Safety Integrity Level (SIL), as the sole numerical measure of evaluation for Safety Instrumented Systems (SIS’s), has always been overlooked for its authentic existence. Everyone leaves no effort during the engineering phase, but the fact that the integrity shall be maintained to stay on the initially obtained level, is most of the time ignored. 

It is like maintaining a balanced scale. On the right side, your SIL target is based on process specifications, hazard studies, regulations, etc. The target is built up by adding all of the blocks on top of each other.

If you obey those rules on the right side, and choose qualified partners to realize your SIS’s, they can be easily built at the same level as the SIL target. However, this is only the beginning of a long journey because the building blocks on both sides are never inert. The right side is changing slowly due to activities like updating the standards and revising the regulations. As to the left side, the changes are more active. Carelessness tampers with the dynamic balance which leads to several (negative) possibilities, and consequently collapses.  

For example, if you buy a new car, usually some issues start to occur as the odometer increases. This is mainly because while you are driving the car, the car is also degrading. Materials like rubber may become worn, metal may become more brittle, etc. and larger problems will emerge when many small issues accumulate.

These degrading processes are inevitable. But we can hold off the occurrences of problems in a systematic way by implementing inspections at regular intervals. Intrinsically not all of the issues can be detected and fixed. Therefore, the demand for inspections is becoming more frequent and the scope becomes wider along the lifecycle of a car. In the Netherlands, for example, the first inspection of a car occurs after just four years of driving, and the next two inspections occur every second year thereafter. Then annual inspection is required until the limit has been reached.

In the world of functional safety, i.e. SIS’s, the inspection is called a proof test. During the proof test, safety engineers shall examine the whole loop by simulating the hazardous signal, and verifying if all the elements are reacting as specified. The proof test interval constitutes the integrity level together with other factors. Therefore not executing the proof tests degrades the integrity directly.

Besides inspection, maintenance is also an important part of a car’s lifecycle. For example, the engine oil should be checked regularly to guarantee the engine’s performance.  If one chooses improper engine oil, it could lead to issues like additional fuel consumption, or problems like engine wear and tear.

In general, you need to familiarize yourself with the engine specification, environmental conditions like average temperature and how many kilometers are shown on the odometer. This kind of homework will provide necessary information to choose the proper oil. But it is always better to let a trustworthy garage do it for you. Because from there you can find professional mechanics, expertise, maintenance records and thus a warranty.

Same goes to your SIS’s. It might be a modification in the logic or simply a replacement of a wire. In order to find the “trustworthy garage”, you need to check at least three aspects: competent personnel, clear working procedures and auditable trails.

Regarding SIS’s modification, competent personnel should have sensitive thinking of potential hazards and risk analysis. Skepticism is even encouraged sometimes since safety systems look the same as other process equipment is on site, valves for example. Being competent in this case means one should recognize the protection layer and the potential hazards that are attached to those valves.

Based on experience and qualifications, some safety authorities nowadays can provide safety certification for not only personnel competence, but also company procedures, management systems, etc. It is always recommended to verify their certificates as evidence for being a “trustworthy garage.”

Unfortunately, current technology cannot promise us 100% certainty in what we do. In our car analogy that is to say the design cannot be flawless. Although we may be able to achieve 99.99%, unknown issues are still hiding in the 0.01% based on our current understanding of the world.  Cars that are produced on the same process line have different lifetimes and ending stories. Some are running in a colder environment while others are running in a drier environment. With the same measures of evaluation, cars behave differently due to environments and driving behavior. From steam engine to electric cars, emerging technologies provide us as many challenges as benefits. But we should prepare a systematic way to gain the knowledge from trial and error.

In respect to SIS’s, those unknown factors can be anywhere in the lifecycle, like in the design specifications, manufacturing processes, installation, etc. and the diversity of technology has made it more challenging. As an example, cyber security for the process industry is relatively new and its impact on SIS is not completely understood yet. To cater for such challenges, proper company policy and management procedures should be in place, which enables studying of the issue and initiating appropriate follow-ups.

However, no matter how expensive and advanced your car is, it can never stop you from driving off a bridge.

What kind of car will you choose? How will you drive it? How often will you take it to inspection? The answers solely depend on your choices. One with reactive thinking behaves totally different from one with proactive thinking. That is why “people” are the bottom block on the scale.

The mindset behind safety thinking is the most enduring and profound challenge for us to overcome. To achieve the ultimate goal which is to bring safety into the corporate culture, various methods like training are well-developed nowadays. But before forcing the people to be motivated, employing safety consultancy can be very effective since this kind of service normally gives a customized roadmap which avoids unnecessary barriers. Of course, it is only trustworthy when this roadmap is drawn by certified safety personnel.

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