HAZOP

HAZOP stands for “hazard and operability studies.” This is a set of formal hazard identification and elimination procedures designed to identify hazards to people, process plants, and the environment. The techniques aim to stimulate in a systematic way the imagination of designers and people who operate plants or equipment so they can identify potential hazards. In effect, HAZOP studies make the assumption that a hazard or operating problem can arise when there is a deviation from the design or operating intention. Corrective actions can then be made before a real accident occurs.

Some studies have shown that a HAZOP study will result in recommendations that are 40 percent safety-related and 60 percent operability-related. HAZOP is far more than a safety tool; a good HAZOP study also results in improved operability of the process or plant, which can mean greater profitability.

The primary goal in performing a HAZOP study is to identify, not analyze or quantify, the hazards in a process. The end product of a study is a list of concerns and recommendations for prevention of the problem, not an analysis of the occurrence, frequency, overall effects, and the definite solution. If HAZOP is started too late in a project, it can lose effectiveness because:

1.               There may be a tendency not to challenge an already existing design.

2.               Changes may come too late, possibly requiring redesign of the process.

3.               There may be loss of operability and design decision data used to generate the design.

HAZOP is a formal procedure that offers a great potential to improve the safety, reliability, and operability of process plants by recognizing and eliminating potential problems at the design stage. It is not limited to the design stage, however. It can be applied anywhere that a design intention (how the part or process is expected to operate) can be defined, such as:

•            Continuous or batch processes being designed or operated

•            Operating procedures

•            Maintenance procedures

•            Mechanical equipment design

•            Critical instrument systems

•            Development of process control computer codeThese studies make use of the combined experience and training of a group of knowledgeable people in a structured setting. Some key concepts are:

•            Intention—defines how the part or process is expected to operate.

•            Guide words—simple words used to qualify the intention in order to guide and stimulate creative thinking and so discover deviations. Table 26-2 describes commonly used guide words.

Deviations—departures from the intention discovered by 

•            Causes—reasons that deviations might occur.

•            Consequences—results of deviations if they occur.

•            Actions—prevention, mitigation, and control —Prevent causes.

—Mitigate the consequence.

—Control actions, e.g., provide alarms to indicate things getting out of control; define control actions to get back into control.

The HAZOP study is not complete until response to actions has been documented. Initial HAZOP planning should establish the management follow-up procedure that will be used.

The guide words can be used on broadly based intentions (see Table 26-2), but when intentions are expressed in fine detail, some restrictions or modifications are necessary for chemical processes, such as:

No flow

Reverse flow

Less flow

More temperature

Less temperature

Composition change

Sampling

Corrosion/erosion

This gives a process plant a specific HAZOP guide-word list with a process variable, plant condition, or an issue.

HAZOP studies may be made on batch as well as continuous processes. For a continuous process, the working document is usually a set of flow sheets or piping and instrument diagrams (P&IDs). Batch processes have another dimension: time. Time is usually not significant with a continuous process that is operating smoothly except during start-up and shutdown, when time will be important and it will resemble a batch process. For batch processes, the working documents consist not only of the flow sheets or P&IDs but also the operating procedures. One method to incorporate this fourth dimension is to use guide words associated with time, such as those described in Table 26-3.

HAZOP studies involve a team, at least some of whom have had experience in the plant design to be studied. These team members apply their expertise to achieve the aims of HAZOP. There are four overall aims to which any HAZOP study should be addressed:

1.               Identify as many deviations as possible from the way the design is expected to work, their causes, and problems associated with these deviations.

2.               Decide whether action is required, and identify ways the problem can be solved.

3.               Identify cases in which a decision cannot be made immediately and decide what information or action is required.

4.               Ensure that required actions are followed through.

The team leader is a key to the success of a HAZOP study and should have adequate training for the job. Proper planning is important to success. The leader is actually a facilitator (a discussion leader and one who keeps the meetings on track) whose facilitating skills are just as important as technical knowledge. The leader outlines the boundaries of the study and ensures that the design intention is clearly understood. The leader applies guide words and encourages the team to discuss causes, consequences, and possible remedial actions for each deviation. Prolonged discussions of how a problem may be solved should be avoided.