Hello everyone!,
Currently, I'm involved in the design of a closed drain system; it consists on, as usual, of drain connections from several equipment discharging to a header which collects all the fluids and directs them to a Closed Drain drum. This drum is equipped with pumps to discharge the liquids once a certain level within the vessel is reached and it is connected to the LP flare through an open vent. The LP flare system was designed based on the scenario of a fire occuring on this vessel (fire case). Our client is asking for verification of gas blowby scenarios to check if the vent of the vessel is correctly sized; this is understandable, however, I have the following concerns:
1) Not a single piece of equipment (mainly pig launchers, headers and manifolds) is connected to the closed drain system has an automatic level control; all drain connections are manually valved (or equipped with motorized valves) that will be opened once each system is depressurized. Is gas blowby scenario realistic to consider in this case?.
I ask this because I've always thinked about gas blowby when considering malfunction in the level control of a vessel or failure of a control valve (including bypasses), but: is it reasonable to consider it when the connections will be opened manually as well?.
2) Several equipment (pig launchers) are provided with mechanical interlocks to ensure the proper operation, avoiding opening the pigtrap doors when the equipment is still pressurized, etc. Can I take credit the mechanical interlock to rule out gas blowby scenario on these connections?.
3) Considering the fact that this is a vessel designed to collect the largest liquid inventory from a single source (one equipment, one header, or one manifold at a time) and that fire case in this vessel was considered for the design of the LP flare system; isn't it reasonable to think that the vent will be able to handle any peak of gas coming in case of a gas blowby scenario?.
I would love if you share your opinions and insights on this, as well as any other concern that should be considered and is not mentioned in my explanation. Thanks in advance!.
Cheers,
Fr3dd