Good till the last drop: How much is too much valve leakage?

Validating the effectiveness of safety instrumented systems (SISs) is an integral and vitally important part of maintaining protection layers and preventing a hazardous condition. However, deciding on the basis for what constitutes sufficiently safe can be difficult. For example, when considering valves used as the final element in SISs, many in industry are basing the Maximum Allowable Leakage Rate (MALR) on the valve tightness specification instead of the hazardous condition that is being prevented when these valves are closed. This article will review a pilot conducted at The Dow Chemical Company to compare using the valve tightness class as a basis for MALR versus a safety-based calculated MALR. Economics and safety aspects are evaluated and the general types of safety based calculations used are reviewed. Key questions answered include: (1) what exactly is the requirement for estimating MALR, (2) how is MALR calculated using a safety basis, (3) are there differences in cost when basing MALR on valve tightness class versus a safety based calculation, (4) are time efficiencies realized when basing MALR on the safety case versus on the valve tightness class, and (5) which is usually more conservative, a valve tightness class-based MALR or a safety-based MALR? (c) 2015 American Institute of Chemical Engineers