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Relief device inlet piping: Beyond the 3 percent rule

11.01.2011  |  Smith, D.,  Smith & Burgess LLC, Houston, TexasBurgess, J.,  Smith and Burgess LLC, Houston, TexasPowers, C.,  Smith and Burgess, Houston, Texas

With careful consideration, an engineer can be certain that an installation will not chatter

Keywords: [inlet piping pressure] [piping] [OSHA] [API] [ASME] [relief devices] [fluids] [vapors] [relief valves]

Good engineering practices (API STD 520 and ASME B&PV Code Section VIII) have long specified/required that inlet piping pressure drop from the vessel to the safety relief device should be limited to no greater than 3% of the safety relief valve’s set pressure. Many companies have taken a more lenient approach to the inlet pressure loss limits; consequently, many installations do not meet the 3% design guideline, as the prevailing company logic assumed that existing installations were “safe” as long as the inlet losses were less than the safety relief device’s blowdown with some built-in safety margin. Up until recent fines by OSHA, there have been no hard and clear industry requirements or penalties for companies to adhere to the 3% inlet pressure loss rule. However, OSHA recently rejected this argument and has now begun levying fines against companies violating this 3% rule. In an April 2010 letter to the API STD 520 Committee, OSHA stated that higher inlet losses may be considered acceptable if safety relief valve stability could be assured with an engineering analysis.

This monumental shift has added serious financial consequences for violations of this rule, making compliance no longer an academic argument. This article details a procedure to assist facilities to ensure that existing relief devices with inlet losses greater than 3% are properly designed and will not chatter. It is not the goal of this article to confirm the criteria for an installation to chatter, but instead to give engineering guidance as to which installations are acceptable as they are not expected to chatter. To ensure that this methodology actually solves problems associated with real installations, an entire refinery was subjected to the methodology, and it was found that over half of the installations that have inlet pressure losses greater than 3% are acceptable as is and are not expected to chatter.

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