Orifice sizing for pressure vessel blowdown

[engr2GW]

Hello,

I have a pressure vessel (3-phase separator) with a blow down line connected between it and a atmospheric storage tank. the separator can operate anywhere from 80psi to 450psi, and the tank from 0oz to 16oz.
I have never done this before, I just need help on a rule of thumb, equation, etc. on how to size the orifice to blow down this separator to the tank without over pressuring the tank.
The fluid to be blown is natural gas (may contain a bit of condensate sometimes).

Or a book/page that I can look to get an idea

Thank you.

As much as possible, do it right the first time...

 

[Latexman]

This and similar subjects have been discussed here many times. Look in FAQs, and use the Search function between Forum and FAQs.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[georgeverghese]

Standard oil and gas practice is not to use API 620 or 650 tanks as vent or flare knockout drums. Minimum design pressure on the KOD should be 700kpag to withstand detonation in the event of flashback due to loss of purge. At the very least, design pressure for an LP flare KOD with assurance of constant purge gas may be 350kpag with Engg department approval on acceptance of the incremental risk on going down from 700kpag to 350kpag.

 

[LittleInch]

engr2gw,

This sounds like a wholly bad idea to me. I'm not sure what you mean by 0 to 16 oz, but most "atmospheric" tanks have a pressure rating of a few inches of water column only.

Even if the volume flow is somehow less than the vent system of the tank, you risk sending a pressure pulse down the vent line on activation of the relief system before it gets to any steady state and the velocity going into your tank could be very high unless you have a very big pipe. Your gas might also be at very low temperatures after the de-pressurisation and contain liquid droplets at high veleocity.

Do what is suggested and use a purpose built KO drum prior to your vent / flare.

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