A new edition of the UK’s pressure vessel code has just been published. This blog post discusses what the code does and why it matters.
A pressure vessel is a housing designed to contain fluids or gases under pressure. Such vessels are found throughout industry and are indispensable. However they’re also a potentially lethal hazard: they store huge amounts of energy and can fail without warning. When they do the consequences can be devastating. A 2010 report to the World Congress on Engineering recorded that between 1992 and 2001, there were 23,338 pressure vessel related accidents, that’s an average of 2,334 per year. The number of fatalities over this 10-year period was 127.
Incidents that the report highlights include that of a 22,680 kg pressure vessel that exploded at a chemical plant in Houston, Texas in 2004. Individual pieces of shrapnel weighing over 450 kg travelled at high speed for distances of up to half a mile before landing on highways, railway lines, cars, a church and other nearby buildings. A large portion of the plant was destroyed and three workers were killed.
Closer to home, in 2005 43 people were injured and over 2,000 people evacuated when a pressure vessel failed at the Buncefield oil storage facility in Hertfordshire, England, causing a vapour cloud of evaporated leaking fuel. The timing of the explosion – at 6am on a Sunday morning – likely saved lives. However twenty petrol tanks caught fire, the initial blast was heard up to 125 miles away, and smoke from the fire could be seen from space.
More recently, RIDDOR  data from 2014 to 2020 shows that ‘failure of a pressure system’ was reported as a “dangerous occurrence” (incidents with a high potential to cause death or serious injury) 387 times. That’s an average of more than 60 incidents a year in the UK over the last six years.
Critical for safety
It’s because pressure vessels can be so dangerous that they need standards and codes that set out good safety practice. In the UK, since 1976, the national code has been PD 5500. It replaced the UK’s various sector-specific pressure vessels standards that had been in existence since the 1950s. The latest version of PD 5500 is PD 5500:2021 Specification for unfired fusion welded pressure vessels.
This code specifies the requirements for the design, manufacture, inspection and testing of unfired pressure vessels made from carbon, ferritic alloy, and austenitic steels. It also includes material supplements containing requirements for vessels made from other materials such as aluminium, copper, nickel, titanium, and duplex. The 2021 edition starts the latest three-year publishing cycle that will see updates to the code released in September 2021, 2022 and 2023.
Changes and enhancements
Meanwhile this new edition brings together all of the changes and enhancements introduced during the 2018 publishing cycle that made the code more accurate and user-friendly during the last three-years. These include the updating of various internal and external references; a new Enquiry Case for cylindrical shells of varying thickness under external pressure; new requirements for load combinations on pressure vessels, for flanges with swing bolts or clamp bolts and for permanent joints other than welding. As well the code was clarified in some areas and revisions made to enhance the document’s accuracy and usability.
The result is an invaluable reference tool for the design and assessment of pressure vessels. PD 5500 is predominantly used to produce pressure vessels for the UK market, but sometimes also used in other places throughout the world – particularly in Commonwealth/former Commonwealth countries, the Middle East, and Asia (China, Malaysia). In all cases the end goal is to produce safe and cost effective pressure vessels.
 See: http://www.iaeng.org/publication/WCE2010/WCE2010_pp1120-1123.pdf
 Under The Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013 (RIDDOR), “Pressure systems” covers “The failure of any closed vessel or of any associated pipework (other than a pipeline) forming part of a pressure system as defined by regulation 2(1) of the Pressure Systems Safety Regulations 2000(1), where that failure could cause the death of any person.” See: https://www.hse.gov.uk/riddor/dangerous-occurences.htm