“We know we need civilization and laws and procedures, but isn’t it frustrating? Wouldn’t it be great if we could just do what we needed to do?”  — Lee Child

When someone thinks about a Process Hazard Analysis, a PHA, they usually think about a Hazard and Operability Review, a HazOp. And when they think about a HazOp, they usually think about taking a set of Piping and Instrument Diagrams (P&ID’s), splitting them into nodes consisting of either vessels or lines connecting vessels, evaluating each node in terms of a standard set of deviations to identify causes, consequences, safeguards, and risk, and when necessary, recommending additional risk reduction measures. The essential document in these HazOps is the P&ID because it is by understanding the P&ID that the process is understood, and hazards are discovered.

But what if the P&ID is not the essential document to understanding the process?

Processes That Don’t Depend on P&IDs

There are operations that use mobile equipment, very little fixed piping, and even less automation. They are mostly manual operations run in a set of equipment configured for the process being run. We think of the equipment in these operations as “pots and pans.” While depictions of process flows are useful, P&IDs are not particularly relevant. Instead, it’s the recipe that matters. Just as the P&IDs are not particularly relevant to understanding the process, the standard set of deviations of a traditional HazOp is not particularly relevant to discovering the hazards of the process.

Instead, these batch processes—they are always batch processes—reveal their hazards in a review of the batch procedure. A tool for discovering the hazards of these processes is the Procedural HazOp.

The Procedural HazOp

In the early 1990s, several organizations presented ideas about applying the original process HazOp methodology to procedures.  They used the same approach: taking the essential descriptive documents, splitting them into nodes, evaluating each node in terms of a standard set of deviations to identify causes, consequences, safeguards, and risk, and when necessary, recommending additional risk reduction measures.

The obvious difference between a Procedural HazOp and the traditional process HazOp is that a process HazOp is based on complete and accurate P&IDs, while a Procedural HazOp is based on a complete and accurate procedure. From that, there are also differences in what constitutes a node and in what the standard set of deviations should be.

Nodes were defined as steps in the procedure rather than as sections of process equipment.  Deviations were still defined as a combination of parameters and guide words, but they were relevant to procedural steps rather than the deviations typical of a traditional process HazOp.

Steps – The Nodes of a Procedural HazOp

The nodes of a Procedural HazOp are the steps in the procedure rather than sections of process equipment.

Steps consist of changes to the state of the process.  This includes changes in the orientation of a valve or set of valves, e.g. going from closed to open; starting or stopping a set of motors, particularly pump or agitator motors; or changing the set point of a control loop to achieve a new process objective. It is important to note that, depending on how the procedure is written, instructions in a procedure may include more than one step.

Nodes also includes assumed steps—those steps, typically at the beginning or end of a procedure that must be done but are not explicitly stated.  Examples of assumed steps include draining, cleaning, and drying equipment, initially lining up valves, and cooling or preheating equipment.

A Procedural HazOp only considers steps where the state of the process changes.  Many procedures include other directions that are not intended to change the state of the process, but to confirm that the process is in the correct state.  These directions are not steps, but safeguards, and should be listed in the Procedural HazOp worksheet as safeguards.  They typically include directions that begin with the words “verify”, “check”, “confirm”, “make sure”, “prove”, and “ensure”.

The Deviations in a Procedural HazOp

Like the deviations in a process HazOp, the deviations in a Procedural HazOp are a combination of parameters and guidewords. Instead of the familiar parameters of pressure, temperature, flow, level, and composition, however, the parameters of a Procedural HazOp are

  • Order
  • Timing
  • Duration
  • Action

The guidewords for a Procedural HazOp are similar to those for a process HazOp: Low, High, Other than, and Not. The relevant deviations, then, are

  • Out of order (Order, other than)
  • Begin/Start too soon (Timing, low)
  • Begin/Start too late (Timing, high)
  • End/Stop too soon (Duration, low)
  • End/Stop too late (Duration, high)
  • Partial completion (Action, low)
  • Additional action (Action, high)
  • Repeated (Action, high)
  • Wrong (Action, other than)
  • Omitted (Action, not)

We have discovered that it is helpful to start with “Omitted”. Once the team has considered the consequences of simply not doing the step, the other deviations are easier to tackle. We also save “Wrong” to the end, where it serves to prompt consideration of hazards not already addressed.

Human Factors

Because Procedural HazOps are usually applied to highly manual processes, it is the nature of a Procedural HazOp to consider human factors. For instance, a typical cause of omitting a step is a lapse—the operator forgets to do it. It is also easy to see how human factors would play into each of the deviations. That said, the Procedural HazOp methodology also prompts the PHA team to consider control failures as potential causes.

Using Procedural HazOps

If you are struggling with performing PHAs on your batch processes because P&IDs don’t seem particularly relevant and yet are essential to process HazOps, don’t despair. Many have found themselves in the same circumstances and have successfully turned to Procedural HazOps. Dividing a procedure into nodes takes the same kind of discipline as dividing P&IDs into nodes, and as with anything, you will get better at it with practice. Once you’ve done that, though, and populated your HazOp worksheets with the appropriate list of deviations, you will find that it is a productive way to review a batch process. As a facilitator, you will use the same skills you have already developed for the more traditional HazOp, and you will find that the method will identify process hazards and prompt your PHA team to make good recommendations.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.