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Ortiv
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Join Date: May 2010
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Practical Significance of a Valve Positioner

07/11/2010 1:12 AM

Hi guys,

I have come across valve positioners in a plant and have read about them but can't seem to understand their practical significance. Need someone to explain it for me.
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Mikerho
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#1

Re: Practical Significance of a valve positioner

07/11/2010 4:17 AM

Hi Ortiv,

Consider a process where you must control the flowrate of a liquid into a vessel. It has been determined that the flowrate must be 100 litres/hour. The system has been set up with a controller that will monitor the flowrate and send a signal to a valve positioner of a control valve to achieve that exact flowrate. If the flowrate is only 90 litres/hour, the controller reads this and consequently sends a signal to the valve positioner to open the control valve more so that the flowrate will increase to achieve the required 100 litres/hour.

Does that help? If so, please click on the "Rate" button and give me a "Good Answer".

Mike

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Ortiv
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#2
In reply to #1

Re: Practical Significance of a valve positioner

07/11/2010 7:54 AM

Hi Mike, Thanks for your answer. You have taken me to a new level of understanding with regards to this topic. In line with your answer, I would like to ask what the effect of removing the positioner will be ie metering instrument air directly to the diaphram. Ortiv
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Carl_E
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#3
In reply to #2

Re: Practical Significance of a valve positioner

07/11/2010 9:24 AM

I would agree that a controller calculates an output to drive a valve to compensate to a given position to satisfy a controller setpoint. but there are reasons why the flow might not achieve a given value even though the valve at a certain position could provide that flow rate.

A controller sending an electrical 4-20mA control signal to an I/P which converts the 4-20mA to 3-15 psi (air pressure) can directly drive an actuator diaphragm on a modulating valve. The HVAC people do this, on millions of valves. The actuator and the valve have to be sized for this operation (3-15 psi) and be able to close against the line pressure. However, doing so provides no feedback as to the actual valve position; it is assumed that the valve position gets to its control position. Cooling water and low pressure steam service valves can provide adequate service in many cases without a positioner.

In the process world, however, there are line pressures, environmental conditions, medium conditions and control requirements such that 3-15 psi (or some other respanned I/P output) to an acutator will fail to drive the valve to the position it should be at. Reasons are generally called "sticktion" a combination of sticking and friction, that prevent the valve stem from moving to the position that an applied I/P pressure should take it to. Sometimes extra push is needed to get the valve stem where it belongs.

A position uses mechanical linkage to sense the position of the valve stem, uses this position as feedback to apply whatever force necessary (up to the limits of the supply) to get the valve position to equal the control system demand point.

A positioner is a valve position controller. Its setpoint is the controller's demand signal. Its process variable is the actual valve position. The position's output applies pressure to the actuator to get demand = valve position; SP = PV.

As to the impact of removing a positioner, something has to meter air to the valve actuator. Just using an I/P air signal reflecting the output of the controller could as simple as the loss of the improved control accuracy of the positioner due to sticktion.

Or it could be more drastic, failure to move the valve throughout some or much of its range because the I/P isn't scaled to overcome the valve spring force. I/P has to be matched to the valve spring/acutator assembly, just like a positioner does. The modern smart positioners 'self-learn', the older pneumatics required a bit of tweaking.

Some control systems are wired and configured to expect feedback from a positioner. The removal of a positioner providing feedback would leave the control system blind at that point.
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Mikerho
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#4
In reply to #3

Re: Practical Significance of a valve positioner

07/11/2010 1:40 PM

Carl,

Very good answer. Thanks for posting!

Mike

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LAA_Lucke
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#5
In reply to #2

Re: Practical Significance of a valve positioner

07/12/2010 3:53 AM

You should have given a rating to Mike (GA---yes ?) since he answered your question correctly.

on your second question: removing the air supply from the actuator (I presume the valve positioner (= actuator) is air pressure controlled), will either open the valve fully or close it completely!

Feed back means that you get an image of the process value your valve is controlling (whether fluid flow or fluid temperature etc) and send it back into a controller {usually, a voltage or a current value proportional to it}, (different types but most common are Proportional Integral differential as PID) where it will be processed and a signal is sent to move the actuator one way or the opposite to increase or decrease the value you are monitoring.

Maybe the above is very new to you but then you need to read some books or the instruction manuals to get more grasp of the subject.
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HiTekRedNek
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#6
In reply to #5

Re: Practical Significance of a valve positioner

07/12/2010 8:54 AM

One could consider a positioner as a high pressure output relay that uses the actual physical position of the valve stem as feedback.

3-15 psi input to a positioner could equal up to the supply pressure out, depending on valve stem position.A positioner gives a quicker more precise output than just an I/P transducer alone.

One calibrates a valve to be 100 percent open at 15 psi,and fully closed at 3 Psi.

The I/P is calibrated to 4/20 Millamps = 3-15 psi, respectively.A 50 percent output to the I/P would be 12 milliamps, outputting 9 Psi, and should equal 50% travel on the valve.IF everything works according to plan, in a perfect linear system,this would equal a 50% flow thru the valve.However, in the real world, when 9 psi is applied, the valve must first build pressure at the 9 psi pressure to get to the 50 % position.This takes time,especially on long runs and small(1/4)" tubing.The positioner, however, is very low volume on the input, and can respond immediately to any change in command value, and in turn, can supply whatever pressure is required to achive the desired position of the valve.The result is improved performance of the control loop by reducing delays in the pneumatic system and compensating for any small leaks in diaphram,or sticking of seals.The PID control function is easier to tune by reducing this delay and other variables that may occur over time, such as wear on the shaft stem.This is especially important on flow control where things happen very fast.(High " P" (low gain) High I(reset) and medium D *(preact)).(The pneumatic terms are included for reference).

Postioners are seldom used on slow processes, like temperature control of vats, etc.

Hope this is helpful.

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Mikerho
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#7
In reply to #5

Re: Practical Significance of a valve positioner

07/12/2010 9:22 AM

You should have given a rating to Mike (GA---yes ?) since he answered your question correctly.

He did, I saw it there yesterday, but it looks like someone else snatched it away. I have no problem with that if someone really feels that way. What I don't like is that they can do this anonymously. This feels mean-spirited and malicious.

Anyway, good input, and good detail!

Mike

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#8
In reply to #5

Re: Practical Significance of a valve positioner

07/12/2010 4:56 PM

It appears you also need to do furthur study on positioners and control theory.
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LAA_Lucke
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#9
In reply to #8

Re: Practical Significance of a valve positioner

07/12/2010 7:13 PM

I don't know who this guest is.?

In any case, on what are you basing your statement?

I do not pretend that I know everything about positioning control, but can guide and answer specific questions when properly asked and defined (I could even let you know that " I don't know" an answer to your case).

But for you to hide behind a wall (Guest) and give an uneducated opinion, leaves much to be desired about you. Have you designed or handled anything to do with positioners and their controls? if yes please educate us by just giving your opinion and show your face first.
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Anonymous Poster
#11
In reply to #5

Re: Practical Significance of a valve positioner

07/12/2010 9:18 PM

Actually, feedback is a representative sample of the output that is monitored by the controller to compare to the setpoint.The feedback can be in the form of air pressure, electrical signal or many other variable forms.

The controller responds to the deviation from setpoint by generating an output signal to correct the deviation with as little overshoot or undershoot as possible.

A proportional controller will always incur an offset betweeen setpoint and process.(P)

A 2 mode control (P+I) applies an output after the initial proportional output to drive the process back to the setpoint to eliminate the offset, however, it is slow to respond to a rapid change in the deviation.

A 3 mode control(Proportional, Integral and Derivative)(not Differential) responds also to the rate of change of the process.A faster change yields a faster response.

When properly applied in the right combination, the result is a process that will respond quickly to an upset with minimal overshoot, undershoot, and no remaining offset.

Many methods of loop tuning exist, icluding 1/4 wave decay, sustained oscillation, etc. to mention only a few.But the end result is the same.

Most modern electronic controllers are self tuning, but even they can benefit from some good starting numbers given by an experienced loop tuning instrument technician.This reduces the time needed for the loops to self tune.

As for a valve positioner, it can be considerd an amplifier at the valve itself.A diaphragm valve may require many cubic inches of volume to move the valve stem.If the controller is 300 feet away, this can take a while.This can introduce a delay in response above and beyond the process variables themselves.A positioner has a very small volume for input signal, thus can respond very quickly to commands from the controller.It can apply whatever force is available(supply air pressure) to move the valve to the proper position regardless of external friction(ice on shaft, sticky seals, etc).

In some cases, when the valve must close quickly for emergency situations, a quick exhaust valve may be added.If it senses a sudden decrease in input pressure, it dumps the air from the diaphragm thru a large port, closing the valve almost instantly.

Hope my appearing as guest does not offend anyone, but it is not always convenient to log in.I do not want to risk my password in public places.

HTRN
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LAA_Lucke
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#13
In reply to #11

Re: Practical Significance of a valve positioner

07/13/2010 11:09 AM

Thank you for correcting The D definition but believe me it was an error only. I know that it should be Derivative.

On the other hand, you are welcome to explain in detail the way you did and it will be beneficial for whoever wanted to know more about the subject.

What annoyed me is you barging in and judging someone's competence without knowing his back ground: I did not ask for an explanation about servo controls or open or closed loop controls or about actuators or positioners of different type for you to send me about gathering more lectures etc. Suffice it to say that i am sure that I can design and trouble shoot very complicated systems as you have described.

The point here is not to lecture but to give answers and guide someone who asks about a specific issue. He can then elaborate or request more info but eventually, he should seek further education in a proper institution etc if he so wishes, without being bullied or treated with contempt.

I do not see the point about you wanting to remain anonimous for the reason you stated: We all take the same risk (which is not so critical). What is the point of registering if you still want to remain anonimous. Anyway this is your prerogative and you are free to do so.

Thank you for coming back and hope you will be more co-operative ...
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Ortiv
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Join Date: May 2010
Posts: 8
#10

Re: Practical Significance of a Valve Positioner

07/12/2010 7:57 PM

Much appreciation to you all for your posts. You have aided my understanding of positioners in no small measure. Please keep up the good work, even when distractions come in the form of unwelcome 'guests'.
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wailinharp
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#12
In reply to #10

Re: Practical Significance of a Valve Positioner

07/13/2010 3:08 AM

As a retired instrument tech. I can say you have received some of the best info. possible from all those posted, especially our guest. But my opinion for what it's worth - is the question is much too generic for an accurate response, especially in regards to safety. Valve Positioners over the years have served many,many functions. The Options offered in regards to specific pressures, valve seat arrangements, and the process being controlled are way too numerous to get into. They have also at times been very manafacturer specific. Please look very closely at the job this particular valve and positioner's (components) are meant to perform before making modifications, because often there is much more than one might initially see involved.

Best of Luck
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Mikerho
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#14
In reply to #12

Re: Practical Significance of a Valve Positioner

07/13/2010 11:28 PM

Hi wailinharp,

Good Answer. I agree that different positioner/actuator setups are almost as numerous as their use in different applications. Thanks for bringing your perspective to the table!

Welcome to CR4!

Are you a harmonica player, by chance?

Mike

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wailinharp
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#15
In reply to #14

Re: Practical Significance of a Valve Positioner

07/14/2010 1:59 AM

Most definitely harp player.......35+years. Play harp,brass,and some vocals with a group on the west coast. Blues and classic rock. You?

Not sure how to respond except off topic.....so excuse me.
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Anonymous Poster (2); Carl_E (1); HiTekRedNek (1); LAA_Lucke (3); Mikerho (4); Ortiv (2); wailinharp (2)

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