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What to do when a blocked valve is closed accidentally?

asked Jul 12, 2016 in Process by TPham (5,010 points)
The described situation happens from the inadvertent closure of a block valve or failure of a control valve in the closed position on an outlet line.  Typical overpressure application, where this form of protection is required, includes production separators, compressor discharge piping, or pump discharge piping.  The safety relief valves are sized to handle 100% of the anticipated upstream flow.  If the feed stream is multi-phase, the relief rate is the total inlet flow (gas plus liquids).

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answered Jul 12, 2016 by TPham (5,010 points)

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.

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