Eccentric Orifice Plate is orifice which bore position is offset from center line of the pipe.
It is used in service if there is secondary fluid phase (gas contains liquid or liquid contains gas) in which the application of vent hole or drain hole on the concentric orifice plate may adversely affect the accuracy in case the vent or drain hole size requires large area to cater the secondary phase.
Eccentric Orifice Plate is also used in service where severe entrainment may occur such as in dirty gas or liquid, which in this case the solid or slurries could plug the vent or drain hole.
If the primary phase is liquid which contains gas as secondary phase, the location of eccentric bore is offset upward. If the primary phase is gas which contains liquid as secondary phase or if primary phase is liquid which contains solid/slurries, the location of eccentric bore is offset downward.
Below picture shows the arrangement of eccentric bore of orifice plate.
