Differential Pressure Effect on Control Valve Opening

"Direct acting acutator and reverse acting actuator will encounter different effect of valve opening (valve lift or valve travel distance) when control valve is installed in the field. We may get an ideal curve indicating relationship of % of valve lift versus Air pressure applied to the actuator. However, we may get different result of mentioned curve if the same control valve is installed in the field.
The reason of different curve relationship is due to the effect of differential pressure between upstream pressure (control valve inlet port) and downstream pressure (control valve outlet port) exerted by the fluid flowing inside the valve. This differential pressure will try to push up the valve plug. As a result, the Air-To-Open valve (reverse acting actuator) will need lesser than the required pressure (0.2 bar or 3 psi) to push up (open) the valve plug while the Air-To-Close valve (direct acting actuator) will need more than the required pressure (0.2 bar or 3 psi) to push down (close) the valve plug."
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"Direct acting acutator and reverse acting actuator will encounter different effect of valve opening (valve lift or valve travel distance) when control valve is installed in the field. We may get an ideal curve indicating relationship of % of valve lift versus Air pressure applied to the actuator. However, we may get different result of mentioned curve if the same control valve is installed in the field. The reason of different curve relationship is due to the effect of differential pressure between upstream pressure (control valve inlet port) and downstream pressure (control valve outlet port) exerted by the fluid flowing inside the valve. This differential pressure will try to push up the valve plug. As a result, the Air-To-Open valve (reverse acting actuator) will need lesser than the required pressure (0.2 bar or 3 psi) to push up (open) the valve plug while the Air-To-Close valve (direct acting actuator) will need more than the required pressure (0.2 bar or 3 psi) to push down (close) the valve plug."