Information on pressure relief beyond the API 520 and API 521. Learn from Practical examples.

In a previous post I addressed the issue of not designing a relief valve for liquid overfill in relation with the requirements for instrumental protection. Taking this subject a little bit further, there is also a relation between liquid overfill and design pressure.

Not taking this relationship into account during early design stages may lead to surprises at later stages as safety reviews may require corrective action to safeguard the design. What could have been an easy solution (selecting a slightly higher design pressure) will then no longer be an option as it would usually mean massive re-work and schedule impact.
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waterAPI standard 521 provides a possibility to take credit for operator response to avert relief. It says that a commonly accepted time range for the response is between 10 min and 30 min, depending on the complexity of the plant. In practice this criterion is often used to rule out liquid overfill as relief contingency for large vessels like for instance distillation columns. A ‘nice’ fringe benefit of this action that we do not have to account for liquid discharges to the flare system (at least not on paper that is!).

Is Operator Intervention Safe?

An ‘interesting’ situation will occur if the shut-off pressure of a charge pump significantly exceeds the design pressure of the downstream vessel while the installed relief valve is not sized for the pump capacity (taking credit for operator intervention). This would then mean that catastrophic failure would occur in case the operator “fails on demand” to stop the overfill.

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