Vacuum Relief Sizing

[Mark Suplee]

Hello,

I am trying to size a Vacuum Relief Valve for a Chemical Storage Tank at atmospheric pressure. I have looked through API 2000 but have only found specifications for selecting flow rate, nothing to do with pressure setting. From the Emerson Pressure Relief Valve Engineering Handbook, all I am seeing is that the valve should be set at a pressure lower than the tank design pressure.

Is it really as simple as that? If the tank is designed to withstand 10 psi do I simply set it at ... say 8 psi with a flow rate of 60 SCFH(per API 2000)?

Thank you

 

[RVAmeche]

The tank will have (or need) a MAWV (maximum allowable working vacuum). You will then use this value, along with process parameters such as pump discharge flow rates, to determine how to size your vent. Note that many vents require 100% overpressure (or underpressure for vacuum) to achieve full flow. So if you have a vent set at 5" WC, the rated flow capacity is at 10" WC. Therefore you have to plan your set point accordingly or choose a vent with a tighter operation (and therefore more expensive).

Example -
Tank MAWV = 15" WC
Tank Vacuum Vent SP = 7" WC
Tank Vacuum Vent Full Flow = 14" WC (100% underpressure)

 

[Mark Suplee]

So the set pressure then would be arbitrary, as long as the full flow pressure is less than the MAWV?
by "arbitrary" I mean there is no given formula....

 

[RVAmeche]

No, there's not really an equation per se. You have to make sure that the vent:
1) Capacity is adequate for the worst case scenario
2) Tank vacuum level doesn't exceed the MAWV
3) Vent set point doesn't open prematurely based on normal operating/process parameters

 

[JStephen]

The allowable vacuum is based on the tank design.
For an API-650 tank, if no vacuum design is specified, then the standard specifies that it is good for 1" water column.
Refer to the tank standards to see if the "design pressure" or vacuum is the set point or is maximum pressure/vacuum while flowing.
For a small or heavily built tank, it may be possible to rerate it at considerably higher vacuum than was originally specified.

 

[Mark Suplee]

The tank is XLPE built to ASTM D 1998 13.
Vendor is suggesting venting at 2" WC.

 

[LittleInch]

The vendor sounds like he knows what he's talking about.

Most "atmospheric" tanks really really don't like negative pressure.

But yes it is a s simple as that. If you don't set your protection equipment within the design envelope then don't expect the tank to survive.

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