We have found the exist of the valve stiction and quantized it. Then what should we do if we want to compensate it? The dithering signal and the so-called Knocker signal may work, but in that case, the life of the valve is shortened. Any other ways exist?
What is your application? Is the valve air or some other fluid? Is it controlling an actuator, or flow of some fluid?
One trick I have used is to ignore the stiction on the first cycle or two and then once it gets going the stiction effects sometimes aren't as prominent (This may not be true of all types of stiction on all devices). This relies on a regularly cycling valve and may not work well if the unpredictable nature of the first few cycles will cause you problems. If its just an actuator you may be able to figure out a place in the sequence where you can safely pulse the valve on an off several times on the very first cycle.
The valves are both air and fluid. I just want to make sure there may be ways to compensate the valve stiction. What you have said is useful to me, thanks for that.
Valve stiction is a physical problem, and is best resolved by a physical solution - not by a software fix.
Valve stiction is largely due to the frictional forces around the valve stem, primarily from the valve packing. Environmental concerns about fugitive emissions have exacerbated the problem, since many personnel will simply tighten the valve packing to try to reduce emissions. The resulting "stiction", combined with PI or PID control, means that the valve stick and slip, stick and slip.
As for solutions - I recommend that you FIRST perform a physical inspection of the valve & positioner. Check the tightness of valve packing, air supply pressure, air vents, positioner size, presence of sticky fluids, flow forces, etc. Note that many of these problems can be fixed even while the process is running. Adding a positioner or even a volume booster can help to reduce the size of the stiction cycle, but may not eliminate it.
The following software solutions may be useful, but are ONLY recommended if you cannot resolve the physical problem:
1. Dithering (overlay of a small, back-and-forth signal) may be useful if viscous/sticky fluids are the cause of the stiction. Note that dithering will not be helpful if the problem is due to air supply pressure or positioner sizing issues. I agree, this may result in increased valve wear.
2. Alternate control algorithm, such as an "integral deadband". This algorithm eliminates the integral action when the PV is within some deadband (say 1%) of SP. After all, it is the integral action on the controller that forces the back-and-forth stiction cycle.
For more information on valve diagnostics, follow this link:
-George Buckbee, P.E.
Hi, George, thanks for your useful suggestion.
Yes, valve stiction is a physical problem, so the best way to 'compensate' the valve stiction, which is also as you said, is a physical solution.
When we find the stiction, maintenance should be undertaken. However, it is often necessary to stop a major part of a production line in order to perform this maintenance. Therefore, the maintenance is often postponed until the next planned production stop, and the production is continued even after a detection of stick-slip motion. But, as a matter of fact, normal production stops are typically between every 6 months and 3 years. So the software solution may be a choice, even it would do harm to the valve.
Thanks very much for the alternate control algorithm and the link, I will have a look.
Did you try to find out what is causing valve sticion ? There may be several reasons - either the valve design inherently has high friction e.g. a high temperature tight leak packing design, or a tight leakage seal. Sometimes thermal expansion of the internals can cause friction.
Is stiction only a recent problem with this valve or has it been this way all throughout ?
Identifying the root cause and fixing it might be better rather than trying to compensate for it externally.
jronalddeepak [at] gmail.com
I've got to agree with George's comments.
However, certain valve actuators can have a user programmable hysteresis effect, giving a response that can look like the familiar stick-slip characteristic attributed to physical stiction. It is worth checking, as it could be a simple fix.
"Identifying the root cause and fixing it might be better rather than trying to compensate for it externally." I agree with you.
Thanks for your attention and suggestion.
First time I hear that word "stiction". I had to look it up to find out what you are talking about. Anyway English is my second language so it is to be expected I suppose.
Personally I have never had to use any special compensating software to overcome a sticking valve problem and I believe that is not the way do it. You need to RESOLVE the problem not just FIX it.
There are other ways to compensate for a sticking valve, you do not have to use special software. I will tell you about that once you have fixed the valve and actuator and then come back to us and said that the valve is still sticking after you have done everything as laid out below. The chances are that it will not be necessary since the procedures and checks below will solve most sticking and binding problems.
In most applications you should have a bypass in the line so removing/working on the valve should not be to much of a problem especially if you can justify that the bind is causing erratic control problem to the plant. The repair will not take more than a couple of hours if you get all the spares together before hand and prepare properly for the job. The bypass valve can be controlled by the production operators for those couple of hours so you should be able to do this repair without to many problems from the production department.
To first try and fix the sticking problem on the valve and actuator:
The best is to remove it and take it to the workshop but the repair can be done on site as well if you need to do that. These sticking or binding problems are normally caused by only a few things and you need to try and fix these first before you can say there are other reasons why the valve is sticking. The main cause are the packing that is to tight or damaged, so strip the valve and replace the packing with a new set and tighten the packing flange until hand tight and one turn with a spanner. Test the valve action by hand.
If this didn't work you need to go deeper and strip the valve completely and inspect the trim and see if all is well and that there are no chips or marks anywhere. Disconnect the plug from the stem and inspect it to see of it is straight, since that is another major cause of valve sticking. If possible replace the stem with a new one. If you need to use the old stem inspect it for marks and rough surfaces. Clean it and smooth it with a fine grinding paste if some areas are suspect. Running your hand over the entire surface will be a good enough test to test for smoothness of the stem. So the stem itself and the packing are two major causes for valve sticking. Spend some time on these two and get it right.
Other possibilities are dryness/straightness/roughness of the actuator piston stem. This stem can be tested by disconnecting the valve from the actuator and see where the bind is, in the actuator side or the valve side or both. Move both the valve and actuator stems by hand to test and fine out where the sticking problem is. Rust inside the actuator or on the actuator stem or even moisture inside the actuator are all possibilities that needs to be checked and repaired. The O-rings (perished) on the actuator piston/disk is another possibility, as well as the actuator stem guide in the actuator. If it is a diaphragm actuator you can inspect the diaphragm and replace it is it is very old and show sighs of cracks and hard surface areas in the rubber.
Basically all areas that will have friction when the valve is in operation needs to be checked, replaced, repaired or lubricated. Working the stems by hand (loosen yoke nut to release the spring if it is a FO or FC valve) is the simplest but the most effective way to test if the sticking problem still exist after your repairs so do that as a final check before connecting the valve and actuator again. Do some testing after wards with air and a regulator and move the connected valve (without the positioner) very slowly since that is when the sticking problem will be most apparent. Stop the movement and increase the air pressure very slowly until the valve starts to move and see is this movement is smooth or does it start to move by jumping to a new position. If it does the sticking problem has not been resolved yet, and you need to try again. The valve action need to be smooth from stop to just starting to move and once you have achieved that the sticking problem will be resolved as well. Remember not to use the positioner during these tests with the regulator.
Valve binding and sticking problems, in my experience, is 99% of the time a mechanical problem and not software related. If yours is, it will be the exception to the rule so get the mechanical side fixed first then start thinking about other possibilities if the problem still exist.
Hi Sam. thanks for your attention and suggestion.
The word stiction is a combination of the words STIck and friCTION, created to emphasize the difference between static and dynamic friction.
Stiction is the resistance to the start of motion, usually measured as the difference between the driving values required to overcome static friction upscale and down scale. Hope I say it clear.
What you have said is useful for me, and I really appreciate it.
I do not believe that the below derivation of the word "stiction" is correct:
"The word Stiction comes from Static + friction which is the force acting on stationary bodies."
I believe that it comes from the word combination which is somewhat descriptive of what is occurring:
Stiction = Stick + Friction
You can find out more about valve stiction at:
William (Bill) L. Mostia, Jr. PE
SIS-TECH Solutions, LP
Any information is provided on Caveat Emptor basis.
i am doing project work on stiction identification, quantification and compensation.
is there any idea to do it.
I'm busy doing research on valve faults. can anyone tell me how to create hysteresis on a fisher control valve?
Why not try a ceramic ball valve which is hard-seal,both of valve ball and seats are made of structure ceramics. The valve will not these problems with long service-life. Maybe you can refer to this link:
It can be last a long service-life.
If you haven't searched the site, take a look a similar thread from last March.