Vent size on atmospheric tanks 1

[MRLatLSC]

I am currently trying to determine the adequacy of an existing vent in a tank for a pressure relief scenario. Is there a particular equation or calculation to verify what size vent is needed?

I have looked through API 2000, NFPA 30 and Crane TP 410 (the most difficult to interpret) and thus far have come up with sizing tables given the wetted area and flow rate into the tank. What I want to know is where those vent size numbers come from for a given capacity.

Thank you in advance for your help.

 

[fegenbush]

The capacity of the tank would be the wetted area multiplied by the high liquid level. The high liquid level value alone is not a useful value from which to determine vent sizing.

The vent sizes per wetted area are determined based on the allowable pressure or vacuum in the tank, the inbreathing or outbreathing requirement based on the flow rates out of and into the tank, thermal effects from tank contents evaporating, and emergency conditions such as a fire.

 

[IFRs]

API 2000 Annex D "Basis of sizing equations" and Annex E "Basis for normal out-breathing and normal in breathing" should shed some light on your question of how the numbers were arrived at. Once you understand the basis for and have calculated the venting requirements it is only necessary to review vendor data curves to select the proper size and quantity of vent(s) for your situation.

 

[aamir2003]

IFR, possible for you to share any calculation sheets as per API 2000?

 

[IFRs]

The vent capacities are given by the vent manufacturers. They should supply curves of flow versus the pressure across the vent with families of curves for actuation pressure. They are most likely test values, not calculations but I really don't know how they were arrived at. See section 7.1 of API 2000. Section 6 describes how to test a vent to determine its capacity.

 

[chicopee]

Beside the NFPA 30 code, there is the NFPA 30 Handbook which goes into further details and explanations about the contents of the related code.