Final Thoughts

• Above I discuss the fact that the rupture disk needs to be able to pass the certified flow capacity of the relief valve! But which flow capacity, the stand-alone relief valve or the relief valve/rupture disk combination? Unfortunately, the way ASME³ reads, there is plenty room for interpretation. For example, paragraph UG-127 (a) (3) (b') (5) basically says the rupture disk must be able to pass the certified capacity of the relief valve/rupture combination. However, for the rupture disk installed in the tail pipe, paragraph UG-127 (a) (3) (c') (4) says the rupture disk must be able to pass, "...the rated capacity of

  the attached pressure relief valve without exceeding the allowable overpressure." Now, for individual cases where the rupture disk is installed only upstream of the relief valve or only downstream of the relief valve, I can buy into this as not being contradictory, i.e. use rated capacity of the relief valve/rupture combination for the inlet line or use the rated capacity of the stand-alone relief valve for the tail pipe. But what about the case where the rupture disk is installed both upstream and downstream of the relief valve?
  
  The flow used to evaluate the inlet line is to be the same flow used to evaluate the tail pipe. And, the 3% Rule clearly wants you to use the certified capacity of the stand-alone relief valve with the rupture disk being treated as just another piping component.
  
  So which do I suggest we Process Design Engineers use? The certified flow capacity of the stand-alone relief valve in all instances; it will be a little more conservative.

• The code requirements discussed above help to emphasize the importance of the material presented in Parts 3 and 4 of this series, i.e. the maximum allowable specified burst pressure, the Manufacturing Range, the Burst Tolerance, the Operating Ratio, and superimposed, built-up and variable backpressures; especially as they relate to the relief valve/rupture disk combination

Summary

• Rupture disks may be installed upstream and/or downstream of a relief valve.

• The rupture disk acts to de-rate the relief valve capacity. This de-rating factor is called the Combination Capacity Factor. Standards call for the use of this factor in determining relief valve area and in de-rating the stand-alone relief valve's certified capacity. Code only requires the use of this factor in de-rating the stand-alone relief valve's certified capacity.

• The size of the rupture disk in this application is totally dependent on relief valve sizing.

• The rupture disk must be able to pass the certified flow of the relief valve.

• The size of a rupture disk installed at the inlet of the relief valve should have minimal affect on the 3% Rule and must have a flow area of at least equal to the inlet flow area of the relief valve.

• The size of a rupture disk installed at the outlet of the relief valve should provide minimal contribution to the built-up backpressure.

• Code governs how a rupture disk is applied to a relief valve installation and the general type of rupture disk to use (non-fragmenting).

• Code addresses rupture disk bursting requirements.

• Code addresses backpressure affects and what must be done to avoid it.

• When specifying a rupture disk, especially in combination service with a relief valve, the maximum allowable specified burst pressure, the Manufacturing Range, the Burst Tolerance and the Operating Ratio all must be considered very carefully.

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), ASME Combination Capacity Factors, Catalogue 1-1111
5. Fike (www.fike.com), Technical Bulletin TB8103, July 1999