Two types of block valves are used in Pipelines.
1. Sectionalising Valves
2. Emergency Shutdown Valves.
It may not always be possible to achieve complete leak tight isolation, but flow should in any case be severely limited so that the main objective is achieved, i.e. the fluid within the isolated sections is contained.
Valves which are necessary for the routine operation of the pipeline, such as pig trap valves, are not considered as block valves.
The use of fittings between block valves, such as flanges and instrument taps, should be minimised.
In some situations, e.g. at the downstream side of plants or major river crossings, a check valve may be used instead of a block valve, since it is simple, reliable and self-actuating. The valve will normally need to be piggable, either by using a piggable flapper or by allowing the flapper to be temporarily secured in the open position.
Check valves shall not be used to provide isolation for maintenance of equipment.
