Required Ammonia Pressure Relief Valve Capacity

[jbkettler]

Hello - please be patient as this is my first post.
I am having trouble finding the basis for the required discharge capacity of a relief valve as stated in ASHRAE 15 (C=fDL). Are the D & L portions to reflect the vessel volume or the vessel surface area? I would initially think volume since that is what needs to be relieved, however there is a note in ASHRAE 15 below the equation that states the formula is based on fire conditions (possible surface area reference).

First - The reason I am inquiring is because I need to calculate the required discharge capacity for a Silo Jacket. I believe plugging in the D and L from the ASME U form will yield inaccurate results since the vessel is not of a cylindrical design - rather it is like a plate and frame heat exchanger. If I could find the basis of the equation (volume or surface area) I feel that I could justifiably calculate a required flow rate that is defensible. If someone has another approach please feel free to share.

Second - for the Silo Jackets that can not be isolated from their respective ammonia pressure vessels (accumulators), the equation seems inadequate for the volume of ammonia. For a normal plate and frame HX with a HX accumulator, the accumulator volume >> the HX volume and the RV is calculated from the vessel dimensions. On these Silo Jackets and Vessels, the volumes are very near each other and I think that calculating the RV capacity from the vessel dimensions would result in a significant error. This goes back into the - is it based on Volume or Surface Area question...any input or ideas?

 

[jbkettler]

I have located additional information, but would like some experienced opinions to weigh in on this topic in general.

From ANSI/IIAR-2008 11.2.7 "...In the case of a plate and frame heat exchanger, replace the DxL portion of the equation with a term equal to half the overall external surface area. In the place of a double piped condenser, replace the DxL portion of the equation with a term equal to half the overall external surface area."

This means that the minimum required relief capacity is based on surface area of the vessel.

So my question now becomes - how to address a jacketed silo. Would I simply look at the surface area facing outward from the tank (in the event of a fire it would be unlikely that the fire would be both outside and inside the tank - especially since the inside of the tank contains non-flammable liquid)?