Thursday, June 25, 2026

Considerations for inadvertent valve opening are similar to the inlet control valve failure; however, the calculation methodologies for pipe flow as described in CCPS GPREH 2nd Edition §4.4¹ are typically employed. Any normally closed valve can have the potential to open, though additional measures (such as locking the valve closed) may be taken to reduce the likelihood of that initiating event.² Risk assessment may then deem the potential overpressure scenario non-credible; however, the potential for leakage across the valve should be considered.

Control valve bypass. When evaluating the control valve failure case, the potential for some operations to partially or fully open the bypass valve around a control valve is important to recognize, particularly during startup or in response to increases in unit throughputs. In these cases, the failure open of the control valve should not be considered independent of the opened bypass valves. In addition, bypass valves around control valves tend to be selected as full-port valves based on the nominal size of the control valve as opposed to the equivalent maximum flow; therefore, the excess flow introduced by the opening of a bypass valve can be much greater than that for the control valve.

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.

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