Hazard and Operability Study

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(PD) Photo: U.S. Chemical Safety Board
Fire at Delek Refinery in November 2008, Tyler, Texas

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.[1][2][3] 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 (HE) 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.[4] In the United States of America 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.

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.[5]

Most importantly, it 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:

Table 1: Terminology and Definitions
Term Definition
Hazard A source, situation, event, circumstance or condition with the potential to cause harm,
including ill health, injury or death, production losses, or damage to the environment.
Study nodes
(or process sections)
Section of the process equipment with definite boundaries (for example, the piping
from a process vessel to a pump) within which process parameters are investigated
to determine the effect of deviations from the design intention and the potential of a
hazard being created in that section.
Process parameter A physical or chemical property involved in a study node, including items such as
temperature, pressure, pH, concentration, reaction or volume.
Deviation Departures from the design intention that are discovered by systematically applying
the pertinent guide words applicable for each study node.
Design intention Definition of how the study node is expected to operate in the absence of deviations from
the process design.
Guide words
(or guidewords)
Simple words used to qualify the design intention and to guide and stimulate the
brainstorming process for identifying potential process hazards.
(or protection systems)
Administrative controls (such as operating manuals and work procedures) or engineered
systems designed to prevent deviations or to mitigate the consequences of deviations.

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:[5][6][7][8]

Table 2: Example Matrix of Guide Words and Process Parameters
Guide word
More Less None Reverse Part of As well as Other than
Flow x x x x x x x
Temperature x x
Pressure x x x x
Level x x x
Volume x x x x
Mixing x x x
Composition x
Reaction x x x x x
pH x x
The letter x denotes which guide words are applicable for each of the process parameters.

As noted, the above Table 1 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.[9]

A HAZOP team typically meets daily for sessions of 3 to 6 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 8 to 9 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.

Process designer(s): One or more representative(s) of the team that designed the facility to explain the process design and provide other information that may be 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.


(PD) Diagram: Milton Beychok
Figure 1: The sequence of steps in performing a HAZOP[5]

As shown in the adjacent Figure 1, the sequence of steps in the HAZOP methodology are typically these:[5][9]

  • 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

Table 3: Risk Ranking Matrix[5]
   Severity    Likelihood
 Frequent   Probable  Occasional     Remote    
Major Unacceptable Unacceptable Unacceptable High
Moderate Unacceptable High High Medium
Small High Medium Low Low
Insignificant Medium Low Low low
Frequent = Once or more per year
Probable = Between once or more per year and once per 10 years
Occasional = Between once per 10 years and once per 30 years
Remote = less than once per 30 years

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: Should be mitigated to a medium risk within a reasonable period of time.
  • Medium: Verify that controls, procedures and policies, and safeguards are in place.
  • Low: 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.


  1. 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. 
  2. Trevor Kletz (2006). Hazop and Hazan, 4th Edition. Institution of Chemical Engineers. ISBN 0-85295-506-5. 
  3. Hazard & Operability Studies (HAZOPS) Excellent explanation of HAZOPs by a HAZOP software company
  4. All of these hazard evaluation methods are defined in Chapter 5 of the CCPS book named in reference 1
  5. 5.0 5.1 5.2 5.3 5.4 Personal communication from Dr. Chandra Roy, Consulting chemical engineer, 2010
  6. Chemical Process Hazards Analysis, Tables 4.14 and 4.15 U.S. Department of Energy Handbook-1100-2004, pages 45-46
  7. Laird Wilson and Doug McCutcheon. Industrial Safety and Risk Management, 1st Edition, publisher=University of Alberta, year=2003. ISBN=0=88864-394-2. 
  8. HAZOP Terminology, Guide Words, Process Parameters & Deviations
  9. 9.0 9.1 HAZOP Team Selection and Management]