November 2016

Special Focus: Plant Safety and Environment

Useful tips for a successful HAZOP study

The products we use on a daily basis comprise a variety of chemicals.

Kunte, V. A., Sakthivel, S., TATA Consulting Engineers Ltd.

The products we use on a daily basis comprise a variety of chemicals. The products increase the living standards of billions of people across the world. Many technologies have been developed to manufacture these chemicals. In any type of industry, such as petroleum refining, petrochemicals or fertilizer production, specialty chemical manufacturing, or pesticide production, the handling, processing, storing and transferring of huge quantities of volatile and flammable hydrocarbons and chemicals are inevitable. Since hydrocarbons and chemicals are vulnerable to fires, explosions or toxic release, they can lead to various types of accidents. The consequences of an accident could be catastrophic, which affects capital investment, personnel safety, equipment and the environment.

These incidents can be avoided by proper assessment and management of risk. The process of risk assessment involves not only leadership, but also those who are involved in planning, design, operations, maintenance and safety. This process includes managers, engineers, supervisors, operators and laborers. Typically, risk reduction involves the selection and design of proper equipment for processing and storing chemicals. This operation involves selecting suitable piping, instrumentation and controls, electrical aspects, risk assessments and evaluations, safety audits, onsite and offsite disaster management plans, and safety-related training throughout the lifecycle of the plant.


Risk management involves process hazard analysis (PHA) as the first step to commence the process of hazard identification. Many methods exist for conducting PHA, such as:

  • What-if checklist
  • Fault tree analysis (FTA)
  • Failure mode and effect analysis (FMEA)
  • Cause–consequence analysis
  • Event tree analysis (ETA)
  • Hazard and operability analysis (HAZOP).

Of the above, a HAZOP study is a powerful technique for the identification of hazards. It requires that a systematic and comprehensive procedure be followed throughout the study, and it utilizes team efforts of experienced persons in the areas of design, operations, maintenance and safety. This article provides a number of useful tips for carrying out a successful HAZOP study. These tips are based on the authors’ experience and subsequent analysis of a variety of HAZOP studies attended as a participant and as a HAZOP study team leader.


HAZOP is a systematic and structured technique commonly used in identifying potential hazards and/or problems with plant operability, recognizing consequences arising from various causes and providing recommendations for safety improvements in design and operations. This exercise is performed by a properly selected, experienced and interdisciplinary team. Experienced professionals have the ability to identify potential hazards that may otherwise escape attention during individual desk reviews. Although this technique was originally developed and used for chemical plants, it has also been used for many other facilities, such as water treatment plants, sewage treatment plants, water, chemical, hydrocarbon storage and transfer systems, and pilot plants. Moreover, techniques such as SAFOP, e-HAZOP, c-HAZOP, software HAZOP and procedure HAZOP have also been developed to identify possible errors in software, procedures and operational sequences. In such cases, the guidewords vary, but maintain the same intent—i.e., identifying hazards and causes for operational problems. The following points should be considered.

Requirement of updated documents

Documents for a new or existing facility must be available long before a HAZOP study. The availability of these documents saves time during these sessions:

  1. Process flow diagrams, along with heat and material balances
  2. Process descriptions with interlocks descriptions
  3. Piping and instrumentation diagrams (P&IDs)
  4. Equipment layout drawings
  5. Unit plot plans
  6. Material safety data sheets (MSDS) for hazardous chemicals
  7. Equipment and instrument datasheets
  8. Project control philosophy and cause-and-effect diagrams
  9. Provisional operating instructions, and startup and emergency shutdown procedures
  10. Utility specifications (as applicable).

HAZOP is usually carried out once final P&IDs from the design team are available. The diagrams are subjected to questioning with the use of proper guidewords and visualizing possible failure modes. Before undertaking HAZOP of a new facility, it is essential that the latest design considerations during the design review stages are already incorporated in the P&IDs and other documents.

If a HAZOP study (TABLE 1) is to be carried out for an existing facility, it is imperative that the P&IDs (FIG. 1) are up to date. Wherever required, an operator should be involved in updating P&IDs to reflect the as-built status.

TABLE 1 provides a sample record sheet prepared during a HAZOP study session using different deviations for a continuous process. FIG. 1 provides a schematic representation of the P&ID for a continuous process. Typical guidewords used for a HAZOP study of a batch process are provided in TABLE 2.

TABLE 3 provides the sample of a typical recording done during a HAZOP study session using different deviations for a batch process. FIG. 2 provides a schematic representation of the P&ID for a batch reactor.

FIG. 1. Schematic diagram of a chemical plant for a HAZOP study of a continuous process.
FIG. 1. Schematic diagram of a chemical plant for a HAZOP study of a continuous process.

Realization of adequate time by management

Since the technique of carrying out HAZOP is systematic and thorough, it is a time-consuming activity. Hence, the identified resources that will participate during HAZOP study sessions must be provided in a timely manner. Since making these resources available for the HAZOP sessions involves drawing experienced personnel away from their routine activities, along with travel expenses and other costs, management may find it difficult to allocate the required resources for an adequate time. Therefore, these sessions are expected to be completed by the HAZOP team during a very limited time period. This is not an adequate amount of time, considering the number of P&IDs, their complexity and the volume of work. Therefore, management must understand the time duration for such studies and should ensure that unrealistic deadlines are not set.

Selection of proper resources

The intention is that questions raised during the meeting should be answered immediately, instead of having to resort to outside expertise. Therefore, depending on the type of process, personnel with suitable backgrounds that are skilled and knowledgeable in the design, operation, commissioning, safety, shutdown and maintenance of the plant should be part of the HAZOP study team.

If HAZOP is performed by a less-experienced team, all hazards may not be identified. The team could generate recommendations that management may find difficult to accept.

Desired qualities of a HAZOP study team leader

The responsibility of the team leader includes triggering the discussions using guidewords and parameters for each of the nodes identified, ensuring the completeness of the analysis and maintaining the progress according to the schedule and agenda.

A HAZOP study leader should be technically strong and experienced. HAZOP study team participants come from different fields of knowledge and experience. It is important that the HAZOP study team leader is able to manage effectively during the HAZOP study sessions, keeping in mind the HAZOP study objectives and the set timelines. The team leader should ensure that the atmosphere during the HAZOP study sessions is open to a free exchange of ideas and various possibilities without fear of hierarchy by the participants. The participants should be able to express their ideas and convince other participants of possible scenarios, causes and consequences. With the knowledge and skills of the team leader, the team should feel that a balanced and constructive approach is maintained during the study.

FIG. 2. Schematic diagram of a batch reactor.
FIG. 2. Schematic diagram of a batch reactor.

The team leader should be an independent person with no responsibility for the process and/or the performance of the operations. Therefore, often with a new facility, a third-party experienced HAZOP study team leader is considered.

Selection of scribe/secretary

The scribe is an important member of the team. The scribe should be able to quickly write down numerous causes identified by various team members for any deviations during brainstorming sessions. Thereafter, each and every cause is transcribed to ensure its credibility, identify consequences and arrive at recommendations. With the help of the team leader and participants, the scribe should properly record everything into simple and unambiguous language. At the end of each deviation, the scribe should read to the group the transcribed material. The leader and the participants must ensure that the write-up is revised, wherever necessary, and agreed upon. The approved write-up should convey to each member of the group a consistent and clear meaning, as well as provide the proper context in which the recommendations are considered by the team.

Clear recordings are essential, as these are read at a later stage by other personnel and management for consideration of implementation of recommendations. Often, it is preferable to add more words, sentences or a small sketch to improve clarity and further define potentially ambiguous meanings.

At present, multiple software applications are available for recoding during HAZOP study sessions. It is important that both the team leader and the scribe are well skilled with the usage of such software before HAZOP study sessions. This knowledge will allow the recording to be completed quickly and accurately. This approach helps prevent these sessions from being boring and avoids team demoralization, which can affect the quality of the study.

Avoiding poor recordings of HAZOP study findings

During the HAZOP process, many ideas can be identified as a probable cause to a deviation. These ideas must be captured. Each cause needs to be considered separately to determine its credibility, and consequences should be identified for each credible cause, without initial consideration of the safeguards provided. It is a common observation that some enthusiastic participants try to jump to conclusions and offer recommendations without going through the systematic process.

Consequences may comprise both process hazards and operability problems. More than one consequence from a single cause may exist, or several causes may have the same consequence. Therefore, the consequences need to be recorded separately against each cause. Combining all consequences with all causes for a particular deviation should be avoided. Moreover, it is necessary to clearly identify and record whether the consequences lead to hazardous situations, loss of production, financial losses, damage to the local or external environment, safety of personnel, etc. The team should consider, identify and record all possible upstream and downstream consequences associated with a given cause.

The findings of HAZOP study reports are important for further safety studies, such as safety integrated level (SIL) studies, or for consideration by management for the approval of implementation of recommendations. Therefore, the HAZOP study team leader and the participants must be meticulous in properly and effectively recording all information and data so that they can be used for further studies and analysis.

Written recommendations should not be open-ended, but should be as specific as possible. The participants must clearly identify the actionable items and the agency responsible for taking the action.

HAZOP study sessions should be time conscious

Time constraints on the team leader and team members to complete a HAZOP study of a large number of P&IDs are always present, due to non-availability of resources and the costs involved. To complete a large volume of work, as well as follow the systematic methodology of a HAZOP study, the HAZOP sessions may have to be conducted for eight to 10 hours each day. Since HAZOP is a brainstorming technique, fatigue can reduce the overall effectiveness of the team. In such cases, there is a tendency to rush the HAZOP process and adopt some short-cut methods to more quickly complete the study.

Therefore, it is essential to schedule adequate time for the given volume of work, and to conduct the HAZOP study sessions accordingly.

Properly executed design reviews

At times, it has been observed that preliminary HAZOPs are performed on P&IDs for normal design reviews. HAZOPs conducted under such considerations take a longer time. The HAZOP team is seeing these diagrams for the first time, and must address issues to improve designs. Therefore, such HAZOPs generate a large number of recommendations that could have otherwise been picked up during normal design review stages.

In such cases, the final project/process HAZOP study must be undertaken only when the final design is completed and the required documents are updated.

Substituting HAZOP for the design review process compromises the very concept of HAZOP, which is a separate and an independent process hazard identification technique.1

HAZOP startup and shutdown procedures

It is essential that the P&IDs that will be used for HAZOP have been updated and have considered piping and controls for normal startup, normal shutdown and emergency shutdown, bypass connections and purge connections. The process control philosophy, startup and shutdown procedures must be well documented and understood before HAZOP sessions. This process ensures that these procedures can be explained in-depth during the HAZOP process. Accordingly, consequences can be identified and recommendations can be made by the team.

The use of proper guidewords for continuous and batch processes

Since the nature of operations for continuous processes and batch processes are different, it is important to use a different set of guidewords for these processes.

A deviation can be defined as process conditions departing from their intention. A deviation is a combination of “guideword” and “process parameter.” Guideword is a short word to define a deviation from the intention (e.g., more, less, no, reverse, other than). A process parameter is a relevant parameter for the conditions of the process (e.g., pressure, temperature, flowrate, composition, etc.).

Deviations stimulate creativity and generate new ideas from the participants, which are useful in brainstorming sessions involving multi-disciplinary, experienced and knowledgeable team members in that field.

Proper identification of nodes

Prior to the commencement of a HAZOP study, the process plant or the portion identified for the study is to be divided into “nodes.” Nodes are small, manageable and logical portions into which the process is divided.

Consider the following guidelines for the identification/selection of nodes on a P&ID:

  • Input streams to the equipment
  • Output streams from the equipment
  • Utility connections to/from the equipment
  • Vent lines, drain lines, overflow lines
  • Equipment, such as a reactor, tank, heat exchanger, dryer, centrifuge, etc.

Each node from each input/output stream should be marked in the P&ID, preferably with different color codes. These nodes are normally identified by a HAZOP study team leader, with assistance from the process engineer, well before the HAZOP study session. If time does not permit, this identification exercise can be done at the beginning of the first HAZOP study session.

Long nodes running into two or more P&IDs, consisting of a number of lines and equipment within the same node, are sometimes identified by team leaders. This should be avoided, as a HAZOP study is likely to miss some probable causes and consequences, decreasing the study’s overall effectiveness.

Individuals involved in the selection of the HAZOP study team, and participants in the complete HAZOP process, must adhere to these tips during the planning and execution stage. These steps will help ensure the effectiveness of the HAZOP study. The suitable recordings made during HAZOP study sessions, along with the preparation of a HAZOP study report, should serve as a beneficial tool that can be used for future reference. These tools can also be useful for subsequent safety studies, commissioning, troubleshooting and training activities. HP


  1. Jones, D., “Lessons from HAZOP experiences,” Hydrocarbon Processing, April 1992.

The Authors

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