November 2011

Special Report: Plant Safety and Environment

Relief device inlet piping: Beyond the 3 percent rule

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

Smith, D., Smith & Burgess LLC; Burgess, J., Smith and Burgess LLC; Powers, C., Smith and Burgess

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 indu

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