CONTROL VALVE ACCESSORIES
“In many control systems, a properly sized spring and diaphragm actuator will do an excellent job without the use of a positioner or booster”. This is quoted from the Fisher Control valve Handbook.
Valves all seem to come with a positioner now although they may not be really necessary. Fisher recommends their use only where the stem packing stiction is unusually high. For a fast pressure process, they recommend the use of a booster.
17.2 What does a positioner actually do?
A positioner is generally mounted on the side or top of an actuator, and positions the valve stem accurately.
It does this by means of a mechanical linkage attached to the valve stem, this gives a “Position Feed back”, so that valve stem position can be compared to the incoming signal from the controller output, and positioned accordingly.
The relationship between the controller output signal and the actual valve position can be changed by calibrating the positioner to a different input to output relationship.
In addition, two or more valves can be operated differently on the same controller signal to form a “Split-Range”, system with either Air-to-open or Air-to-close or both if required.
17.3 Positioner types
There are currently several different types of positioner in service.
Some examples are–
- The Fisher 3570 series (top of Valve positioner type)
- The Fisher 3582 series (stem position transmitters)
17.4 The Fisher 3582 Positioner
17.5 The Fisher 3570 Positioner
17.6 Masoneilan Valve and Positioner
Boosters are used to improve the response time of the controller to valve part of a control loop, due to long transmission lines from the controller to the valve.
17.8 Transducers (I/P)
A current to pressure (I/P) transducer changes the industry standard electrical transmission signal (m.A) into the industry standard pressure signals of, usually 3 to 15 p.s.i and sometimes 6 to 30 p.s.i.
Solenoid valves are electrically operated devices, which either supply an air signal to a valve or vent the air signal to a valve, usually used in conjunction with E.S.D. systems.
17.10 Limit switches
Limit switches are usually operated mechanically by use of Push Rods ; Cams; and springs are used to send signals to operate lights and or ESD system trips or permissives.
They can be set so as to indicate a valve Fully Open; Fully – Closed or still in transit from one of these positions to the other.
Electric relays, alarms or solenoid valves, can also be operated by the switches to set up a sequence of events in a start-up or shutdown system, a good example of this is the valve sequence on the process valves of a gas compressor
Cam operated types are available with up to six individual switches operated by valve stem movement.
18.0 The Position Transmitter or Motion Transmitter
Position transmitters show us the actual position of the valve, no matter what the controller output says!
There is a false assumption among operators that the output of the controller gives us the position of the valve.
Well it should, but what if the air supply was turned off or recent maintenance change a split range back to normal?
The output of the controller would still stay the same. On some DCS installations, position feedback is employed from a position or motion transmitter.