Thursday 10 March 2011

HAZOP STUDIES


INTRODUCTION

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

1)                 Specify the purpose, objective, and scope of the study. The purpose may be the analysis of a yet to be built plant or a review of the risk of unexisting unit. Given the purpose and the circumstances of the study, the objectives listed above can he made more specific. The scope of the study is the boundaries of the physical unit, and also the range of events and variables considered. For example, at one time HAZOP's were mainly focused on fire and explosion endpoints, while now the scope usually includes toxic release, offensive odor, and environmental end-points. The initial establishment of purpose, objectives, and scope is very important and should be precisely set down so that it will be clear, now and in the future, what was and was not included in the study. These decisions need to be made by an appropriate level of responsible management.
2)                 Select the HAZOP study team. The team leader should be skilled in HAZOP and in interpersonal techniques to facilitate successful group interaction. As many other experts should be included in the team to cover all aspects of design, operation, process chemistry, and safety. The team leader should instruct the team in the HAZOP procedure and should emphasize that the end objective of a HAZOP survey is hazard identification; solutions to problems are a separate effort.
3)                 Collect data. Therefore has listed the following materials that are usually needed
Ø     Process description
Ø     Process flow sheets
Ø     Data on the chemical, physical and toxicological properties of all raw materials,, intermediates, and products.
Ø     Piping and instrument diagrams (P&IDs)
Ø     Equipment, piping, and instrument specifications
Ø     Process control logic diagrams
Ø     Layout drawings
Ø     Operating procedures
Ø     Maintenance procedures
Ø     Emergency response procedures
Ø     Safety and training manuals








HAZOP Guide Words and Meanings

Guide Words
Meaning
No
Less
More
Part of
As well as
Reverse
Other than
Negation of design intent
Quantitative decrease
Quantitative increase
Qualitative decrease
Qualitative Increase
Logical opposite of the intent
Complete substitution


4)                 Conduct the study. Using the information collected, the unit is divided into study "nodes" and the sequence diagrammed in Figure, is followed for each node. Nodes are points in the process where process parameters (pressure, temperature change between nodes as a result of the operation of various pieces of equipment' such as distillation columns, heat exchanges, or pumps. Various forms and work sheets have been developed to help organize the node process parameters and control logic information.

When the nodes are identified and the parameters are identified, each node is studied by applying the specialized guide words to each parameter. These guide words and their meanings are key elements of the HAZOP procedure. They are listed in Table (9.1).

Repeated cycling through this process, which considers how and why each parameter might vary from the intended and the consequence, is the substance of the HAZOP study.

5)                 Write the report. As much detail about events and their consequence as is uncovered by the study should be recorded. Obviously, if the HAZOP identifies a not improbable sequence of events that would result in a disaster, appropriate follow-up action is needed. Thus, although risk reduction action is not a part of the HAZOP, the HAZOP may trigger the need for such action.

The HAZOP studies are time consuming and expensive. Just getting the P & ID's up to date on an older plant may be a major engineering effort. Still, for processes with significant risk, they are cost effective when balanced against the potential loss of life, property, business, and even the future of the enterprise that may result from a major release.

HAZOP Study of Storage Tank for Naphtha

A HAZOP study is to be conducted on naphtha storage tank, as presented by the piping and instrumentation diagram show in fig (9.2).
In this scheme, naphtha is unloaded from tank trucks into a storage tank maintained under a slight positive pressure until it is transferred to the process. Application of the guide words to the storage tank is shown in Table (9.2) 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.

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