Safety Instrumented Systems design tips for Instrumentation and Control engineers,automation engineers,safety engineers

Introduction

Modern chemical and hydrocarbon processing plants, oil & gas production facilities, power plants and other similar process plants all have some instrumentation and automation that ensures safety. These are known as Safety Instrumented Systems (SIS for short). These systems also are known by various other names such as Emergency Shutdown Systems (ESD for short), Safety Shutdown Systems, High Integrity Pressure Protection Systems (HIPPS) and so on. But all of them belong to the class of systems that are referred to as SIS.

Designing a Safety Instrumented System

No, here we are not talking about designing the next breakthrough in a great logic solver (also commonly referred to as a "Safety PLC"). We are addressing the situation in which many Instrumentation and Control engineers find themselves in, when assigned a job to design the SIS for a process plant. Here, the entire process involves finding out what kind of systems and devices to use in the application that the client or user wants. These design tips should make the task somewhat easier.

Note: If you would want to understand the whole process completely in depth, I would suggest downloading the Safety Instrumented System e-learning course from this site. It covers all aspects of Safety Instrumented systems including the entire process starting from hazard assessment to partial stroke testing of safety shutdown valves and everything in between, including concepts such as SIL, HIPPS and all the other jargon that leaves many people intimidated and confused.

Design Tip 1

Keep the big picture in mind. An SIS is a Risk Reduction measure, not an end in itself.

Any large processing plant has a certain degree of inherent risk that is associated with operating it. There is nothing alarming about it. The principle applies to any voluntary human activity, like say driving a car. Driving a car has some risk and to counter this risk, one takes some safety measures (wear seat belts, have air bags, keep tire pressure OK,etc). Similarly one reduces the risk of running a processing plant by employing safety measures, one of which is by having an SIS. Thus an SIS is not the only risk reduction measure.

Secondly the goal of any safety measure (including an SIS) is to reduce the inherent risk of a process to an acceptable level. Keep this principle in mind before jumping straightaway into SIL calculations, quad redundant PLCs, etc. Will this system reduce risk to an acceptable level? Is this the only way to reduce the risk? Will it work? are some of the questions that you should ask.

Design Tip 2

Quantify the inherent risk and the acceptable risk.

Make sure that you know what is the inherent risk of your process (either by calculations, or historical records, or other data). This may be expressed in a variety of ways including FAR (Fatal Accident Rate), Undesired Events per year, reportable accidents per year, worker injuries per year and so on. Now also make sure, that you know what is the acceptable level of risk in the same units. This information can be sourced from your corporate safety department, or risk management team.

Now use the equation

Risk Reduction = Inherent Risk-Acceptable Risk

to give you a measure that will define the amount of risk reduction that your system has to be able to do.

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