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2 phase flow in PSV?

2 phase flow in PSV?

2 phase flow in PSV?

Dear All,

One of the PSVs in the plant is installed on an expansion vessel, whereby high pressure liquid is expanded from 70 bar.g to 5 bar.g (as attached). The PSV is sized for failure open of the level control valve in the upstream gas-liquid separator. The inlet nozzle is extended below the normal operating level to minimise noise in level control, whereas the portion of nozzle exposed above normal liquid level has openings to release the gas phase.

For a failure open of the level control valve in the upstream gas-liquid separator, could it result to 2-phase flow into the PSV?

Is there any calculation that could be done to evaluate the criteria of critical fluid velocity that would potentially lift the liquid phase into the PSV inlet in such a case?

Thank you and best regards,


RE: 2 phase flow in PSV?

You've probably got a shutdown valve on the G/L sep liquid exit to close the exit valve on loss of level.
What happens ( from an instrumented trip point of view)when you've got coincidental high high level in this expansion vessel? Does it isolate this feed through some other mechanism?

RE: 2 phase flow in PSV?

Thanks a lot for your reply.

Yes, we have already installed a shutdown valve at the outlet of the G/L separator. However, from the perspective of Safety Integrity Level, this single shutdown valve is not sufficient to mitigate the risk. A PSV is required in this case, and that is when the question about the 2-phase flow into the PSV arose.

Yes, in the event of high level in this expansion vessel, the level control of the expansion vessel will over write the level control of the G/L separator.

RE: 2 phase flow in PSV?

You may want to get a copy of R. Darby, F.E. Self and V.H. Edwards on "Properly Size Pressure-Relief Valves For Two Phase Flow." written in Chemical Engineering dated 6/2002. I suggest that you research more recent Ch.E. articles on the subject.

RE: 2 phase flow in PSV?

Calculations can be done to estimate two phase flow onset and disengagement. I refer you to Emergency Relief System Design Using DIERS Technology. It is the Design Institute for Emergency Relief Systems (DIERS) Project Manual of the AICHE.

It is a dynamic situation, but you know the volume fraction of the vessel at key operating stages, like at the normal liquid level. Then you could look at reasonable increments as the scenario progresses.

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: 2 phase flow in PSV?

The high level control override on the expansion drum to throttle incoming feed from the G/L sep is fine from an operational perspective, but it wont hold ground from a process safety view.

Having to cater for 2phase feed flow to a PSV may be stepping on a hornets nest if you ask me. See if you can convince your colleagues to install the independant trip loops (LSHH and PSHH) on this expansion with adequate reliability so you only need to deal with all vapor or all liquid flow to the PSV.

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