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Chemical Technology • July 2015
10
Layers of protection
and safety integrity
T
hus, if a hazardous event should occur, it will have to
break through the layers of protection before people
andproperty couldbeharmed, as illustrated inFigure1.
Layers of protection can be achieved by manual actions,
by mechanical devices or by instrumentation. The more reli-
able each protection layer is and the more of them there
are, the more difficult it will be for a hazardous effect to
penetrate through to hurt people or damage assets. There-
fore the integrity of a protection layer is important, ie, its
availability, which is a function of its reliability and maintain-
ability. Lately, through modern technology, a lot of emphasis
is put on the implementation of instrumented protection,
like trips and interlocks, in processes and operations.
Independent layers of protection
The concept of an Independent Protection Layer (IPL) which
is an independent safety system devised to stop the pro-
gression of an event to the hazardous state, is used. This
can be illustrated by referring to Figure 2 depicting a simple
example where an operator has to fill a tank with a corrosive
and toxic liquid. If the main hazard identified is pollution,
then in this situation the causes could be overfilling of the
tank or tank failure, eg, cracking. In this case there are no
layers of protection and pollution is extremely likely.
Referring to Figure 3, the following layers of protection
can be added:
1. Design integrity, ie, specification of a non-corrosive mate-
rial of construction for the tank.
2. Providing the operator with procedures and training to
monitor the tank level visually and close the valvewhen full.
3. Provision of a level indicator so that the operator does not
need to climb onto the tank to observe the level.
4. Add a level control loop to automatically control the level
in the tank avoiding the need for the operator to be in
attendance.
5. Add a high level alarm so that whenever the control loop
fails, the operator is alerted to take corrective action, eg,
close the valve.
6. Add a high level interlock using a high level switch to
automatically close an actuated valve, should any of the
above protection layers fail.
7. Finally, provide an overflow pipe on the tank and a bund
to contain any spillage should any of the above layers of
protection fail.
It is important that each layer of protection is capable of
acting independently of any other protection layer.
In industry there are hazards which can
lead to loss of life and property. To avoid
these consequences, it is essential to
prevent them from happening in the
first place or, as a last resort, mitigate
their effects by means of protection.
If prevention is included in the term
protection, then one may visualise
an installation with various layers of
protection around it.
by Daniel J E Rademeyer, ISHECON, Johannesburg, South Africa