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

Health and Safety
Executive
tanks. Large vents will be needed to avoid damaging the tanks. Freezing of any
condensate lodged in the system can block or fracture equipment.
13 Air may be used to purge non-flammable gases. It is unlikely to effectively
remove all liquid from long pipelines/pipework systems. Some systems may be
designed to allow the use of partial vacuum techniques.
A final air purge may be required to ensure no asphyxiation hazard at a worksite.
14 Direct purging is a technique which is applicable to certain plant in the natural
gas industry and involves the displacement of air by fuel gas or vice versa. Details
of the technique and its application are found in the Institute of Gas Engineers
Safety Recommendations IGE/SR/22 Purging operations for fuel gases in
transmission, distribution and storage. 28
Flushing activities
15 Flushing is the continuous or cyclic filling and draining of the system with a
liquid, such as water, to wash out hazardous substances.
16 Flushing can be important when:
■
■
■
draining or venting could leave hazardous process residues trapped in less
accessible parts of the plant (for example in internal fittings, instruments, deadlegs
in pipework, valve-body cavities, legs and structures of floating roof tanks,
roof fittings or vents, and on surfaces);
process residues remaining within a plant could release trapped gases due to
disturbance or changes in ambient temperature. High temperatures (eg from
welding) may also cause chemical changes or ignition; or
pipelines (eg off-plot chemical and oil installation) where hazardous residues
can present particular problems are not amenable to pigging.
17 Water is often used for flushing, or to float out light substances from vessels.
It is ideal for the removal of water-soluble materials, but is less effective for the
removal of petroleum-type substances. Medium- or high-pressure water can
be used to scour the surface of a vessel or pipeline. However, water must be
totally removed from hot oil vessels because steam can be generated following
recommissioning. In winter, freezing can cause blockages or equipment fractures.
Water can increase corrosion risk, especially where the plant contains halogenated
compounds. Pipework, pipework supports and vessels need to be designed to
bear the weight if water-flushed. This is particularly important for large diameter
systems that normally contain gases or low specific gravity liquids, eg flare
systems.
18 Chemical solutions can be used to neutralise residual hazardous chemicals. A
solvent or water flush may also be required.
The safe isolation of plant and equipment Page 74 of 81