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study. The design intent for a given part of a system is expressed in terms of elements which
convey the essential features of the part and which represent natural divisions of the part.
The selection of elements to be examined is to some extent a subjective decision in that there
may be several combinations which will achieve the required purpose and the choice may
also depend upon the particular application. Elements may be discrete steps or stages in a
procedure, individual signals and equipment items in a control system, equipment or
components in a process or electronic system, etc.
In some cases it may be helpful to express the function of a part in terms of:
· t he input material taken from a source; · a n activity which is performed on that material; · a p roduct which is taken to a destination. Thus the design intent will contain the following elements: materials, activities, sources and
destinations which can be viewed as elements of the part.
Elements can often be usefully defined further in terms of characteristics which can be either
quantitative or qualitative. For example, in a chemical system, the element “material” may be
defined further in terms of characteristics such as temperature, pressure and composition. For
the activity “transport”, characteristics such as the rate of movement or the number of
passengers may be relevant. For computer-based systems, information rather than material is
likely to be the subject of each part.
The HAZOP team examines each element (and characteristic, where relevant) for deviation
from the design intent which can lead to undesirable consequences. The identification of
deviations from the design intent is achieved by a questioning process using predetermined
“guide words”. The role of the guide word is to stimulate imaginative thinking, to focus the
study and elicit ideas and discussion, thereby maximizing the chances of study completeness.
Basic guide words and their meanings are given in Table 1.
Table 1 – Basic guide words and their generic meanings
Guide word Meaning NO OR NOT
MORE Quantitative increase LESS Quantitative decrease AS WELL AS Qualitative modification/increase PART OF Qualitative modification/decrease REVERSE Logical opposite of the design intent OTHER THAN 10
01 Complete negation of the design intent Complete substitution © BSI 28 1002−80ISB ©
August 2001 BS IEC 61882:2001
6 1882 Ó I EC:2001 – 12 – Additional guide words relating to clock time and order or sequence are given in Table 2.
Table 2 – Guide words relating to clock time and order or sequence
Guide word Meaning
Relative to the clock time LATE Licensed Copy: Puan Ms. Norhayati, Petroliam Nasional Berhad 4397000, 01 October 2003, Uncontrolled Copy, (c) BSI EARLY Relative to the clock time BEFORE Relating to order or sequence AFTER Relating to order or sequence There are a number of interpretations of the above guide words. Additional guide words may
be used to facilitate identification of deviation. Such guide words may be used provided they
are identified before the examination commences. Having selected a part for examination, the
design intent of that part is broken into separate elements. Each relevant guide word is then
applied to each element, thus a thorough search for deviations is carried out in a systematic
manner. Having applied a guide word, possible causes and consequences of a given
deviation are examined and mechanisms for detection or indication of failures may also be
investigated. The results of the examination are recorded to an agreed format (see 6.6.2).
Guide word/element associations may be regarded as a matrix, with the guide words defining
the rows and the elements defining the columns. Within each cell of the matrix thus formed
will be a specific guide word/element combination. To achieve a comprehensive hazard
identification, it is necessary that the elements and their associated characteristics cover all
relevant aspects of the design intent and guide words cover all deviations. Not all
combinations will give credible deviations, so the matrix may have several empty spaces
when all guide word/element combinations are considered.
There are two possible sequences in which the cells of the matrix can be examined, namely
column by column, i.e. e lement first , or row by row, i.e. g uide word first . The details of
examination are outlined in 6.5 and both sequences of examination are illustrated in
Figures 2a and 2b. In principle the results of the examination should be the same.
4.3 Design representation 4.3.1 General An accurate and complete design representation of the system under study is a prerequisite
to the examination task. A design representation is a descriptive model of the system
adequately describing the system under study, its parts and elements, and identifying their
characteristics. The representation may be of the physical design or of the logical design and
it should be made clear what is represented.
The design representation should convey th...
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