Drain hole is a small hole which is provided in the lower region of orifice plate. Drain hole is required in gas flow service where liquid entrainment may occur.
Drain hole size could affect the accuracy of flow measurement. However, if the diameter of the drain hole is less than 10% of the orifice bore, then the unmeasured flow is less than 1% of the total flow.
Drain 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.
See also, Vent Hole