The safe isolation of plant and equipment (HSG253) - Health and ...

Health and Safety
Executive
161 The necessary extent of purging and flushing will depend on the substance
concerned and the nature of the intrusive activity. For example, very stringent
control is required where intrusive hot work is to be carried out on systems that
have contained flammable substances. 10 Precautions may include:
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depressurise to atmospheric pressure;
cool (or heat, for cryogenic systems);
drain;
water flush and/or fill;
nitrogen purge;
others such as air movement, high-pressure water jetting, back flushing,
detergent wash; and
demonstrating a non-explosive atmosphere (substantially below lower explosive
limit (LEL)), internally, immediately before the work starts.
162 Specify arrangements for control, calibration and checking of test equipment
in your procedures. Where necessary, ensure that gas testing equipment is suitable
for use after purging (ie that measurements will not be adversely affected by the
presence of inert gas or depleted levels of oxygen).
163 When testing plant and pipelines to prove that they are totally gas or vapourfree,
ensure that a representative sample is taken. For large items of plant, you
may need top, middle and bottom samples to ensure that no light or heavy gases
remain.
164 Testing of pipelines requires special care, as the point of isolation may be
some distance from the sampling point. In such circumstances you should consider
installing a sample point closer to the isolation point to verify its gas or vapour-free
status.
Testing and monitoring effectiveness of the isolation
165 Prove the integrity of all isolation points of an isolation scheme before
proceeding with intrusive work (unless your risk assessment has indicated that use
of non-proved isolation is acceptable):
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each part of the isolation should be proved separately, eg prove each valve in a
double block and bleed scheme;
each part should be proved to the highest pressure which can be expected
within the system during the work activity. Particular care is required when there
is a low differential pressure across valves where the sealing mechanism is
activated by pressure; and
where possible, each part of the isolation should be proved in the direction of
the expected pressure differential.
166 For a positive isolation scheme, there are two stages of testing. Prove both
the initial (valved) isolation and the final isolation.
167 Your procedure for installing the isolation should clearly specify the
arrangements for proving the isolation, the test success criteria and, for positive
isolations, the pressure to be applied (see paragraphs 180-183).
168 Do not rely on an isolation that is unproved because the facilities are not
present, or where you lack confidence in the test. Extend the isolation boundary or
defer the work until shutdown. You should also consider plant modification to install
facilities for testing that isolation point in future (see paragraphs 138-139).
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