The valve positioner is an instrument working on force balance principle to position the control valve stem in accordance to a pneumatic signal received from a controlling system, with out the interface of the protecting box, and even actuator swing or unequal forces applied on the valve by nature. For neutralizing this a positioner will balance all the forces to operate a control valve. Valve cv sizing calculator for liquids and gases flow systems.
Model PVP-1 Single acting, Direct action.
Supply connection ¼” NPT, (F)
Supply air pressure 1.4 to 3.5 kg/cm^2
Input 0.2 to 1 kg/cm^2
Stroke 14 mm to 100 mm
Stroke speed 10 mm/sec
Hysteresis, linearity within +/- 1% of FS
Air consumption 7.0 NL / min (Normal), 200 NL / min (Maximum).
PRINCIPLE OF OPERATION
The instrument signal is applied to the signal diaphragm. An increasing signal will drive the diaphragm and flapper-connecting stem to the right. The flapper-connecting stem will then open the supply flapper admitting supply pressure in to the output, which is connected to the actuator diaphragm. The exhaust flapper remains closed when the flapper-connecting stem is deflected to right. The effect of increasing signal is to increase the pressure in the actuator. This increased pressure in the actuator drives the valve stem downwards and rotates the positioner lever clockwise. This clockwise rotation of the lever results in a compression of range spring through cam. When the valve stem reaches the position called for by the controller, the compression in the range spring will give a balance force resulting the closure of both the flapper.
If the control signal is decreased the force exerted by the signal diaphragm will also decrease and the force from the range spring will push the flapper-connecting stem to the left, opening the exhaust flapper. This causes a decrease in actuator diaphragm pressure and allows the valve stem to move upward until a new force balance is established.
PVP-1 model is available with bypass system. Bypass system facilitates to cut off the positioner and to pass the signal pressure to valve actuator directly. This enables to short time servicing of positioner, without affecting the working of the valve.
To bypass the positioner
1 Loosen the bypass knob fixing screw
2 Rotate selector knob clockwise and set to “bypass” position
3 Tighten the bypass knob fixing screw. The positioner is ready for “Bypass”.
Replacing pilot relay assembly:
Pilot relay assembly is factory calibrated and sealed, field disassembly is not recommended. If required replace factory calibrated spare pilot relay assembly in following steps:
a) Before removing pilot relay assembly, note whether action of positioner is direct or reverse to avoid misalignment when reassembly.
b) Remove three screws fastening assembly to body
c) Inspect existing gasket and replace if necessary
d) Check the action by reversing the positioner relay assembly and mount in the desired position
e) Fix the 3 screws to securely fasten assembly to the body
When manifold or pilot assembly removed, if the gasket found to be worn out, change the same
The positioner is mounted on the control valve and calibrated in factory. Hence calibration is not required. However if positioner has been disconnected for some reasons and if it is to be reconnected then calibration is required.
1 Apply regulated air pressure to actuator and adjust pressure until stem is positioned at midstroke.
2 Adjust motion connector assembly until positioner lever and motion connector are parallel and horizontal at midstroke.
3 Disconnect regulated air supply to diaphragm and connect tubing from output port of positioner to diaphragm.
4 Check numbers stamped on the positioner lever and move stroke-adjusting pin to desired stroke and tighten.
5 To set positioner’s start point, apply desired starting pressure (usually 3 PSI) and adjust the spring adjuster until valve stem starts moving.
6 Set desired maximum pressure of signal range (usually 15 psi) and check for the stem stroke. If stroke is less, move stroke-adjusting pin to the hgher value i.e. right. If stroke is achieved at pressure lesser than desired pressure , move to lower value i.e. left.
7 Recheck the start point.
8 Repeat step 5 & 6 until travel is found within acceptable limits.
CHANGING POSITIONER ACTION
PVP positioner can be changed from direct action to reverse action or vice versa without additional parts or special tools. The same does not demand repositioning of positioner or tubing.
To change the action, follow the steps given below:
1 Release feed back spring and remove travel pin.
2 Remove the 3 Nos screws, which fasten the pilot assembly to the housing.
3 Rotate the pilot by 1800 and remount it to the housing. Make sure that DA or RA marking is properly lined with arrow mark.
4 Remove the cam and fasten it in the opposite direction.
5 Readjust the travel pin and feedback spring.
6 Recalibrate as illustrated in “Calibration”.