Pressure relief systems are often the last line of defence in protecting hazardous process plants against overpressure. However, they are rarely tested in operation, so how can you be sure that they will perform when needed?
In order to ensure that relief systems work correctly both the design and maintenance systems have to be robust. The design must ensure that the worst case relief rate has been identified and that the whole system is designed to handle this rate. Maintenance and inspection systems have to ensure that the relief pipework, in-line items and safety valves are regularly inspected and are functioning properly. Finally any changes and modifications to the system have to be carefully controlled to make sure that they don’t compromise the original design.
Many operators find it difficult to guarantee the effectiveness of their
relief systems, for the reasons stated above, but one area they feel more confident about is the reliability of relief valves themselves. However recent data from a study of oil and gas industry relief valves suggests that this confidence may be misplaced.
Nearly 7000 relief valves were tested and some of the key findings were:
* only 66 per cent lifted within ±10 per cent of their set pressure.
* 4 66 per cent had not lifted at 130 per cent of set pressure.
* 49 per cent failed to reseat and hold pressure within 10 per cent of their set pressure.
* 30 per cent failed to reseat and hold pressure at all.
Clearly this data is concerning and operators should be vigilant in checking their own safety valve test results and acting on the conclusions.
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