A Human independent protection layer (IPL) can be a valid and valuable IPL. A human can be combined with an SIS-based alarm, but the human would then be part of the Logic Solver (decides what to do) and is part of the final control element. The criteria for counting human IPLs will be more completely defined in the upcoming textbook, Guidelines for Independent Protection Layers and Initiating Events, CCPS/AIChE, 2012 (pending early 2012). The human IPL needs to be specified and validated similar to other IPLs; part of the validation is testing of the hardware functioning up to the annunciation and part is testing (drills) of the human, who must complete the action (1) within the maximum allowable response time (MART) and (2) be out of harm’s way by then as well, since a human IPL is not valid if it places the responder (trouble-shooter) in harm’s way.
About the Author: Bill Bridges
William G. (Bill) Bridges is President of the Process Improvement Institute (PII). He has over 40 years of experience, including more than 30 in senior management and senior PSM advisory roles. Bill is considered one of the leading authorities on process safety engineering, risk management, and human error prevention.
Can a final element, such as an air-to-open control valve, be shared between a BPCS control loop that is the cause of a scenario, and an SIF that is an IPL for the same scenario?