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# Need help about sizing PSV for loss of liquid seal2

## Need help about sizing PSV for loss of liquid seal

(OP)
I have an upstream vessel (A)  that has a letdown valve at the bottom outlet which connects to another downstream vessel (B).  I have to size a PSV for the downstream vessel (B) for loss of liquid seal from the  upstream vessel (A).

How do I determine the required flow capacity for the PSV?  The letdown has a wide open capacity of 1.5 x the normal flow rate, and this flow contains a mixed of  liquid and gas.

I saw a similar situation from an old job, but I am not sure how they got the required flow capacity and the relieving temperature, which is  somewhere in between the operating and design temperature of the downstream vessel (A).

I am not too familiar with the "loss of liquid seal" basis.  I would appreciate any inputs.  Thanks.

### RE: Need help about sizing PSV for loss of liquid seal

ruwl,
Don't get too "hung up" over the term loss of liquid seal.  What it means is to just consider your system (vessels and piping) to be empty of liquid, as if you had an interruption in liquid flow but without any loss of vapors flowing through your system.  Consider the letdown valve to be wide open, and calculate the vapor flowrate from knowing the valve's Cv and the pressure differential.  If I understand the situation, then that's the amount of vapor that must escape from vessel B.  Size the PSV on that basis.
Doug

### RE: Need help about sizing PSV for loss of liquid seal

ruwl,

I call this the "run dry blow thru scenario".

I'm sure you can create several scenarios that cause this.  Vessel A is in a fire and all the liquid boils off, letdown valve fails wide open, or both happen at same time.  There could be others.

I wouldn't waste too much time going thru every possible scenario.  If Vessel A is at 1.21 x it's MAWP and Vessel B is at 1.1 x it's MAWP, what is the vapor only flow from Vessel A to Vessel B?  If the seal fluid will still flow into Vessel A when it is at 1.21 x it's MAWP, I would consider two phase flow, but if it won't, you are finished!

Good luck,
Latexman

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