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PD 5500:2009 - Benefits and challenges of unfired fusion welded pressure vessels


PD 5500:2009 Specification for unfired fusion welded pressure vessels.pdf




Introduction




PD 5500:2009 Specification for unfired fusion welded pressure vessels.pdf


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If you are involved in the design, manufacture, inspection or testing of unfired pressure vessels, you might have heard of PD 5500. But what is it exactly and why is it important? In this article, we will explain what PD 5500 is, what are unfired fusion welded pressure vessels, and why you should use PD 5500 as a reference tool for your projects.


What is PD 5500?


PD 5500 is a published document by the British Standards Institution (BSI) that specifies requirements for the design, construction, inspection and testing of new unfired fusion welded pressure vessels. It was first published as BS 5500 in March 1976 and then as PD 5500 in January 2000. The latest edition is PD 5500:2009, which was published in January 2009 and amended in September 2012.


PD 5500 covers unfired pressure vessels made from carbon, ferritic alloy and austenitic steels, as well as material supplements containing requirements for vessels made from aluminium, copper, nickel, titanium and duplex. It also includes guidance on the use of alternative materials and design methods.


PD 5500 is not a British Standard, but it is widely recognized and accepted as a code of practice for unfired pressure vessels in the UK and internationally. It is also harmonized with the European Pressure Equipment Directive (PED) and can be used to demonstrate compliance with its essential safety requirements.


What are unfired fusion welded pressure vessels?


Unfired pressure vessels are containers that are designed to hold fluids (liquids or gases) under pressure without being exposed to direct heat from a combustion process. They are used for various applications such as storage, transport, processing and power generation.


Fusion welding is a process that joins two or more metal parts by melting them together using an electric arc, a gas flame or a laser beam. Fusion welding creates a strong bond between the metal parts and reduces the risk of leakage or failure.


Unfired fusion welded pressure vessels are therefore unfired pressure vessels that are constructed using fusion welding techniques. They can have various shapes and sizes depending on their purpose and design criteria.


Why use PD 5500?


PD 5500 is an invaluable reference tool for the design and assessment of unfired fusion welded pressure vessels. It provides comprehensive and detailed requirements for all aspects of their design, construction, inspection and testing. It also offers best practices and recommendations based on the latest research and experience in the field.


By using PD 5500, you can benefit from:



  • Safer and more reliable pressure vessels that meet the highest standards of quality and performance



  • More cost-effective and efficient pressure vessels that optimize the use of materials and resources



  • Increased trust and confidence from your customers, regulators and stakeholders



  • Easier access to new markets and trade opportunities by demonstrating compliance with the PED and other international codes and regulations



  • Better management of risk and liability by following a recognized and accepted code of practice



Design requirements




General


The design of unfired fusion welded pressure vessels should be based on sound engineering principles and practices. The design should consider all the relevant factors that affect the safety and performance of the pressure vessels, such as:



  • The nature and properties of the fluid to be contained



  • The operating conditions and environment of the pressure vessels



  • The loads and stresses imposed on the pressure vessels



  • The corrosion, erosion and protection of the pressure vessels



  • The fabrication, inspection and testing methods to be used



  • The documentation and certification to be provided



The design should also comply with the applicable laws and regulations in the country or region where the pressure vessels are intended to be used.


Application


The design of unfired fusion welded pressure vessels should take into account the following parameters:



  • Design pressure: The maximum pressure that the pressure vessel is designed to withstand under normal operating conditions. It should be determined by the designer based on the fluid properties, the operating temperature, the safety devices and the allowable stress.



  • Design temperature: The maximum and minimum temperatures that the pressure vessel is designed to withstand under normal operating conditions. It should be determined by the designer based on the fluid properties, the heat transfer, the thermal expansion and contraction, and the material properties.



  • Thermal loads: The changes in temperature and pressure that occur during start-up, shut-down, normal operation, emergency situations and transient conditions. They should be considered by the designer to ensure that the pressure vessel can accommodate them without exceeding its design limits.



  • Wind and earthquake loads: The external forces that are applied to the pressure vessel due to wind or seismic events. They should be considered by the designer to ensure that the pressure vessel can resist them without compromising its stability or integrity.



Materials selection




General


The materials used for unfired fusion welded pressure vessels should be suitable for their intended purpose and service conditions. They should have adequate strength, ductility, toughness, corrosion resistance, weldability and compatibility with the fluid to be contained.


The materials should also conform to the specifications and standards referenced in PD 5500 or approved by the Inspecting Authority. The materials should be identified, tested, certified and traceable throughout their supply chain.


Materials for pressure parts


The materials for pressure parts are those parts of the pressure vessel that are directly exposed to the fluid under pressure, such as shells, heads, nozzles, flanges, supports and attachments. The materials for pressure parts should meet the following requirements:



  • They should have a specified minimum yield strength (SMYS) not less than 165 MPa.



  • They should have a specified minimum tensile strength (SMTS) not less than 1.15 times their SMYS.



  • They should have a specified minimum elongation at fracture (A) not less than 14% for carbon and low alloy steels, 20% for austenitic steels, 12% for aluminium alloys, 15% for copper alloys, 20% for nickel alloys, 10% for titanium alloys and 25% for duplex steels.



  • They should have a specified minimum Charpy V-notch impact energy (CVN) not less than 27 J at -20C for carbon and low alloy steels, 40 J at -196C for austenitic steels, 15 J at -80C for aluminium alloys, 15 J at -40C for copper alloys, 40 J at -196C for nickel alloys, 40 J at -60C for titanium alloys and 40 J at -50C for duplex steels.



  • They should have a specified maximum carbon equivalent (CE) not greater than 0.43% for carbon and low alloy steels, 0.35% for austenitic steels, 0.40% for aluminium alloys, 0.30% for copper alloys, 0.35% for nickel alloys, 0.10% for titanium alloys and 0.40% for duplex steels.



Construction and workmanship




General


The construction and workmanship of unfired fusion welded pressure vessels should be carried out in accordance with the approved design and welding procedures. The construction and workmanship should ensure that the pressure vessels are free from defects, distortions and residual stresses that could impair their safety and performance.


The construction and workmanship should also comply with the applicable laws and regulations in the country or region where the pressure vessels are intended to be used.


Welding


Welding is a crucial part of pressure vessel construction, as it joins the various components of the pressure vessel together. Welding should be performed by qualified welders using suitable welding techniques and equipment. The welding process should ensure that the welds are strong, durable and compatible with the base materials.


Some of the common welding techniques used in pressure vessel construction are:



  • Shielded Metal Arc Welding (SMAW): This welding process uses a flux-coated consumable electrode. This is often thought of as the default form of arc welding. It is versatile and can be used for various materials and positions. However, it produces slag that needs to be removed after welding, and it may not be suitable for thin materials or high-quality welds.



  • Gas Metal Arc Welding (GMAW): This welding process uses a continuously fed solid wire electrode and an inert gas shield. It is also known as metal inert gas (MIG) welding. It is fast and efficient, and it produces clean welds with minimal spatter. However, it requires a constant power source and gas supply, and it may not be effective for thick materials or vertical positions.



  • Flux-Cored Arc Welding (FCAW): This welding process uses a continuously fed tubular wire electrode filled with flux and an external gas shield. It is similar to GMAW but with more flexibility and penetration. It can be used for various materials and positions, and it can tolerate some contamination on the base metal. However, it also produces slag that needs to be removed after welding, and it may generate more fumes than GMAW.



  • Gas Tungsten Arc Welding (GTAW): This welding process uses a non-consumable tungsten electrode and an inert gas shield. It is also known as tungsten inert gas (TIG) welding. It is versatile and can be used for various materials and positions. It produces high-quality welds with excellent appearance and strength. However, it is slower and more complex than other welding processes, and it requires a high level of skill and precision.



  • Plasma Arc Welding (PAW): This welding process uses a non-consumable tungsten electrode and a plasma gas shield. It is similar to GTAW but with more heat and speed. It produces high-quality welds with deep penetration and low distortion. However, it is more expensive and less flexible than other welding processes, and it may require special equipment and safety precautions.



The choice of welding technique depends on various factors such as the material type, thickness, shape, position, joint design, quality requirements, cost and availability. The welder should select the most appropriate technique for each situation and follow the approved welding procedure specification (WPS).


The welder should also ensure that the welds are properly prepared, aligned, cleaned, preheated, post-heated, cooled and inspected according to PD 5500 requirements.


Heat treatment




General


Heat treatment is a process of heating and cooling metal parts to alter their physical and mechanical properties. Heat treatment can be used to improve the strength, ductility, toughness, hardness, corrosion resistance or stress relief of metal parts.


Non-destructive testing