Pilot Positioners

MikeyB

What is the proper procedure for calibrating a Pilot Positioner for a Pneumatic Control Valve?

BillW

Positive Positioners

Positive positioners are found on certain pneumatic valve and damper actuators.  They compensate for pressure changes in the controlled medium (water or air), allow you to fine tune the system, eliminate the need for different spring ratings, and allow for sequencing.  They are usually grey and black in color, have a dial on the top, and are found on Honeywell MP953E direct acting and MP953F reverse acting valve actuators and on MP918 damper actuators.

To calibrate, you need to know what the start point value was on the job drawings.  Use a 5/64 allen wrench to unlock the startpoint knob screw.  Each "click" adjusts the startpoint  by 1/4 psi.  Tighten the screw back down to lock.

Install 0-30 pressure gauges in pilot and main air lines. Slowly apply pilot pressure, and note at what pressure the valve stem begins to move.  This pressure should be within +/- 3/4 psi of the start point setting on the drawings.  Increase the pilot pressure until the valve stem travel is complete, and note that this pressure should be within +/-3/4 psi of the start point pressure plus the range setting  (3,5 or 10 psi).  This is determined by the feedback spring in the actuator.

Air connections are "P", piped to the "B" port on the controller (TP Series thermostat or RP920 receiver/controller), "M" for main air, and "B" to the actuator. Unless someone has really played with the positioner, there isn't a lot of need to change the settings.  As far as I know, parts are still available from commercial distributors.

MikeyB

Pilots

Thank you Bill W. We have (2) Siemens control valves for our  B&G heat exchangers (Steam to water Heat exchangers), one valve is a 1/3rd and the other is a 2/3rd valve (that's how it is labeled on our BMS) I blew a diaphragm on the 2/3rd steam valve actuator, replaced the diaphragm then noticed that both of the valves are all over the map (hunting up and down, but maintaining set point) thinking it might be a Siemens programming issue, but wanted to double check the Pilot positioners as well. Im on vaca next week and will take another look at the valves when i get back

BillW

steam/hot water HX

Those valve actuators are probably reverse acting (air-to-open), since if the air supply fails, or some other control issue occurs, the valves will spring-return closed, cutting off the steam and preventing overheating.  The positioners work the same way, but the calibration methods may be different from the Honeywell units I mentioned. 

Wesman

Positioners

With Siemens positioners the span and startpoint are both adjustable.

In your case doing 1/3, 2/3 normally closed valve sequencing you would set them up as follows.

The smaller 1/3 valve is designed to open first and do most of the work most of the time. You would typically choose a span of 5 psi and a start point of 3 psi. The larger valve is set with a 5 psi span and 8 psi start point.

With no call for heat, both valves would receive a 0 psi signal and be closed. As demand increases and the pilot signal increases to 3psi the smaller valve will start to modulate open. With a 5 psi span, the small valve will be fully open at 8 psi (3 psi start point + 5 psi span = 8psi) while the larger 2/3 valve is still closed. As demand increases, the small valve will remain fully open and as the signal rises above 8 psi the large valve will start to open. The large valve will be completely open at 13 psi (8psi start point + 5 psi span = 13psi).

Follow the installation instructions and markings on the positioner to determine the the placement of the feedback spring to achieve these settings. If the valves are sized correctly you should achieve excellent control and maintain setpoint without any quick hunting from either valve.

Heat exchangers use normally closed valves but if the application had normally open valves, the small 1/3 would be set 8-13 and the large 2/3, 3-8 psi.

With Honeywell MP953 you must determine the span ( 3, 5 or 10 psi)before purchasing the actuator as the span is preset and non adjustable, only the start point is.