To exempt emergency valves from the application of Sentence 4.5.7.1.(1) if the valves conform to a certain standard, on the basis that the standard provides an acceptable level of safety for such valves.
This is to limit the probability that operating temperatures and pressures will lead to failure of the valves, which could lead to the escape and spread of liquid, which could lead to the ignition of vapour from a nearby ignition source, which could lead to damage to the building or facility.