The Basis of Safety

Article by Phil Eames

Why your plant is safe, and how to maintain It

My name is Phil Eames and I’m a chemical engineer with 36 years experience in the process industries in a variety of engineering and management roles. I have a long-held interest in process safety and have practised as a process safety consultant for the last ten years, attaining Professional Process Safety Engineer status in 2015.

I have experienced the transformation in the profile of process safety over the last 15 years and am particularly interested in the differences between the management of process and occupational safety, which I will explore in this series of articles.

The previous article in this series emphasised that in order to manage process risks effectively we need to understand them well. The basis of safety for the process – the explanation of why it is safe –  can be produced following an effective process hazards identification study, and then maintained through an active assurance programme.

The human cost

When I was a young plant manager on a major hazards operation, an experienced colleague under my supervision suffered what was believed to be a lethal exposure to hydrogen cyanide during preparations for an overhaul (turnaround). He somehow survived but never worked again.

I will never forget the shock, emotional turmoil and stress throughout the period of his illness and the subsequent investigation. I was responsible for the operations he was performing, and the investigation showed that they were far from adequate.

We were clearing the liquid hydrogen cyanide supply line to the plant using nitrogen to blow it into the plant’s reactor, where it could be drained and subsequently treated. The operation had been carried out in preparation for the plant’s two-yearly overhaul for many years; the supervisor involved had performed it a number of times. The operation utilised a 3 barg nitrogen utility supply to blow the contents of the line into the reactor; 3 barg had been shown to be sufficient pressure. However, unbeknown to us this time, the nitrogen supply had been modified since the previous overhaul to provide a nitrogen blanket to a storage tank elsewhere on the plant; a pressure reducer had been installed to reduce the pressure to 30 mbarg. So when this supply was used, it failed to dislodge the contents of the line, so when the nitrogen was disconnected pure hydrogen cyanide liquid was immediately released.

The procedure for the task contained no requirement to verify the nitrogen pressure as adequate, no specific location for the point of connection to the nitrogen system, no expectation of how long the blowing should take, no requirement to verify that the contents had actually been transferred, and no specific requirement to disconnect the nitrogen at a point within the area of the plant that required the wearing of chemical protection suit and full breathing apparatus. It had not been reviewed since the previous overhaul, nor subject to any kind of hazard identification assessment such as procedural HAZOP.

These requirements for connection, blowing and verification, and the calculations that should have underpinned them, are what we now often call the basis of safety –  a justification for why the process or operation is safe, based on a description of the barriers that are in place to prevent, control or mitigate potential major process incidents.

Basis of safety

The basis of safety for an operation is usually generated from hazard identification, typically from hazard Identification (HAZID), or hazard and operability (HAZOP) studies (that can include procedural HAZOP in relation to activities like the one described above). To summarise and communicate the basis of safety in a usable form, it is often extracted from the detailed study worksheets into a separate document, from which it can be represented in user-friendly formats such as those shown in Figures 1 and 2.

Figure 1: Reaction runaway basis of safety in list format
Figure 2: Chlorine drum basis of safety in bow tie format

Capturing the basis of safety in one of these formats provides a helpful start point for spot-checking on the plant that barriers are in place and functioning correctly; that alarms are active, protective system proof testing is being carried out, procedures and training are up-to-date etc. Assurance processes such as these – called variously basis of safety assurance 1, barrier assurance or deep dive assurance – have increased in popularity in recent years. They provide a direct and efficient way to provide assurance to management and to generate process safety performance indicators (PSPI) based on the challenge to “show me why it’s safe”. They are also useful vehicles to involve employees in process safety activities and to deepen understanding of the basis of safety throughout the organisation. And they are also more interesting and fun than traditional management system auditing, but can still provide evidence of systemic weaknesses. For younger engineers, they can provide a great way to get out on the plant and into the engineering workshops, meeting front-line staff involved in operations, maintenance and inspection and learning about how the organisation manages process safety.

So next time you are involved in a discussion relating to issues that could be impacting process risk on your plant, remember the question: “what is the basis of safety?” and the challenge: “Show me why it’s safe…let’s go and see”.

In the next article we will examine the concept of operational discipline and its fundamental role in maintaining the basis of safety.


1. "Are You in Control of Process Safety? Basis of Safety Assurance Can Provide The Answer", IChemE Loss Prevention Bulletin 231, June 2013.

IChemE’s Hazard Identification Techniques Leadership and Process Risk Assessment courses provide more detailed explorations of this topic.

Article by Phil Eames

An IChemE Fellow and an independent consultant. Eames has worked on process safety in most sectors of the process industries including oil and gas, petrochemicals, pharmaceuticals, power generation and water. He is also a trainer for IChemE, delivering a range of courses including the HAZOP Leadership and Management course

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