+2 votes
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Electrostatic treater vessel pressure safety valve sizing criteria

asked Feb 9, 2015 in Process by rajprasad (870 points)
Does RP 521 require the pressure safety valve on our electrostatic treater to be sized for two-phase relief in a fire case?

This question is related to an offshore facility processing oil with density from 27 to 31°API. The facility comprises a horizontal crude oil electrostatic desalter with an internal diameter of 5 meters, a tangent-to-tangent length of 20 meters and a design pressure of 12 barg. The electrostatic desalter is provided with a pressure safety valve. The governing relief scenario for sizing of the pressure safety valve is a fire case for the liquid filled electrostatic desalter. For this relief scenario, section 3.15.3.2 of API Recommended Practice 521 applies.

2 Answers

+3 votes
answered Feb 9, 2015 by ChemKB (8,210 points) | selected Feb 9, 2015 by rajprasad
 
Best answer
Based on past experience for typical processes, two-phase relief device sizing is not normally required for the fire case provided that the relief device is located in the vapor space or on the top of the vessel. Two-phase relief device sizing should be considered for fire cases involving unusually foamy materials or reactive chemicals.
+2 votes
answered Feb 9, 2015 by cobiesmith (3,310 points)
Some referenece for your review:
1) H. G. Fisher and H. S. Forrest, “Protection of Storage Tanks from Two-Phase Flow Due to Fire Exposure”, Process Safety Progress, Volume 14, No. 3, July 1995, pp 183-199.

2) H. S. Forrest, “Emergency Relief System Design for Fire Exposure with Consideration of Multiphase Flow”, 1995 International Symposium on Runaway Reactions and Pressure Relief Design, ISBN 0-8169-0676-9, Published by American Institute of Chemical Engineers, pp 604-630.

3) H. Mott, B. Sparrow, “A Simple Relief Valve Rating/Sizing Method for Fire Relief from Liquid-filled Vessels”, presentation to API RP-521 Subcommittee, September 30, 2002.

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