VALVE  POSITIONER

VALVE POSITIONER CALIBRATION SET UP AND PROCEDURE / MAINT

1. INTRODUCTION OF VALVE POSITIONER

The positioner  is  a  high-gain  plain  proportional  controller  which  measures  the  valve  stem position (to within 0.1 mm), Compares that measurement to its set point (the controller output signal),  and,  if  there  is  a  difference,  corrects  the  error  .The  open-loop  gain  of  positioners ranges  from  10 to 200 (proportional band of 10% to 0.5%) . In  other  words, the positioner is a very sensitively tuned proportional –only Controller.

The main purpose of having a  positioner  is  to  guarantee  that  the  valve  does  move  to  the position where the controller wants it to be. By adding a positioner, one can correct for many variations, including changes in pausing friction due to dirt, corrosion, or lack of  lubrication; variations  in  the  dynamic  forces  of  the  process;  dead  band;  or  nonlinearities  in  the  valve actuator.  The  dead  band  of  a  Valve/actuator  combination  can  be  as  much  as  5%;  with  the addition of a positioner it can be reduced to less than 0.5% .It is the job of the  positioner to protect the controlled variable from being upset by any of the above variations.  In addition, the  positioner  can  allow  for  split-  ranging  the  controller  signal  between  more than one valve, can increase the actuator speed or thrust by increasing the pressure and /or volume or the actuator air signal, and modify the valve characteristics by cams.

Valve Positioners are not used for on-off control applications.

Positioners must have an air supply of 3/8” tubing as a minimum, and each one should have its own supply filter-regulator (PRV).  Control valves use substantial  amounts  of  air,    and severe control interaction can occur if common air supply is used.

2. Fisher Valve Positioner

Fisher Valve Positioners are available with or without bypass assembly. When the instrument pressure increases, the bellows expands to move the beam, causing the flapper to restrict the nozzle. The nozzle pressure increases and moves the relay diaphragm assembly to open the Supply valve.  The  output  pressure  to  the  diaphragm  case  of  the  control  valve  increases, Moving the actuator stem downward. Stem movement is fed back to the beam by means of The cam which causes the flapper to move away from the nozzle. Nozzle pressure decreases and  the  relay  supply  valve  closes  to  prevent  any  further  increase  in  output  pressure.  The positioner is once again in equilibrium but at a higher instrument pressure and a new valve plug  position.  When  the  instrument  pressure  decreases,  the  bellows  contracts  to  move  the beam and uncover the nozzle. Through relay operation, the exhaust valve in the relay opens to release diaphragm pressure to atmosphere, permitting the actuator stem to move upward. Stem position is fed back to the beam by the cam to reposition the beam and flapper. When equilibrium conditions are obtained, the exhaust valve closes to prevent any further decrease in diaphragm case pressure.

3. DESCRIPTION OF OPERATION

A  typical  application  of  the  Bailey  Positioner  is  shown  in  the  above  Figure. The  power operator  to  which  the  Positioner  is  applied  is  a  double  acting  air  cylinder  and  piston assembly.  The  piston  rod  may  be  connected  through  suitable  linkage  to  position  a  valve, damper, or other regulating device. The positioner consists essentially of two opposing forces balanced against each other. When the positioner  is  balanced,  the  force  exerted  upward  on  the  balance  beam  by  the  loading bellows depends upon the control loading pressure established by the system in relation to the demand for the controlled medium. The force exerted by the positioning spring depends upon the position of the power operator piston and the shape of the positioning cam. The position of the power operation piston is tied-back to the  Positioner drive arm  thru the positioner drive rod (a drive rod is the usual means used to tie back the motion of the power operator, however other methods may be used depending on the application). Therefore, for very  position  of  the  power  operator,  the  positioner  drive  arm  assumes  a  corresponding position. The drive arm is geared to the positioning  cam  which  is  shaped  to  give  a  desired characteristic of power operator position V/S control loading pressure.

4. Automatic Positioning

When the forces exerted by the loading bellows and positioning spring are balanced against each other, the balance beam holds the pilot valve stem in its “neutral” position, i.e. the pilot valve stem lands are centered at the pilot ports, creating equal pressure on either side of the piston. Thus, the position of the power operator is maintained. When the control system indicates that the power operator should be repositioned, the force exerted on the balance beam by the loading bellows increases as a result of the increase or decrease  in control loading pressure. Movement of the beam raises or lowers the pilot valve stem, increasing or decreasing the air pressure applied to either side of the piston. As the piston  is  moved  by  the  differential  in  pressure  thus  created,  positioning  spring  tension increase  or  decrease  until: 

(1)  the  forces  exerted  by  the  bellows  and  spring  are  again  at balance;

(2) the pilot valve stem returns to its “neutral” position; and

(3) the piston assumes a new position.

5. CONTROL LOADING ARRANGEMENTS

The  positioner  is  normally  furnished  mounted  on  the  power  operation  with  all  tubing connections  made  between  the  operator  and  Positioner.  Depending  upon  the  specific application, the positioner may be adjusted for either direct or reverse loading operation as described below. Direct Loading  is that arrangement where by upward movement of the piston rod causes a valve damper, or other regulating device to open as control loading pressure to the Positioner bellows increases.  Control air pressure from the upper port of the  Positioner  pilot  valve  is supplied to the underside of the piston .The  positioning cam is so that when the cam turns in a counterclockwise direction, tension on the positioning spring will increase. Reverse loading is that arrangement where by downward movement of the piston rod causes a valve, damper, etc. to close as control loading pressure to the Positioner bellows increases. For applications requiring reverse loading, it is necessary  to  switch air line connections to the cylinder and reverse the positioning  cam (red concentric and radial lines visible) so that when the cam turns in a clockwise  direction, tension on the positioning spring will increase. To reverse the positioning cam:

1.  Remove cam .

2.  Remove locating set screw.

3.  Reassemble set screw in tapped hole in other side of cam hub.

4.  Reassemble positioning cam so that opposite side of cam faces outward.

6. CALIBRATION ADJUSTMENTS

The Positioner adjustment described below may be used to improve the operation of the power operator system either by it self or in relation to other systems or parts of a multiple system.

1.Zero  Adjustment  – By means of the zero adjustment  an initial tension may be imposed upon  the  positioning  spring  so  that  the  piston  will  not  start  to  move  from  its  minimum position until the control loading pressure has increased  (from 3 psig) to any value up to 15 (or 20)psig.

2.Range Adjustment – The range adjustment   affords a variation of power operator motion for  a  given  control  loading  pressure.  The  amount  of  variation  extends,  roughly,  from  full piston travel for a 3 to 15 psig change in control loading pressure to one-half piston travel for a 3 to 15 psig control loading pressure change

MAINTENANCE OF VALVE POSITIONER

General

1.  Keep air connections tight to prevent leakage which may indicate improper functioning of the unit. Check all connections for leakage, while under pressure, with  soap solution.

2.  Maintain a clean air supply (free of dirt, oil, or moisture) for satisfactory operation of Positioner and power operator.

Routine Maintenance

1. Whenever power operator is out of service (or when required) , remove pilot valve stem and inner liners and clean with a common solvent. Never use files, or abrasives on valve stem lands  or  valve  liners.  If  liners  stick  in  valve  body  upon  removal,  push  them  free  with  a wooden stick or pencil: never use a metal rod for this purpose.

2.Once each year or whenever  Positioner  supply  or  bypass  valves  begin  to  show  signs  of sticking, remove valve and lubricate with Bailey Petcock. To remove valve, leave handle in present position and turn valve nut out of pilot valve body.

CALIBRATION PROCEDURE: –

1.  Check the fully closed position of the Valve by applying pressure and mark or adjust the Position indicator to close position.

2.  Check the fully open position of the valve by applying pressure for the maximum travel of the stroke.

3.  Make the Calibration Set up.

4.  Check the air supply pressure.

5.  Take the As found readings with 0,25,50,75 & 100% of the input signal in increasing and Decreasing order.

6.  Calibrate the Control Valve with the Valve positioner.

a.  Give the input signal (start slowly) and check with finger on the Stem for starting of operation. When the  Ste+m  starts  moving,  note  down  the  input  reading  it  should  be 4ma dc / 3 PSI. If not, adjust zero on the Valve positioner.

b.  Increase  the  input  slowly  and  check  the  stem  for  it  stops  traveling.  Note  down  the reading, it should be 20 ma DC/15 PSI). If not adjust the span/range adjustment on the Valve positioner.

c.  Repeat the steps a & b till both are OK.

d.  Take As left readings with 0, 25, 50, 75 & 100% of the input signal in increasing and decreasing order.

e.  Fill up the Calibration Sheet.

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