Free vent for API 650 Tank 1

[chemter]

Hello. I am a process engineer (fairly new on the job) and I had a question pertaining to a free vent. The vessel in question is a plastic tank (HDPE) and falls under the API 650 standards. The tank is rated for 0 pisg at 100°F. Does anyone know where it states in API 650 (or another standard) what the allowable pressure/vacuum pressure is? I was thinking 1 or 2" w.c., but I wanted to verify. Thank you.

 

[JStephen]

API-650 is for steel tanks, so if it is a plastic tank, it doesn't fall under API-650.

Normally, for venting steel atmospheric tanks, I figure around 5 PSF, which is about 1" water column. You can actually come up with a maximum allowable pressure from from Appendix F of the standard. This is just slightly less than the weight of the roof plates.

For a plastic tank, I'd try to run down the manufacturer and/or the standard it was built under. It could be as simple as finding the right catalog.