Why I Don't Like the Coefficient of Discharge Model

• It's too restrictive! During the basic design phase of a project, actual piping configuration is unknown. You may think you are within the "8 & 5" rule at first but may not be when the final details are worked out. Remember, the "5" means 5 pipe diameters. For a 3" line, that is only a nominal 15". For a 6' vertical vessel with a rupture disk discharge being piped to a drain hub on the floor, the 15" maximum length is exceeded without even thinking.
  

• Using the Resistance to Flow Method is valid for all cases. All sizing calculations can be standardized.

• The Kr used in the Resistance to Flow Method is obtained by actual flow data for a given model of rupture disk and holder. Its use will provide a much more accurate calculation. The 0.62 coefficient of discharge used in the Coefficient of Discharge Method is very general and independent of rupture disk manufacturer model and type, holder, disk bursting characteristics and flow restrictions of the total relief system.

• Two-phase flow can be a major concern when using this method. The coefficient of discharge was established mainly for true vapors. Its application to liquids is questionable and its application to two-phase flow is totally fictitious. Granted, for the Resistance to Flow Method the Kr is not particularly applicable to two-phase systems either but one can easily compensate for this in the system calculations. Also, the rupture disk is only a part of an entire piping system and its overall contribution to the system frictional losses may not be greatly significant. Therefore, errors in Kr may not be very significant. In the Coefficient of Discharge Method, it is the only device considered. If the coefficient of discharge is grossly in error, the MNFA calculated will also be grossly in error.

• The same argument can be made for highly viscous liquid systems such as polymers.

In Summary

• Obtain the required relieving rate using good sound "what can go wrong" scenario analysis.

• Use the Resistance to Flow Method to calculate the size of the rupture disk (use the Coefficient of Discharge Method if you really must and you fall within the "8 & 5" rule).
  
• For the Resistance to Flow Method, try to choose the manufacturer and model of rupture disk you intend to purchase ahead of time or at least have a list of acceptable manufacturers and a good idea of the model you intend to use from each.
  
• For the Resistance to Flow Method use the ASME Kr value of 2.4 if you have no idea who the manufacturer(s) will be at the time of sizing.

References

1. API (www.api.org) Recommended Practice 520, "Sizing, Selection, and Installation of Pressure-Relieving Device in Refineries, Part 1-Sizing and Selection", 7^th Edition (January 2000)
2. API (www.api.org) Recommended Practice 521, "Guide for Pressure-Relieving and Depressuring Systems", 4^th Edition (March 1997)
3. ASME (www.asme.org) "Boiler and Pressure Vessel Code, Section VIII, Division 1" (1998)
4. Continental Disc Corporation (www.contdisc.com), Certiflow^TM Catalogue 1-1112
5. Fike (www.fike.com), Technical Bulletin TB8104, December 1999
6. Another good rupture disk manufacturer to investigate would be Oseco (www.oseco.com).
7. A good reference source for calculating flow through the system for liquids and gas/vapors is CRANE Technical Paper 410, "Flow of Fluids Through Valves, Fittings, and Pipe"
8. A great source and one that I feel should be the bible on two-phase flow is: Leung, J.C. "Easily Size Relief Device and Piping for Two-Phase Flow", Chemical Engineering Progress, December, 1996