Control Valve Positioners and Bench range

[Instnapp05]

Hello Everyone,

I'm a second year student doing my degree now in Instrumentation and would like to get some additional information on valve positioners.As part of our practical assessment,I need to install an 8012 model masoneilan positioner on a control valve with a NEW bench range of 6-30 PSI.Previous bench range was 3-15 PSI on an 87 masoneilan actuator.The springs were change out to achieve the NEW bench range 6-30 PSI and also the positioner was replace(not for changing the bench range)but as part of the assesment from a 4700E to an 8012 masoneilan model.

My question to you'll, if the positioner was previously 3-15 PSI(25 PSI supply)which means to say at output 4(mA)3 PSI the valve plug should start to move(if bench tested correct) and at 20(mA)the plug touches the seat and the actuator gets the full supply pressure 20 PSI.When the bench range is change now from 6-30 PSI(30 PSI supply),what's inside the positioner to determine at 4 mA the output should be 6 PSI for the valve plug to start to move(ATC) and at 30 PSI to stop as compared to the signal range 3-15 PSI.

In summary;
How does the positioner knows to adjust itself to an output of 6 PSI and not 3PSI for the NEW bench range?
Is it that the positioner senses the valve position at 0-100% and when calibrating and the I/P adjust the output as necessary from 6-30 PSI?

Appreciate your feedback.

 

[danw2]

Your last statement sort of heads in the right direction.

The positioner senses the stem position and provides whatever air pressure is necessary for the stem position to equal what the control signal position demands.

A positioner doesn't know what pressure it provides (it has no pressure sensing), it knows only the position of the stem (feedback) and the stem position the control signal is asking for. Its internal pneumatics are designed to make the feedback position equal the control signal 'position'.

I hope that your have at least at 35 psi supply line for a 6-30 benchset. Air supply is always higher than the maximum bench set.

 

[Instnapp05]

Thanks danw2.
Can you advise on a good Instrumentation book which I can purchase for additional knowledge on control valve besides the Emerson control valve handbook?

Thanks
Naresh

 

[danw2]

Tony Kuphaldt's massive tome on instrumentation, "Lessons In Industrial Instrumentation, version 2.13, 3200 pages and the price is free.

opens in a browser at

http://www.ibiblio.org/kuphaldt/socratic/sinst/book/liii.pdf

or downloads (right click, save link) from

http://www.ibiblio.org/gsearch/?cx=...f=FORID:11&ie=UTF-8&q=Tony+Kuphaldt&sa=Search

 

[Ch9117]

Please find the attached 87-88 manual. Bench range change information refer to 13 page.
You additional need more 3 pring required.