I apologize for being late in responding to this very interesting and important topic. I believe we have already had some of the top brains within our Forums already contributing to this thread and my contribution is not very different from what has already been discussed.
I would not relieve excess liquid level in the reflux drum with a PSV. I would, instead, relieve the excess liquid level in the reflux drum with a level control & dump valve to a safe, closed drain if there is sufficient pressure or head to drive it.
If the PSV on the crude column top head is sufficient to protect both the column and the reflux drum, it should suffice in the event of an over-pressure. I assume - as fallah - that there is no way to block the overhead line + the air-cooled condenser + the liquid line to the reflux drum and that the capacity of the PSV is not reduced for the worse case scenario.
I would ensure that the source (cause) of the excess liquid reflux in the reflux drum be mitigated first. This could be one - or a combination - of the following:
- excess reboil heat;
- reflux pump failure - electrical or mechanical;
- reflux control valve(s) fail in closed position;
- reflux block valves erroneously closed.
I would never shut down or cut off my protection against an over-pressure and subsequent flare release: the overhead condenser. This is not a cause of the excess liquid in the reflux drum. I would do the following - or a combination of the following:
- Stop all reboil heat ( or reduce it considerably) with an automatic block on the source or an automatic reduction of heat - depending on the ability to react with operator remedial action;
- Automatically have a standby reflux pump startup upon the High level in the reflux drum (I identify the high level as the point of remedial action and the High-High level as the point of relief action) - as Ankur mentions;
- Automatically activate the LC dump valve upon reaching the High-High level in the reflux drum;
- The automatic dump can be done back to the crude column sump if the reflux drum is located with sufficient head or height; if not, the dump should go to a closed drain for recovery;
- The crude feed to the column should be activated to the blocked position once the High-High level is reached in the reflux drum and reboil heat is blocked or reduced.
There should be no vapor release to the flare or activation of the PSVs in order to bring the column down to a safe, operable condition where the reflux accumulation problem can be resolved and the column started up again.
Additionally, the size (liquid inventory) of the reflux drum should be carefully strapped and identified vis-à-vis the crude column's sump capacity. In other words, if the column is being fed on flow control, any excess liquid in the reflux drum represents a level drop in the column sump - and an effect on the reboiling ability. This should be studied and identified as to the effect on the scenario. The time required for operator reaction to the scenario should be identified with regards to the capacity of the reflux drum to store and dump its inventory. The capacities and normal residence times in the column sump and reflux drum should be calculated and identified. (In fact, I would add this data to the P&ID for the operator's attention.) Certainly, the mechanical design and size of the reflux drum should incorporate some residence time to allow for operator reaction to a reflux failure.