3.4.8 Vent valve and related piping

Vent valves shall be installed on the topmost part of the pipe to ensure that all the gas is vented from the dead spaces not swept by the displacer. This requirement ensures that the piping, the pipe prover and the meter being verified/calibrated remain completely filled with liquid. Provisions shall be made for the draining of liquids and for venting vapours from the prover. If the prover is intended to be calibrated using the water draw method or the master meter method, correct connections shall be provided.