Process Automation Reliability vs. Safety

Is It Possible to Have Safe Systems That Aren't Considered Reliable?

By William L. Mostia

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This is a consortium of companies coming together with a vision to benchmark current performance of instrumentation and controls in process industry applications, define a common failure taxonomy to support consistent collection of quality data from maintenance and proof-test activities, and share lessons learned in improving instrumentation.

Control system availability is related to equipment and functional reliability. While higher-level control equipment can contribute to downtime, control system availability is a function of the cumulative availability of the individual loops in the system, which depends in part on the reliability of the loop equipment hardware. It's also a function of the unavailability of the loop functionality, such as the time in manual, in bypass, bad tuning and systematic errors that lead to loss or degradation of loop functionality, etc. If you're running control valves with their bypasses open, have out-of-tune loops, are not keeping up with your instrument maintenance due to poor management, lack resources due to budget cuts, you might also be suffering from a lack of availability. Human beings are often expected to fill gaps in control systems lacking availability, but they can have varying degrees of success.

Free from Danger, Risk or Injury

Safety can be defined as the condition of being free from danger, risk or injury. Process safety management can be defined as the control of recognized hazards to achieve an acceptable level of risk to people.

One of OSHA 1910.119 PSM regulation's 14 elements is mechanical integrity—to ensure that critical process equipment is designed and installed correctly and operates properly. This sounds like reliability is probably in there somewhere, but you will probably not find a reliability engineer on the PSM staff nor a PSM engineer on the maintenance staff, and maybe not even cross-pollenization of duties. Hopefully, there will be some direct coordination between these functionalities to assure that safety-critical equipment is reliable and is maintained appropriately.

How else are safety and reliability related? Is it possible to have safe systems that aren't considered reliable? Due to redundancy designs to achieve high safety reliability (e.g., 1oo2), the reliability equation has minimization of the potential unavailability of the safety system as a primary consideration, with process availability secondary. Safety systems will not be considered "reliable" if they trip often and cause process outages, but this is many times a function of poor design rather than any inherent limitation of safety in regards to reliability.

Safety in a process plant is generally divided into worker safety (e.g., reduction in lost-time accidents and recordables) and process safety (e.g., reducing the risk of a loss-of-containment (LoC) event). People safety is improved by reliable equipment by reducing the man-machinery interaction. With the introduction of LOPA in the late 1990s, emphasis has been placed on independent protection layers (IPLs), which can include instrumented and non-instrumented systems. Emphasis has been largely on safety instrumented systems to reduce the risk of an LoC event.

The importance of other non-SIS IPLs has come to the forefront recently, along with the realization that reducing the frequency of initiating causes (i.e., reliability) provides a practical reduction in risk (i.e., fewer demands on the safety systems equals fewer potential incidents). The Center for Chemical Process Safety ( has published a book on the subject, Guidelines for Independent Protection Layers and Initiating Events. In addition, the S84 committee has recognized that instrumented protective systems other than SIS play an important part in process safety and has moved to address them in the ANSI/ISA-84.91.01-2012 standard, "Identification and Mechanical Integrity of Safety Controls, Alarms and Interlocks in the Process Industry."

It seems fairly obvious that safety systems should be reliable, or at least tolerate faults or failures. From the design perspective, improving reliability can be considered an inherent safe design principle. Essentially, the more reliable a facility is, the safer it is.

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