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Why SIL is the best thing that has happened to process safety – and why it needs to go further

30 March 2019

By continuously monitoring critical parameters, instruments can highlight potentially dangerous conditions and ensure that processes are safely shut down. For this reason, instruments need to offer maximum protection against unsafe events escalating into dangerous failures. One way to ensure this is to select SIL-rated instruments. In this Q&A, David Bowers of ABB explains more about SIL and its impact on process safety. 

What are the main changes in process safety since you started your career?

Having worked in this industry for many years, I can confidently say that the introduction of Safety Integrity Levels, or SIL, in 1998 has been the most significant change in process safety. 

Up until 1998, there was a growing need for improvements in health and safety to prevent dangerous process failures and reduce the risk of hazardous events.

SIL, as defined by IEC 61508, was introduced to rate the efficiency of a safety system within a process and its probability of failure on demand (PFD), when dealing with dangerous fluids and chemicals.  SIL ratings range from SIL 1 up to SIL 4, with the lower number indicating the lower chance of system failure.  As the numbers increase so does the cost of maintaining the safety of a complex system and the risk of process failure that could lead to injury and even death.

When dealing with SIL, it is important to understand that the SIL rating relates not to the instrument itself but rather the process it is being used in.  Individual safety instrumented functions (SIF) and safety instrumented systems (SIS) within a process can obtain SIL ratings but instruments can only be perceived as being compatible with a particular SIL-rated application.

The whole idea behind SIL is redundancy and the ability to mitigate risk if a measuring instrument of some kind fails in place.  Having either a back-up function or spare instrument in case the primary device fails makes perfect sense and provides an added safeguard against process failure.

Even then, it is not a case of ‘job done’. If these instruments are not checked, recalibrated and maintained, then all safety measures go out of the window, highlighting the vital contribution that a thorough maintenance regime can make to process safety.

How recognised is SIL in industry?

The most surprising element of SIL is the fact that so few people are aware of it.  Even after events at Buncefield in 2005, many companies do not appreciate the relevance of applying the SIL approach to their own operations when it comes to measurement.

Throughout most industrial sectors we have instruments measuring water quality, steam distribution in power plants and custody transfer but there are no legal requirements to have redundant or fail-in-place instruments installed.  A good example is chemical dosing in potable water treatment applications. Many of these chemicals can be harmful to human health if administered incorrectly. Although doses of these chemicals are monitored extremely closely, the instruments making the measurements are not often monitored to ensure they haven’t failed in place.

In addition to helping safeguard against human injury or loss of life, having instruments with back-up protection in place can also help to protect against the loss of vital process data. The loss of data due to an instrument failing with no redundancy measure in place can cause a company enormous financial damage, especially where processes are subject to traceability requirements. Both can harm a brand’s reputation, not to mention incurring huge revenue losses due to delays or heavy fines for breaching legislation.

Taking all this into consideration, why wouldn’t you want to know if your instruments aren’t working correctly?

What factors do you think have acted as a barrier to the adoption of the SIL approach across industry?

In my opinion, there are three key factors that have presented a barrier to the widespread adoption of SIL, namely cost, skill and knowledge gaps, and the burden of responsibility.

As with most businesses, expenditure is closely monitored and if there are opportunities to cut spending they are usually taken.  When it comes to SIL-rated instruments, the initial outlay may be hard to justify but these costs pale into insignificance over the entire instrument lifecycle.  While there are SIL rated continuous measurement instruments, point measurement devices, which are only called into action when needed, can help to extend the total lifespan significantly and reduce lifetime cost. To ensure these instruments will function properly when called upon, it is advisable that they are frequently checked, with additional maintenance and recalibration if necessary. 

Unfortunately, this occasional use can also support the idea that SIL instruments are an unnecessary expense. When you consider though the £750 million fine Total had to pay after the Buncefield disaster, SIL approved instruments might not seem so expensive after all.

SIL skill and knowledge gaps have also proved to be a major barrier.  SIL is a complex topic and cannot be learned overnight.  Training courses are time consuming and costly, which means there are very few experts with the required knowledge to implement and approve SIL-approved safety systems.  Businesses can also be wary of training employees to be SIL accredited, well-aware of the high premium that SIL-accredited status offers, as well as the risk of losing out to companies willing to pay more for the employee’s services.

The other main barrier concerns the burden of responsibility.  With huge implications if a life-threatening incident occurs, including potentially large fines and prosecution of responsible individuals, plus the lengthy training process, it is not surprising so few people put themselves forward for this type of work.

What more could be done to improve understanding/awareness?

Most concerning is the use of SIL-rated instruments is still not a more common legal requirement. There needs to be more support from Government in the first instance, but this looks unlikely since not much has happened in terms of promoting SIL and its importance even after Buncefield.  Expanding SIL requirements to measurement instruments across all industries would be a positive step.  Although most sectors aren’t as dangerous as the petrochemical industry, there is no reason why stricter safety measures should not be applied across the board as a matter of best practice.

How would you like to see things develop over the next five years?

The greatest achievement would be for SIL-rated instruments to be used more frequently and become commonplace.

Ideally, if we could get to a point where SIL became standard, no other alternatives would exist.  With larger awareness and take up, these standards would become the norm, which would ultimately drive down cost when implementing safety systems in the future, helping to greatly enhance the overall management of safety throughout industry.


About the interviewee

David Bowers is the Process Flow and Pressure product manager for ABB Measurement and Analytics division, joining the company over four years ago. He has nearly 30 years’ experience in the industry and is a member of the Institute of Measurement and Control.

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