API 12F Condensate Dump Tank Overpressure Protection Orifice 1

[Wascator]

Hi, we replaced a 150 bbl tank due to corrosion, with an almost identical tank built to API 12F. The tank has a pressure-vacuum relief (thief hatch) plus a vent in the top with a flame arrestor. It receives pipeline condensate from a dump valve on a filter-seperator unit via a 2 inch line.
The old tank did not have an orifice in place on the 2 inch line, and I understand it is usual practice to utilize an orifice to protect the tank from pressure if the scrubber dump hangs open and vents gas to the tank. This is natural gas service.
I am looking for advice on how to size the orifice, rather than guess at the size.

 

[SNORGY]

What sizes are the body and the port on the condensate dump valve and what is the highest filter-separator pressure when it dumps? You might not need the orifice plate, depending on the pressure setting on the thief hatch and the outbreathing pressure drop through the flame arrestor on the tank.

These links will help you with the calculation related to the dump valve:

http://www.isa.org/Content/Microsit...ent_Board/Home119/Ballots/ISA_750101_SPBd.pdf

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

http://en.wikipedia.org/wiki/Choked_flow

http://www.scribd.com/doc/19215896/Valve-Sizing-Calculations

The Universal Gas Sizing Equation will be of the most benefit to you for the valve; the Choked_Flow equation will help with the orifice plate.