Vent hole is a small hole which is provided in the upper region of orifice plate. Vent hole is required in liquid flow service where gas entrainment may occur.
Vent hole size could affect the accuracy of flow measurement. However, if the diameter of the vent hole is less than 10% of the orifice bore, then the unmeasured flow is less than 1% of the total flow.
Vent hole is not recommended in dirty fluid service or slurries as the hole could be plugged. In this application, the use of eccentric orifice plate becomes alternative.