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The contlol valve works efficiently with posotioner assoon as signal comes from controller to the I/P converter,having the pneumatic supply in positioner it provide the3-15 psi air pressure thus valves takes action of open/close position as per controller signal ,but without positioner it works as on/off control with delay action ,further draw back is change in Set point and Increase in Dead band of controller .
Control valve can be used without positioner only when its actuator is designed according to line pressure. and can also be used as on/off valve.
The output of the i/p converter is3 to15 psi. The positioner is used for accurately controlling the desired parameter. In this case if the spring range of the control valve is3 to15 psi and the valve is of small size, the positioner need not be connected and i/p converter output can be directly provided to control valve.
Of course you'll need an I/P converter but if its a small valve and the pressure is not too high the diaphram can probably actuate the valve without a positioner. Changing fluid conditions will cause the position of the valve to change even with a constant pneumatic signal which may cause some disturbance in control. I've had success without a positioner. In most cases the control is fast enough and the hysterisis due to starting friction on the packing is mild.