Control Valve Accessories

8/8/2019 Control Valve Accessories

http://slidepdf.com/reader/full/control-valve-accessories 1/11

Control Valve Accessories

Sep, 30th 2010



Positioner

1. The positioner is a high gin plain propotionalcontroller which measures the valve stemposition (to within 0.1 mm), compares thatmeasurement to its set-point and if there is adifference correct the error

2. The open loop gain of positioners ranges from10 to 200 (proportional band of 10% to 0.5%)and their periods of oscillation range between

0.3 and 10 seconds (3-0.1 Hz)3. In the other words, the positioner is a very

sensitively tuned proportional-only controller



Positioner (2)

1. The main purpose of having a positioner is to

guarantee that valve does move to the position

where the controller wants it to be

2. By adding positoner, one can correct for many

variations, including changes in packing friction

due to dirt, corrosion, or lack of lubrication

3. The dead band of a valve-actuator can be asmuch 5% with the addition of a positioner it can

be reduced to less than 0.5%



Positioner Characteristic

When the valve is in remote manual (open loop)operation, it is always benefit from the additionof a positioner, because it will reduce the valves

hysteresis and dead band when increasing itsresponse

When the valve is under automatic (closed loop)control, the positioner will be helpful when the

loop response is not very fast Actuator without springs always require

positioners



Positioner Characteristic (2)

A controlled process can be considered slow

if its period of oscillation is three times the

period at which the positioned valve oscillates



Limit Switches

To describe switches installed to signal when avalve is at or beyond a predetermined position

These switches are used for operatorinformation, interlock inputs, or computerfeedback

Its necessary to consider mounting problems,

electrical classification of area, electricalcharacteristics of circuit, over-travel of actuating and corrosive nature of the area



Limit Switches (2)

Becaused of enviromental problems, some

users have been using sealed magnetically

actuated or proximity switches



Regulator or Airset

Is the air regulator with filter and drip out used tosupply air to the positioner or other instrumeny

The regulator must have an appropriate pressure

range for the positioner and actuator and itshould have a built-in overpressure vent

Capacity is a problem only with very large valves

Some piston actuators and positioners willoperate with up to 150 psig (1MPa) supply andthere is a trend toward eliminating the regulatorbut retaining the filter



Regulator or Airset (2)

The risk in providing airsets for high-pressure actuatorssuch as air cylinders is that field operator can thereby limitthe supply pressure, which in turn reduces actuatorstiffness (resistance to the dynamic forces of the process)

Naturally, when the pressure rating of the actuator orpositioner is less than the air supply pressure, an air setmust be used

The use of air filters is always recommended

They should be installed in the air supply serving the

positioner and should be designed for the pressure of theair supply

Their purpose is to remove moisture, oil and all particlesthat are 5 microns or larger



Solenoid

Control valve accessory is used to operate on/off

pneumatic actuator or interrupt the action of

modulating valves by switching air pressures

Two philosophies are in common use

± The 3 to 15 Psi (0.2-1 bar) signal to the positioner is

blocked and the downstream tubing either vented or

connected to some other preset pressure ± Its installed in the tubing between the positioner and

actuator



Thx

Documents

Control Valve Accessories