Many different sectors, such as the chemical, fertilizer, and manufacturing industries apply batch processes. For batch processes involving hazardous substances, focusing on process safety is key to protecting personnel, the environment, and assets. Hazard and Operability (HAZOP) studies are one of the most used methods for identifying hazards and operability issues related to processing designs. The method was devised in the 1960s. Since then it has seen wide adoption and has been of great use when designing new process facilities. It ensures that hazards are identified, and barriers are included in the design to mitigate or prevent these hazards from occurring.
HAZOP studies are normally based on reviewing the Piping and Instrumentation Diagrams (P&IDs) and Cause and Effect Diagrams (C&Es). This is usually combined with a process description. What's good is that you can use them for almost any process design. The overall approach to HAZOPs remains the same for any process design (read more about it in this insight). However, performing Hazard and Operability of a batch process differs somewhat from performing HAZOP of a continuous process. This insight highlights some of these differences. It also aims to provide some guidance for ensuring an optimal application of the HAZOP method to batch processes.
Main differences between continuous and batch processes from a HAZOP perspective
In a continuous process, as the name implies, the process effects occur continuously. This means that as long as the process is running within its operating envelope, the conditions (fluids, temperatures, flow rates, pressures, etc.) remain more or less constant, “independent” of time. In a continuous process, this allows a single node in a Hazard and Operability to cover a specific part of the process design.
A batch process, on the other hand, is a process design where time is a factor, and where the process effects are based on a stepwise approach. This means that for a certain part of the design, there might be significant changes to the process conditions. There might even be different fluids in the system. This depends on which step of the process recipe the system is currently in. This means that a specific part of the process design needs to be part of multiple nodes.
To get the most out of the HAZOP for batch systems, we recommend the following main steps. Let's go through all of them.
In addition to the P&IDs, C&Es, and process narratives normally forming the basis for all HAZOPs, additional preparation for a batch HAZOP should include ensuring that Material Safety Data Sheets (MSDS) are available for all raw materials, products, intermediate products, and by-products from the process. Further, you should clearly identify and describe the permissive conditions in place (automatic or manual) for going from one process step to the next.
Establishing the boundaries for the HAZOP
As for HAZOPs of continuous process designs, it is equally important for HAZOPs of batch processes to clearly delimit the scope of the review, and to understand the limitations of the review method. The HAZOP should focus on hazards related to the process design, hence “external events”, or issues not directly linked to the process design as represented on the P&IDs may be better to cover with other reviews. This could be leak sources, ignition sources, impacts / dropped objects, equipment failure mechanisms, etc.
These lines may often be somewhat blurry. Therefore, it is important to clearly specify upfront which types of events the HAZOP should include. Additionally, to which extent the Hazard and Operability should cover for example “leaks” and “ignition sources” in general. However, it is not necessarily feasible to list all leak points or ignition sources in a HAZOP. You can certainly cover failure of equipment e.g., agitators and pumps in general in the HAZOP. However, it is not feasible to list all failure mechanisms of an agitator or a pump, raw materials can be contaminated, but this is not necessarily directly linked to process design, etc.
Batch HAZOP Nodes
In a HAZOP of a continuous process, the design is usually split into nodes based on the piping and equipment design. For the reasons highlighted in the above section, a HAZOP for a batch process should be different. It should split the design into nodes based on the steps in the process recipe. This ensures that all variations in process conditions are covered. This means that some parts of the piping and equipment will be part of several nodes. To reduce the scope of the review, you can perform a screening of the various process steps before the HAZOP. This way you can exclude simple and familiar steps of low hazard potential.
A design may e.g., consist of a reactor with a pump system for round-pumping (for mixing) and for emptying the reactor after the reaction has finished. It could also consist of cooling and heating systems, and systems for measuring and adding the various reagents.
To reduce the amount of repetition, it may also be a good idea to cover the auxiliary systems in detail. In this case that would be the pump system, the heating/cooling systems, and the systems for measuring reagents. You could do this only in the first process step where you use them. You could also do it as separate nodes, where you identify specific causes and hazards within these utility systems. For subsequent process steps where you use these systems again, it may then be sufficient to cover issues in these systems on a higher level (e.g., too much heating / too little heating, too much of material A / too little of material A, etc. rather than the detailed causes of this), and then assess the specific consequences of this for the process step in question.
Batch HAZOP Guidewords
In addition to the standard guidewords used in HAZOPs to cover the operational parameters (e.g., flow, pressure, temperature, level, phase, composition, etc.) and guidewords related to operational modes and maintenance, the list of guidewords for a batch HAZOP must consider the stepwise process recipe. Hence the list of guidewords for a batch process should include aspects such as:
Timing (Before/Sooner/Early, Late/after)
Failure of (Agitator, Power, Vents, Fuel, Other utilities (e.g., water, blanketing gas, air, including failure modes of valves)
Read also our article about: "HAZOP vs HAZID – when is one more useful than the other?"
Want to know more about how you can tailor HAZOPs for batch processes?
ORS Consulting has supported multiple companies in the chemical and manufacturing sectors with process safety matters. This includes functional safety (SIL), Process Hazard Analysis (such as HAZOP), barrier management, risk assessments, and other technical risk management methods.
Please contact us if you want to discuss the application of process safety for your facility. We will get back to you as soon as possible.