Pressure relief systems 'neglected'

Paul Boughton
It is most likely your existing pressure relief systems are installed to protect your plant and equipment from the consequences of under and overpressure in accordance with relevant design codes and best practice expectations. Such relief devices will be installed as an integral part of your basis of safety when fitted to equipment such as a pressure vessel or storage tank.

Essentially such devices are used to protect people and the environment, mitigate business and financial risk and are usually perceived as the last line of defence when all other layers of protection fail.

However over time, end user operators assume that because there is a relief system installed, that part of the process is adequately protected, no matter how long ago the relief stream was designed or installed. This belief is usually based on the assumption that nothing has really changed within the process, which in itself may well have been developed more than some 20 years earlier.

A recent process industries site survey by ABB Engineering Services compared the valve set pressure against the pressure at which they actually operated. Of the 341 valves tested, 68 (20 per cent) failed to lift within 10 per cent of the cold set pressure, and 13 valves (4 per cent) failed to lift at 1.3 times the cold set pressure.

With regard to your safety management performance criteria it would be noted that this range of statistics would generate real cause for concern. They suggest that a significant percentage of relief devices are simply not designed or installed correctly and will provide little or no protection when they are called upon in a hazardous situation.

As pressure relief systems appear to have been neglected over recent years, there will be a need for more and more companies to address this key area of vulnerability and to commence a strategy to remedy the situation. Also it should be noted that regulatory bodies are focusing more on relief stream design, maintenance and verification.

To assist in this matter, ABB have a proven methodology to address the requirements for identifying the credible relief scenarios, the sizing basis for the device itself, the requirements for the management of correct inspection, maintenance and history of the overall relief system (not just the specific device) and the assurance that confidence exists that the plant and equipment is is protected from under and overpressure.

ABB Global Consulting Ltd is based in Warrington, Cheshire, UK.

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