"Atmospheric" MAWP for a API 650 Tank

[AskOR]

Hi- I'm sizing conservation vents and emergency vents for several older API 650 tanks and need some advice. The mechanical drawings for the tanks say the MAWP is "atmospheric", but there is a noted test pressure of 2-3 psig.

These tanks are NOT API 650 appendix F tanks, so the MAWP is limited to the basic calc of roof weight / cross sectional area of the tank (or approximately 1.5 inWC every time). However, the tanks have been operated for a number of years with a nitrogen blanket at 3 inWC and the current valves are set anywhere from 28 - 45 inWC for pressure relief. So, I don't think the real MAWP is 1.5 inWC.

Is there a guideline anywhere that allows the test pressure to be used as the MAWP in the calculation for the valve size?

Thanks!

 

[darthsoilsguy2]

the test pressure is likely for emergency situations where a plant fire evacuates the (assumed to be flammable) materials out the emergency vent at a rate that prevents an increase pressure above 2.5psig inside the tank. if the e-vent doesn't work to keep psi below 2.5 or the tank can't take 2.5 to start with, the tank becomes a bomb and kills firefighters. MAWP for atmospheric tanks is 2.5 psig.

API 2000 is for vent sizing of 650 tanks.

i believe the maximum pressure allowed for normal venting is 1.0 psig and usually occurs during filling/discharge. but i could be wrong on that number.

Conservation vent manufacturers (i.e. protectoseal) give away software to size their products.

 

[JStephen]

If the tanks are very small (shop-built), they may have been tested with air instead of using a hydrotest, and that may be the test pressure referenced.

You may need to re-rate the tanks for increased pressure if there is a specific need. The current API-650 includes some requirements on set pressure vs overpressure of vents and emergency vents, but I don't have it here in front of me. You would also need to confirm if current code or older code of construction would be used for this- the requirements relating to uplift and anchorage have changed through the years.

 

[Samm17]

The MAWP for an atmospheric tank is not 2.5 psi as stated above.

Without knowing anything else about your tank the MAWP would equal to the weight of the roof plate as you originally stated. Just because the tank has been operated above its original design condition doesn't mean it should be. If required I would have the tank re-rated.

 

[IFRs]

I agree with Sammm