Building barriers against major accidents

We all have someone we love – a life partner, kids, friends, family or even a dog. These are the most important things in our lives – and we care deeply about the wellbeing of these special people (and animals) in our lives. We trust employers to make workplaces safe such that our most important ones can come back safely from work every day. Some workplaces have inherent dangers that are exposing people to unacceptable risks unless handled in a good way. How do we manage the most severe accident risks, such as explosion risk on an offshore oil platform, nuclear accidents, or releases of toxic chemicals, such as the horrific 1984 Bhopal accident?

When we build and operate such plants we need to know what the hazards are, and we need to plan barriers to avoid accident scenarios from developing. Risk management is thus integral to all sound engineering activity. A good description of a risk management process is given in ISO 30001 – such a process consists of several steps that should be familiar to practicing engineers and plant managers.

In the figure you can see this risk process explained. First of all, it is necessary to establish the context so that we can understand the impact of the risk – this is we need to ask questions such as;

  • What is the business environment we are operating in?
  • Who will be present and exposed to the risk?
  • What type of training do these people have?
  • Where is the plant located?
  • Etc., etc.

Then, we work to identify risks. In a process plant this activity is typically done in a number of workshop meetings such as design reviews, and maybe the most important, the HAZOP (a Hazard and operability study). The risks identified are then analyzed, to see what the overall risk to the asset and the people operating it are. Based on the risk analysis, the risk is evaluated up against acceptance criteria; is the risk acceptable, or do we need to devise some scheme to lower the risk?

In most cases where major accident hazards are possible, some form of risk treatment is necessary. In fact, an overall principle for barriers against major accident hazards (MAH’s) that is included in many legislations is:

“A single failure shall not lead directly to an unacceptable outcome.”

This leads us directly to our next natural line of thought; we need to build barriers into our process to stop accidents from happening, or to at least make sure an accident development path is changed to avoid unacceptable outcomes.

Common practice in process engineering is to require two barriers against accident scenarios, and these shall be different in working principle and be able independently to stop an accident from occurring. In practice, one of these barriers would typically be mechanical system not relying on electronics at all – such as a spring-loaded pressure relieving valve. The other barrier is typically implemented in an automation system as a safety trip. It is to this latter barrier type, the Safety Instrumented Function (SIF) we apply the concept of safety integrity levels (SIL) and the reliability standards IEC 61511 and IEC 61508.

Taking overpressure in a pressure vessel as an example, we see how these barriers work to stop an accident from occurring. Assume a pressure vessel has a single feed coming from a higher pressure source, but where the pressure is reduced before entry into the vessel by using a pressure reduction valve (a choke valve). As long as the design pressure (the maximum allowable working pressure, MAWP) of the pressure vessel is below the pressure of the source, we have a potential for overpressurizing the tank. This is always dangerous – and particularly so if the contents are flammable (hydrocarbon gases, anyone?) or toxic (try googling methyl icocyanate). Clearly, in this situation, a single error in the choke valve can lead to a large release of dangerous material. Such errors may be due to material failure of the valve (e.g. fatigue), maloperation or a control system error if the valve is an actuated valve used as final element in a control system, for example for production rate control. Process safety standards, such as ISO 10418 or API RP 14C require such pressure vessels to be equipped with pressure safety valves, that will release the pressure to a safe location when the design pressure is exceeded (typically the gas is burnt in a controlled flaring process). That is one barrier. Another barrier would be to install a pressure transmitter on the tank, and a safety valve that will shut off the supply of the gas from the pressure source. This valve and measurement should be connected to a control system that is independent of the normal process control system – to avoid a failure in the control system from also disabling the barrier function.

To sum it up; by systematically identifying risks and evaluating them against acceptance criteria we have a good background for introducing barriers. All accident scenarios should be controlled with at least two independent barriers, where one of them should be instrumented and the other one preferably not. Instrumented functions should be in addition to the basic control system to avoid common cause failures. The Safety Instrumented System (SIS) should be designed in accordance with applicable reliability standards to ensure sufficient integrity. Finally – the design must comply with local regulations and required industry practice and guidance – such as applicable international or local standards.

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