Hi,

May I know when we are talking about Overfilling scenario for liquid relief sizing, do we consider the drum pump to be running to bring the drum liquid level down? Relief load for PSV sizing would be maximum inflow - outflow (i.e. pump capacity)?

Thanks for answering my question.

Credit on the outflow should not be taken for overfilling scenario.
Reason being the pump (outflow) could failed stop and with continue inflow into the tank.

may i ask then what is the distinction between overfill and block outlet? i think i am confused.

Lets take an example...
Liquid feeding a tank with proper control valve and control loop. And liquid is pump out from the tank feeding user with proper control valve and control loop.

i) Block discharge
The control valve on the pump discharge closed due to control loop failure results no flow.
This may apply to vent line.

ii) Overfill
The control valve on the feeding line failed (to FULL OPEN position) leading to liquid level increase. An overflow line may be a potential ultimate safeguarding measure for tank ovefill.

Hope this help.

I think the distinction can be somewhat semantic too. A vessel or tank that is not normally run full, can be overfilled. A vessel that is normally run full, cannot be overfilled, but can be blocked-in.

thanks for your explanation, it helps!