Control valve positioner

You must calibrate the I/P converter so that there is 1 to 1 relationship between input and output. If I/P is disturbed and you calibrate the control valve, you may solve the problem temporarily, but your colleagues & successor may not know what you have done and he may have to re calibrate the valve again if he go for change of I/P converter.
 
Hello to all!

I got very intrested in this forum when i saw the rich replies on positioners. Pls my question is this "ARE POSITIONERS PRIMARY SENSING ELEMENTS"? i am trying to group them in my "technical object type." i am a cmms engineer and we do build data base for asset maintenance.
 
Not necessarily, although most analog input/output positioners now have HART communications. Unless they have Profibus, or Foundation fieldbus.

Look up HART, Profibus and Foundation fieldbus on Google for more information. It's all there, in copious quantity.

You might also look up the manufacturer and model number of the positioned in question. Their websites usually have more information on their products than you can possibly absorb.

Best,
Walt

Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
[email protected]
 
A
Ronald,

Thanks for your explanation. As a follow up, please confirm if a control valve equipped with a HART positioner DVC6200 function properly with a HART-to-Fieldbus Converter connected to a Delta V DCS where a DVC6200f is not readily avalaible?
 
Thanks for info

could you please inform me in this case "we don't need any actuator and positioner is complete and assembled on valve" that's mean valve + positioner worked as control valve
 
Sir,

can you please explain me the difference between positioner and actuator....
And when to go for an actuator and when to go far a positioner?..

Whether actuator can completely replace positioner?
 
Nice explanation, except that for some positioners like Fisher, the feedback arm is line up to the travel mark of the arm when the valve is fully open or close. Plastic alignment pin has to be inserted fist; if you are doing it as per the book.
 
The Positioner is a force-balance instrument that provides a control valve position proportional to a pneumatic input signal. The balance of opposing forces in the positioner occurs at the summing beam. One force applied to the summing beam is developed from the input signal pressure on the diaphragm. The other force is from the feedback spring and is proportional to the position of the feedback lever. When the input pressure is increased to the diaphragm of the input module, the diaphragm strokes down, increasing the effective force from the input module and compressing the feedback spring. The summing beam moves the spool down in the spool body, opening output port B to supply air to the left side of the actuator. At the same time, output port A of the spool valve opens, allowing the right side of the actuator to vent to atmosphere.

The piston in the actuator moves to the right, rotating the feedback shaft and cam counterclockwise. This rotation causes the feedback lever to rotate clockwise, increasing the compression on the feedback spring. These rotations continue until the additional force from the spring balances with the input module force on the summing beam. When the forces are equal, the summing beam returns to its steady state or neutral position and the actuator is held at a new position. U can have lot of information from the below URL

http://www.documentation.emersonprocess.com/groups/public/documents/bulletins/d200437x012.pdf
 
> A positioner without I/P converter is known as HART? Please give me some idea how it works.

It May be called as smart positioner...
 
Hi Ronald,

Thank you so much for the explanation.This really helps me up in understanding positioner better. I do keep in touch for further queries.

Thanks,
Akash
 
> Yes, you are right. it is helpful for me. thanks.

I have other question?

1. how to change DVC positioner online with Double acting Control valve (also valve has hand wheel)? is it possible to change positioner or isolate the control valve and then change(offline)?

2. please explain how to do calibration.
 
Question in reply to BC on 11 May, 2012 - 11:59 am??

Would you happen to have any written procedure for #1 below? This manual is hard to understand for #1 below.

1. The specific procedure I am looking is setting the range +(fwd/rev) , zero, span, flapper/nozzle.

2. D-RING procedure would be helpful too, but the manual seems to be decent on D-RING alignment.

I am pretty sure I have been instructed wrong for #1 above.

jmnyoung
 
Valve Body with plug is installed in the process line. That plug is operated up/down or in angular position by actuator. Actuator may be diaphragm type or cylinder type.

The actuator moves from 0-100% position in according to minimum and maximum pressure supply. This pressure is supplied by positioner.

If we see the flow path then DCS to IP convertor to positioner to Actuator to valve. In case the valve malfunctions due to positioner and you do not have spare positioner then you can also connect the output of IP convertor to the actuator of the actuator spring range is 3-15 PSI. One more point i want to discuss here is that if the calibration of IP is out then its better to change the IP rather than calibrate it.
 
One problem i'm facing calibrating with DVC6200 is when the valve is set to fully closed, there are often small pressure in the actuator left that i can't get fully shut off of the valve. the certain pressure that left in actuator at 0% will let the CV didn't achieve better seat leakage result. did anyone encounter this problem?
 
Hi,

was there an answer to this question? I have recently installed a Fisher 6205 / 6215 set on a plant critical system where we do not have the luxury of stroking the valve post installation. We understood that it was possible to copy the entire config over from one positioner to the next - particularly as the feedback device is a remote hall-effect and therefore the positioner has no physical connection to the process.

> 1. how to change DVC positioner online with Double acting Control valve (also
> valve has hand wheel)? is it possible to change positioner or isolate the control
> valve and then change(offline)?

> 2. please explain how to do calibration.
 
R
An actuator and a positioner are completely two different components meant to carry out different functions.

The trim portion consisting of a plug, seat and suitable sealing arrangement for leak tightness when the valve is closed 100%, is connected with a stem for opening the plug up or down to open or close the valve.

The valve stem is connected to an actuator. Actuator can be pneumatic, hydraulic or electrical. An electrical actuator is nothing but a bi-directional motor connected with a suitable gear mechanism to the valve stem. Depending on the direction and magnitude of the error, the controller whose output drives the motor enerigises either of the two relays (K1/K2 i.e., Open/Close) until the error becomes zero. The relay is then de-energised.

A hydraulic actuator is nothing but a cylinder actuator whose piston is coupled to the valve stem. Extension or retraction of the stem shall close or open the valve. The K1/K2 (open/close) relays explained earlier shall energise either of two solenoid valves meant to supply oil at pressure at either side of the piston depending on the direction of the error to extend or retract the piston.

Spring return actuators shall have only one solenoid valve which energises to supply oil pressure against the spring to extend the piston. On de-energising, the oil supply port is closed and oil in the cylinder is connected to drain (to the tank) to retract the piston on spring force.

A pneumatic actuator can also be of cylinder type as explained above. Dry instrument air being used as the medium instead of oil.

Another type of pneumatic actuator is the diaphragm type actuators in which air pressure on top of the diaphragm moves the stem connected to the centre of the diaphragm down against a spring force; pressure of the air applied again depending on the direction and magnitude of the error.

As explained actuator helps give the required command to position the valve as desired by the controller. How far and how well the desired position of the valve is achieved depends on factors like, inertia, friction between valve stem and gland packing, losses in transmission of the signal, static pressure or pressure of the fluid flowing in the process pipeline acting on the plug, etc.,

A valve positioner compares the signal (or, input) from the controller with the mechanical feedback from the valve stem and helps to balance / match them so that the desired valve position is achieved. As explained to another question in the forum, a valve without positioner can be compared to a open loop control while a valve with positioner is like, closed loop control.

A positioner can never do the role of an actuator. In other words, simply fixing a positioner to the valve stem isn't going to make the valve operable. It requires an actuator to actuate (open/close) a valve.
 
The guys have pretty much covered it all. as a shift instrument tech myself, i just wanted to give you a tip. most DCSs use an analog output of 24v to power a transmitter (rosemount preferably), and the transmitter acts as a sink and pulls voltage down depending on the output of the transmitter (3-15). if the gauge on the I/P happens to be broke and you don't have a handheld calibrator, you can use a volt meter and measure voltage across the leads landed on the pos and neg in the I/P. if it's in working order, you will read 1 to 5 volts DC. 1 volt being 4ma and 5 being 20 ma. just a quick way to see what you have if you're in a pinch, and always remember, you wont read 24v unless the leads are lifted.
 
I/P is basically working on the flapper and nozzle principle. And the positioner is working on the Forced balance principle I/P Converter , converts 4-20 mA signal from controller to .2 to 1.0 Kg/cm2 pneumatic signal.

Positioner consist of air supply port , signal input port, output port. When positioner get input signal, it compare with stem position and output is generated and signal boosted sufficiently to operate the valve. It uses force balance system.

It works on force balance principle. A coil is suspended in a magnetic field on a flexible mount. At the lower end of the coil is a flapper valve that operates against a precision ground nozzle to create a backpressure on the servo diaphragm of a booster relay. The input current flows in the coil and produces a force between the coil and the flapper valve, which controls the servo pressure and the output pressure.

Zero adjustment of the unit is made by turning a screw that regulates the distance between the flapper valve and the air nozzle. Span adjustment is made by varying a potentiometer, which shunts input current past the coil.

mA ---> Kg/Cm2 ---> % Valve Open

4 ---> 0.2 ---> 0%
8 0.4 25%
12 0.6 50%
16 0.8 75%
20 1.0 100%
 
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