Hazard Analysis

Hazard analysis is a critical component of planning for hazardous material releases. It is identification of undesired events that may lead to the realization of the hazard; the analysis of the mechanism by which those undesired events could occur and the estimation of the extent, magnitude & relative likelihood of any harmful effects.

To analyze the safety of a major hazard installation as well as its potential hazards, a hazard analysis should be carried out covering the following areas :

Which toxic, reactive, explosive or flammable substances in the installation constitute a major hazard;
Which failures or errors could cause abnormal conditions leading to a major accident;
The consequences of a major accident for the workers, people living or working outside the installation, or the environment;
Prevention measures for accidents;
Mitigation of the consequences of an accident.

Preliminary Hazard Analysis (PHA):

As a first step in hazard analysis, a PHA should be carried out.

A PHA should be used to identify types of potential accident in the installation, such as toxic release, fire, explosion or release of flammable material, and to check the fundamental elements of the safety system.

The PHA should be summarized in documentation covering, for each accident considered, the relevant component (storage vessel, reaction vessel, etc.) the events initiating the accident and the corresponding safety devices (safety valves, pressure gauges, temperature gauges, etc.).

The results of a PHA should indicates which units or procedures within the installation require further and more detailed examination and which are less significance from a major hazard point of view.

Hazard And Operability Study (HAZOP):

A HAZOP is a technique commonly used by chemical process facilities to identify hazards and difficulties that prevent efficient operation. There are two versions of the technique, one which deals with “deviations “ and the other with “disturbances”. “Deviations” are caused by malfunction or mal operation of a specific production system. “Disturbances” include problems caused by influences outside the specified system, including other activities and the environment.

The first version of HAZOP to be developed and the most widely known was aimed at deviations and is called a “Guide word” HAZOP. Each element of the process is evaluated separately. The purpose of the element is specified and notational deviations are generated by associating this purpose of the element with distinctive words or phrases called “
guide words”. These guide words are “no” or “not”, “more”, “less”, “as well as”, “part of”, “reverse”, and “other than” which, broadly speaking, cover all possible types of deviation.

For each notational deviation, a determination must be made whether this is a possible situation (e.g., no flow or reverse flow in a transfer line that should have forward flow). If this is possible, the conditions in which that situation might occur and the possible hazardous consequences must be identified. The guide words are applied to all materials and all operating parameters (e.g., flow, temperature, pressure). The guide words are applied not only to the equipment but also to the operating procedures. All phases of operation (e.g. start up, operation, shut down, backwash) must also be included. As would be expected, this approach can be time-consuming and the time taken can vary from several days for a small production unit, to several months for a complex facility.

The second version of the HAZOP studies is called a “creative checklist” HAZOP. This version has been developed as a compliment to the guide word HAZOP to cover “disturbances”. It is of particular value in two situations. These are to enable a HAZOP study to the carried out very early in design process, even before the detailed design necessary for a “guide word” HAZOP is available; and to cover hazards which may be caused by interactions between units which could be perfectly safe if built in isolation, but may be capable of adverse interactions. This second method uses a checklist of known major hazards and nuisances. The checklist would contain words such as “fire”, “explosion”, “toxicity”, “corrosion”, “dust”, and “smell”. The checklist is initially applied to every material likely to be present; raw materials, intermediates, finished products, byproducts and effluents. This establishes quantitative data base of the numerical intensities of different hazards. Thus “fire” would result not only a note that a material is flammable but numerical measurements such as “flash point” and “flammable limits”. Any missing data are pinpointed and timely steps taken to collect such data.

The second method continues with the association of the same checklist with each item of then equipment. The materials present in such equipment, together with the inventories, are known as the “materials hazards”. As the analysis proceeds , the potential for all major hazards including interactions between units or the unit and its environment are identified. The flow of hazards can be in both directions. For example, the environment may pose hazards to the unit (e.g., flooding and earthquakes), which would have to be considered in the site selection, design, and layout of the unit. Although less well known than the guide word HAZOP, the creative checklist HAZOP has been found to be a quick and valuable complementary approach.

to be continued....