Is Pipe Stress Analysis Required for Cold Water Piping Systems from Pump Stations? This is perhaps one of the most frequently asked questions in the water piping industry. Engineers often argue over this question which results in a dual answer. Some engineers say stress analysis is not required and others say it is required. In this article, we will explore the answer to the above question.
The answer to the above question is not straightforward. In general, as for water piping systems with pump stations, the difference between the maximum working temperature and installed temperature (generally 10 to 20 Degrees) is less, hence it is believed that pipe stress analysis of such water piping systems is not required. This could be valid up to a certain point if good engineering judgments are followed and the piping system is provided with sufficient flexibility.
But before we can conclude, a high level of assessment must be conducted to understand if pipe stress analysis is required. The parameters that substantiate the need for pipe stress analysis are:
How an increase in pipe temperature affects the system:
Even though it is believed that the water piping system will be near to installed temperature. Still, there could be variation due to the following reasons:
Seasonal variation of temperature from winter to summer.
Empty pipe exposed to sunlight.
Full pipe exposed to sunlight.
Temperature variation between day and night.
How external forces and displacements affect the water piping and Pump system:
Forces and displacements due to seismic, wind, and structural settlements may impose unacceptable levels of stress on the system. These stresses can not be estimated without proper pipe stress analysis.
Limitation of Pump Nozzle Loadings:
It’s difficult to estimate the nozzle loads without performing pipe stress analysis.
Support optimization:
Cold water piping systems usually have very large sizes along with large valves that result in huge structural loading. Without proper stress analysis, those loads can be overestimated and support optimization may not be possible.
Design of Expansion Bellows or Expansion Joints:
The use of rubber bellows or steel expansion joints at pump stations for water piping systems is quite common. In general pump stations are routed quite straight without much flexibility. So, these expansion joints are used to absorb thermal expansion which in turn reduces pump nozzle loads.
However, there is a concern as the subject of expansion joints is one of the most misunderstood components in the piping industry. In the usual case, simple untied bellows are used which is simply a flexible element between two flanges. It is true that they easily absorb the thermal expansion by compressing the bellow elements in all axial, lateral, angular, and torsional directions. However, a pressure thrust force is generated in untied expansion joints which must be calculated and considered in pipe stress analysis to get the actual effect. This pressure thrust force will be transferred to the pumps. In some situations, the pressure thrust force, itself, will be more than the allowable pump nozzle loads.
Another option is to use a tied expansion bellows. However, even though the tie rods will take care of the pressure thrust load, they will not compress to absorb axial movement and the thermal effect will be transferred to the nozzles. So, their position has to be such that the thermal movement is lateral.
A common misconception is to use tied bellows to a pump line with a gap in the tie nut and believe that the tie rods will take care of the pressure thrust force and the gap between the tie rod nuts will help in absorbing the thermal force by compressing. But as soon as any thermal expansion occurs, the tension in the tie-rods reduces and the pressure thrust load is transferred to the nozzle. So, this has to be carefully considered.
Typical Example of a Pump Station Cold Water Piping System
Fig. 1 below shows a typical example of a water piping system pump station.
Fig. 1: Cold Water Piping from Pump Station
From the image, you can easily understand that the configuration does not have sufficient flexibility, and nozzle loads may increase the allowable.
We all know that a hydrostatic test is performed in all piping systems to ensure the integrity of all the joints. It is a code requirement before the commissioning of the piping/pipeline system. However, in certain situations, hydrostatic testing may not be technically feasible or may be hugely cost-intensive. For example, When a new piping component is connected to an already existing pipe. The situation may not always be favorable for hydro tests. In such a situation, the golden joint comes as a savior to construction professionals. The joint is called “golden” because of its highly critical nature and the importance of its quality. In this article, we will learn the meaning and applications of golden joints in piping.
What is a Golden Joint?
Golden Joint is a pipe/pipeline joining procedure where hydro testing is not performed after the joining. Extensive NDT methods like radiographic test/ultrasonic testing methods are used to ensure that the joint is defect-free, design is in line with codes and standards, and fit for intended service. A golden joint is also known as a golden weld or closure weld.
Examples of Golden Joints
Some typical examples of golden joints are:
Golden joints in tie-ins from existing pipe parts.
During Hot tapping from running/working/operating pipelines.
When only a little repair work is performed on the system at one or two locations, and hydro-testing of the complete system is difficult, costly, and time-intensive.
Golden Joint Procedure
The requirements of golden weld joints are identified by the construction contractor. Then they mark up the isometric/alignment drawings and get permission from the client. Next following a proper golden joint fabrication method statement, the welding is performed. Next NDE and final inspection clearance is taken and the golden joint is approved. The golded weld or closure weld must be performed by a highly skilled welder.
NDE requirements for Golden Joints
Non-destructive examination or NDE is performed after the PWHT if required by the piping class, thickness, and service limitation. In general, the following NDT tests are performed for golden piping joints.
Radiographic Testing:Radiographic testing can be performed as per the guidelines provided in Article 2 of ASME BPVC Code section V.
Ultrasonic Testing:Ultrasonic testing can be performed as per the guidelines provided in Article 5 of ASME BPVC Code section V.
Magnetic Particle Testing:Magnetic Particle Testing is performed as per the guidelines provided in Article 7 of ASME BPVC Code section V.
Dye Penetration Test:Dye Penetrant testing is performed as per the guidelines provided in Article 6 of ASME BPVC Code section V.
Code requirements for Golden Joints
ASME B31.3 Requirements for Golden Joint
As per clause 345.2.3 (c) of ASME B31.3, The final weld connecting piping systems or components need not be leak tested (golden weld) provided the weld is examined and passes with 100% radiographic examination or 100% ultrasonic examination.
Clause 437.1.4-b of ASME B31.4 provides requirements for golden welds. It states:
Testing Tie-Ins. Because it is sometimes necessary to divide a pipeline into test sections and install test heads, connecting piping, and other necessary appurtenances for testing, or to install a pretested replacement section, it is not required that tie-in welds be tested; however, tie-in welds and girth welds joining lengths of pretested pipe shall be inspected by radiographic or other accepted nondestructive methods in accordance with para. 434.8.5(b) if the system is not pressure tested after tie-in. After such inspection, the joint shall be coated and inspected in accordance with para. 461.1.2 before backfilling.
ASME B31.8 requirements for Golden Joints
Clause 841.3.1-a of ASME B31.8 specifies the requirements for golden welds. As per this code:
The circumferential welds associated with connecting pretested assemblies, pretested repair pipe lengths or sections, and welded tie-in connections are not pressure tested after construction in accordance with para. 841.3.2 or para. 841.3.3 or leak tested after construction in accordance with para. 841.3.4 or para. 841.3.5 shall be examined by radiography, ultrasonics, magnetic particle, or other comparable and accepted non-destructive methods in accordance with paras. 826.3 (c) and 826.3(d).
API 570 Requirements for Golden Weld Joints of Piping
As per API 570, when performing a pressure test of a final closure weld that joins a new or replacement section of piping to an existing system is not practical, all of the following four requirements need to be satisfied.
a) The new or replacement piping section is pressure tested and examined by the applicable code governing the design of the piping system, or if not practical, welds are examined with appropriate NDE, as specified by the authorized piping inspector.
b) The closure weld is a weld between any pipe or standard piping component of equal diameter and thickness, axially aligned (not miter cut), and of equivalent materials.
c) Any final closure butt weld shall be of 100 % RT; or angle beam ultrasonic flaw detection may be used, provided the appropriate acceptance criteria have been established.
d) MT or PT shall be performed on the root pass and the completed weld for butt welds and on the completed weld for fillet welds.
The owner/user shall specify industry-qualified UT angle beam examiners for closure welds that have not been pressure tested and for weld repairs identified by the piping engineer or authorized piping inspector.
API RP 14-E Requirements for Golden Joint
API RP 14E recommends that when hydrostatic or pneumatic tests create any adverse effect on the piping or operating fluid, a golden joint can be used.
Applications of Golden Joints
As explained above, Golden Joints are commonly encountered in the following situations:
Pipeline/Piping Tie-Ins: When connecting new sections of pipeline or piping to existing systems.
Facility Modifications: During the integration of new equipment or piping into an operational system, such as through Hot Tapping.
Closed-Loop Systems: In scenarios where internal pressure testing is impractical.
Final Welds in Inaccessible Locations: Where the completion of the last welds is in areas that are difficult to access.
Features of Golden Joints
Common features of Golden Joints include:
Mandatory Non-Destructive Examination (NDE): Since pressure testing is not used, thorough NDE methods like Ultrasonic Testing (UT) and Radiographic Testing (RT) are required to check the weld for any flaws.
Strict Welder Qualification: Only highly qualified and certified welders are allowed to perform Golden Joints.
100% NDE Coverage: Every layer of the weld must be carefully inspected using NDE techniques.
Detailed Documentation: Comprehensive records of the welding procedure, welder qualifications, and NDE results are required for future reference and traceability.
Piping Materials vs Pipeline Materials: Major Differences
Both pipelines and piping systems play a pivotal role in the transportation of fluids, gases, and other materials across various industries. Even though the terms “piping” and “pipeline” are often used interchangeably, there are significant differences between the two. The major differences between piping and pipeline are already discussed here. Note that, there are distinctions between the materials used in piping and pipelines. In this article, we’ll learn some of the differences between piping and pipeline materials while exploring their characteristics, applications, and considerations.
What are Piping Materials?
Piping refers to a system of pipes used to convey fluids within a facility or plant. Piping materials are carefully selected based on factors such as the type of fluid being transported, temperature and pressure requirements, corrosion resistance, and cost considerations. Some of the materials used in Piping are:
Metals:
Common metals used in piping systems are:
Carbon steel,
LTCS (low-temperature carbon steel)
Stainless steel,
Copper, aluminum, and various other alloys.
Carbon steel is a popular choice for its strength and affordability, while stainless steel is preferred for its corrosion resistance, making it suitable for corrosive environments.
Plastics:
Plastic materials are chosen for their corrosion resistance, flexibility, and ease of installation. Common plastic materials in piping are:
Polyvinyl chloride (PVC),
Polyethylene (PE)/HDPE, and
Polypropylene (PP)
Composite Materials:
Fiberglass-reinforced plastic (FRP) and reinforced thermosetting resin (RTR) are composite materials increasingly utilized in piping. These materials offer a balance of strength, corrosion resistance, and lightweight characteristics.
Piping vs Pipeline Materials
What are Pipeline Materials?
Pipelines are long-distance transportation systems designed to carry large volumes of fluids or gases over extended distances. Pipeline materials must withstand the challenges posed by external environmental factors, such as soil conditions, temperature variations, and potential ground movement. Common materials used in pipelines are:
Carbon Steel: Carbon steel remains a predominant material for pipelines, especially in the oil and gas industry. It provides a balance of strength, durability, and cost-effectiveness. Pipeline materials use API 5L code for the pipes and API 6D code for the valves.
High-Strength Alloys: In high-pressure and extreme temperature applications, pipelines may incorporate high-strength alloys such as X65 or X70 to ensure structural integrity.
Corrosion-Resistant Alloys: For corrosive environments, corrosion-resistant alloys like duplex stainless steel or nickel alloys are employed to prevent degradation over time.
Non-metallic pipelines: Pipelines can also use PE/HDPE/PP materials or FRP/GRP/RTR/GRE materials for fluid transportation.
Differences between Piping Material and Pipeline Materials
The major differences between Piping and Pipeline Materials can be discussed considering various parameters as mentioned below:
Material Selection Criteria
Piping materials are decided considering the following parameters:
The type of fluid that will be transported through the pipes
The design and operating temperatures and pressures of the pipes
The flow medium characteristics, such as viscosity, density, corrosiveness, abrasiveness, etc.
The fracture tendencies of the pipes, such as fatigue, creep, thermal expansion, etc.
The corrosion and erosion of the pipes, such as galvanic, pitting, stress corrosion cracking, etc.
The environment where the pipes will be working, such as indoor, outdoor, underground, underwater, etc.
On the other hand, pipeline materials are selected considering the following factors:
The fluid transportation efficiency (the ability of the pipeline to deliver the fluid with minimal losses, such as friction, leakage, etc.)
Safety and reliability (the ability of the pipeline to prevent or withstand failures, such as rupture, explosion, fire, etc.)
The environmental impact (the effect of the pipeline on the surrounding environment, such as soil, water, air, wildlife, etc.)
Temperature Resistance
Piping materials have the capability of carrying fluids at very high temperatures and pressures. While the temperature capability of Pipelines is limited to 232 degrees C as per ASME B31.8 (120 degrees C as per ASME B31.4).
Design Allowable Stresses
Piping materials are limited by the design’s allowable stresses which is governed by the term Sc and Sh which are much lower than the material yield strength. On the other hand, the pipeline material’s design allowable stress is governed by SMYS (Specified minimum yield strength). So, pipeline materials have a very low safety factor than piping materials.
Material Thickness
For the same pressure rating, pipeline materials usually have less material thickness as compared to piping materials. Hence, the weight of the piping materials is more than that of pipeline materials for the same pressure classes.
Hydrotest Pressure
Piping materials are usually hydrostatically tested at 1.5 times the design pressure whereas pipeline materials are hydrotested at 1.25 times the design pressure.
Corrosion Resistance
In most cases, the piping materials have to resist the corrosion generated by the fluid it carries means internal corrosion. While pipeline materials need to withstand external corrosion from soil and environmental factors.
Temperature and Pressure Ratings
Piping materials must meet specific temperature and pressure requirements based on the application, while pipeline materials need to account for the varying conditions along their extensive routes.
Cost Considerations
Piping materials are often selected based on factors such as installation costs and ease of maintenance within a confined space. Pipeline materials, on the other hand, must balance upfront costs with the long-term durability required for extended transportation routes.
Environmental Factors
Pipelines traverse diverse landscapes, and materials must be chosen considering factors like soil conditions, seismic activity, and temperature fluctuations.
Here’s a comparative table that outlines the major differences between piping materials and pipeline materials:
Feature
Piping Materials
Pipeline Materials
Scope
Used for smaller, often individual sections or components
Used for larger, continuous segments of transportation systems
Size Range
Typically smaller diameter, up to a few inches in diameter
Generally larger diameter, from several inches to several feet
Generally less expensive due to smaller size and simpler applications
Typically more expensive due to size, material requirements, and installation complexity
Durability
Suitable for a range of applications with varying durability needs
Designed for long-term durability and resistance to harsh conditions
Corrosion Resistance
Varies by material; options for corrosion-resistant materials like stainless steel or PVC
Often designed with materials or coatings that resist corrosion in challenging environments
Installation
Easier to install in small sections; often done manually
Requires heavy machinery and specialized techniques for installation
Maintenance
Generally easier and less costly to maintain and replace
Maintenance can be more complex and costly due to the scale and potential for more significant issues
Standards and Codes
Subject to various industry and local standards for residential and commercial use
Governed by stringent industry standards and codes for large-scale infrastructure, such as API, ASME, and ASTM
Temperature Range
Varies by material; some designed for high temperatures, others for cold
Designed to handle extreme temperatures and environmental conditions. However very high design temperatures are usually not expected.
Table 1: Piping Materials vs Pipeline Materials
In a nutshell, Piping materials are often constructed with a wide range of materials and components, such as cast iron, carbon steel, low-temperature carbon steel, alloy steel, stainless steel, duplex stainless steel, nickel alloys, aluminum, titanium, etc. Piping materials also use different codes and standards for the pipes, fittings, and valves, such as ASTM, BS, API 5L, ASME B31, ANSI, DIN, ISO, etc. Pipeline materials have to mainly consider fluid transportation efficiency, safety and reliability, and environmental impact. Pipeline materials are typically constructed with a single type of material, such as steel, plastic, or composite. Pipeline materials use API 5L code for the pipes and API 6D code for the valves.
Pressure Vessels vs Storage Tanks: Major Differences
Pressure vessels and storage tanks are both types of containers that are used to store fluids. However, they have different characteristics, functions, and applications. In this article, we will explore the major differences between pressure vessels and storage tanks.
What is a Pressure Vessel?
A pressure vessel is a mechanically designed container that holds gases, vapors, liquids, or two-phase fluids at pressures different from the ambient pressure. Usually, they are equipped with provisions for the introduction or removal of heat from the container. To avoid the risk of bursting, the pressure vessels are designed, fabricated, installed, and maintained following industry codes, standards, and procedures. Pressure vessels have various parts including shell, head, nozzle, saddles, skirts, etc.
The ASME BPVC is used for rules and guidelines for the design, construction, inspection, testing, and certification of pressure vessels. It ensures the safe operation of vessels under various pressure and temperature conditions. Pressure vessel manufacturers, operators, and regulators worldwide use the ASME BPVC code to ensure the integrity and safety of pressure vessels used in various industries.
What is a Storage Tank?
A storage tank can be defined as a container designed to store liquids or gases at atmospheric or low pressures. Storage tanks are usually made from thin-walled materials so they don’t hold as much pressure as pressure vessels. They’re often found to hold fuel for cars, which has less than 10 psi but is still high enough to create problems if not properly contained. Storage tanks consist of some of the important parts like shell, roof, bottom, nozzle, manhole, etc.
The API 650 and API 620 standards provide rules and guidelines for the design, construction, inspection, testing, and certification of storage tanks to ensure their safe operation under various pressure and temperature conditions. These standards are widely recognized and followed by manufacturers, operators, and regulators worldwide to ensure the integrity and safety of storage tanks used in various industries.
Storage Tank vs Pressure Vessel
Differences between a Tank and a Pressure Vessel
Pressure vessels and storage tanks differ in several aspects, such as shape, purpose, construction, orientation, mounting, material, size, cost, and heating method. Here are some of the key differences between them:
Shape:
Pressure vessels can have various shapes, such as cylindrical, spherical, rectangular, or conical, depending on the design requirements and the type of fluid stored. Cylindrical pressure vessels are one of the most common types. They have a cylindrical shape with flat or dished ends, offering a simple and effective design for containing fluids under pressure. These vessels are used in applications such as storage tanks, air receivers, and hydraulic accumulators.
Spherical pressure vessels have a spherical shape, offering the best design for containing fluids under high pressure. These vessels are used in applications such as gas storage, liquefied natural gas (LNG) carriers, and nuclear reactors.
Rectangular pressure vessels have a rectangular shape, offering a compact design for containing fluids under low to moderate pressure. These vessels are used in applications such as water heaters, boilers, and autoclaves.
Conical pressure vessels have a conical shape, offering a tapered design for containing fluids under varying pressure. These vessels are used in applications such as rocket engines, nozzles, and diffusers.
Storage tanks, on the other hand, are usually cylindrical in shape, with flat or conical roofs and bottoms, offering a simple and economical design for storing fluids under low to moderate pressure. These tanks are used in applications such as fuel storage, water storage, and oil storage. Spherical tanks and cylindrical bullet tanks are also found for low-pressure storage applications.
Purpose:
Pressure vessels and storage tanks have different purposes, depending on the type of fluid stored and the process involved. Pressure vessels are primarily used for processing, separating, or transporting fluids under pressure. They often serve as reactors, heat exchangers, distillation towers, separators, reboilers, knock-out drums, scrubbers, dryers, coalescers, etc for various industries, including chemical, petrochemical, oil and gas, power, pharmaceutical, and food and beverage.
Storage tanks are primarily used for storing liquids or gases at a specified pressure. Storage tanks often serve as reservoirs for various industries, including petrochemicals, pharmaceuticals, and water treatment plants.
Construction:
Pressure vessels and storage tanks have different construction methods, depending on the design requirements and the type of fluid stored. Pressure vessels are usually made of thick-walled materials, such as carbon steel, stainless steel, alloy steel, or composite materials, to withstand high pressure and temperature. They are also welded, bolted, or riveted together, depending on the size and shape of the vessel. Pressure vessels are also equipped with various accessories, such as valves, gauges, safety devices, insulation, etc, to control and monitor the pressure and temperature of the fluid inside.
Storage tanks are usually made of thin-walled materials, such as carbon steel, stainless steel, aluminum, or plastic, to store low to moderate-pressure fluids. They are also fabricated, erected, or assembled on-site, depending on the size and shape of the tank. Storage tanks are also equipped with various accessories, such as vents, overflows, drains, ladders, platforms, etc, to facilitate the filling and emptying of the fluid inside.
Orientation:
Pressure vessels and storage tanks have different orientations, depending on the design requirements and the type of fluid stored. Pressure vessels can be horizontal, vertical, or inclined, depending on the space available and the flow direction of the fluid. Horizontal pressure vessels are used when the fluid has a low-density difference or a high viscosity, such as oil, water, or air. Vertical pressure vessels are used when the fluid has a high-density difference or a low viscosity, such as gas, steam, or liquid. Inclined pressure vessels are used when the fluid has a moderate density difference or a moderate viscosity, such as two-phase fluids, slurries, or mixtures.
Storage tanks are usually vertical, with flat or conical roofs and bottoms, depending on the type of fluid stored and the environmental conditions. Vertical storage tanks are used when the fluid has a low vapor pressure or a high specific gravity, such as water, oil, or diesel. Flat-roofed storage tanks are used when the fluid has a moderate vapor pressure or a moderate specific gravity, such as gasoline, kerosene, or ethanol. Conical-roofed storage tanks are used when the fluid has a high vapor pressure or low specific gravity, such as propane, butane, or ammonia.
Mounting:
Pressure vessels and storage tanks have different mounting methods, depending on the design requirements and the type of fluid stored. Pressure vessels can be supported by legs, skirts, saddles, lugs, or brackets, depending on the size, shape, and weight of the vessel. Legs are vertical supports that are attached to the bottom of the vessel, providing simple and stable support. Skirts are cylindrical or conical supports that are attached to the bottom of the vessel, providing a rigid and uniform support. Saddles are curved supports that are attached to the sides of the vessel, providing flexible and adjustable support. Lugs are protruding supports that are attached to the sides of the vessel, providing convenient and portable support. Brackets are angular supports that are attached to the sides of the vessel, providing compact and lightweight support.
Storage tanks can be supported by foundations, piers, columns, or piles, depending on the size, shape, and weight of the tank. Foundations are concrete or steel structures that are built on the ground, providing solid and durable support. Piers are vertical supports that are built on the ground, providing simple and economical support. Columns are vertical supports that are built above the ground, providing flexible and adjustable support. Piles are vertical supports that are driven into the ground, providing strong and stable support.
Size:
Pressure vessels are generally smaller in size than storage tanks, as they have to contain higher pressure and temperature. Storage tanks are larger in size than pressure vessels, as they have to store larger volumes of fluid at lower pressure and temperature.
Cost:
Pressure vessels are more expensive than storage tanks, as they require more material, labor, and quality control to ensure their safety and performance. Storage tanks are less expensive than pressure vessels, as they require less material, labor, and quality control to ensure their functionality and durability.
Below is a tabular format outlining the main differences between a pressure vessel and a storage tank:
Pressure Vessel
Storage Tank
Pressure vessels contain gases or liquids at high pressure.
Storage tanks hold liquids or gases at atmospheric pressure or low pressure.
Operating under high pressure (above atmospheric pressure) is the main characteristic of every type of pressure vessel.
Storage Tanks operate at or near atmospheric pressure or low pressure.
Pressure vessels are engineered and designed to withstand significant internal pressure.
The design of storage tanks focuses on storing and retaining fluids with minimal pressure resistance.
Pressure vessels are utilized in various industries like chemical, petrochemical, power generation, mineral, food, pharmaceutical, etc.
Storage tanks are commonly used in storage applications to store fuel, water, crude oil, etc.
The design of pressure vessels requires rigorous stress analysis and design calculations
Storage tank generally has less complex stress analysis and design requirements
Pressure vessel usually has thicker walls to handle high pressure.
In most situations, storage tanks have thinner walls due to lower pressure requirements.
Safety relegation of pressure vessels is subjected to strict requirements and codes
The storage tank follows safety regulations but usually has fewer requirements
Most pressure vessels require pressure relief devices to prevent overpressure.
Storage tanks are usually equipped with vents for pressure equalization.
Pressure vessels can be mobile or stationary depending on the application.
Typically the storage tanks are stationary and not designed for mobility.
Table 1: Storage Tanks vs Pressure Vessels
Note that pressure vessels and storage tanks have distinct differences in design, purpose, and applications. However, there may be some overlap in their usage based on specific industry requirements and operating conditions.
Pressure vessels are integral components in a wide range of industries, ensuring the safe storage, transportation, and processing of fluids under pressure. Understanding the different types of pressure vessels and their specific applications is vital for engineers, designers, and professionals working with these critical components. Whether it’s cylindrical, spherical, storage, heat exchangers, or reactors, each type serves a distinct purpose and plays a significant role in various industrial processes, contributing to the advancement and safety of modern technology and infrastructure.
Pressure vessel fabrication refers to the process of manufacturing containers designed to hold fluids or gases at varying pressures, often under high or elevated pressure conditions. These vessels are found in a wide range of industries, including petrochemical, pharmaceutical, aerospace, nuclear, food, and more. The process of pressure vessel fabrication involves planning, studying pressure vessel general arrangement and fabrication drawings, selecting appropriate materials, assembling, welding, testing, and inspecting these vessels to ensure they meet specific safety and performance standards.
Pressure vessels are crucial components in various industrial processes where the containment of substances under pressure is essential for the successful operation of various applications. Pressure vessel fabrication must adhere to strict engineering and quality control standards to ensure the vessels’ integrity and safety in their intended use at different pressures and temperature conditions. It is a complex and highly specialized field that requires precision, skill, and adherence to strict quality standards. In this blog post, we will explore the process and key considerations of pressure vessel fabrication. Fig. 1 below shows an example of typical pressure vessel fabrication.
Fig. 1: Pressure Vessel Fabrication
Key Considerations in Pressure Vessel Fabrication Process
Pressure vessels often work under a range of varying pressure and temperature. So, its fabrication must be proper so that the equipment performs the intended operation for its design life without failure. The fabrication of a pressure vessel is a complex process and it involves several critical considerations.
Pressure Vessel Fabrication Drawing
Pressure vessel fabrication drawings are detailed technical drawings that provide a visual representation of the design, dimensions, and specifications of a pressure vessel. These drawings serve as a crucial reference for the designers, inspectors, and fabricators during the manufacturing and quality assurance process. Pressure vessel fabrication drawings are typically created by experienced engineers and designers using Computer-Aided Design (CAD) software to ensure precision and adherence to design codes and standards.
The fabrication drawing shows all details required for equipment fabrication such as dimensions, part detailing, weld details, nozzle schedules, bill of material, design data, equipment plan view, elevation view, sectional views, etc. Here’s what is typically found in a pressure vessel fabrication drawing:
Pressure Vessel Design Code
Vessel Geometry that includes detailed illustrations of the vessel’s shape, dimensions, cross-sections, and overall geometry. The fabrication drawings provide the vessel’s diameter, height, and specific features like nozzles, manholes, or flanges.
Material Specifications to inform the pressure vessel fabricator about the material, grade, and thickness to be used in the fabrication process.
Welding Details like the type of welds, welding locations where welding is required, weld joint details, and weld size requirements are typically included.
Information related to Pressure Vessel Nozzles and Openings like nozzle positions, sizes, and types of nozzles and openings, including details like flange ratings and orientations.
Pressure and Temperature Ratings for the vessel’s design
Reinforcement and Supporting details
Instrumentation and Accessories such as pressure gauges, safety valves, sight glasses, and level indicators that may be attached to the vessel.
Once the pressure vessel fabrication drawing is generated, the fabricators produce the pressure vessel following the below-mentioned steps:
1. Material Preparation:
Material preparation is a crucial first step in pressure vessel fabrication. It involves the following activities:
Material Selection: The material requirement of each component is mentioned in the vessel fabrication drawing. The selection of appropriate materials is done by the material engineers based on factors such as the type of substance to be stored, operating temperature, and pressure conditions. Some of the common pressure vessel materials are carbon steel, stainless steel, LTCS, alloy steel, and exotic alloys.
Cutting and Shaping: Selected materials are then cut, shaped, and prepared for assembly according to the vessel’s design specifications. This includes cutting sheets or plates into the required shapes and sizes.
Edge Preparation: The edges of the material pieces are often prepared to ensure proper fit-up and weld quality. This may involve beveling or chamfering the edges.
2. Welding:
Welding is a critical process in pressure vessel fabrication. Skilled welders are responsible for joining the prepared material pieces to create the vessel’s shell and heads. The welding process includes:
Welding Techniques: The use of appropriate welding techniques such as shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), or submerged arc welding (SAW), depending on the materials and design.
Weld Joint Preparation: Preparing the joints for welding, which may involve bevelling or chamfering, and ensuring proper alignment.
Quality Control: Continuous monitoring of the welding process to ensure high-quality and defect-free welds.
3. Heat Treatment:
Heat treatment is often applied to pressure vessel components after welding to relieve residual stresses and enhance material properties. The heat treatment process usually involve:
Stress Relieving: Controlled heating and cooling to reduce stresses induced during welding, which can help prevent cracking and improve material properties.
Normalizing: A heat treatment process that refines the grain structure and improves material uniformity.
4. Testing:
Testing is a critical aspect of pressure vessel fabrication to ensure the vessel’s integrity and safety. Common tests include:
Non-Destructive Testing (NDT): Some of the common NDT methods are radiography, ultrasonic testing, magnetic particle testing, and dye penetrant testing. They detect defects and ensure the welds are sound.
Hydrostatic Testing: Filling the vessel with a liquid, usually water, and pressurizing it to verify its performance and to check for leaks or deformations.
5. Surface Finishing:
After testing, the pressure vessel undergoes surface finishing, which includes:
Cleaning: Removing any contaminants, oils, or residues from the vessel’s surface to prepare it for surface treatments.
Painting or Coating: Applying paint or protective coatings to the vessel’s exterior to prevent corrosion and enhance its appearance.
6. Final Inspection and Documentation:
Before the vessel is ready for delivery, it undergoes a final inspection to ensure compliance with design specifications and industry standards. This step includes:
Visual Inspection: A thorough visual examination of the vessel’s components and welds.
Documentation: Preparing comprehensive documentation that includes material certificates, welder qualifications, NDT reports, and other quality-related documents.
7. Delivery and Installation:
Once the pressure vessel has passed all quality control checks and inspections, it is prepared for delivery to the client’s site. This involves:
Transportation: Safely moving the vessel to the installation site, which may require specialized equipment and transportation methods.
Installation: Installing the pressure vessel at the intended location, often involving the connection of piping, instrumentation, and safety mechanisms for it to be fully operational.
To ensure the successful fabrication and safe operation of pressure vessels, each of these steps plays a crucial role, especially when industries prioritize safety and performance. Adherence to design specifications and industry standards is essential throughout the entire fabrication process.
Video Tutorial on Fabrication of Pressure Vessel
Refer to the following video, that explains the process of a typical vessel fabrication process clearly.
Pressure Vessel Fabrication Video Tutorial
Hydrostatic Test of Pipelines | Hydrostatic Test Equipment
Hydrostatic testing of pipelines ensures the overall integrity of the system. It is a leak testing method performed in all liquid and gas pipelines before the first content throughput. The term hydro is related to water, so pipeline hydrostatic testing is performed using water as the primary test media. The water inside the pipelines is pressurized and the joints are then inspected for any possible sign of leakages. In this article, we will learn the following:
Definition of pipeline hydrostatic testing
Pipeline hydro test pressure
Pipeline hydro test procedure
Definition of Pipeline Hydrostatic Test
Pipeline hydrostatic testing or hydro testing of pipelines is a non-destructive testing method to evaluate the strength and integrity of pipelines. It ensures that the pipelines can safely operate at their maximum operating pressure. Hydrostatic Testing is a standard practice in the oil and gas, water, and chemical industries wherever liquid, gas, or water pipelines are used to verify that pipelines can withstand their intended service conditions without leaks, ruptures, or other structural failures.
So, the main purpose of Hydrostatic testing pipelines is to ensure the pipeline’s operational fitness. The pipeline hydrostatic test is also known as pipeline pressure test, hydrostatic pressure test, or pipeline pressure testing.
Hydrotest Pressure of a Pipeline
The pressure at which the pipeline hydrostatic test or pipeline pressure test is performed is known as the test pressure. The hydrostatic test pressure for pipelines is designed based on the guidelines provided in the ASME B31.4 code for liquid pipelines and the ASME B31.8 code for gas pipelines. In general, the test pressure for the pipeline system is selected as the minimum of
1.25 times the design pressure
the pressure that creates a hoop stress of 90% SMYS of the material.
the test pressure value of the weakest component in the pipeline system.
Pipeline Hydrostatic testing is performed once the pipeline is erected and constructed at the site. As the pipeline profiles vary based on the soil/land elevations, the pressure will not be constant at each point of the pipeline. So, the elevation profile must be plotted to check that the test pressures at the low and high points will not result in pressures or combined stresses outside the above limits.
Working Principle of Hydrostatic Testing of Pipelines
The working philosophy of pipeline hydrostatic testing is as follows:
The pipeline section is filled with treated water and then gradually the pressure is increased till it reaches the hydrotest pressure as defined above. This pressure is more than the pipeline’s design pressure. Once the test pressure is reached, the pressure is held for a certain timeframe and then all joints are visually inspected for any possible leakages. If leaks are found in any section, those are marked. Once the full section is inspected, the pipeline is depressurized and water is removed. Vent and Drains must be installed in the pipeline to help in air escape while water filling and draining water once the test is complete.
Sequence Steps for Pipeline Hydrostatic Test Procedure
Fig. 1 below shows the sequence of hydrostatic pressure testing operations of pipelines.
Fig. 1: Pipeline Hydrostatic Test Sequence
The hydrostatic testing must be performed by a highly experienced pipeline hydro test engineer. Each organization usually has its own procedure for pipeline pressure testing operations. The procedure for pipeline hydrostatic test includes:
Detailed list and schedule of the main activities, tests, and responsible personnel.
Details of test sections with approved hydro test diagram.
Potential safety and environmental hazards Identification
Details of water source, treatment methods, disposal, permit requirements, etc.
Test section preparation details, including temperature stabilization.
Hydrostatic test details
Post-test activity details
Rectification activity details (Leak rectification when a leak is observed, dewatering, etc)
Hydrotest water disposal plan.
Hydrostatic Testing for Buried Pipelines
Hydro testing of aboveground pipelines is comparatively easy as the test section can be visually inspected. But for buried or underground pipelines, the same is not quite possible.
For buried pipelines, the complete hydrostatic pressure test is performed in two steps. Initially, the pressure is increased to the hydro test pressure and left for a certain period of time and its strength is tested. In the next step, a leak tightness test is performed to assess any possibility of leaks. A leak tightness test is performed immediately after the strength test using the same water in the pipeline. To allow for pressure variations due to temperature changes during the test duration, the hydrostatic test pressure is set at a level of 1.1 times the design pressure. For all buried pipelines and flowlines the minimum duration for the hydrostatic pressure test is generally 4 hr strength test followed by a 24 hr leak tightness test.
During the leak tightness test, the pressure is continuously recorded to find any variation. The pipe, soil, and air/sea-water temperature is also recorded at 1-3 hour intervals for calculating pressure-temperature variation.
If the recorded pressure is found to be stable after accounting for the pressure-temperature variation, it can be concluded that there are no leaks.
Hydrostatic Test Equipment
All the temporary fittings, piping, test heads, pumps, pigs, materials, equipment, and consumables used for the pressure testing operations are part of hydrostatic test equipment. The term hydro test equipment does not mention a single equipment, but it refers to all the components required for a successful hydrostatic testing process.
In general, the hydrostatic test equipment consists of the following equipment/components:
Filling and pressurizing pumps
Filters
Valves
Break Tanks and Piping
Chemical Injection Pumps
Air-conditioned or heated test cabin unit
Deadweight tester
Pressure data logger
Filling/Pressurising flowmeter and temperature probe
A schematic hydrostatic test equipment layout for a typical pipeline test section is shown in Fig. 2 below:
Fig. 2: Layout of a Hydrostatic Test Equipment
The following video clearly explains the hydrostatic test procedure of a natural gas pipeline
Hydro-test procedure for a pipeline
Frequently Asked Questions
Here are some of the frequently asked questions related to hydrostatic testing of pipelines for your ready reference:
What are the requirements for hydrostatic testing of pipelines?
All new pipelines must be hydro-tested before commissioning. For existing pipeline systems, if any repair or alteration is made, then a hydrostatic test needs to be performed before bringing them into operation.
What is the hydro-test pressure for pipelines?
In general, the pipeline hydro test pressure is calculated as 1.25 times the pipeline design pressure. However, there are other considerations as mentioned in the article above.
What is the ASME standard for hydrostatic testing of pipelines?
Pipeline hydrostatic testing is performed by following the guidelines given in ASME B31.4 for liquid pipelines and ASME B31.8 for gas pipelines.
What is the purpose of a pipeline hydrostatic test?
The hydrostatic testing of pipelines ensures the structural integrity and leak tightness of the pipeline system.
What is the duration of the pipeline hydrostatic test?
The duration of the pipeline system is decided based on various factors like the type of pipelines (aboveground or buried), the total number of pipeline joints, the length of the test pipeline section, hydrostatic testing equipment capacities, manpower employed for the test and inspection, temperature stabilization time, etc. In general, a minimum of 4 hours is required for aboveground pipeline hydrostatic testing and 24 hours is required for underground pipeline systems.
