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

Appendix 4: Valve types and
issues
Health and Safety
Executive
1 Table A outlines the characteristics of common valve types. Select isolation
valves carefully to ensure successful performance in service, based on isolation
circumstances. Key factors are:
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sealing ability;
security (potential for accidental or deliberate reopening); and
reliability (potential for partial or total loss of seal).
2 Limitations on the use, for isolation, of valves with other primary functions
include:
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control or choke valves are not generally suitable for isolation;
emergency isolation valves may be used for plant isolation, provided they are of
a suitable type for the fluid and conditions being isolated;
relief valves should not be used unless there is a purpose-designed method of
securing the valve in the closed position and if it is possible to pressure test
the valve to the maximum anticipated differential pressure in the direction of the
applied pressure. Particular care is required to ensure proper reinstatement of
relief valves to their required operating status and correct set pressure.
3 Valves are likely to provide a tight seal ‘on demand’ only where you select:
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an appropriate type of valve; and
trim materials compatible with the process fluid conditions.
4 Valve selection should take account of accumulated experience of a particular
valve/trim combination performing satisfactorily under specific process conditions.
5 Use commissioning tests to confirm the initial sealing performance of the
installed valve.
6 Prove the leak tightness of a valve every time it is used for isolation.
7 Once a valve has been proven leak tight, the likelihood of a leak then
developing during the isolation is low, but this could be an issue for long isolations
(eg greater than a month). In such situations, consideration should be given to the
periodic testing of the upstream valve of the isolation.
8 Careful consideration is required before using valves which provide a double
seal in a single-valve body with a bleed in between (eg double-wedge gate, parallel
expanding gate or double-seal ball valves) for isolation. In some applications,
both barriers cannot be easily tested. Also, the security of the isolation depends
upon the immobilisation of a single valve operating stem. Such valves should be
used in preference to a double block and bleed isolation method only after full
consideration of these increased risks.
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