To illustrate the principles of the examination procedure, consider a plant in which chemicals A and B react together to form a product C.
Figure 2: Example of a Simple Process Plant
Chemical A
Chemical B
Product C Pump 1
Pump 2 Valve 3
Valve 1
Valve 4 Valve 2
To Process
Suppose that the chemistry of the process is such that the concentration of raw material B must never exceed that of A otherwise an explosion may occur.
Referring to Figure 2 start with the pipeline extending from the suction side of the pump which delivers raw material A to where it enters the reaction vessel.
The intention is partly described by the owsheet and partly by the process control requirements to transfer A at some specied rate. The rst deviation is that obtai ned by applying the guideword NO or NOT to the intention. This is combined with the intention to give:
No Transfer of A
The owsheet is then examined to establish the causes which might produce a complete cessation of the ow of ‘A’. These causes could be:
▪ Supply tank is empty; or
▪ Pump fails to turn due to:
▪ mechanical failure;
▪ electrical failure;
▪ pump being switched off, or
▪ Pipeline is fractured; or
▪ Isolation valve is closed.
Clearly some at least of these are conceivable causes and so this is a meaningful deviation.
Next the consequences are considered. Complete cessation of ow of ‘A’ would very soon lead to an excess of ‘B’ over ‘A’ in the reaction vessel and consequently to a risk of explosion.
Therefore a hazard in the design is discovered and this is noted for further consideration.
The next guideword, which is MORE, is now applied. The deviation is:
More ‘A’ is passed into the Reaction Vessel
The cause would be that the characteristics of the pump might, under some circumstances, produce excessive ow rate. If this cause is accepted as realistic, the consequences should be considered:
▪ The reaction produces ‘C’ contaminated with an excess of ‘A’ which goes on into the next stage of the process; and
▪ The excess ow into the reaction vessel means that some will leave the vessel by the overow.
Figure 2: Example of a Simple Process Plant (repeated from previous page)
Chemical A
Chemical B
Product C Pump 1
Pump 2 Valve 3
Valve 1
Valve 4 Valve 2
To Process The next guideword, which is LESS, is now applied. The deviation is:
Less ‘A’ is passed into the Reaction Vessel
The causes are a little different from those when the deviation was the complete cessation of
ow of ‘A’:
▪ Low level in the supply tank;
▪ The isolation valve is slightly closed;
▪ The pipeline is partly blocked; or
▪ The pump fails to produce full ow because:
▪ the impellers are eroded; or
▪ the valves are worn, etc.
Clearly these are conceivable causes and so this is a meaningful deviation.
The consequence is similar to no ow and so the potential hazard is of a possible explosion.
The other guidewords are applied in a similar way.
As well as:
▪ The transfer of some component in addition to ‘A’, e.g. the supply tank is contaminated.
This may cause a chemical reaction or dilute ‘A’;
Part of:
The other related deviation is that which occurs when the design intention is incompletely achieved. The guidewords are PART OF and the deviation PART OF TRANSFER ‘A’. This could mean:
▪ A component of ‘A’ is missing. Here a knowledge of the composition of ‘A’ is required so the effects of the missing component can be assessed, e.g. ‘A’ decomposes in the process or incorrect quality of supply.
Reverse:
The guideword is REVERSE and the deviation REVERSE TRANSFER OF ‘A’. This means ow from the reactor back though the pump. The owsheet is examined to see if this is possible and the consequences are assessed.
Other than:
Lastly, there is the complete substitution of the design intention by something else.
The guidewords are OTHER THAN and the deviation is OTHER THAN TRANSFER.
This could mean the transfer of a different material. The owsheet is examined to see if this is possible. Substitution could arise in a number of ways. For example, the wrong material could be delivered or change in the nature of the activity, for example, can ‘A’ solidify instead of being transferred?
These are recorded on a HAZOP form such as Table 4.
Table 4: Completed HAZOP Study Results
D e v
When the pipeline which introduces raw material ‘A’ has been examined, it is marked on the
owsheet as having been checked. The next part of the design is then chosen for study and this could be the pipeline which introduces raw material ‘B’ into the reaction vessel. This sequence is repeated for every part of the design, each line, the vessel auxiliaries such as stirrers, any services to the vessel such as the provision of heating and cooling and the vessel itself. This particular approach is sometimes called the ‘line by line’ method.
Only under exceptional circumstances is a written record made of every step of the examination.
It is more usual to carry out the steps mentally and verbally in discussion and to write down only the potential hazards and their causes.
The proposed action is also noted if it can be agreed straight away. If there is some doubt about the action or if further information is required, the matter must be brought forward to a subsequent meeting.