Control valve positioner

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Thread Starter

Hemanth

Hi, I am a fresher and just joined Essar, would like to know the operation of positioner. Like 4 to 20 milli amps is converted to 3 to 15 PSI by I/P. this 3 to 15 psi is the control signal for positioner, I would like to know the exact working as I am into projects.
 
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Abhijit Goswami, Haldia

A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. An I/P converter only sends the opening/closing request to valve but can not confirm its position.

Positioner senses the valve opening through a position feedback link connected to valve stem which is its input signal. I/P converter output is its setpoint input. The difference between these two is the error signal based on which the positioner positions the valve to correct position to reduce error to zero. Hence positioner is nothing but a pneumatic feedback controller. Controlled external supply air to positioner provides power to positioner to position a valve. Also positioner is used in a valve when valve operating signal range is different from I/P converter output range.

In recent days software configurable digital positioner are being used in valves which do not require I/P converter and has many features like advanced valve diagnostics, partial stroke testing, remote communication etc.
 
Dear Hemanth,

Without you knowing the concept of Postioner, who selected you in ESSAR? Pls return to your college and study once again and join... I think it would be better for you. MR

Bye.
 
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Ashwanth Narain

Dear Mr.Anonymous person,

Nobody knows everything (even if it is basics)

If some body needs to go back to college, if he/she does not know a basic concept, then everybody will be sitting in college

What is really required, is only the attitude,
to learn and keep learning
 
Dear Mr.Anonymous person,
you are right with your question i learn detail about the positioner usally i use to only calibrate now i know the detail thankyou please do share your knowledge and problem you face during calibration i will love to read it.
 
The positioner is designed for controlling control valves by producing analog pneumatic signal to control the percentage of opening or closing operation for valve.

Example:
Inputs to positioner
Air supply (feeding of positioner)
Control pneumatic signal: it comes from I/P equipment

Output: pneumatic signal

Numerical example:
Air supply: 35 psi
Control signal: 3-15 psi
Output : 6-30 psi
 
A valve positioner is a device which exactly positions a control valve to the required position as per the signal given to it.

For example, if a process change requires the valve to open/close a further 10%, and accordingly a 1.2 psi change has been given in the signal to the valve diaphragm (without a positioner), there's no guarantee that the command is obliged. Due to factors like valve friction, static pressure, etc., stem may assume only a 7% or 8% change in its position and thereby, an error in control.

Whereas in case of a valve with a positioner, the signal from the controller is given to the positioner. The positioner then compares it with current valve position and gives an output suitable to the valve diaphragm until the desired position is achieved. The output from the positioner stabilizes when the desired position (signal matches feedback from valve stem) is achieved.

In short, a valve with positioner means a closed loop control (with feedback) and without it means an open loop control.
 
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I stumbled upon this website in search of some knowledge, and I just want to thank you for your explanation. It helped to find the equivalent in my language.
 
Mr. Anonymous
Scientist Newton was debarred from school as he used to remain mentally absent, But during your studies you learnt lot from his inventions and you know why freshers are not appointed as General manager? Because they do not have practical experience. It not possible to know every thing in the world, and no Man is PERFECT in the world. And this is right time for him he can ask silly question (silly for Me & You). But this is the way he is marching towards PERFECTION.
Thank you
CPPguy
 
well, that was a harsh comment, if you want to learn more specific for the use of an valve positioner, check fisher website at emerson.com find brand fisher. because as far as i know they have an good explanation for the use of that thing. and as an addition an pneumatic positioner who not use I/P converter (or not installed) are have other use as amplifier signal (pneumatic) for the actuator. since some of the actuator are actually doesn't use exact 3-15 psi for moving the diaphragm or piston (for some valve actuator they even installed an pressure accumulator tank), because the needs for higher pressure to actuated it and some has characterizable function such as for valve that have quick opening, equal precentage or linier. well i forgot the definiton of the characteristic above but the point is the positioner are have more usable function than just an feedback control for the control valve. same as the pneumatic the "modern" positioner also have some other function who already mention by others gentlemen at this topics.

plus the HART and Fieldbuses Protocols who made it easier (and looks very cool if you are configuring an control valve from remote desktop or laptop) to monitor, configuring, troubleshooting, even calibrates the position of the valve. to make it sure, you still need to go on sites or do it bench test. so go find details about the positioner, and don't shame if you "unfamiliar" with that thing (to be honest, until now I cannot even understand what is "Facebook")
 
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Ronald Deepak

Dear All,

I work in the Fisher control valves division of Emerson.

I would like to explain few things about the working of a positioner.

The positioner is a device mounted on a control valve that receives control signal from a DCS or any host system. The signal can be a 4-20mA/HART/Fieldbus, etc. The positioner receives the signal and understands the desired (target) position of the valve.

E.g.,A positioner working on a 4-20mA signal range receives a 12mA means the valve has to be positioned at 50% open. Without a positioner, the valve might not be positioned at 50% due to several factors such as fluid forces, friction, etc. The positioner sends pressure to the actuator in order to position the valve at 50%. The positioner is also physically connected with the valve stem, so it recieves feedback about the current position of the valve. Based on the feedback, the positioner adjusts the output to the actuator if required. In short, the ultimate function of the positioner is to ensure that the Desired opening of the valve is achieved in response to the control signal received from the host system.

Advanced positioners which are often referred to as SMART positioners have several other useful features that improve the performance. They have the abiliity to transmit diagnostic data about the valve back to the host system.

Positioners also have tuning features that allow users to set gain values based on the required performance criteria like stroking time, over-shoot, etc.

Positioners used in special applications like Compressor anti-surge control , Emergency shut-down valves, etc. have added features which are essential to meet the performance specifications in those applications.

I hope this helps. If you would have any specific questions regarding a control valve Positioner, please contact me.

Ronald Deepak
jronalddeepak [at] gmail.com
 
Is it a proportional controller or a PI controller. Somewhere I read that it is a P only controller. This would mean that there will be an offset associated with it like every other P only controller. So how the Offset is being removed.

Another question:

For spring diaphragm actuators: whether the positioner output is proportional to 0.2 to 1 kg signal when the spring range is different for example 1-4 kg output. For example 0.2 signal corresponds to 1 and 1 kg signal corresponds to 4 kg output.
 
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George Buckbee

Most positioners are P-only controllers, usually with a fairly large gain. This provides enough kick to the actuator to overcome valve restriction.

Keep in mind that the positioner is effectively the inner/slave loop in a cascade control scheme. The integral action comes from the outer control loop, which will keep adjusting the desired valve position to overcome any offset with the process variable (i.e. temperature).

For good control with a cascade system, you must ensure that the inner and outer (slave and master) control loops have different response times. This has 2 major implications:

1. Do not apply positioners on very fast control loops (i.e. most flow loops).

2. Ensure that the outer loop tuning is not overly aggressive, as this could result in interaction (cycling) of the controls.

I will be hosting two webinars on Cascade control tomorrow, Oct. 27. These are free 60-minute webinars, and all are welcome to register at:

http://www.expertune.com/r2.asp?f=AList&l=webinarcascade.asp

-George
 
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Ronald Deepak

Yes, you are right. The positioner in fact has an integral gain component. However, there is an dead zone within which the valve travel is allowed to vary. The dead zone can typically be 1%. Only if the travel varies outside of this zone, the error is integrated. This is done in order to avoid travel oscillation.

To answer your second question, the span of air pressure for which the valve moves from 0% to 100% open is referred to as "BENCHSET". The benchset defines the range of pressure to move the actuator spring between 0 and 100% valve travel (without considering valve forces). The benchset is arrived at, when actuator sizing is performed. When the positioner is mounted on to the valve/actuator assembly and calibrated, the positioner understands the end-points of valve travel by moving the valve from 0 to 100% and measuring the pressures sent to the actuator corresponding to the end points (0 & 100).

In short, the pressure output from the positioner to a Spring & Diaphragm actuator is not fixed, but rather depends on the spring being used.

A simple equation relating the pressure span and spring rate is as follows :

(Pu - Pl)*A = K*T

where (Pu-Pl) - pressure span (psig)
Pu - Upper benchset pressure (psig)
Pl - Lower benchset pressure (psig)
A - Actuator diaphragm area (in2)
K - Actuator spring rate (lb/in)
T - Travel (in)

Hope this helps.

Please let me know if you have questions.

Ronald Deepak
jronalddeepak [at] gmail.com
 
There is nothing wrong in asking a basic question if you that, you do not know much about it. Since you are in project, you must understand basic question, otherwise it may cost your company a lot in later stage. So keep asking questions, don't keep mum.
 
Hello,
now, I am confused. One reply stated that positioner is a P-controller only, while other reply stated that it has only integral gain.
Which one is true?

I think that it must have an integral component to ensure that set point from outer loop is achieved.
 
Deepak

since you are from the CV industry some more questions for U.

What is the effect of DP across the valve on CV/ Actuator sizing.
Whether DP and DP-shut off of the valve are different. In my opinion DP across the valve is at the normal flow and DP -shut off obviously at valve fully closed position. But why then these 02 things are normally included in CV data sheets. I also hope that these 2 values are used only for actuator sizing and not the valve part.

Regards
NIK
 
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hi hemanth,

i am also a fresher now joined in a power plant. your questions help me to understand the basics. thanks for every one who appreciates freshers like me and hemanth.

and Mr.Ananymous, guide us if you like. otherwise don't demotivate us..
 
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Ronald,

Your reply is excellent, but I suggest one slight change in wording. I think you should not use the terms "valve opening" and "stem position" interchangeably. A signal from the controller of 50% is a request for a 50% stem position, not 50% valve opening, since the valve opening must also take into account the valve characteristics, which is not the subject of this thread, but may be a good subject for another thread.

Harold Wade
hlwade [at] aol.com
 
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