Sunday, 10 April 2011


A HAZOP survey is one of the most common and widely accepted methods of systematic qualitative hazard analysis. It is used for both new or existing facilities and can be applied to a whole plant, a production unit, or a piece of equipment It uses as its database the usual sort of plant and process information and relies on the judgment of engineering and safety experts in the areas with which they are most familiar. The end result is, therefore reliable in terms of engineering and operational expectations, but it is not quantitative and may not consider the consequences of complex sequences of human errors.
          The objectives of a HAZOP study can be summarized as follows:
1)                  To identify (areas of the design that may possess a significant hazard potential.
2)                  To identify and study features of the design that influence the probability of a hazardous incident occurring.
3)                  To familiarize the study team with the design information available.
4)                  To ensure that a systematic study is made of the areas of significant hazard potential.
5)                  To identify pertinent design information not currently available to the team.
6)                  To provide a mechanism for feedback to the client of the study team's detailed comments.

 A HAZOP study is conducted in the following steps:

Table 7.1: HAZOP Guide Words and Meanings
Guide Words
Part of
As well as
Other than
Negation of design intent
Quantitative decrease
Quantitative increase
Qualitative decrease
Qualitative Increase
Logical opposite of the intent
Complete substitution
 7.1:HAZOP Study of Storage Tank for Ethylene Oxide

  A HAZOP study is to be conducted on ethylene oxide storage tank, as presented by the piping and instrumentation diagram shown in fig.
In this scheme, ethylene oxide is unloaded from tank trucks into a storage tank maintained under pressure until it is transferred to the process. Application of the guide words to the storage tank is shown in Table along with a listing of consequences that results from process deviation. Some of the consequences identified with these process deviations have raised additional questions that need resolution to determine whether or not a hazard exists.

Piping and instrumentation diagram

Deviations from operating conditions
What event could cause this deviation
Consequences of this deviation on item of equipment under consideration
Process indications





Tank runs dry

Rupture of discharge line
V-3 open or broken
V-1 open or broken
Tank rupture (busting of vessel)
Unload too much from column
Reverse flow from process

Temperature of inlet is colder than normal
Temperature of inlet is hotter than normal 
External fire

Pump cavitates

Reagent released

Reagent released
Reagent released
Reagent released

Tank overfills

Tank overfills

Possible vacuum

Region released

Tank fails




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