Relief Requirements

Design Institute for Emergeney Relief Systems (DIERS) Institute under the auspices of the American Institute of Chemical Engineers founded to study relief requirements for reactive chemical systems and two-phase flow systems. 

Other Utihties - Failure of other uOhties, such as inert gas to seals and pinge systems, or compressed air (when used by the process) may in some cases determine pressure relief requirements. These cases are evaluated on a single contingency failure basis similar to the above. 

Partial Condensing - The relief requirement is the difference between the incoming and outgoing vapor rates at relieving conditions. The incoming vapor rate should be calculated on the same basis as stated earlier. If the reflux is changed in composition or rate, the incoming vapor rate to the condenser should be determined for the new conditions. 

Pumparound Flow Failure - The relief requirement is the vaporization rate caused by an amount of heat equal to that removed in the pumparound circuit. The latent heat of vaporization would correspond to the temperature and pressure at PR valve relieving conditions. "Pinchout" of steam heaters may be considered. 

Absorbent Flow Failure - For lean oil absorption generally, no relief requirement results from lean oil failure. However, in a unit where large quantities of inlet vapor may be removed in the absorber, loss of absorbent could cause a pressure rise to relief pressure, since the downstream system may not be adequate to handle the increased flow. In such cases, the effect of this additional vapor flow into downstream equipment must be analyzed. 

Grolmes, M. A., Pressure Relief Requirement for Organie Peroxides and Otlier Related Compounds, Int. Symp. on Runaway Reaetions, Pressure Relief Design and Effluent Elandling, AIChE, pp. 219-245, Mareh 11-13, 1998. 

Leung, J. C., Simplified Vent Sizing Equations for Emergeney Relief Requirements in Reaetors and Storage Vessels, AIChE Journal, Vol. 32, No. 10, pp. 1622-1634, 1986. 

Total pressure relief requirement = 6000 -t 371 = 6371 SCFH free air... 

SCFH free air 14.7 psia and 60°F Total vacuum relief required ... 

Leung, J. C., et a.L, Emergency Relief Requirements for Reactive Chemical Storage Tanks, I. Chem. E. Symposium Series 110, 1988. 

The procedure for specifying the location of reliefs requires the review of every unit operation in the process and of every process operating step. The engineer must anticipate the potential problems that may result in increased pressures. Pressure relief devices are installed at every point identified as potentially hazardous, that is, at points where upset conditions create pressures that may exceed the MAWP. 

Figure 9-1 Required vent area as a function of overpressure for two-phase flow. The vent area is decreased appreciably as the overpressure increases. Data from J. C. Leung, Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels, AICHE Journal (1986), 32(10).

J. C. Leung, Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels " AICHE Journal (1986), 32(10) 1622. 

A 1-in (internal diameter) pipe is used to supply water to a low-pressure tank with an MAWP of 20 psig. The water is supplied through a regulator from a source with a maximum pressure of 100 psig. Determine the diameter of a conventional spring-operated relief required to protect the vessel from a regulator failure. Be sure to state clearly any assumptions and your justifications for them. 

API RP 521 Section 3.15 contains extensive guidance relative to relief requirements for external fire (as discussed earlier in this book). 

The external fire relief requirements were determined using equations presented in Appendix D of API RP 520, but which also depend on the adequacy of... 

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