API 650 Allowable Overpressure? 2

[KernOily]

Hi guys. Could someone please point me to the section in API 650 that establishes the allowable overpressure beyond the tank MAWP, for example during venting? A search of posts here turns up "For API 650 tanks, overpressure must not exceed MAWP of the tank. That means the set point should be lower than the Design pressure." but I couldn't find that in 650 or in API 2000. The only thing I find in 650 is Appendix F, and 2000 refers you back to the applicable tank design code.

Thanks very much in advance! Pete

 

[JStephen]

That is a shortcoming in the standard. Under the current standard, if no design pressure is specified, the tank is designed as an atmospheric tank and there is no requirement to establish an allowable maximum pressure, even if this is needed for sizing vents.
That said, there are a couple of paragraphs in the body of the standard that address venting, but I don't have it in front of me.
One thing to note is that seemingly in each of the last dozen addendums or so, the overturning criteria have been adjusted each time. So if you have an existing tank, and wish to know what pressure it could have been rated for when built, that may be a challenge to determine. Establishing maximum pressure under the current standard is not so difficult if you have the tank drawings and design calculations.
One other note- if a tank is not designed as an Appendix F/ Annex F tank, design pressure is limited to the weight of the roof plates, and may be further limited by overturning criteria.

 

[LittleInch]

Section 5.8.5 on roof venting would seem to give you what you need and refers through to Annex F with various factors applied.
There are different clauses for anchored and unanchored tanks.

This is from API 650 12th edition 2013 with addendum 1 of sept 2014

a) For unanchored tanks, the pressure relief devices shall be adequate to prevent internal pressure from exceeding
the tank design pressure as determined in F.4.1 (subject to the limitations in F.4.2 and F.4.3, as applicable). In
calculating limitations per F.4.2, use M = 0.



b) For anchored tanks, except those designed to F.1.3, the pressure relief devices shall be adequate to prevent
internal pressure from exceeding the tank design pressure as determined in F.4.1 (subject to the limitations in
F.4.3, as applicable).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

Yes, that is the section. The item noted above is misleading- that is the the pressure for emergency venting only. For standard venting, "Normal venting shall be adequate to prevent internal or external pressure from exceeding the corresponding tank design pressures..." But, as I mentioned, there is no requirement for the tank design pressure to be established or to be anything other than zero. But presumably, you can go back through the design and rerate it for whatever pressure capability is inherent in the design. Note the decision tree in Figure F.1. Also, the overturning criteria for non-Annex F tanks in Section 5.11 (wind) and E.6.2.1.1.1 (seismic), either of which may limit allowable pressure.

 

[LittleInch]

Section 1.1.1 backs up your previous statement:

...tanks for internal pressures approximating atmospheric pressure (internal pressures not exceeding the weight of the roof plates)....

Agree the previous sections quoted were for emergency venting.

If a standard does not allow you to specifically exceed the MAWP or design pressure then you are not permitted to do so. A standard doesn't have to / won't generally go around saying this or that is not permitted other than in some specific areas.

Section f2 implies that design pressures less than 1 kPa (4in water) don't require any modifications.

Also the term MAWP or even the full wording, doesn't appear in API 650, only design pressure, therefore the quote in the OP is not correct "For API 650 tanks, overpressure must not exceed MAWP of the tank. That means the set point should be lower than the Design pressure."

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

Section F2 says that the shell and appurtenances affected by hydrostatic loading do not need modification when p is less than 4" of water column, but doesn't exempt roofs or overturning criteria.