HAZOP Study

General Overview of HAZOP Method

A HAZOP (HAZard and OPerability) study identifies hazards and operability problems in a process plant. It is a tool for the identification of hazards due to process parameter deviations. The concept involves investigating how the plant might deviate from the design intent. 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. Although the HAZOP study was developed to supplement experience-based practices when a new design or technology is involved, its use has expanded to almost all phases of a plant's life. The “Guide-Word” HAZOP is the most well known of the HAZOPs; however, several specializations of this basic method have been developed.

The HAZOP concept is to review the plant in a series of meetings, during which a multidisciplinary team methodically “brainstorms” the plant design, following the structure provided by the guide words and the team leader's experience.
The primary advantage of this brainstorming is that it stimulates creativity and generates ideas. This creativity results from the interaction of the team and their diverse backgrounds. Consequently the process requires that all team members participate (quantity breeds quality in this case), and team members must refrain from criticizing each other to the point that members hesitate to suggest ideas.

The team focuses on specific points of the design (called "study nodes"), one at a time. At each of these study nodes, deviations in the process parameters are examined using the guide words. The guide words are used to ensure that the design is explored in every conceivable way. Thus the team must identify a fairly large number of deviations, each of which must then be considered so that their potential causes and consequences can be identified.

The success or failure of a HAZOP study depends on several factors, to name a few:

Ø The completeness and accuracy of drawings and other data used as a basis for the study

Ø The technical skills and insights of the team

Ø The ability of the team to use the approach as an aid to their imagination in visualizing deviations, causes, and consequences

Ø The ability of the team to concentrate on the more serious hazards which are identified.

In the process of identifying problems during a HAZOP study, if a solution becomes apparent, it is recorded as part of the HAZOP result; however, care was taken to avoid trying to find solutions which are not so apparent, because the prime objective for the HAZOP is problem identification. 

Various Terms used in HAZOP Study

The HAZOP process is systematic and it is helpful to define the terms that are used in the study:
A. Study Nodes - The locations (on piping and instrumentation drawings and procedures) at which the process parameters are investigated for deviations.
B. 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., flow-heets, line diagrams, P&IDs.
C. Deviations - These are departures from the intention which are discovered by systematically applying the guide words (e.g., "more pressure").
D. 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.
E. Consequences - These are the results of the deviations should they occur (e.g., release of toxic materials). Trivial consequences, relative to the study objective, are dropped.
F. Guide Words - These 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 1 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. These guide words are applicable to both the more general parameters (e.g., react, transfer) and the more specific parameters (e.g., pressure, temperature). 

Table: HAZOP Guide Words and Meanings

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

With the general parameters, meaningful deviations are usually generated for each guide word. Moreover, 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. 

Finally, when dealing with a design intention involving a complex set of interrelated plant parameters (e.g., temperatures, reaction rates, composition, or pressure), it may be better to apply the whole sequence of guide words to each parameter individually than to apply each guide word across all of the parameters as a group. Also, when applying the guide words to a sentence it may be more useful to apply the sequence of guide words to each word or phrase separately, starting with the key part which describes the activity (usually the verbs or adverbs). These parts of the sentence usually are related to some impact on the process parameters. 

HAZOP Methodology

The concepts presented above are put into practice in the following steps:

1.   Define the purpose, objectives, and scope of the study

2.   Select the team

3.   Prepare for the study

4.   Carry out the team review

5.   Record the results.

It is important to recognize that some of these steps can take place at the same time. For example, the team reviews the design, records the findings, and follows up on the findings continuously.

Risk and Risk Matrix in HAZOP study

In the context of process safety management, Risk is defined in terms of the likelihood and consequences of incidents that could expose plant personnel, property, assets, process and environment to the harmful effects of a hazard. According to Centre for Chemical Process Safety of the American Institute of Chemical Engineers, hazards are potential sources of harm, including chemical or physical conditions or characteristics that can damage people, property or the environment. Incident likelihood encompasses frequency and probability; consequences refer to outcomes and impacts.

Risk ranking uses a matrix that has ranges of consequence and likelihood as the axes. A typical risk matrix is a 4 x 4 grid.

Design of Risk Matrix in HAZOP study

Although there are many risk matrices that have been developed, effective risk ranking tools in day-to-day operations, such as during hazard and operability (HAZOP) studies are very limited. Layer of Protection Analysis (LOPA) approach is one of them. It is simple to implement and easy for most HAZOP participants to understand.

Various Layers of Protection in a Process Plant

HAZOP study deals with the identification of hazards due to process parameter deviations. When a failure occurs due to deviations, it may take the process outside of its normal operating ranges. In general, there are several layers of protection measures in a plant in response to a process deviation. The basic process controls, alarms, safety valves, operator supervision etc. are the typical protection measures against any harmful consequences due to deviation of process parameters as shown below:

ø Process equipments are designed for process operating limits.

ø Basic process controls, alarms and operators are adjusted to process deviations.

ø Presence of Critical Alarms along with Speedy Response of Operators.

ø Safety Interlock System/Emergency Shut Down at operating limits.

ø Relief Systems that activate at equipment design limits.

ø Mitigation systems that contain the effects of incident.

ø Plant emergency response to control the effects of incidents (On-site Control Arrangement).

ø Emergency response to protect the public from the effects of an incident (Offsite Control Arrangement).