+1 vote
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how to calculate relief valve size

asked Mar 10, 2016 in Engineering by Oliver Colemn (260 points)
In need of a means to calculate needed relief valve for an ammonia storage tank.
commented Mar 12, 2016 by dauod (1,080 points)
Do you have any more background on this?
commented Mar 15, 2016 by Oliver Colemn (260 points)
Have multiple storage tanks. Two of which are 75k gallons each, bullet tanks, with max working pressure of 250 psi. That is the relief valve setting. Very much like propane storage tanks.
commented Mar 15, 2016 by dauod (1,080 points)
Oliver, That's great. Would you happen to know how the Tanks are fed? Are they product Tanks from  an ammonia processing facility? What is the product rate into the Tanks? 1000 T/D?
commented Mar 16, 2016 by Oliver Colemn (260 points)
Storage tanks only which are filled as needed. When filling with pumps rate is 40 T/hr. Trucks load out of them at that rate also. Storage and handling only.

1 Answer

+1 vote
answered Mar 11, 2016 by dauod (1,080 points)
First determine the potential overpressure scenarios. These are usually liquid overfilling from the source - a pump on a truck, rail car or barge, for example. Or back-flow from a user of the ammonia - a compressor or pump leading to the refrigeration system or process. A smaller fraction of the time, a fire-size case is the determining scenario. This means taking the external surface area of the tank, below 25 ft elevation, and typically and applying a radiant heat flux. The heat input into the tank, divided by the latent, which may be 500 BTU/lb for ammonia, for example provides a vapor relief rate. The fluid latent heat is different for aqueous ammonia than dry ammonia. These mass flow rates are compared, to find the maximum case. The mass flow can be used in a relief valve vendor software, or formula, to provide a preliminary relief valve size.
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