Thursday, June 25, 2026

The steady-state techniques discussed in CCPS GPREH 2nd Edition §3.3¹ are usually conservative in nature when applied correctly. In general, some conservatism is desirable due to the variables involved in abnormal events that lead to overpressure; however, excess conservatism has other consequences, such as oversizing of pressure relief valves, which can lead to cycling or chattering of the relief valve opening. In addition, some situations cannot be modeled appropriately using the steady-state techniques. As a result, there are some cases where a dynamic analysis can be employed, even for nonreactive scenarios.

One challenge encountered in employing the dynamic analysis is the number of variables that need to be characterized appropriately. As an example, consider a distillation tower where there are many variables involved that can affect the relieving requirements, such as the efficiency of mass and heat transfer on the trays. While there are techniques available to estimate this information for design purposes, they may not be able to be extrapolated to the conditions that can occur during the overpressure event. Testing is the best means to provide information for the characterization of these variables. In the absence of testing, care should be taken to use conservative assumptions and to validate the models used. ISO 23251:2008 §5.22 recommends the use of sensitivity analyses for factors that affect the venting requirements to ensure the dynamic modeling is an appropriate representation of the design basis for the relief system.²

Blog series information. This blog is part of a series on the proposed updates to the CCPS Guidelines 2nd edition §3.3 Venting Requirements for Nonreacting Cases that were removed during final editing. See the general CCPS Guidelines for Pressure Relief and Effluent Handling 2nd Edition review for more information.

 

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[1] AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.

[2] ANSI/API Standard 521 / ISO 23251 (Identical), Petroleum and natural gas industries — Pressure-relieving and depressuring systems. 5th Edition, January 2007 (incl. Errata June 2007 and Addendum May 2008). American Petroleum Institute, Washington DC, USA.

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