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 Hazard and Operability Study (HAZOP)
A Hazard and Operability Study (HAZOP) is a simple, structured methodology for identifying, evaluating and prioritizing potential hazardous occurrences in an existing process facility or a proposed new facility. The HAZOP methodology is a safety analysis that uses and encourages imaginative thinking (or brainstorming) and was first developed by Imperial Chemical Industries (ICI), a British chemical company. It is performed by a multi-disciplinary HAZOP team and entails the use of guide words to stimulate the brainstorming. For a proposed new process facility (such as a petroleum refinery, natural gas processing plant or chemical plant), a HAZOP may require many weeks to perform.

Although the HAZOP methodology was originally developed to study chemical process facilities, it has been extended to other types of facilities and complex operations.

There are many other hazard evaluation techniques, some of which are simpler than a HAZOP and some of which are more complex. For example, the Checklist and What-If methodologies are simpler than a HAZOP, and the Failure Mode Effects and Analysis (FMEA) and Fault Tree methodologies are more complex. In the Unoited States as well as some other nations, governmental regulations require some type of hazard evaluation be performed for certain, specified types of existing or proposed new process facilities.
Purpose of a Hazop
The primary purpose of a HAZOP is to improve the safety of the plant personnel as well as any nearby populated areas by identifying the potential for accidents to occur and taking steps to reduce the risk of such accidents.
Safety and reliability in the design of an industrial plant initially relies upon the application of various design codes and standards.  These represent the accumulation of knowledge and experience the industry as a whole.  Such application is usually backed up by the experience of the engineers involved, who have usually been involved in the design or operation of a similar plant.
Although design codes and standards are valuable, it is important to supplement them with a study of operational deviations that might occur because of events such as equipment malfunction or operator error. A HAZOP is an opportunity to identify such deviations and to make design and operational changes to prevent them from occurring and/or to mitigate the consequences of such deviations.
What a HAZOP is not
A HAZOP is not an method to determine how far one can go in physically removing or mitigating all risks nor is it a method for defining detailed engineering or procedural solutions for eliminating sources of risk. Most importantly, utilizing HAZOP methodology is not a guarantee that adverse consequences will not occur.
Table 1 provides a list of the terms pertinent to a HAZOP along with their definitions:
Matrix of guide words and process parameters
Table 2 presents an example matrix of which typical guide words are applicable to each of a typical set of process parameters:
As noted, the above Table 2 is merely a typical example. The HAZOP team will select the appropriate sets of guide words and process parameters that are to be used for the process or operation being studied.
The HAZOP team
The team that will perform the HAZOP should consist of personnel with a good understanding of the process facility to be studied. The team members should be people from a range of disciplines and that is one of the strengths of the HAZOP methodology.
A HAZOP team typically meets daily for sessions of three to six hours each and, as noted above, it may require many weeks to perform the HAZOP. A typical team should be limited to no more than about eight to nine members and include:
A team leader: It is important that the team leader be someone from outside the immediate organization of the facility being analyzed and, ideally, come from a completely independent company. The leader should be experienced in the HAZOP methodology and have in-depth knowledge of how chemical process facilities work even though he or she may not have an intimate knowledge of the technology involved in the particular facility being studied.
A scribe or recorder: The primary function of the scribe is to record and document the proceeding of the HAZOP sessions as well as any recommendations made by the team. The scribe is a full member of the team and participates in the discussions. The scribe must have the experience  and qualifications to occasionally switch roles with the team leader.
The scribe must screen the discussions and only record what is relevant to the topic being analyzed. The scribe is also usually responsible for preparing interim and final reports under the direction of the team leader.
Process designer(s): One or more representative(s) of the team that designed the facility to explain the process design and provide other information as needed. If the process design of the facility involved an outside contractor or process licensor, then they should provide a knowledgeable process design representative.
Facility operator: A representative of the workers selected to operate the facility.
Process control expert: A process control expert to provide expertise on the instruments and control systems, as well as the safety shutdown systems. He or she should be a full-time member of the team.
Maintenance representative: If appropriate, the team should include a representative of the facility's maintenance department.
Specialist(s): At times, specialists may be needed for limited time periods. For example, if the team is studying some issues involving corrosion, it may need the help of a corrosion expert for that part of the HAZOP.
Pre-HAZOP preparations
Before the HAZOP commences, the team leader should:
  • Identify and locate up-to-date process flow diagrams (PFDs), piping and instrumentation diagrams (P&IDs), a facility site layout (plot plan) and all equipment design specifications and construction drawings. Also, locate the startup, operating and shut-down manuals and procedures.
  • Participate in selecting the appropriate team members, orient them as to the HAZOP methodology and provide some basic training in HAZOPs if needed.
  • Prepare a schedule for the HAZOP session meetings and distribute it to the team members.
  • Develop a list of the guide words to be used for the HAZOP and make sure that the team members agree with the list.
As shown in the adjacent Figure 1, the sequence of steps in the HAZOP methodology are typically these:
  • Steps 1 and 2: Select the first section of the facility to be studied and the P&ID sheet that includes that section. Define and mark the first study node on the P&ID. The size of the selected node will depend on the experience of the team members, the degree to which similar process systems may have already been discussed, the complexity of the process and the judgment of the leader.
  • Step 3: Define the process design intent of the study node and select the process parameters (e.g., flow, temperature, pressure, etc.) that apply to that node.
  • Step 4: Select one of the agreed upon process parameters.
  • Step 5: Select a guide word (e.g., more, less, reverse, etc.) and discuss any significant deviations in the node's process intent that might occur in the selected parameter as described by the selected guide word. For example if the selected parameter is flow, and the selected guide word is more, determine if any significant deviation might occur in the process intent of the study node.
  • Step 6: Identify the cause of the deviation in the node's process intent and record it.
  • Step 7: Identify the consequence of the deviation and record it.
  • Step 8: Evaluate and record the HAZOP team's agreed upon estimate of the severity of the consequence (see the risk ranking matrix in the next section below).
  • Step 9: Identify and records any safeguards (if any) provided in the facility design to reduce the severity or the likelihood (see the risk ranking matrix) of occurrence for the identified consequence.
  • Step 10: Evaluate and record the HAZOP team's agreed upon estimate of the likelihood of occurrence for the consequence.
  • Step 11: Using the recorded severity and occurrence likelihood of the identified consequence (as per steps 8, 9 and 10), determine the risk level from the risk ranking matrix.
  • Step 12: Identify and record any agreed upon recommendations for preventing or mitigating the identified consequence.
  • Step 13: Repeat for each guide word applicable (as per Table 2 above or a similar table) to the selected parameter.
  • Step 14: Repeat for each parameter of the selected study node. Mark the study node as completed on the P&ID when all of the process parameters have been studied.
  • Step 15: Repeat steps 3 through 14 for each study node selected from the selected P&ID sheet and mark that P&ID sheet as done.
  • Step 16: Repeat for each P&ID of the facility.
Risk ranking matrix
The adjacent Table 3 provides an example of a typical risk ranking matrix defining four levels of risk:
  • Unacceptable risk: Must be mitigated to a medium risk of 3 as soon as possible.
  • High risk: Should be mitigated to a medium risk within a reasonable period of time.
  • Medium risk: Verify that controls, procedures and policies, and safeguards are in place.
  • Low risk: Acceptable as is, and no action is necessary.
As shown in Table 3, each of the four levels of risk are defined by their severity and likelihood of occurrence as identified and recorded during the step-by-step HAZOP methodology.
Table 3 is merely an example matrix and the HAZOP team may agree upon the use of a variation of Table 3 that uses more risk levels as well as different names and different action requirements for each level.
  • Center for Chemical Process Safety (CCPS), AIChE (2008), Guidelines for Hazard Evaluation Procedures, 3rd Edition, Wiley-American Institute of Chemical Engineers, ISBN 0-471-97815-9.
  • Trevor Kletz (2006), Hazop and Hazan, 4th Edition. Institution of Chemical Engineers, ISBN 0-85295-506-5.
  • Hazard & Operability Studies (HAZOPS) An excellent explanation of HAZOPs by a HAZOP software company.
  • Personal communication from Dr. Chandra Roy, Consulting chemical engineer, 2010
  • Laird Wilson and Doug McCutcheon (2000), Industrial Safety and Risk Management, 1st Edition, University of Alberta Press, Canada.ISBN 0-88864-394-2.