage-3:

Auxiliary Steam Lines covered in steam blowing are listed below.

  • 3a) Main steam line to Aux PRDS
  • 3b) CRH to Deaerator
  • 3c) CRH to HPH-6.

All auxiliary steam lines will be steam blown by the continuous blowing method

Stage-4:

The following Aux steam lines, connected to the deaerator are steam blown using auxiliary steam from the Auxiliary PRDS header.

  • 4a)PRDS to Deaerator.
  • 4b)Extraction 4 to
  • 4c) PRDS to Gland steam header

Continuous Steam Blowing Procedure

  • The initial procedure is the same as the puffing method except:
  • Continuous firing till the completion of steam blowing. No need to shut off the firing during blowing.
  • Maintain constant pressure during the blow

Recommended blowing parameters for continuous steam blowing

  • Dynamic steam pressure = 3.5 MPa
  • MS temp = 420(not to exceed)
  • HRH temp = 480( not to exceed)
  • Steam flow = 845 TPH
  • Corresponding Drum pr. = 40 Ksc
  • Furnace load ≈ 39%

Method of Continuous Steam blowing

  • Set the Temporary purging valve, by-pass valve, drain valve, and granulating device behind the temporary pipes
  • Check the tightness, support, and expansion of the temporary system
  • Section by section rinse of condensate water piping, feedwater piping, and boiler through the start-up system to the CW system
  • Circulation begins when Fe+ in the water of the main feedwater pipe and separator outlet will be less than 100 ppm.
    • Maintain the circulation flow or start-up flow at a minimum set value
  • Start oil firing and raise the temp and press. according to the cold start-up.
  • rise rate of water wall = less than 2OC / min
  • rise rate of main steam = 4 ~ 5 OC / min
  • Before MS press reaches 1.0 Mpa, open the bypass valve of the temporary purging valve to warm the piping with all water drain valves of the system open.

-The blowing system is divided into two parts

  1. A) Preliminary Steam Blowing
  2. B) Final Steam Blowing

Preliminary Cleaning

PURPOSE-

  • Primary cleaning out sundries and bulky grain deposited in the RH and main steam system
  • To ensure the fastness of supports and hangers with proper expansion
  • Know well about the operation property of the oil-burning system, condensate water, and feed water systems

Preliminary Steam Blowing Procedure

  • Start the oil firing with max. rating of 15%
  • Control the gas temperature at the furnace outlet below 500 OC (max.538OC )
  • Raise the SH outlet press to 1.6 – 1.8 Mpa with a steam temperature of around 350 OC.
  • Open the temporary purging valve for 15-20 min to blow in the MS system
  • Maintain K around 0.5.
  • SH & RH desuperheater water system also cleaned

Final Steam Blowing Procedure

  • The main steam and reheated steam system are purged in series
  • Start 2-3 coal mills when the oil burner hits the rated firing rate of 15%
  • Increase the SH outlet press. to 3.5 Mpa.
  • Maintain the MS temp < 420 OC.
  • Keep HRH steam temp around 480 OC.
  • When the steam line blowing parameter reaches, open the temporary purging valve gradually and increase the fuel and feed water volume to sustain parameter stability.
  • Establish the MS flow around 40% of the MCR (max.- 50%)
  • Ensure CFR (K) should be 1.25 to 1.3 for MS and 1.05 to 1.1 for reheated steam
  • Blow under this operating condition for 20 to 30 min
  • Gradually reduce coal firing and close the purge valve at 0.5MPa.
  • Remove the target plate and check it
  • By estimation, the target may satisfy the requirements after 15 to 20 times in-series purging
    • When the steam line temp is above the sat. temp of MS press, close all drains and Open the boiler MSSV fully.
  • Maintain the press. through controlled firing
  • Insert a reference target
  • Check the CFR
  • Allow running for 30-60 min

Target Plate in Steam Blowing

  • Generally Stainless steel panel
  • Target plate set at the first, sixth, ninth, and twelfth purging, thereafter, set a target for each purging until purging results qualified.
  • Width about 8% of the steam vent tube inner diameter (ID) and length equal to the ID
  • Brinell hardness < 90
  • Steam velocity – 258 m/sec
  • Target plates are to be introduced just before steam blowing/light up—to be removed soon after blowing is completed.

Online Target Plate Change Arrangement

Debris Filter

Steam Blowing Completion Criteria

  • At least 2 continuous target plates should not have ORIFICE GRANULARITY on the target and shall be no larger than 0.8 mm.
  • CFR should be 1.25 to 1.4 of an orifice, granularity shall not surpass 08 nos.

Determining Cleanliness After Steam Blowing

To assess the cleanliness of a piping system after steam blowing, “target plates” are used. These plates, typically made of aluminum or copper, help gauge the effectiveness of the cleaning process. The evaluation involves three key steps:

  1. Placement and Impact Assessment: Position target plates at the outlet points of the main piping system during the steam blowing operation. Assess the impact on these plates to evaluate the cleaning action.
  2. Evaluation of Results: The cleaning process is deemed effective if two consecutive target plates show fewer than two pits per square inch, with each pit having a maximum diameter of 0.3 mm.
  3. Reinstallation and Recommissioning: After steam blowing, reinstall any removed steam traps and other equipment. Following these steps, the piping system can be reheated and recharged for operational use according to its intended purpose.

Advantages of Steam Blowing

  • Required less time for completion of the total process
  • Less time is required to normalize the system for final light-up to synchronization
  • This reduces the reactionary forces on the temporary pipes
  • Stresses on the boiler system are lower

Comparison between Puffing & Continuous Method of Steam Blowing

PUFFING METHODCONTINUOUS METHOD
—  More time is required for complete steam blowing due to stage-wise blowing(8-10 days)  
—  More time is required for stage-wise temporary pipe erection and shifting of the blowing device
—  No mill required
—  Quality of cleanliness is better than a continuous process
—  Thermal shock is the driving force behind cleaning
—  More thermal stress on tube material and sudden loading on supports
—  Repeated light-up and shutdown
—  There is a time gap between the blows to make up DM water
—  System normalization time after steam blowing is more
—  Silencer use is optional  		—  Less time required for completion (3-4 days)    
—  Less time is required as only valves are to be opened for different systems  
—  Minimum 02 nos. of mill required
—  The quality of cleanliness is slightly less than Puffing.
—  Steam velocity or Removal force is the driving force
—  Less thermal stress on tube material  
—  Light up only once at the beginning of the steam-blowing
—  DM water makeup to the system during steam blowing is a challenge
—  System normalization time after steam blowing is less.
—  Silencer use is compulsory.  
Difference Between Puffing and continuous Method of Steam Blowing

Calculation of Steam Blowing Thrust force at the end of Pipe Exit

The thrust force (F) of the jet at the pipe exit is estimated as

Steam Blowing Thrust Force Calculation
Steam Blowing Thrust Force Calculation

Hazards of Steam Blowing

Steam blowing poses several hazards, primarily due to the high-pressure steam used in the process. The intense force of the steam can cause severe burns, injuries, or fatalities if proper safety precautions are not followed. Additionally, the sudden release of steam and debris can create dangerous projectiles and high noise levels, which can further pose risks to personnel. The high-pressure environment also presents risks of equipment failure or malfunction, potentially leading to catastrophic failures or explosions if the system is not properly maintained and monitored. Implementing stringent safety measures, including protective gear, safety barriers, and thorough training, is crucial to mitigating these hazards and ensuring a safe steam-blowing operation.

Pipe Expansion Loops on the Piping or Pipeline Systems

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

Expansion loops are fundamental in various piping industries involving higher temperatures including oil and gas, chemical processing, power generation, and HVAC systems, where temperature fluctuations can cause significant stress on piping networks. Even in pipeline systems where the length of the pipe is greater, expansion loops play a significant role in proper design even though the temperature is not higher. One of the most common methods used to manage thermal expansion in piping and pipeline systems is the expansion loop.

Why do we need Piping Expansion Loops?

Piping systems are often subjected to varying temperatures, either due to the fluid being transported or environmental conditions. As the temperature of a material increases, it expands, and when the temperature decreases, it contracts. This expansion and contraction can result in significant stress on the piping material.

All piping engineers are well acquainted with expansion loops in the piping systems. Whenever thermal displacements are more than a certain value these expansion loops are added to absorb the displacement inside the expansion loop. These are mainly required in any piping system design to

  • Reduce system stress,
  • Limit thermal displacements, or
  • Limit Support Loads.

How Piping Expansion Loops Work

An expansion loop is a section of pipe that is intentionally bent into a U-shape, Z-shape, or similar configuration (Refer to Fig. 1) to absorb the thermal expansion and contraction of the pipeline. By introducing flexibility into the system, expansion loops allow the pipe to expand or contract without causing undue stress on the pipe or surrounding supports and structures.

Piping expansion loops are used to increase the flexibility of the piping system. To reduce the generated expansion stress and displacement caused by thermal expansion or contraction, legs perpendicular to the main piping system are provided. This perpendicular length is known as the length of the expansion loop. The more this expansion loop leg length, the better for the piping system. However, this leg length is limited by support feasibility, vibration tendency, and cost. That is the reason the length of the absorbing leg in an expansion loop is decided only to meet the requirement of stress qualification.

Piping expansion loops are widely used in long runs of pipes running over the pipe racks or sleepers. Pipeline expansion loops are usually provided at every 500 m length as the design temperature of pipelines is normally less as compared to piping systems.

Fig 1 shows typical loops used in the piping system.

Expansion loops
Fig. 1: Typical Expansion loops

Functions of Piping Expansion Loops

Expansion loops serve various purposes as listed below:

  • Piping Expansion Loops provide the necessary leg of piping in a perpendicular direction to absorb the thermal expansion. They are safe when compared with expansion joints but take up more space.
  • Load due to axial expansion causes bending stresses to be developed, increasing upwards in the vertical pipes and becoming a maximum at the loop elbows.
  • That bending moment stays at that maximum bending moment level for the entire length of the top horizontal pipe until it gets to the next elbow and starts’ reducing until it reaches the bottom pipe on the other side of the loop.
  • As the loop gets higher, both axial resultant stress in the horizontal pipes and the bending moments in the loop are reduced.

Types of Piping Expansion Loops

Piping Expansion loops are categorized into different styles:

1. Symmetric loop vs Nonsymmetric loop (Fig. 2):

Ideally, loops shall be located centered between pipe anchors with equal legs on either side of the anchor. Symmetrical loops are advantageous in absorbing an equal amount of expansion from both directions.

When this isn’t practical make legs on either side of the anchor as equal as possible.

Friction Forces are determined by the number of pipes supporting a line crosser. By making these legs equal, the forces at the anchor should remain nearly balanced.

Symmetric and non-symmetric loops
Fig. 2: Symmetric and non-symmetric loops

2. 2-D vs 3-D Expansion Loops (Fig. 3)

Expansion Loops may be 2-D (Two-dimensional loop) or 3-D (three-dimensional loop) types. Normally for steam lines, flare lines, condensate lines, and sloped lines, where there is the possibility of two-phase flow, 2-D expansion loops are preferred. Otherwise, the 3-D loop can be provided.

2-D vs 3-D Loops.
Fig. 3: 2-D vs 3-D Loops.

Requirements of Multiple Expansion Loops

More than one expansion loop may be required when:

  1. It is impossible to make branch connections flexible enough.
  2. Spacing between branches and neighboring lines or steel is limited.
  3. When the loop becomes too large to support or fit into the space available.
  4. Anchor forces become too unbalanced and steel cannot be economically braced.
  5. More than one expansion loop may be required when the forces required to bend the loop are too great, and the anchors cannot be economically reinforced.
  6. When the thermal displacements exceed the project’s specifically allowed displacements.
  7. When the length of shoe supports due to high thermal displacement is becoming too high it looks odd.
Multiple Expansion loops in a piping system
Fig. 4: Multiple Expansion loops in a piping system

Placing Expansion Loops/ Expansion Loop Placements

  1. Loop width should always be based on utilizing existing supports.
  2. Thermal expansion must be allowed when spacing adjacent loops.
  3. Loop width does not have to be near 20 feet just because the loop nomographs happen to use that number. Loop width has only a secondary effect on results.
  4. Minimum loop height depends on the berthing of the line with respect to the location of the loop support.
  5. Piping Expansion Loops cannot extend too far beyond existing support or the overhang will cause the loop to “lose its balance.” This sets the maximum allowable loop height.
  6. The first two points have more influence on loop design than stress formulas, from the piping point of view.
  7. Three-dimensional expansion loops are widely used because this arrangement does not block the routing of low-temperature lines under the loop.
  8. Vertical loops are placed at road crossings and sometimes are nonsymmetrically located due to the location of the road.

Method for Sizing Pipeway Expansion Loops

Anchor lines near their center to determine which lines require loops by checking the allowable expansion at each end of the run. If the thermal displacement at each end is within the project-specific limit and will not clash with other lines, no expansion loop will be required. However, if the line spacing cannot be adjusted to take the movement, expansion loops need to be added.

Determine which of the lines requiring loops need the largest loop, second largest, etc., by the following:

  • Multiply the total expansion of each line between its proposed anchors by the pipe’s moment of inertia (E). (The stiffness of a line is measured by its “Moment of inertia.”).
  • The line with the largest of these calculated numbers will require the largest loop, the next smaller number, the next smaller loop, etc.
  • The above rule does not check stress. This is checked after the loops are roughly dimensioned.

Fit the expansion loops between two pipe supports using the minimum spacing plus allowance for line expansion and bowing. Make the loops as wide as possible, but keep the height to a minimum. If stress or force is extremely high, check with the stress engineer for the height of the loop.

Send finished pipe way to stress for accurate calculation of anchor forces for transferring to Structural and accurate evaluation of stresses in the piping.

Estimating Expansion Loop Leg Requirement / Expansion Loop Calculator

A preliminary estimate of the expansion loop absorbing leg requirement can be made from the equations derived from the guided cantilever method. As per that formula, the required absorbing leg, L (Refer to Fig. 5) for the piping expansion loop is given by

L=√(3ED∆/S)

where E=Elasticity modulus, D=Pipe OD, S=Allowable Stress at the maximum temperature, and ∆=thermal expansion.

Piping Expansion Loop Leg Requirement
Fig. 5: Piping Expansion Loop Leg Requirement

Providing an expansion loop in a piping system needs additional space and additional elbows (additional cost) which may not be possible in some instances. In such a scenario piping expansion joints are used to absorb the thermal growth.

I have created an expansion loop calculator for preliminary loop sizing using the above-mentioned formula which can be downloaded by clicking here.

Piping-Expansion-Loop-CalculatorDownload

Frequently Asked Questions

Now that you have studied all the relevant sections regarding piping and pipeline expansion loops above, it is time to learn the following frequently asked questions:

What is a piping or pipeline expansion loop?

A piping or pipeline expansion loop is used in the systems to control the thermal expansion and contraction of pipes due to temperature variations. An expansion loop consists of bends/elbows and pipe sections designed in a specific way to absorb the movement and prevent stress on the piping system. They are usually located between two pipe anchors or buried parts in the aboveground piping/pipeline system.

Expansion Loops
Fig. 6: Example of Expansion Loops in a Process Plant

What factors should be considered when designing an expansion loop?

When designing a thermal expansion loop, the factors that must be taken into consideration are:

  • Pipe Material
  • Temperature Differential (Install temperature to maximum and minimum design temperature)
  • Pipe Diameter: With an increase in pipe diameter, it becomes more rigid which may call for more flexibility and thus increase in expansion leg length.
  • Piping Layout: The layout of the piping system, including the placement of bends, elbows, and supports, affects how the thermal expansion is distributed. In a straight pipe run, thermal expansion will cause the pipe to elongate in the direction of the run. However, introducing bends and loops into the layout provides flexibility, allowing the pipe to expand and contract without imposing excessive stress.
  • Loop Dimensions: The size of the expansion loop is determined based on the amount of thermal expansion expected and the allowable stress on the pipe.
  • The required range of movement shall be considered.

What are the alternatives to expansion loops in piping and pipeline systems?

In a piping system, expansion loops can be avoided by providing suitable offsets. In critical systems where space is limited and offsets are not sufficient, expansion joints, bellows, or compensators are used as alternatives to piping expansion loops.
In large-size pipeline systems, the zig-zag pipeline route is followed to avoid pipeline expansion loops to optimize space requirements by pipeline loops.

What are the Advantages of Piping Expansion Loops?

The main advantages of expansion loops are that they are:

  • Cost-effective
  • Simple in design
  • Can be easily installed
  • Low Maintenance.

What are the Disadvantages of Expansion Loops in a Piping System?

Expansion loops need more space requirements for their installation and to work properly. So, sometimes where there is a space constraint, expansion loops may not be feasible.

Video Tutorial on Piping Expansion Loop

For further doubts kindly refer to the following video tutorial

Video Tutorial: What is an Expansion Loop?

Few more resources for you…

Expansion Loop and Rack Piping for a Piping Stress Engineer
Basics of Pipe Stress Analysis
Piping Design and Layout
Piping Materials
Piping Stress Analysis

11 Questions & Answers from ASME B31.3 that a Piping Stress Engineer must know

Written by Anup Kumar Dey in Piping Design Basics,Piping Stress Analysis,Piping Stress Basics

ASME B31.3 is the bible of process piping engineering and every piping engineer should frequently use this code for his knowledge enhancement. But to study a code similar to B31.3 is time-consuming and also difficult because the contents are not at all interesting. Also every now and then it will say to refer to some other point of the code which will irritate you. But still, every piping engineer should learn a few basic points from it. The following literature will try to point out 11 basic and useful points from the code of which every piping engineer must be aware of.  

ASME B31.3-2020

1. What is the scope of ASME B31.3? What does it cover and what does not?

Ans:  The Process Piping Code, ASME B31.3 is usually applicable for the piping systems in petroleum refineries; chemical, petrochemical, pharmaceutical, paper, textile, ore processing, onshore and offshore petroleum and natural gas production facilities; semiconductor, cryogenic plants; food and beverage processing facilities; and related processing plants and terminals. ASME B31.3 provides design, fabrication, erection, test, inspection, assembly, and material requirements for piping systems that carry the following fluids:

  • Petroleum products
  • Raw, intermediate, and finished chemicals.
  • Refrigerants
  • Gas, steam, air, water
  • Fluidized solids
  • Cryogenic fluids

Packaged equipment piping design is also covered by the B31.3 code.

The process piping code ASME B31.3 typically does not cover the following:

  • The piping systems designed for internal gage pressures at or above zero but less than 105 kPa (15 psi), provided the fluid handled is non-flammable, non-toxic, and not damaging to human tissues as defined in 300.2, and its design temperature is from −29°C (−20°F) through 186°C (366°F).
  • Power boiler and power system piping following ASME B31.1
  • Fired heater internal piping.
  • Pressure vessels, heat exchangers, pumps, compressors, and other fluid handling or processing equipment, including internal piping and connections for external piping.

Alternatively, refer to the below-attached figure ( Figure 300.1.1 from code ASME B31.3)  

B 31.3 Scope

2. What are the disturbing parameters against which the piping system must be designed?  

Ans: The piping system must stand strong (should not fail) against the following major effects:  

  • Design Pressure and Temperature: Each component thickness must be sufficient to withstand the most severe combination of temperature and pressure.
  • Ambient effects like pressure reduction due to cooling, fluid expansion effect, the possibility of moisture condensation, and build-up of ice due to atmospheric icing, low ambient temperature, etc.
  • Dynamic effects like impact force due to external or internal unexpected conditions, Wind force, Earthquake force, Vibration, discharge (Relief valve) reaction forces, cyclic effects, etc.
  • Component self-weight including insulation, rigid body weights along with the medium it transport.
  • Thermal expansion and contraction effects due to resistance from free displacement or due to thermal gradients (thermal bowing effect) etc.
  • Movement of pipe supports or connected equipments etc.

3. How to calculate the allowable stress for a carbon steel pipe?  

Ans: The material allowable stress for any material other than bolting material, cast iron, and malleable iron is the minimum of the following:

  1. one-third of tensile strength at maximum temperature.
  2. two-thirds of yield strength at maximum temperature.
  3. for austenitic stainless steels and nickel alloys having similar stress-strain behavior, the lower of two-thirds of yield strength, and 90% of yield strength at temperature.
  4. 100% of the average stress for a creep rate of 0.01% per 1000 h
  5. 67% of the average stress for rupture at the end of 100000 h for temperatures up to and including 815°C.
  6. For temperatures higher than 815°C (1,500°F), (100 × Favg)% times the average stress for rupture at the end of 100000 h. Favg is determined from the slope, n, of the log time-to-rupture versus log stress plot at 100 000 h such that log Favg = 1/n. Favg shall not exceed 0.67.
  7. 80% of the minimum stress for rupture at the end of 100000 h
  8. for structural grade materials, the basic allowable stress shall be 0.92 times the lowest value determined (1) through (7) above.

4. What is allowable for Sustained, Occasional, and Expansion Stress as per ASME B 31.3?  

Ans:  Calculated sustained stress (SL)< Sh (Basic allowable stress at maximum temperature) Calculated occasional stress including sustained stress< 1.33 Sh Calculated expansion stress< SA = f [ 1.25( Sc + Sh) − SL] Here f =stress range factor,   Sc =basic allowable stress at minimum metal temperature and SL=calculated sustained stress. The sustained stress (SL) is calculated using the following code formulas:

Code Equations

   Here,

  • Ii=sustained in-plane moment index. In the absence of more applicable data, Ii is taken as the greater of 0.75ii or 1.00.
  • Io=sustained out-plane moment index. In the absence of more applicable data, Io is taken as the greater of 0.75io or 1.00.
  • Mi=in-plane moment due to sustained loads, e.g., pressure and weight
  • Mo=out-plane moment due to sustained loads, e.g., pressure and weight
  • Z=sustained section modulus
  • It=sustained torsional moment index. In the absence of more applicable data, It is taken as 1.00.
  • Mt=torsional moment due to sustained loads, e.g., pressure and weight
  • Ap=cross-sectional area of the pipe, considering nominal pipe dimensions less allowances;
  • Fa=longitudinal force due to sustained loads, e.g., pressure and weight
  • Ia=sustained longitudinal force index. In the absence of more applicable data, Ia is taken as 1.00.  

5. What are the steps for calculating the pipe thickness for a 10-inch carbon steel (A 106-Grade B) pipe carrying a fluid with a design pressure of 15 bar and a design temperature of 250 degrees centigrade?  

Ans: The pipe thickness (t) for internal design pressure (P) is calculated from the following equation.

Here,

  • D=Outside diameter of the pipe, obtain the diameter from pipe manufacturer standard.          
  • S=stress value at design temperature from code Table A-1 
  • E=quality factor from code Table A-1A or A-1B      
  • W=weld joint strength reduction factor from code        
  • Y=coefficient from code Table 304.1.1 Using the above formula calculates the pressure design thickness, t.  

Now add the sum of the mechanical allowances (thread or groove depth) plus corrosion and erosion allowances if any with t to get the minimum required thickness, tm.  

Next, add the mill tolerance with this value to get the calculated pipe thickness. For seamless pipe, the mill tolerance is 12.5% under tolerance. So calculated pipe thickness will be tm/(1-0.125)=tm/0.875.  

Now accept the available pipe thickness (based on the next nearest higher pipe schedule) just higher than the calculated value from manufacturer standard thickness tables.  

6. How many types of fluid services are available for process piping?  

Ans: In the process piping industry following fluid services are available..  

  • Category D Fluid Service: nonflammable, nontoxic, and not damaging to human tissues, the design pressure does not exceed 150 psig (1035kPa), the design temperature is from -20 degree F to 366 degrees F.
  • Category M Fluid Service: a fluid service in which both of the following conditions apply:
    • highly toxic fluid such that a single exposure to a very small quantity of the fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken.
    • after consideration of piping design, experience, service conditions, and location, the owner determines that the requirements for Normal Fluid Service do not sufficiently provide the leak tightness required to protect personnel from exposure.
  • Elevated Temperature Fluid service: a fluid service in which the piping metal temperature is sustained equally to or greater than Tcr (Tcr=temperature 25°C (50°F) below the temperature identifying the start of time-dependent properties).
  • Normal Fluid Service: a fluid service pertaining to most piping covered by this Code, i.e., not subject to the rules for Category D, Category M, Elevated Temperature, High Pressure, or High Purity Fluid Service.
  • High-Pressure Fluid Service: a fluid service for which the owner specifies the use of Chapter IX for piping design and construction.
  • High Purity Fluid Service: a fluid service that requires alternative methods of fabrication, inspection, examination, and testing not covered elsewhere in the Code, with the intent to produce a controlled level of cleanness. The term thus applies to pipe systems defined for other purposes as high purity, ultra-high purity, hygienic, or aseptic.

7. What do you mean by the term SIF?  

Ans: The stress intensification factor or SIF is an intensifier of bending or torsional stress local to a piping component such as tees, and elbows, and has a value great than or equal to 1.0. Its value depends on component geometry. ASME B31.3 Appendix D, up to edition 2018 used to provide provides formulas to calculate the SIF values which are reproduced in the following figure. However, From ASME B31.3 Edition 2020 onwards Appendix D is deleted from the code and the code now suggests using ASME B31J or FEA calculations to find the values of Stress intensification factors.

Appendix D ASME B31.3 SIF
Appendix D of B 31.3

8.  When do you feel that a piping system is not required formal stress analysis?  

Ans: Formal pipe stress analysis will not be required if any of the following 3 mentioned criteria are satisfied:

  1. if the system duplicates or replaces without significant change, a system operating with a successful service record (operating successfully for more than 10 years without major failure).
  2. if the system can readily be judged adequate by comparison with previously analyzed systems.
  3. if the system is of uniform size, has no more than two points of fixation, no intermediate restraints, and falls within the limitations of the empirical equation mentioned below:

  Here, D = outside diameter of the pipe, mm (in.) Ea = reference modulus of elasticity at 21°C (70°F), MPa (ksi) K1 = 208 000 SA/Ea, (mm/m)2 = 30 SA/Ea, (in./ft)2 L = developed length of piping between anchors,m (ft) SA = allowable displacement stress range U = anchor distance, straight line between anchors,m (ft) y = resultant of total displacement strains, mm (in.), to be absorbed by the piping system  

9.  How will you calculate the displacement (Expansion) stress range for a piping system?

Ans: Expansion stress range (SE) for a complex piping system is normally calculated using software like Caesar II, Start-Prof, Rohr-II, or AutoPIPE. However, the same can be calculated using the following code equations:

here
Ap = cross-sectional area of pipe
Fa = range of axial forces due to displacement strains between any two conditions being evaluated
ia = axial stress intensification factor. In the absence of more applicable data, ia=1.0 for elbows, pipe bends, and miter bends (single, closely spaced, and widely spaced), and ia =io
it = torsional stress intensification factor.
Mt = torsional moment range between any two conditions being evaluated.
Sa = axial stress range due to displacement strains= (ia X Fa )/Ap
Sb = resultant bending stress range due to displacement strains.
St = torsional stress range= (it X Mt )/2Z
Z = section modulus of pipe
ii = in-plane stress intensification factor
io = out-plane stress intensification factor
Mi = in-plane bending moment
Mo = out-plane bending moment

10. What do you mean by the term “Cold Spring”?

Ans: Cold spring is the intentional initial deformation applied to a piping system during assembly to produce the desired initial displacement and stress. Cold Spring is beneficial in that it serves to balance the magnitude of stress under initial and extreme displacement conditions.

When cold spring is properly applied there is less likelihood of overstraining during initial operation; hence, it is recommended especially for piping materials of limited ductility. There is also less deviation from as installed dimensions during initial operation so that hangers will not be displaced as far from their original settings.

However, nowadays most EPC organizations do not prefer the use of the Cold Spring while analyzing any system.

11. How to decide whether Reinforcement is required for a piping branch connection or not?

Ans: When a branch connection is made in any parent pipe the pipe connection is weakened by the opening that is made in it. So it is required that the wall thickness after the opening must be sufficiently in excess of the required thickness to sustain the pressure. This requirement is checked by calculating the required reinforcement area (A1) and available reinforcement area (A2+A3+A4) and if the available area is more than the required area then no reinforcement is required. Otherwise, additional reinforcement needs to be added. The equations for calculating the required and available areas are listed below for your information from the code. Please refer to the code for the notations used:

RF Pad

Few more piping stress interview questions with answers for you..

Spring hangers: Common Interview Questions with Answers
Interview questions with Answers for Jacketed Piping Stress Analysis
Piping Stress Job Interview questions and answers

Pipelines Material Selection in the Oil & Gas Industry

Written by Anup Kumar Dey in Engineering Materials,Pipeline

Types of Pipelines in the Oil & Gas Industry

In the pipeline industry, various names are provided for the lines depending on the function and location of the pipelines. They are

  • Injection lines: Pipelines injecting water/steam/polymer/gas into the wells to improve the lift.
  • Flow lines: Pipelines from the wellhead to the nearest processing facility.
  • Trunk lines / Inter field lines: Pipelines between two processing facilities or from pig trap to pig trap or from block valve station to block valve station.
  • Export lines / Loading lines: From the processing facility to the loading or export point.
  • Transfer lines / Spur lines: Branch line exiting into trunk line or export line.
  • Gathering lines: One or more segments of pipelines forming networks and connected from the wells to processing facilities.
  • Disposal lines: Pipeline which disposes of normally produced water into disposal wells (shallow/deep).
  • Subsea pipelines: Pipelines connecting the offshore production platforms to onshore processing facilities.

Pipeline Typical Flow Scheme – Export Crude (Fig. 1)

Figure showing Pipeline Typical Flow Scheme – Export Crude
Fig. 1: Figure showing Pipeline Typical Flow Scheme – Export Crude

Pipeline Typical Flow Scheme – Export Gas (Fig. 2)

Figure showing Pipeline Typical Flow Scheme – Export Gas
Fig. 2: Figure showing Pipeline Typical Flow Scheme – Export Gas

Pipeline Typical flow scheme – Offshore (Fig. 3)

Figure showing Pipeline Typical Flow Scheme – Offshore
Fig. 3: Figure showing Pipeline Typical Flow Scheme – Offshore

Codes used in the Oil & Gas Industries

Design and Construction

Different codes and standards used for the design and construction of pipelines are

  • ASME B31.4 – Pipeline Transportation Systems for liquid hydrocarbons and other liquids
  • ASME B31.8 – Gas Transmission and distribution piping systems
  • ISO – 13623 – Petroleum and Natural gas industries  Pipeline transportation systems
  • DNV –F-101 – Offshore Standard for submarine pipeline systems

Sour Applications

The pipeline codes and standards used for sour service applications are

  • NACE MR-01-75 – Sulphide stress cracking resistant materials for oilfield equipment
  • ISO 15156 – Materials for use in H2S-containing environments in oil and gas production

Materials

Pipeline codes and standards that define the material requirements are

  • API 5L – Specification for line pipe
  • API 5LC – Specification for CRA line pipe
  • API 5LD – Specification for CRA clad or lined pipe
  • API 5LE – Specification for Polyethylene line pipe
  • ISO 3183 – Petroleum & Natural gas industries – Steel Pipe
  • ISO 14692 – Petroleum and Natural gas industries – Glass Reinforced plastic piping
  • AWWA M – 45 Fiberglass pipe design

Pipeline Fittings

  • ISO – 15590 – 1 Pipeline Induction bends
  • ISO – 15590 – 2 Pipeline Fittings
  • ISO – 15590 – 3 Pipeline Flanges
  • MSS – SP 75 – Specification for High Test Wrought Butt welding fittings
  • MSS – SP 44 – Steel Pipeline Flanges
  • ASTM A 694 – Steel forgings for high-pressure transmission service

Valves

Other Pipeline Components

Corrosion Threats in Oil & Gas

  • CO2 Corrosion (Sweet Corrosion) – General metal loss due to the presence of CO2 in the process fluid.
  • H2S Corrosion (Sour Corrosion) – Localized metal cracking and corrosion due to the presence of H2S in the process fluid.
  • Chlorides and Bicarbonates – Cracking in the metal due to the presence of stress and chlorides in the process fluid.
  • Corrosion due to Oxygen – Oxidation and metal loss due to the contact of metal with oxygen in the process fluid.
  • Microbiologically induced corrosion – Bacteria that induces corrosion, particularly within H2S
  • Erosion (Abrasion) corrosion – Corrosion due to the fluid flow and velocity within the pipe environments.
  • Corrosion (External) Threats in the facilities – External atmospheric corrosion on above-ground lines and corrosion due to soil for buried lines.
  • Corrosion Under Insulation – External corrosion due to water ingress under the insulating materials.

Material Selection Process

  • Identify corrosion threats
  • Define the corrosion circuits
  • Calculate the corrosion rate per year
  • Calculate the Service Life Corrosion (SLC) based on design life
  • Consider the materials options
  • Carry out the Life Cycle Costing (LCC) – Capex / Opex / Install
  • Review the selection of the material with respect to the design/operating/constructability
  • Finally, select the chosen materials

Corrosion Agents in Oil & Gas

  • Carbon Dioxide – CO2
  • Hydrogen Sulphide – H2S
  • Oxygen – O2
  • Chlorides – Cl-
  • Water – H2O

Material Options

Metals:

  • CS with a corrosion allowance
  • Stainless Steel
  • Duplex Stainless Steel
  • Super Duplex Stainless Steel

Metals + Lining:

  • CS with internally coated FBE
  • CS with internal PE lining
  • CRA clad/lined materials

Non Metals:

  • Glass Reinforced Epoxy (GRE)
  • Polyethylene (HDPE)

Advantages & Disadvantages of Material Options (Fig. 4)

Advantages and Disadvantages of materials
Fig. 4: Advantages and Disadvantages of materials

Corrosion Control in Oil & Gas Pipeline Chemical Injections

Corrosion inhibitor: Basically meant for CS pipelines, forms a layer of film on the surface and protects the core pipe from corrosion attack. Batch injection or continuous

Scale Inhibitor: Prevents scale formation in the pipelines by dissolving scale-forming salts

Wax  inhibitor: Dissolves the wax  within the crude

Oxygen Scavenger: Reacts and removes oxygen in the fluid

Biocide:  Destroys the bacteria, algae, and fungi in the process fluid.

Coagulant / Antifoam: Normally mixed in the separators to improve mixing and reducing the foam

Demulsifier: Prevents emulsion in the multiphase system

Dehydration  agents: Removes  moisture in the gas normally  Glycol injection

Odorant: Added to the fluid to add  smell and detect the leakage

Online Video Courses related to Pipeline Engineering

If you wish to explore more about pipeline engineering, you can opt for the following video courses

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

Modular Piping Design- How to do a Piping Stress Analysis

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

The idea of modular design is as follows: The complex piping system is divided into several modules that are mounted on a frame structure. Modules can be transported to the site and joined to each other into the entire piping system. One of the modules is shown in the picture below. At the points, “1” external piping or other module piping is connected.

Stress analysis of separate modules for temperature expansion and other loads acting on a whole system is impossible. Only the whole system can be analyzed. Exception: when anchors are added at all connection points between modules. But in this case design for temperature expansion will become extremely hard.

modular design
Sample Module

The practical solution to this problem is the following:

Option 1. Module manufacturer performs its stress analysis using of several, the most unfavorable variants of external piping design. Stress analysis should be successful for all variants. After that manufacturer passes the START-PROF module piping model to the customer. The customer adds the module model to own the piping model and performs an analysis of the whole system.

Option 2. The customer adds anchors or hinged anchors at the points, where his piping model is connected to the module. Performs stress analysis and optimize the design. After that, he passes the START-PROF piping model to the module manufacturer, who joins this model to the module piping model and preforms whole system stress analysis.

Option 1 & 2 can be simplified if you use the overlapping models when one part is included 100% and the second part included just partially.

There’s also Option 3. Too complex and hard. Module manufacturer offers 6 stiffnesses (Kx, Ky, Kz, KRx, KRy, KRz) for each connection point and temperature expansions (point movements). Also, the stiffness matrix 6×6 can be offered. The customer adds these stiffnesses and add support movements. But after analysis, he must pass new movements of connection points back to the manufacturer.

Also, piping systems usually are nonlinear. Some restraints may switch off, friction behavior may be different, therefore the manufacturer should perform new analysis and generate new stiffnesses and new movements. The design process will become iterative. If the piping system has several operation modes with various temperatures, this work becomes almost impossible. Much more easy to offer the whole piping model to counterpart. This option can only be used if the manufacturer wants to keep the design secret, but doesn’t want to use Option 2 for some reason.

Transportation Stage

The transportation stage should be calculated for both options described above. The system should have transportation configuration (locked springs, added temporary supports, detached flanges, etc.) and analyzed for inertial loads and vehicle body deformations during transportation. Added ice, show, wind, impact, and other possible loads.

What is a Piping Elbow? | Types of Pipe Elbows

Written by Anup Kumar Dey in Piping Design Basics

Piping Elbows are very important pipe fitting which is used very frequently for changing direction in the piping, pipeline, and plumbing systems. They are essential pieces of fittings for routing pipes around obstacles or for creating a more efficient and organized pipe layout. Even though the terms “piping bends” and “piping elbows” are often used synonymously, they are not the same. In this article, we will discuss about:

  • Definition of Pipe Elbow
  • Types of Pipe Elbows
  • Features of Piping Elbow
  • Calculating Pipe Elbow Angle
  • Significance of Elbow Radius
  • Pipe Elbow Minimum Thickness Requirement
  • Piping Elbow End Connections
  • Pipe Elbow Material Specification
  • And many more…

What is a Piping Elbow?

A Pipe Elbow or Piping Elbow is a specific, standard, engineered bend pre-fabricated as a spool piece  (based on ASME B16.9) and designed to either be screwed, flanged, or welded to the piping it is associated with. It is a type of pipe fittings. Pipe elbows are usually manufactured to provide a 45-degree or 90-degree direction change from the main pipe direction. There can also be custom-designed piping elbows, although most are categorized as either “short radius” or long radius”.

Types of Piping Elbows

Depending on various piping parameters, pipe elbows can be classified as follows:

Piping elbow types based on direction angle

  • 45-degree elbow
  • 90-degree elbow
  • 180-degree elbow

45-Degree Pipe Elbow:

A 45-degree elbow creates a 45-degree change in direction, providing a slightly less sharp turn than a 90-degree elbow. These types of pipe elbows are preferred when there is space limitation for pipe routing or there is a requirement for a more gradual change in direction to result in less turbulence and pressure drop.

90-Degree Elbow:

This type of elbow creates a 90-degree change in direction, either turning the flow of fluid from horizontal to vertical or vice versa. The majority of piping elbows used in piping or plumbing systems are 90-degree elbows.

180-Degree Elbow:

A 180-degree elbow, also known as a U-bend or return bend, is used to change the direction of a pipe/pipeline by a complete reversal of direction, resulting in a 180-degree turn. Unlike 45-degree and 90-degree elbows, which provide gradual changes in direction, a 180-degree elbow completely flips the direction of flow back on itself. In situations where a complete u-turn of pipe routing is required 180-degree pipe elbows can be used to eliminate the requirement of two 90-degree elbows. In general, they are frequently used in piping/plumbing systems.

Refer to Fig. 1 shows various types of piping elbows

Types of piping elbows
Fig. 1: Types of piping elbows

Types of piping elbows depending on bend radius

  • Long radius elbow
  • Short radius elbow

Long-radius and short-radius pipe elbows differ in terms of their curvature and the radius of the bend.

Long Radius Elbow:

A long-radius pipe elbow has a larger radius of curvature compared to short-radius elbows. The radius of a long-radius elbow is typically 1.5 times the nominal pipe diameter. They provide a more gradual change in direction, resulting in less resistance to fluid flow and reduced pressure drop. Long-radius elbows are often preferred in situations where minimizing flow resistance and maintaining a smooth flow path are important. They are the most widely used piping elbows and are found in applications with higher flow rates, such as in large industrial piping systems or when handling viscous fluids. These elbows are also suitable for applications where space is not a major constraint, as they require more room to accommodate their larger curvature.

Short Radius Elbow:

Short-radius elbows, on the other hand, have a smaller radius of curvature, typically equal to 1.0 times the nominal pipe diameter. They create a sharper change in direction, which can lead to higher resistance to fluid flow and increased pressure drop compared to long-radius elbows. Short-radius elbows are often used in situations where space is limited, and a tighter turn is required to navigate around obstacles or fit within confined spaces. They are usually not preferred by pipe layout engineers or pipe designers but are used when there is limited room for pipe layout routing adjustments. These elbows are commonly found in applications like shipbuilding, process equipment, and tight mechanical rooms.

Piping elbow types considering pipe end connections

  • Butt welded piping elbow
  • Socket welded piping elbow
  • Threaded piping elbow
  • Flanged elbow

Butt Welded Piping Elbow:

A butt-welded piping elbow is a type of pipe elbow that is joined to the pipeline through a butt-welding process. Butt welding involves welding the elbow directly to the pipe by aligning the ends of the pipe and the elbow and then fusing them together using heat and pressure. Butt welded elbows are known for their strength and integrity as the weld provides a continuous, leak-resistant joint. They are commonly used in high-pressure and high-temperature piping systems, especially in industrial piping applications. Refer to Fig. 2 below

Types of Pipe Elbows as per Pipe End Connection
Fig. 2: Types of Pipe Elbows as per End Connection

Socket Welded Piping Elbow:

A socket-welded piping elbow is a type of pipe elbow designed to be joined to the pipeline using socket weld connections. In this process, the pipe is inserted into the socket of the elbow, and then fillet welds are applied to secure the joint. Socket welded elbows are suitable for smaller-diameter pipes and are often used in applications where a strong and reliable connection is needed but without the need for complex welding procedures. They are commonly used in low- to medium-pressure piping systems.

Threaded Piping Elbow:

A threaded piping elbow is a pipe elbow with female threads on both ends, allowing it to be screwed onto threaded pipes or fittings with male threads. The connection is made by threading the elbow onto the pipe threads. Threaded elbows are convenient for assembly and disassembly, making them suitable for applications where frequent maintenance or changes to the piping system may be required. They are commonly used in plumbing and lower-pressure industrial piping systems.

Flanged Elbow:

A flanged elbow is a type of pipe elbow with flanges on both ends. These flanges have holes for bolting the elbow to the matching flanges on pipes or equipment. The joint is sealed using gaskets between the flange faces. Flanged elbows are often used in applications where easy assembly and disassembly are necessary, and they are particularly prevalent in large-diameter pipes and systems with higher pressure and temperature requirements. The flange connection allows for a secure and leak-proof joint.

Other Pipe Elbow Types

Reducing Elbow

A reducing elbow is a type of pipe elbow used in piping systems to change the direction of flow while also reducing the pipe diameter or size in the process. It combines the functions of a standard elbow, which redirects the flow, with that of a reducing coupling or adapter, which reduces the pipe size. Reducing elbows are designed to accommodate situations where two pipes of different sizes need to be connected while maintaining the required change in direction. Reducing pipe elbows are in various end connection types, including butt weld, socket weld, threaded, or flanged, depending on the specific requirements of the piping system.

Features of Piping Elbow

Whenever the term piping elbow is used, it must also carry the qualifiers of type (45 or 90 degrees) and radius (short or long) – besides the nominal size.

Elbows can change direction to any angle as per requirement. An elbow angle can be defined as the angle by which the flow direction deviates from its original flowing direction (See Fig. 3 below). Even though An elbow angle can be anything greater than 0 but less or equal to 90° But still a change in direction greater than 90° at a single point is not desirable. Normally, a 45° and a 90° elbow combinedly are used while making piping layouts for such situations.

piping elbow
Fig. 3: A Typical Piping Elbow with Elbow Angle (phi)

Calculating Elbow Angle

Elbow angle can be easily calculated using a simple geometrical technique of mathematics. Let’s give an example to you.

Refer to Fig. 4. Pipe direction changes at point A with the help of an elbow and again the direction changes at point G using another elbow.

Elbow angle calculation
Fig. 4: Example figure for elbow angle calculation

In order to find out the elbow angle at A, it is necessary to consider a plane that contains the arms of the elbow. If there had been no change in direction at point A, the pipe would have moved along line AD but the pipe is moving along line AG. Plane AFGD contains lines AD and AG and the elbow angle (phi) is marked which denotes the angle by which the flow is deviating from its original direction. Considering right angle triangle AGD, tan(phi) = √( x2 + z2)/y Similarly elbow angle at G is given by: tan (phi1)=√ (y2 +z2)/x  

Elbow Radius | Bend Radius

Elbows or bends are available in various radii for a smooth change in direction which is expressed in terms of pipe nominal size expressed in inches. Elbows or bends are available in three radii,

a. Long radius elbows (Radius = 1.5D): used most frequently where there is a need to keep the frictional fluid pressure loss down to a minimum, there is ample space and volume to allow for a wider turn and generate less pressure drop.

b. Long radius elbows (Radius > 1.5D): Used sometimes for specific applications for transporting high viscous fluids like slurry, low polymer, etc. For radius, more than 1.5D pipe bends are usually used and these can be made to any radius. However, 3D & 5D pipe bends are the most commonly used. In the pipeline industry, a piping bend of up to 60D is quite common.

c. Short radius elbows (Radius = 1.0D): to be used only in locations where space does not permit the use of long radius elbows and there is a need to reduce the cost of elbows. In jacketed piping, the short radius elbow is used for the core pipe.  

Here, D is the nominal pipe size in inches.  

There are three major parameters that dictate the radius selection for the elbow. Space availability, cost, and pressure drop Pipe bends are preferred where pressure drop is of major consideration. The use of short-radius elbows should be avoided as far as possible due to abrupt changes in a direction causing the high-pressure drop.  

Pipe Elbow Minimum Thickness Requirement

Whether a pipe elbow or bend is used the minimum thickness requirement from the code must be met. Code ASME B31.3 provides an equation for calculating the minimum thickness required (t) in finished form for a given internal design pressure (P) as shown below:

bend thickness
Fig. 5: Code Equation for Minimum Elbow Thickness Calculation

Here,

  • R1 = bend radius of welding elbow or pipe bend
  • D = outside diameter of the pipe
  • W = weld joint strength reduction factor
  • Y = coefficient from Code Table 304.1.1
  • S = stress value for material from Table A-1 at the maximum temperature
  • E = quality factor from Table A-1A or A-1B   Add any corrosion, erosion, or mechanical allowances with this calculated value to get the thickness required.

Pipe Elbow Material Specifications

The material specifications for pipe elbows can vary widely depending on the specific requirements of the piping system, the type of fluid being transported, and the environmental conditions. Different materials offer varying levels of corrosion resistance, strength, and temperature tolerance. Below are some common ASTM material specifications used for pipe elbows:

Carbon Steel Piping Elbows:

  • ASTM A234/A234M Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High-Temperature Service.
  • ASTM A420/A420M Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Low-Temperature Service.

Stainless Steel Pipe Elbows:

  • ASTM A403/A403M Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings.
  • ASTM A815/A815M Standard Specification for Wrought Ferritic, Ferritic/Austenitic, and Martensitic Stainless Steel Piping Fittings.

Copper and Copper Alloys Pipe Elbows:

  • ASTM B16/B16M Standard Specification for Free-Cutting Brass Rod, Bar, and Shapes for Use in Screw Machines.
  • ASTM B75/B75M Standard Specification for Seamless Copper Tube.

PVC (Polyvinyl Chloride) Pipe Elbows:

  • ASTM D2466 Standard Specification for Poly(Vinyl Chloride) (PVC) Plastic Pipe Fittings, Schedule 40.
  • ASTM D2467 Standard Specification for Poly(Vinyl Chloride) (PVC) Plastic Pipe Fittings, Schedule 80.

CPVC (Chlorinated Polyvinyl Chloride) Pipe Elbows:

  • ASTM D2846 Standard Specification for Chlorinated Poly(Vinyl Chloride) (CPVC) Plastic Hot-and-Cold-Water Distribution Systems.

Ductile Iron Piping Elbows:

  • ASTM A536 Standard Specification for Ductile Iron Castings.
  • ISO 2531 Ductile iron pipes, fittings, accessories, and their joints for water applications.

Brass Elbows:

  • ASTM B62 Standard Specification for Composition Bronze or Ounce Metal Castings.
  • ASTM B124/B124M Standard Specification for Copper and Copper Alloy Forging Rod, Bar, and Shapes.

Aluminum Pipe Elbows:

  • ASTM B361 Standard Specification for Factory-Made Wrought Aluminum and Aluminum-Alloy Welding Fittings.

Titanium Elbows:

  • ASTM B363 Standard Specification for Seamless and Welded Unalloyed Titanium and Titanium Alloy Welding Fittings.

Nickel Alloys:

ASTM B366 Standard Specification for Factory-Made Wrought Nickel and Nickel Alloy Fittings.

Butt-Welded Pipe Elbows

  • Butt-welded piping elbows are the most widely used pipe connections for high-temperature and pressure lines. They are normally used for pipe sizes with a size of NPS 2 or more. The pipe is connected to butt welded elbow as shown in Fig. 6 by having a butt-welding joint.
  • Butt-welded fittings are supplied with bevel ends suitable for welding to the pipe. It is important to indicate the connected pipe thickness /schedule while ordering. All edge preparations for butt welding should conform to ASME B16.25.
  • Dimensions of butt welded elbows are as per ASME B16.9. This standard is applicable for carbon steel & alloy steel butt weld fittings of NPS 1/2” through 48”.
piping bend
Fig. 6: A Typical Butt-Welded Elbow

Dimensions of stainless steel butt welded fittings are as per MSS-SP-43. Physical dimensions for fittings are identical under ASME B16.9 and MSS-SP-43. It is implied that the scope of ASME B16.9 deals primarily with the wall thicknesses which are common to carbon and low alloy steel piping, whereas MSS-SP-43 deals specifically with schedule 5S & 10S in stainless steel piping.

Dimensions for short radius elbows are as per ASME B16.28 in the case of carbon steel & low alloy steel and MSS-SP-59 for stainless steel.

Butt-welded fittings are usually used for sizes 2” & above. However, for smaller sizes up to 1-1/2” on critical lines where the use of socket welded joints is prohibited, pipe bends are normally used. These bends are usually of a 5D radius and made at the site by cold bending of the pipe.

Alternatively, butt welded elbows can be used in lieu of pipe bends but usually smaller dia lines are field routed and it is not possible to have the requirement known at the initial stage of the project for procurement purposes. So pipe bends are preferred. However, pipe bends do occupy more space and particularly in pharmaceutical plants where a major portion of piping is of small diameter and the layout is congested, butt welded elbows are preferred.

Butt-welded joints can be radiographed and hence preferred for all critical services.

ASME B31.3 allows the application of miter bends subject to meeting its pressure requirements.

Material standards as applicable to butt welded fittings are as follows:

ASTM A234

This specification covers wrought carbon steel & alloy steel fittings of seamless and welded construction. Unless seamless or welded construction is specified in the order, either may be furnished at the option of the supplier. All welded construction fittings as per this standard are supplied with 100% radiography. Under ASTM A234, several grades are available depending on chemical composition. Selection would depend upon the pipe material connected to these fittings. Some of the grades available under this specification and corresponding connected pipe material specifications are listed below:  

ASTM A403

This specification covers two general classes, WP & CR, of wrought austenitic stainless steel fittings of seamless and welded construction. Class WP fittings are manufactured to the requirements of ASME B16.9 & ASME B16.28 and are subdivided into three subclasses as follows:

  • WP-S-Manufactured from a seamless product by a seamless method of manufacture.
  • WP – W These fittings contain welds and all welds made by the fitting manufacturer including starting pipe weld if the pipe was welded with the addition of filler material are radiographed. However, no radiography is done for the starting pipe weld if the pipe was welded without the addition of filler material.
  • WP-WX These fittings contain welds and all welds whether made by the fitting manufacturer or by the starting material manufacturer are radiographed.

Class CR fittings are manufactured to the requirements of MSS-SP-43 and do not require non-destructive examination. Under ASTM A403 several grades are available depending on chemical composition. Selection would depend upon the pipe material connected to these fittings. Some of the grades available under this specification and corresponding connected pipe material specifications are listed below:

ASTM A420

  • This specification covers wrought carbon steel and alloy steel fittings of seamless & welded construction intended for use at low temperatures. It covers four grades WPL6, WPL9, WPL3 & WPL8 depending upon chemical composition. Fittings WPL6 are impact tested at temp – 50° C, WPL9 at -75° C, WPL3 at -100° C and WPL8 at -195° C temperature.
  • The allowable pressure ratings for fittings may be calculated for straight seamless pipe in accordance with the rules established in the applicable section of ASME B31.3.
  • The pipe wall thickness and material type shall be that with which the fittings have been ordered to be used, their identity on the fittings is in lieu of pressure rating markings.

Piping Elbow vs. Piping Bend

A PIPING BEND is simply a generic term in piping for an “offset” – a change in the direction of the piping. It signifies that there is a “bend” i.e.,  a change in direction of the piping (usually for some specific reason) – but it lacks a specific, engineering definition as to direction and degree. Bends are usually made by using a bending machine (hot bending and cold bending) on-site and suited for a specific need. The use of bends is economical as it reduces the number of expensive pipe fittings. The major differences between a pipe elbow and a pipe bend are provided in Table 1 below:

Pipe BendPipe Elbow
A pipe bend is a generic term for a curved section of a pipe used to change the direction of the pipeline.A pipe elbow is a specific type of pipe fitting designed to change the direction of flow at a particular angle, typically 90 degrees or 45 degrees.
Pipe bends can come in various angles, including 90 degrees, 45 degrees, or other custom angles.Pipe elbows are primarily available in two common angles: 90 degrees and 45 degrees, although other angles are possible.
Pipe bends may have a varying radius of curvature depending on the design and application.Pipe elbows can be categorized as either long radius or short radius, each with its own specific radius of curvature.
Pipe bends can be fabricated by bending a straight section of pipe to the desired angle using heat and mechanical force or by welding together straight pipe segments.Pipe elbows are manufactured as dedicated fittings, and their shape is consistent with their specified angle and radius of curvature.
Pipe bends are often used in custom or field-fabricated applications where specific bending requirements exist, or when standard elbows are not readily available.Pipe elbows are standard fittings commonly used in piping systems to achieve predetermined angles for changing direction.
The curvature and angle of a pipe bend may vary depending on the fabrication method and skill of the welder, potentially leading to inconsistencies.Pipe elbows are manufactured with precise angles and radii, ensuring consistency and reliability in pipe routing.
Pipe bends can be compatible with various materials and sizes, depending on how they are fabricated.Pipe elbows are designed to be compatible with specific pipe sizes and materials, conforming to industry standards.
Pipe bends are not standardized fittings and may require custom fabrication for each application.Pipe elbows are standardized pipe fittings available in various materials and sizes, conforming to industry standards.
Pipe bends are used in specialized or custom situations where a standard elbow may not fit or when specific bending requirements exist.Pipe elbows are used as standard fittings in most piping systems to facilitate changes in direction while maintaining flow efficiency.
Pipe bends may require custom fabrication for replacements, making maintenance more complex and time-consuming.Pipe elbows are readily available as standard fittings, simplifying maintenance and replacement tasks.
Table 1: Differences between Pipe Elbows and Pipe Bends

In short “All pipe elbows are bends as they change direction but all pipe bends are not elbows as they are not pipe fittings”

Pipe elbows are used in a wide range of industries, including plumbing, HVAC (heating, ventilation, and air conditioning), oil and gas, chemical and petrochemical processing, shipbuilding, marine, food processing, power plants, and many more.

Few more Resources for you…

Piping Elbow or Bend SIF (Stress Intensification Factor)
Consideration of Flanged Bend while modeling in Caesar II
Piping Design and Layout Basics
Piping Materials Basics
Piping Stress Analysis Basics
Piping Interface Basics