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.
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.
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
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.
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.
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
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.
I have a doubt. Suppose a control valve with a pneumatic positioner receives instrument signal from an IP. If the I to P is disturbed.. Then which is the better way
(1) To Calibrate the I/P with a master gauge separately and then calibrate the positioner
Or (2) calibrate the positioner first and then calibrate the I to P with respect to the positioner..
Please give me a reply..
Consider this, please.....the more error factors that exist within the system components, the greater the overall error will be within the complete system.
I see much very informed advice in this thread, but if you will indulge me a moment, I would like to take you back to some more basic elements of setting up a valve positioner.
First of all, set up the basic v/v and positioner alignment (mechanical alignment) and stroke length for the given v/v.
The safest way to do this is to disable the positioner's ability to control the valve, by removing its supply air and output tubing.
Connect a regulated air supply directly to the diaphragm of the valve. Slowly increase the air feed until the v/v is at 50% of its travel, and maintain it at this position.(Do not exceed the stated max working pressure for the diaphragm!)
Next , check the data plate on the valve for its stroke length. This figure will be used when setting up the Stem coupling drive pin on the positioner's feedback arm graduated scale(for example, it is typically given on a Kent P3300 positioner as being X mm, in multiples of 10mm).
With the v/v still in mid-position, carefully align the positioner feedback arm as horizontal as you can judge it to be, and engage the drive pin in the feeback arm slot , setting it at the required figure derived from the data plate.
Tighten the stem coupling arm, (use a suitably sized spanner to counter the movement of the stem coupling arm as you finally tighten up).
Finally, check the following:
a) that the v/v stem is still at mid-position,
b) that the positioner feedback arm is horizontal
c) that the drive pin is still located in the feedback arm drive slot at the required length as above.
Basic alignment is now complete, and you may remove the regulator and tubing which you used, and re-instate the positioner air supply and output tubing to its original set-up
Next, check and adjust I to P calibration
Finally, calibrate the positioner to your requirements, (assuming full range linear application, a 4 to 20mA input producing 0 to 100% travel)
Check performance/linearity and repeatability in 25% increments upscale and downscale.
Bingo...job done,with all sources of error having been eliminated.(assuming correct cam profile etc are set)
The short of it is , that the more accurately you set up the individual components, then the more accurate will be the overall result
Hope this is of help to you !
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.
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.
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.
Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
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?
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.
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.
> 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...
The positioner is designed for controlling control valves by producing analog pneumatic signal to control the percentage of opening or closing operation for valve.
Inputs to positioner
Air supply (feeding of positioner)
Control pneumatic signal: it comes from I/P equipment
Output: pneumatic signal
Air supply: 35 psi
Control signal: 3-15 psi
Output : 6-30 psi
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.
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")
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.
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.
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.
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:
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.
jronalddeepak [at] gmail.com
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.
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.
What George said earlier is true. The positioner is in fact working in a cascade (inner) loop while the DCS is controlling the process variable (flow/pressure/level,etc.) From what I know, the positioners in fact have integral gains, so the error in the main process variable is not accumulated. However, like I mentioned earlier, there is an integral dead-zone for the valve travel, so the integral action is disabled when travel varies within this zone outside the set-point. So practically, the integral gain is almost non-existent in most positioners because the dead-zone is set high enough so that the valve position does not oscillate.
However, I do not quite agree to the comment about the usage of positioners in fast loops. At present, positioners are used in all kinds of loops and I don't see any problem with using a correctly sized valve and optimally tuned positioner in any kind of flow loop.
To answer your question about pressure drop :
The flowing pressure drop is used for Cv calculation. You might be aware that
Cv (approx.) = Q * Sqrt(SG/dP)
Where, Q is the vol. flow rate, SG is fluid specific gravity and dP is the pressure drop. In case of gases/vapors, the equation is different -dP is replaced by (dP/P1).
The shut-off pressure drop is used in actuator sizing. Like you said, the shut-off pressure drop occurs when the valve is fully closed. So this value is used in calculating the total actuator thrust required to keep the valve closed, because the actuator has to keep the valve closed opposing this pressure drop, in addition to providing the desired leakage class and overcoming friction.
In short, flowing pressure drop is used for Cv calculation and subsequently for sizing the valve. Shut-off pressure drop is used for sizing the actuator.
Hope this helps. If you still have questions, please let me know.
jronalddeepak [at] gmail.com
> 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.
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.
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.
hlwade [at] aol.com
The problem I have is in understanding what I call, the resolution of the unit. How much of a change in current does it take to create actual movement for the positioner, and what is the specification that defines this? Every vendor defines their product by what appears to be whatever spec puts it in the best light. Yes, they do, for the most part, use common terminology, and maybe I'm just stupid, but none of the spec's clearly identifies the 'number of step changes' available for a given 4-20mA signal.
mshopkins [at] dow.com
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?
Thank you so much for the explanation.This really helps me up in understanding positioner better. I do keep in touch for further queries.
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.
Quite correct Dilip...as we say in the North Sea,
"There is no such thing as a daft question...there is only the daftie (fool), who doesn't ask it !"
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..
Thanks a lot for pointing it out. I used the term %opening because it is frequently used these days by valve vendors and end-users to refer to % travel. In fact, the more appropriate term is % travel (this is the term that is used in ISA specification sheets for control valve).
However, I didn't refer to the actual flowing area or the effective opening inside the valve that is available for flow. Only the total effective flow area depends on the valve characteristic.
I would like to answer your questions. First of all, when it comes to the issue of resolution, it is more appropriate to talk about the resolution of the entire control valve assembly (valve,actuator & positioner)than that of just the positioner alone. With the advent of smart digital positioners, the valve&actuator assembly is the one that limits the resolution of the entire unit, not the positioner.
Like you mentioned, the actual movement of a valve in response to a control signal will not be a smooth ramp, but in 'steps'. The question is what is the minimum step change a valve exhibits (this defines the resolution)? Friction inside the valve tends to 'hold' the valve or make the valve stem "stick". Stem packing, seal rings, Actuator O-rings - all of these account for friction and subsequently contribute to dead-band. When there is a significant difference in static & dynamic friction values, the valve tends to exhibit "stick-slip" motion and this affects resolution. In the case of large size valves, the valve & actuator internal components like valve plug/stem, actuator piston (in the case of piston actuators)themselves are very heavy and the sheer forces involved in moving them, in addition to the friction components already mentioned play a significant role in determining the response of the valve.
Having said all this, today's positioners are tuned to handle such large valve constructions to meet typical resolution requirements. Yes, there is no universal specification that specifies an acceptable resolution for a control valve assembly. But several End-Users and Engineering contractors specify the resolution for valve response. Some applications like compressor anti-surge control require precise response to control signals and accordingly the resolution is specified. Most valve assemblies offered these days have the ability to respond to control signal changes as small as 1%.
When highly precise control is required, for ex., in applications like pulp flow control to the paper machine in the pulp & paper industry, basis weight control valves with Electric actuators are used. These valve/actuator assemblies have very high resolutions (somewhere in the range of 10,000 steps for a 0-100% control signal span i.e, a resolution of 0.01%).
However, the resolutions offered by standard globe valves with pneumatic actuators are good enough to handle most other applications encountered across process industries.
Hope this helps.
jronalddeepak [at] gmail.com
How do one calculate the actual air consumption of the positioner?
I know that the amount of air used in general for sizing is approx 0.6 scfm to 1 scfm.
But the actual consumption varies for different manufacturer. What data do one need in order to calculate the actual air consumption of a valve positioner? and how to calculate it.
Thanks in advance
Yes, there is a big difference in air consumption between manufacturers because there are a number of designs, most of which I think got installed in our power plant. We've got 'em all.
The Siemens and ABB digital positioners move the control valve using fast acting valves, similar to fuel injectors. You can hear them rapidly fire their valves when they're maintaining a control valve in a position if the control signal varies or if the valve's pneumatic actuator is leaking.
The advantage of this design is that air consumption goes to zero in an air-tight system, unlike the old nozzle and flapper types that use far more air, particularly when they're applying zero pressure to the actuator. It doesn't shut off the air supply to the actuator--it vents it off.
We've gotten years of reliable service out of the ABB's in indoor applications on small valves that aren't subjected to vibration or water hammer. The Siemens PS2's weren't so fortunate and we've had to replace them in harsh applications. They do offer an upgrade for harsh duty, but we I&C types tend to cull out the weak in favor of the reliable.
We've gotten good, reliable performance out of Masoneilan SVII's on single acting actuators. They use pneumatic relays which use more air and are less precise than the ABB's or Siemens but they make a lot of mounting brackets for a variety of control valves, and that means a lot for the techs who have to install and calibrate them.
One of our plants had problems with double acting SVIIs on drum level control which were never resolved. They were replaced with Flowserves' Logix positioners that are easy to configure, reliable and reasonably priced. They're a definite favorite.
We're impressed with all the gee-wiz firmware and software available that gives you advanced diagnostics and a zillion configurable parameters but for all of its 50 plus parameters on board the PSII's, we prefer the simplicity of the Logix and we haven't got time to play with all the bells and whistles. If a positioner is causing process problems and can't quickly be corrected, it gets replaced, not studied.
PMV's are primitve by todays standards but the only thing that's gone out in 10 years of reliable service on several of them is their position feedback to the DCS which isn't critical.
Fisher DVC 5000's are one of the few that are accepted in nukes because they're rock solid, too, but overpriced and are designed to fit their valves. When applied to non-Fisher products, they tend to require a lot of levers to translate stem travel to rotary motion which creates a lot of linkage slop. We've replaced those with Logix.
Unfortunately, we've got other manufacturer's positioners such as STI on CCI DRAG valves (very robust but difficult to callibrate) to Siemens nozzel/flapper whatever. They all have their pros and cons, but do us maintenance guys a favor: standardize when possible and don't let the gee-wiz factor sway you unless you have lots of play time.
Thanks for the effort, very interesting read.
When you mentioned Siemens PS2 I all off a sudden felt sick. Biggest crap the world have ever seen.
Made me think about how some people went on in another thread about Siemens and their very advanced technological knowledge and better than most PLC's(S7).Might be true but then they should stick to PLC's and stay away from trying to design SMART positioners.
In contrast the Fisher DVC 6000 series from Fisher is the best I ever worked on. We mounted them on Valtech valves as well without any problems.
I suggest you look at Quick Stepper valve positioner at www.processcomponent.se.
It has ZERO consumption and manual mechanical override! Free diagnostic software etc.
I have used the PS1 and PS2 positioners. I recently went through a process of standardizing basic service positioners across a site. Had 6 different types of positioners due to various sections of plant being package supplied. Siemens won as e PS2 was comparable to the DVC5000 on cost and ease of set up - doesn't require a 375. Diagnostics is still new to us but we looking to start utilizing existing diagnostic packages to aid in determining maintenance strategy for valves.
I'm doing a research project on 4 positioners.,two smart which are the Fisher DVC6200 and the Masoneilan SVI IIAP. I'm also doing Fisher 3610J and MAsoneilan 4700P for pneumatic positioners. I was wondering if anyone could give me any insight on what applications pneumatic positioners would be best for. Also i would like to know out of four positioners what one would you recommend for tight shut off applications and throttling.
Both smart valve positioners are available with either HART or Foundation Fieldbus input and are the most adaptable choices. Pure pneumatic positioners with 3-15psi air pressure input can be used with DCS or electronic controllers if a I-to-P (I/P) converter is used to convert the electronic 4-20ma signal from the controller to the requisite 3-15psi air pressure. The only reason I can think of where I would use a pneumatic positioner is where there is an explosive gas mixture present at all times such as in chemical plants making ether or acetylene. Otherwise, smart positioners are always preferred unless their higher cost is a problem. The purpose of all control valves is to throttle the process flow over a narrow range of flow rates.
Tight shutoff cannot be expected with any control valve; the valve itself determines if tight shutoff can be achieved not the positioner; and globe valves used for control are not designed for tight shutoff. If you need tight shutoff, you should use solenoid operated quarter-turn ball valves that are unsuitable for control purposes.
Tight shutoff for conventional pneumatic positioners with 4-20 mA is a functionality driven by the control system it is connected. For eg: if tight shutoff needs to be done the control system will deliver 3.6 mA or sightly less and for full open the current will be 21mA
if you take the case of foundation fieldbus positioners this functionality resides in the positioner and is tuned by appropriately making changes in the parameters of TB01 (tranducer01 block)
Not sure I thinks it can be done by changing
- final cutoff value_HI =110
- final cutoff value_LO =-10
- Stop value_HI =110
- Stop value_LO =-10
Coming back to throttle / Stroke
you can throttle conventional positioner, if you feed mA from 4to 20(4mA=0%,8=25%,12=50%....)
Foundation fieldbus positioners - its done by digital communication and needs a host to execute the commands
Thank you for the insight for what applications would best be suited for pneumatic positioners as it is helping me with my report. With this report i am basically trying to decide and convince my college faculty which positioner is for best for use out in the field and why. This is between the DVC6200 by Fisher and the Masoneilan SVI II Ap. If there is any opinion or features that will help defend my argument that the Masoneilan SVI II AP is better it would help greatly.
Also when looking at the calibration of the DVC6200 i noticed there are two modes of calibration which are travel calibration which consists of manual and auto and sensor calibration. The DVC6200 uses a HART communicator for these modes of calibration. The SVI II AP by Masoneilan uses push buttons and a LCD display for calibration and also can incorporate the HART communicator like the DVC6200. Does this feature give the positioner an advantage over the DVC6200
Also if there are any other advantage or comparisons between the SVI II AP and the DVC6200 that i should be aware of it would help out very much.
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?
Hello to all!
First off: there has been some great discussion with some very qualified individuals in this thread.
I currently work in the nuclear field with the title Field Service Engineer on AOVs. The main advice I can offer to newbies is to tinker with all types of positioners on a bench. Have a qualified individual nearby to assist you if you need it and to also check your work.
In my line of work, we have realized that on a 3-15 positioner, setting the values to 3.2 and 14.8 Psi ends up being the optimal setpoints for an AOV.
3.2psi meaning the diaphragm is completely bled off at this point and valve travel has ceased.
14.8 means the the diaphragm is saturating with air and the valve travel has reached its peak and cannot move anymore.
Also, if you are working with Fisher positioners (3582) and they are straight out of the box, more than likely a beam alignment will need to be performed before calibrating the ZERO and SPAN setpoints. Also, Adjust your span before adjusting the zero. A 3582 positioner is different than most instrumentation and adjusting the span before the zero will eliminate excess time spent calibrating the positioner.
As to Chris's question: the nuclear field in the states uses mostly Fisher and ABB brand positioners. DVCs are making there way but only on the secondary side.
I presume the Fisher 3582 over just about any other positioner, except when I'm dealing with dual acting actuators (Piston actuators). Then an ABB Baileys is a great instrument for the configuration at hand.
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.
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.
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
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%