PVRV Set Points for Atmospheric Tank

[Narbij]

Hello community,

We have some "rig tanks" on site we use for temporary storage of liquids when cleaning our process tanks. We have used these tanks for years - I suspect they are API 12F tanks (pending confirmation). Assuming they are API 12F tanks and Design Pressure is 1 psi (16 osi) - the pressure relief device on the tank is currently set at 1 psi.

API 650 dictates that the set pressure for pressure relief needs to be below the design pressure to allow for accumulation and sufficient venting capacity at pressure. However, API 12F has no such provision - it simply refers to API 2000 where the only thing I see is Clause 3.6.2.2 which states that "it is often necessary that the set (start-to-open) pressure be lower than the design pressure of tank to allow for adequate flow capacity of the devices." - i.e. to allow for accumulation of vapour without exceeding the design pressure of the tank.

For API 650, this makes sense as the standard is for Welded Tanks for Oil Storage and can be of any size, either shop built or field erected. For API 12F, I am skeptical as they are (relatively) smaller, shop built tanks - I believe the set pressure is fine as is; however, I am getting resistance from another engineer saying that the device is not set correctly, i.e. the device should be set at a pressure below 1 psi to allow for 10% accumulation.

Anyone have any experience and/or comments about this? Am I incorrect in saying it is acceptable as is?

Thanks in advance for your help.

 

[EmmanuelTop]

For API 650 tanks, overpressure must not exceed MAWP of the tank. That means the set point should be lower than the Design pressure. How much lower? It depends on the actual (governing) overpressurization scenario.

For API 620 tanks, the overpressure is 10% above Design pressure (non-fire case) and 21% (fire case).

For API 12F tanks, "the opening pressure of relief devices shall not exceed the design pressure of the tank on which the devices are installed. The maximum internal pressure under relieving conditions should not exceed that tabulated in Table C.1, Column 6" (see below)

Dejan IVANOVIC
Process Engineer, MSChE

 

[Narbij]

Thanks EmmanuelTop. Annex C, Table C.1 is for Emergency Venting. I apologize for my lack of precision - I was talking about a PVRV for normal in-breathing and out-breathing - Annex B, Table B.1 of API 12F is the applicable table.

Thanks

 

[EmmanuelTop]

Section 6.1 of API 12F and Annex B, as you noted, define the requirements for normal venting.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Narbij]

Agreed. Section 6.1 points me to API 2000. API 2000 doesn't say if it can be set at Design Pressure or it has to be below it. The only thing API 2000 mentions is in Clause 3.6.2.2 which states that "it is often necessary that the set pressure be lower than the design pressure..." but this doesn't tell me whether it can be at design pressure or it has to be lower - hence my question.

Cheers

 

[LittleInch]

I'll confess I've not heard of API 12F before now, but the basic premise of any design code or standard is that it tells you things you can do and how to do them.

Hence when it says the Design pressures is 16 oz/ sq in / 1/2 oz/sqin vacuum and has no provision, other than appendix C for emergency provisions, to exceed that, then in "normal" operation you should not exceed that design pressure during any normal, anticipated operation by any amount (i.e. zero).

Therefore, in my opinion, you need to allow for the pressure losses incurred through the PVRV at maximum in and out breathing rates. It's sometimes good to think on the dark side and what you would say if the tank suffered any damage and you were asked to justify your decisions based on the codes and standards you have in front of you.

Note that often it's the in breathing rates which cause the problem as you only have 0.5 oz/sqin of pressure to play with before you exceed the vacuum rating of the tank.

At least for the out breathing you could set the device at say 15 oz/sq in and give yourself 1 ox to play with....

So time to find out the pressure drop across your PVRV at max flow.... Also helps sometimes if there are two PVRVs so that when some bird nests on your PVRV you don't suck the tank in.

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

 

[EmmanuelTop]

Which revision of API 12F are you referring to? Mine is 12th edition, effective date April 1, 2009.
Section 6.1 says - in addition to reference to API 2000 - "the pressure setting should be from 2 oz/in.2 to 4 oz/in.2 less than the opening pressure of devices used for emergency venting".

Dejan IVANOVIC
Process Engineer, MSChE

 

[Narbij]

Thanks EmmanuelTop and LittleInch for sharing your understanding with me and educating me. Greatly appreciated.