How Accurate Are Safety Calculations

by | Dec 2, 2019 | Blog, Functional Safety, Process Safety

How accurate are safety calculations: If you have ever sat in a LOPA, inevitably there is someone that questions the accuracy of one factor or another. Usually they are trying to justify making an initiating cause less frequent or take more credit for an Independent Protection Layer (IPL).

As unsatisfying as it is, assessment of Process Safety Systems is a statistical adventure. And when you get down to it, people just don’t like, or understand statistics. They find it a playground in which to argue that their number is the “right” number. Statistics are real, but people don’t like to believe them. Otherwise casinos would not be in business.

Evaluation of protective function performance requirements and performance of the designs for those functions requires establishment of probabilities for things like how often an initiating event may occur and how effective mitigating functions are likely to be. Deciding what probabilities to use is the hard part. The problem when it comes to Process Safety Systems is that these probabilities are very fuzzy numbers. How accurate are safety calculations, unlike a pair of dice, which have very precisely defined odds of 7 or snake eyes coming up, real process related risk and failure data is less than precise.

The Process Safety Standards and Practices used by the Process Industries have developed over the past 20-30 years and the various factors used in Process Safety analysis have tended to converge on consensus values. The AIChE CCPS publication, Layers of Protection Analysis, provides a fairly complete set of values for LOPA related factors, and various publications on Safety Instrumented Systems provide representative failure rates for commonly used devices. In these instances, the publications note that the factors actually used are up to the User.

One of the things these publications generally state, is that absent any hard data supporting another value, all of the factors used should be no more accurate than a power of 10. So, factors used are values of 10, 100, 1000, or their inverse (10-1, 10-2, etc). Attempting to use any values that have more precision is usually an exercise in self-delusion. Even the factor of 10 practice is only an approximation. However, the recommend values in reputable publications are based upon the collective experience and judgement of some very experienced and pragmatic people. Unless you have lots of actual in-service data, you don’t really know anything better. Use the expert’s numbers.

When working with input data that only has a precision of powers of 10, you also have to be cognizant that the answer you get after stringing a bunch of them together in a Risk Reduction or PFD calculation, isn’t going to be any more precise than the input data. So that RRF calculation that tells you need a SIF with an RRF of 186 is giving you a false sense of precision. It’s not actually 186, it could be 100 or could be 1000.

This is why ISA 84 ed.2 and IEC 61511 only recognize Safety Integrity Levels (SIL) specified in decades – RRF’s 10, 100, and 1000.   When you are calculating a PFD of a SIF design, that 186 is often used as a hard requirement, when in reality it is a very, very fuzzy target. There is absolutely no basis to say that that a calculated SIF RRF of 130 doesn’t meet the requirements of a LOPA target RRF of 186. Given the accuracy of the input values used, 130 and 186 are the same number.

This doesn’t say that a practice of requiring a SIF design to meet a higher (but not precise) target is wrong.  It does give a design target and tends to result in more thought about the SIF designs. However, you shouldn’t fool yourself into thinking that you are being very precise. If it’s a major expense to get a SIF from 130 to 186, think about whether that really is making a difference.


 Want to learn more about  SLM®?

Click here to download the free white paper!

Rick Stanley has over 40 years’ experience in Process Control Systems and Process Safety Systems with 32 years spent at ARCO and BP in execution of major projects, corporate standards and plant operation and maintenance. Since retiring from BP in 2011, Rick formed his company, Tehama Control Systems Consulting Services, and has consulted with Mangan Software Solutions (MSS) on the development and use of MSS’s Safety Lifecycle Management software.

Rick has a BS in Chemical Engineering from the University of California, Santa Barbara and is a registered Professional Control Systems Engineer in California and Colorado. Rick has served as a member and chairman of both the API Subcommittee for Pressure Relieving Systems and the API Subcommittee on Instrumentation and Control Systems.