API 650(F) Tank Stamped Design Pressure

[n9ne0hse7en]

Hello, I am a process engineer evaluating the venting systems on a number of tanks designed to API 650(F) in the 1990's-timeframe. I have noticed that a number of tanks have a stamped "design pressure" on the nameplate that is far lower than the MAWP pressure that would be calculated using the formulas provided in API 650 app F. Using the name plate pressure has resulted in a number of undersized vents as it appears the original emergency venting calculations were performed using the Pmax from API 650(F). I am curious if anyone knows why the tank would have such a low pressure listed as design pressure on the nameplate? Thanks in advance.

 

[JStephen]

The pressure design and the interaction of pressure and wind uplift has been repeatedly revised, so don't assume the current standard matches the standard used for design.
If I remember correctly, there is wording in the standard that pressure is limited to that corresponding to the weight of the roof plates unless Annex F is used, so if no design pressure was specified, then the pressure rating would be pretty low. In the current standard, that may be reduced even more by the wind uplift/overturn combinations.
Generally, if the customer specified a pressure, that pressure would be used for design and stamped on the nameplate, even if the actual construction was adequate for a higher pressure.
With an existing tank with a low pressure rating, you may be able to go back and re-rate it for a higher pressure- that would be a "change of service" per API-653.