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Open Vent Sizing Using API 520

Open Vent Sizing Using API 520

Open Vent Sizing Using API 520


Previously I posted about the same topics regarding sizing of open vent (i.e. gooseneck vent) so this time after I did my own research and this is what I wanted to find out from other experts here.

In API 2000 there is not mention about sizing for open vent, only basis of sizing for venting (something like that, i didn't remember). However after reading briefly on API 520 Part-1, I stumbled across this equation for sizing for critical and subcritical flow, and also for steam conditions and liquid phase.

Okay, now my question is that for API 650 which is low pressure atmospheric storage tanks, the design internal pressure is 0.75 kPa(g) and vacuum of 0.25 kPa(g), based on my earlier sizing calculations I checked that for a vent sizing (for critical flow) the downstream pressure shall be less than the upstream pressure in this case, the internal pressure shall be bigger than the outside pressure.

So here, shall I take directly the downstream pressure as -0.25 kPa(g) or should I just consider it as 1 ATM (101.325 kPa(a))? If say I take it as 1 ATM, my pre-requisite checking would end up to be SUB-CRITICAL flow which will be my upstream pressure to be lower than my downstream pressure.

I would really like to know if my assumption here is correct or otherwise. Appreciate the useful answer thanks.

RE: Open Vent Sizing Using API 520

Its 1 atm (corrected for altitude) outside and either 1 atm + 0.75 kPa or 1 atm -0.25kPs inside.

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