Storage Tank - Set Pressure 1

[Chemitofreak]

Hi,

I have some queries regarding storage tanks:

1. As per code, how much pressure accumulation is allowable in storage tanks (both LP (API-620) and ATM Storage Tanks(API-650)).

2. Is there any criteria for specifying the PVRV (Pressure Vacuum Relief Valve) and Blow-Off Hatch set pressure.

 

[TD2K]

API 650 tanks have no allowable overpressure for either pressure or vacuum. You have to select a set point lower than the design pressure so that the device has the required capacity before you exceed design pressure or vacuum.

API 620 has 10% for overpressure, 20% for fire. There is no margin for additional vacuum other than what the tank is designed for.

http://www.aiche.org/sites/default/...ty-site-files/198496/aichemeeting9162014.pptx

 

[Chemitofreak]

@ TD2K:

Thanks for a prompt reply.

The code says the following
1. Set Over Pressure = MAWP (API-650)
2. Set Over Pressure = 1.1*MAWP and 1.2*MAWP for Fire Case (API-620)
3. Set Over Pressure = 1.1 x MAWP or +3 psig, whichever is greater, 1.21 x MAWP for fire case (ASME)

I have seen some engineers using the set pressure for PVRV less than the Design Pressure / MAWP for tanks designed by API-620.

Is it a good engineering practice, or is it o.k. to set the PVRV at 1.1 Design Pressure / MAWP for tanks designed by API-620.

 

[TD2K]

If you are talking about the link I posted, that said that the allowable overpressure can not exceed MAWP for API 650 tanks, not the set pressure.

Tank blanketing valves need a certain overpressure to achieve flow. If you calculate for example that you need X scfh relief capacity then you have to pick a suitable set pressure for that size relief valve that you will get the required capacity before the MAWP is exceeded.

< Is it a good engineering practice, or is it o.k. to set the PVRV at 1.1 Design Pressure / MAWP for tanks designed by API-620 >

No to both questions, it is not good engineering practice or OK to do this.

 

[Chemitofreak]

@ TD2K

Sorry I made a blunder while writing my earlier post. I have rectified the same.

Inconvinience regretted.

My query is as follows:

I have seen some engineers using the set pressure for PVRV less than the Design Pressure / MAWP for tanks designed by API-620. Is this a good engineering practice or is it case to case based on client's requirement, because as per code the set pressure can be equale to Design Pressure / MAWP is allowable.

 

[Chemitofreak]

Hi,

Anyone with a justification for the same.

Thanks in advance.

 

[LittleInch]

Is it good practice to set a pressure relief valve to less than the MAXIMUM - seems like it is good to me so long as it doesn't mean that excess gas is vented or you have spent more than you need to. Stressing anything to less than its maximum gives you a bigger margin over failure and reduces fatigue issues - what's not to like about that?

This is a design consideration and needs to assess the margins available, the accuracy of your PVRV set point, any back pressure at full vent rate, the number of times you think it is going to vent etc. It's usually the accuracy of the valve set point that inhibits a set point at EXACTLY the same figure as the design / MAWP

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

 

[25362]

For atmospheric storage tanks refer to thread123-65538.

 

[gocougs1]

Best practice is to set the PSV at MAWP, and if two PSVs are installed, one at MAWP and the 2nd at 105% of MAWP, per API 520. MAWP is already such a conservative number (about 1/3 of yield point) that lowering your set pressure by 5-10% below MAWP won't help the process, but now you run the risk of the PSV relieving during high normal operation/startup, which could cause a sustained loss of product.

API 650/2000 storage tanks open to atmosphere without a specified design pressure typically have a design pressure of 1.3" WC, with an allowable fire case accumulation of 3" WC, or about 0.1 PSIG.