Hazard and Operability (HAZOP) Study

1.0 Introduction

Hazard and Operability Studies, or in more common terms HAZOP, has been used and developed for ‘identifying potential hazards and operability problems’ caused by ‘deviations from the design intent’ of both new and existing process plants. It is designed to identify potential hazards or safety issues as well as operability or operation issues. In simple terms, one would not like to have any safety associated incidences as well as any disruptions on plant operation.

i. Hazard

Hazard is any operation that could possibly cause a catastrophic release of toxic, flammable or explosive chemicals or any action that could result in injury to personnel.

ii. Operability

Operability is  any operation inside the design envelope that would cause a shutdown that could possibly lead to a violation of environmental, health or safety regulations or
negatively impact profitability.

While issues associated to Hazards are well understood, operability  issues cannot be taken lightly as it significantly influencing the plat profitability. A well-conducted HAZOP study often results in smooth start-ups and operations, hence minimizing process hick-ups and shutdowns.

HAZOP is based on the principle that several experts with different backgrounds can interact and identify more problems when working together than when working separately and combining their results.

The best time to conduct a HAZOP is when the design is fairly firm. At this point, the design is well enough defined to allow meaningful answers to the questions raised in the HAZOP process. Also, at this point it is still possible to change the design without a major cost. However, HAZOPs can be done at any stage after the design is nearly firm. For example, many older plants are upgrading their control and instrumentation systems. There is a natural relationship between the Hazop deviation approach and the usual control system design philosophy of driving deviations to zero; thus it is very effective to examine a plant as soon as the control system redesign is firm.

1.1 History

HAZOP is a technique originated in 1960’s  from ICI, which was then a major British and international chemical company. They started with a technique called critical examination which asked for alternatives, but changed this to look for deviations. The method was developed further to become  hazard analysis procedure that received considerable attention following the Flixborough disaster in 1974.  In 1977 the Chemical Industry Association  published a guide on Hazard and Operability Studies.

1.2 HAZOP Team

The HAZOP process is  a team approach to hazard identification. This is important because when working as a team, more problems can be identified than when individuals working separately combine results. The HAZOP team is made up of individuals with varying backgrounds and expertise. The expertise is brought together during HAZOP sessions and through a collective brainstorming effort that stimulates creativity and new ideas, a thorough review of the process under consideration is made.  This creativity results from the interaction of the team and their diverse backgrounds. Consequently the process requires that all team members participate, and team members must refrain from criticizing each other to the point that members hesitate to suggest ideas.

Ideally, the team consists of five to seven members, although a smaller team could be sufficient for a smaller plant. If the team is too large, the group approach fails. On the other hand, if the group is too small, it may lack the breadth of knowledge needed to assure completeness. The team leader should have experience in leading a HAZOP. The rest of the team should be experts in areas relevant to the plant operation. For example, a team might include:

  • Design engineer
  • Process engineer
  • Operations supervisor
  • Instrument design engineer
  • Chemist
  • Maintenance supervisor
  • Safety engineer

The discussion is by the HAZOP Leader who will keep the team focused on the key task: to identify problems, not necessarily to solve them. There is a strong tendency for engineers to launch into a design or problem-solving mode as soon as a new problem comes to light. Unless obvious solutions are apparent, this mode should be avoided or it will detract from the primary purpose of HAZOP, which is hazard identification.

2.0  The HAZOP Process

The HAZOP team focuses on specific portions of the process called “nodes”. Generally these are identified from the P&ID of the process before the study begins. A process parameter is identified, say flow, and an intention is created for the node under consideration. Then a series of guidewords is combined with the parameter “flow” to create a deviations. For example, the guideword “no” is combined with the parameter flow to give the deviation “no flow”. The team then focuses on listing all the credible causes of a “no flow” deviation beginning with the cause that can result in the worst possible consequence the team can think of at the time. Once the causes are recorded the team lists the consequences, safeguards and any recommendations deemed appropriate. The process is repeated for the next deviation and so on until completion of the node. The team moves on to the next node and repeats the process.

The success or failure of the HAZOP depends on several factors:

  • The completeness and accuracy of drawings and other data used as a basis for the study
  • The technical skills and insights of the team
  • The ability of the team to use the approach as an aid to their Imagination in visualizing deviations, causes, and consequences
  • The ability of the team to concentrate on the more serious hazards which are identified.

Table 2.1: Typical HAZOP Table

Node:                                                                                        Date :

Intention:                                                                                P&ID Page No.:

Deviation Causes Consequence Existing Provision / Protection Action / Recommendation Action Party

2.1 Nodes

The locations (on piping and Instrumentation drawings and procedures) at which the process parameters are investigated for deviations is called Nodes.

2.2  Intention

The intention defines how the plant is expected to operate in the absence of deviations at the study nodes. This can take a number of forms and can either be descriptive or diagrammatic; e.g., flowsheets, line diagrams, P&IDs.

2.3 Deviation

Deviations are departures from the intention which are discovered by systematically applying the guide words (e.g., “more pressure”). Some deviations can be conveniently derived from a combination of Guide-Words and Process Parameter. For example NO (Guideword) FLOW (parameters) produces NO FLOW as deviation.

i. Guide words

Guide words are simple words which are used to qualify or quantify the intention in order to guide and stimulate the brainstorming process and so discover deviations. The guide words shown in Table 2.2 are the ones most often used in a Hazop; some organizations have made this list specific to their operations, to guide the team more quickly to the areas where they have previously found problems. Each guide word is applied to the process variables at the point in the plant (study node) which is being examined. For example:

Table 2.2 Examples of Guide words

Guide Words
No Negation of Design Intent
More Quantitative Increase
Less Quantitative Decrease
Part Of Qualitative Decrease
As Well As Qualitative Increase
Reverse Logical Opposite of the Intention
Other Than Complete Substitution

There are other useful modifications to guide words such as:

  • SOONER or LATER for OTHER THAN when considering time
  • WHERE ELSE for OTHER THAN when considering position, sources, or destination
  • HIGHER and LOWER for MORE and LESS when considering elevations, temperatures, or pressures.

ii. Parameters

These guide words are applicable to both the more general parameters (e.g., react, transfer) and the more specific parameters (e.g., pressure, temperature). With the general parameters, meaningful deviations are usually generated for each guide word.  Examples of parameters are given in Table 2.3 Below.

Table 2.3: Examples of Parameters

General Parameter Specific Parameter Specific Parameter

In practice, it is not unusual to have more than one deviation from the application of one guide word. For example, “more reaction” could mean either than a reaction takes place at a faster rate, or that a greater quantity of product results. With the specific parameters, some modification of the guide words may be necessary. In addition, it is not unusual to find that some potential deviations are eliminated by physical limitation. For example, if the design intention of a pressure or temperature is being considered, the guide words “more” or “less” may be the only possibilities.

Table 2.4: Deviations Generated From Guide words and Parameters

Guide Words Parameter Deviation

2.2.3 Causes

These are the reasons why deviations might occur. Once a deviation has been shown to have a credible cause, it can be treated as a meaningful deviation. These causes can be hardware failures, human errors, an unanticipated process state (e.g., change of composition), external disruptions (e.g., loss of power), etc.

2.2.4 Consequence

The primary purpose of the HAZOP is identification of scenarios that would lead to the release of hazardous or flammable material into the atmosphere, thus exposing workers to injury. In order to make this determination it is always necessary to determine, as exactly as possible, all consequences of any credible causes of a release that are identified by the group.  If the team concludes from the consequences that a particular cause of a deviation results in an operability problem only, then the discussion should end and the team should move on to the next cause, deviation or node. If the team determines that the cause will result in the release of hazardous or flammable material, then safeguards should be identified.

2.2.5 Existing Provision / Protection / Safeguard

Safeguards should be included whenever the team determines that a combination of cause and consequence presents a credible process hazard. What constitutes a safeguard can be summarized based on the following general criteria:

  1. Those systems, engineered designs and written procedures that are designed to prevent a catastrophic release of hazardous or flammable material.
  2. Those systems that are designed to detect and give early warning following the initiating cause of a release of hazardous or flammable material.
  3. Those systems or written procedures that mitigate the consequences of a release of hazardous or flammable material.

The team should use care when listing safeguards. Hazards analysis requires an evaluation of the consequences of failure of engineering and administrative controls, so a careful determination of whether or not these items can actually be considered safeguards must be made. In addition, the team should consider realistic multiple failures and simultaneous events when considering whether or not any of the above safeguards will actually function as such in the event of an occurrence.

2.2.6 Action / Recommendation

Recommendations are made when the safeguards for a given hazard scenario, as judged by an assessment of the risk of the scenario, are inadequate to protect against the hazard. Action Items are those recommendations for whom an individual or department has been assigned.

2.2.7 Action Party

To avoid misunderstanding, action party should be specified for each recommendation derived during the HAZOP session.

3.0 HAZOP Meeting


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