How HAZOP Technique is used to identify Hazards in a Process

In functional safety, hazards are events which have the potential to cause harm such as personal injury, damage to the environment or the business. Hazards in the process industry might include: the pressure of liquid in a vessel (high pressure may result in loss of containment, leaks or vessel rupture), the level of liquid in a vessel (a high level may result in an overflow of liquid into gas streams, or an overspill of a dangerous chemical or flammable liquid; a low level may result in dry running of pumps or gas blow by into downstream vessels). The first step in assessing risk is to identify the hazards like the ones aforementioned. There are a number of techniques used for identifying hazards; nonetheless, the most commonly used methodology is the Hazard and Operability (HAZOP) study.

Why consider HAZOPs?

Even though the design of a plant relies upon the application of codes and standards, the HAZOP process allows the opportunity to supplement these with an imaginative anticipation of the deviations which may occur because of, for instance, process conditions or upsets, equipment malfunction or operator error.

Furthermore, the pressures of project schedules can result in errors or oversights and the HAZOP allows these to be corrected before such changes become too costly.

Since HAZOPs are easy to comprehend, and can be adapted to any process or business, HAZOPs have become the mostly widely utilized hazard identification technique.

The Deviation from Design Intent

All processes, equipment under control or industrial plants have design intent. This might be to attain a target production capacity in terms of an annual tonnage of a particular chemical or a specified number of manufactured items. But key secondary design intent may be to operate the process in a safe and efficient manner and to achieve that; each item of equipment will be required to function effectively.

Let’s consider the example of a cooling water facility as part of a plant production requirement. This cooling facility contains the water circuit with a circulating pump and heat exchanger as illustrated below:

The design intent of this section of the plant may be to continuously circulate cooling water at a given temperature y °C and a rate of x liters per hour. The HAZOP study is typically directed at this low level of design intent. A deviation or departure from the design intent in the case of this cooling facility would be a reduction of circulating flow or an increase in water temperature.

In the example above, an increase in water temperature would be the hazard as it would have the potential to cause harm such as personal injury, damage to the environment or the business.

The HAZOP Technique

HAZOPs are used to identify potential hazards and operability problems caused by the deviation from the design intent of both new and existing process plant and are generally carried out periodically throughout the plant’s life. Definitely an initial or preliminary HAZOP should be carried out early in the design phase. The process should be reviewed as the development progresses and whenever key modifications are proposed, and finally at the end of the development to ensure there are no residual risks prior to the build stage.

A HAZOP is usually conducted in a meeting forum between interested parties with sufficient understanding and experience of the operation and maintenance of the plant. The meeting is a structured brainstorming session whereby guidewords are used to stimulate ideas about what the hazards could be. The minutes of the meeting record the discussion and capture information about the potential hazards, their causes and consequences.

The HAZOP Study Team

It is paramount that a HAZOP team is made up of personnel who bring the best balance of knowledge and experience of the operation and maintenance of the plant being considered to the study.

Normally a HAZOP team is made of the following persons:

Name	Role
Chairman	To explain the HAZOP process, to direct discussions & facilitate the HAZOP. This is someone experience in HAZOP but not directly involved in the design, to ensure that the method is followed carefully.
Secretary	To document the discussion of the HAZOP meeting and provide a visible record of the discussions. To log recommendations or actions.
Process Engineer	The process engineer is responsible for the process flow diagrams (PFDs) and development of the piping and instrumentation diagrams (P & IDs).
Control & Instrumentation Specialist	A person with relevant technical knowledge of control and instrumentation.
User/operator	To advise on the use and operability of the process, and the effect of deviations.
A design team representative	To advise on any design details or provide further information.
Maintainer	A person concerned with maintenance of the process.

The Information used in HAZOP

The following items should be available to view by the HAZOP team:

• Piping and Instrumentation Diagrams (P & IDs) for the facility.
• Process Description or Philosophy Documents.
• Existing Operating and Maintenance Procedures.
• Cause and Effects (C & E) charts.
• Plant layout diagrams.

The HAZOP Methodology

The HAZOP procedure involves taking a full description of the process and systematically questioning every part of it to establish how deviations from the design intent can have a negative effect upon the safe and efficient operation of the plant. The procedure is applied in a structured way by the HAZOP team, and it relies upon them using their imagination in an effort to identify credible hazards.

Guidewords

The HAZOP process utilizes guidewords to focus the attention of the team upon deviations of the design intent, their possible causes and consequences. These guidewords are divided into two sub-sets:

1. Primary guidewords which focus attention on a specific aspect of the design intent or an associated process condition or parameter such as flow, temperature, pressure, level, and so forth.
2. Secondary guidewords which, when combined with a primary guideword suggest possible deviations such as more temperature, less level, no pressure, reverse flow, and so forth.

The use of guidewords must be clearly understood by the team.

Note that, the use of guidewords is to help to stimulate the imagination into what could happen. Thus, not all guidewords will be meaningful, and not all hazards will be credible. Therefore, meaningless or incredible events should be documented as such.

Modes of Operation

Since a HAZOP is a hazard and operability study, it is key to consider not only the normal operation of the process but also other abnormal modes, such as start-up, shutdown, filling, emptying, by-pass, proof test.

This may be achieved by considering each operational mode specified in the scope, as a separate exercise and producing separate HAZOP analyses for each. On the other hand, for relatively simple systems, an additional column can be included in the worksheets to identify the mode. Therefore, a single HAZOP analysis can thus consider all operational modes.

Recording the HAZOP

A number of software tools are available that can guide you through the HAZOP process. Alternatively a simple spreadsheet can be constructed to record the discussions and findings. Spreadsheets allow for easy sorting and categorization, and they also provide visibility and traceability between entries so that cross-referencing with other analyses can be maintained.

HAZOP Worksheet Headings – Identifying Hazards

Consider an illustration of HAZOP as per the table below:

Table 1.1 HAZOP illustration

Ref	Primary Guideword	Secondary Guideword	Deviation	Hazard	Consequence
08	Flow	More	High flow of process liquid into vessel.	High flow into vessel could result in high level, liquid carry over into gas export.	Equipment damage downstream requiring vessel replacement estimated at $ 3M and a process shutdown for 6 months.

Reference

Include a reference column so that each entry can be referenced to from other analyses and also allows traceability to subsequent analysis.

Guidewords

Primary and secondary guidewords should be used. Research on the web can provide lists of guidewords that can apply to different industries and businesses.

Deviation

The deviation is the departure from the design intent prompted by the primary and secondary guidewords and represents the identified hazard.

Cause

The potential causes which would result in the deviation occurring. Include specific information about the cause.

Consequence

The consequences that would arise from the effect of deviation and, if appropriate, from the cause itself, this should be explicit documentation i.e. a full and complete documentation of how the hazard may develop and consequences for example, a consequence may be described as: “compressor damage of up to $3 million and loss of production up to 6 months.”

Safeguards

Existing protective devices which either prevent the cause or safeguard against the consequences are recorded in this column.  In addition to hardware, safeguards may include procedural aspects such as regular plant inspections if they can prevent or safeguard against consequences.

Related articles:

• Hazardous Locations Classifications
• Methods of Protecting Instruments from Explosive Atmospheres
• Safety Function in a Machine Control System
• Functional Safety in Instrumentation Systems (SIS, SIF & SIL)
• What is Safety Related Control System (SRCS)?