Safety Instrumentation and Control Reliability
Angela E. Summers, PhD, PE, President
A site’s risk analysis assumes that a particular level of risk reduction can be provided by the installed safeguards. The fundamental basis for this assumption is that the equipment is designed and managed according to recognized and generally accepted good engineering practices. Safe operation in the field is the goal, so site operation and maintenance records must ultimately demonstrate that the equipment as installed achieves the required risk reduction and is fit for purpose.
The achieved reliability of the process control scheme impacts the safety and profitability of the process unit operation. Higher process control reliability reduces the number of process upsets, shutdowns and restarts. Essentially, the more reliable the process control scheme, the safer the process unit is.
The key process safety objective is to identify failures, gaps or conditions and to correct them before they contribute to a major process safety incident .
The contribution of the process control scheme to abnormal operation can be tracked by automatically saving process safety event data whenever a safeguard is challenged. A process safety event reporter can be configured to flag events and to display important data for root cause analysis. Safety equipment are normally dormant and take specific action only when abnormal operation occurs, so it is a critical site responsibility to assure that safety equipment are not run to failure. An failure discovered during abnormal operation is not only undesirable but potentially dangerous.
Process safety regulations require a proactive maintenance program combined with quality assurance metrics to be applied to safety equipment. Many owner/operators establish a classification scheme to identify and prioritize the equipment that they will more highly manage. A process industry classification scheme can be found in ANSI/ISA 84.91.01 , “Mechanical Integrity of Safety Controls, Alarms, and Interlocks (SCAI).” Safety controls, safety alarms, safety interlocks, and safety instrumented systems (SIS) are frequently implemented as safeguards to address abnormal process operation that potentially leads to loss of containment.
Procedures are needed for gathering information about failures and developing useful metrics regarding failures. The owner/operator must take corrective action to maintain safety if the failure rates exceed those assumed during design. Competent people are necessary to evaluate and analyze the data and then develop and implement plans to improve the instrument reliability. ISA TR84.00.04 Annex R  and ISA TR84.00.03  provide guidance on selecting metrics for SIS, which can be applied equally as well to SCAI.
A database is needed to log service time and other information defined by the chosen failure data taxonomy. This database can be as simple as a spreadsheet or as complex as a computerized maintenance management system. Also needed is a collection method that is easy to follow, technicians motivated to correctly document the information, and people assigned responsibility for improving instrumentation reliability. Once sufficient information has been collected, the good and bad actors can be identified, and plans can be formulated and implemented to eliminate the bad actors and improve reliability.
Good actors are reliable technologies that have been proven through a volume of operating experience that they are fit for purpose. Understanding what makes a device a good actor can help improve the site practices needed across the lifecycle and potentially reduce the overall cost of ownership through better design, specification, construction, installation, operation, and maintenance.
Bad actors are instruments that have repeated failures at a frequency inconsistent with design assumptions or operational needs. They are not only a reliability problem; they also increase operating costs, consume maintenance resources, and impact productivity. Identifying bad actors and resolving underlying problems shifts the instrument maintenance program from one that is reacting to work orders to one that is proactively taking care of problem devices before they affect safe operation.
An instrument reliability program with quality assurance metics provides many benefits to the owner/operator:
- Ensures that maintenance procedures are performed effectively throughout the safety equipment life
- Provides feedback to validate riskanalysis and functional specification assumptions
- Identifies sources of human errors and common cause failures so that the safety equipment can be designed to reduce the impact of these sources
- Demonstrates through prior useevidence (historical performance) that installed safety equipment is fit for purpose and acceptable for continued use
- Ensures that poorly performing safety equipment is identified and that actions are taken to correct deficiencies
- 2010. Guidelines for Process Safety Metrics. New York: AIChE.
- ANSI/ISA 84.91.01. 2012. “Mechanical Integrity of Safety Controls, Alarms, and Interlocks (SCAI)
- 2015. Guidelines for the Implementation of ANSI/ISA 84.00.01- Part 1, TR84.00.04-2015. Research Triangle Park: ISA.
- 2012. Mechanical Integrity of Safety Instrumented Systems (SIS), TR84.00.03-2012. Research Triangle Park: ISA.