When sizing safety relief valves for this situation, do not consider the inadvertent closure of car-sealed and locked-open valves or spectacle blinds. Generally, do not take credit for reduction in relief load because of increased back pressure at relieving conditions except when the following items are all true:
- The flow rate proves to be excessive (e.g., results in overloading of the flare or vent system).
- Reliable performance data for the source of overpressure (compressor, pump, well, etc.) is available.
- The source would actually have a flow reduction due to increasing backpressure. For instance, a reciprocating pump would not, while a centrifugal pump would, respond to increasing backpressure.
- For all of those exceptional cases listed above, rigorous blocked outlet calculations should be performed your Subject Matter Experts.
Assume for blocked outlet cases that the inlet flow is at the maximum rate consistent with normal operation, startup, or shutdown. Note that for snap acting on/off control valve operation that the maximum flow will be controlled by the characteristics of the control valve at 100% open as opposed to the maximum flow from the facility heat and material balance. Downstream pressure should be taken as the system relieving pressure.