Control valve hunting

Apart from control tuning, another problem may be erosion or "wire-drawing" in the valve parts, so that it can no longer modulate smoothly.

ive did travel stabilize/optimise but still same. I dont think thrs nothing to do with the wire-drawing. only around 2% opening valve is hunting at 3% valve is stable. what is it in my mind is maybe due to HP steam that 2% opening cannot hold the pesssure plus steam temp.

Most flow control valves are unstable at low percentage because the gain of the valve is much higher than at larger opening. To tune it properly to operate at 2%, you must reduce the gains that you obtained while tuning at higher percentage.

Once you make it stable at 2%, you will have a very sluggish response at higher opening. This is the nature of the beast unless you use a specially linearized valve aperture.

We usually avoid operating flow controllers at less then 5-10% of valve opening. If we do, we usually install a smaller control valve in parallel to do the fine control while the large one is closed.

Is the valve sized properly for the application?

Is the controller PID, have you tuned the controller?

Hi julius_8,

If your process requires the component supplied by the valve when the valve is open around 2%, it is way oversized - Ace Boeringa was on to something. The valve cannot operate well in this region because the errors at this level will create a very wide range of read errors in your Process Variable.

Sizing of the valve should be optimized for at least 50% open operation. What I mean by this is that; if your process requires around 50 litres/minute, your valve should be capable of providing 0 - 100 litres/minute (this is an example).

It may be that you need to acquire another valve.

Best Regards,

Mike

Normal range of opening of control valve is between 30% to 70%- As in this range it will give you the best control and will maintain the setpoints- so definitely your controlvalve is oversized.

to eliminate this problem, you can provide properly sized Restriction Orifice at the upstream of the controlvalve which gives you some pr.drop and by that way you can have some more opening of the same Control Valve.

For sizing of any controlvalve , you have to provide the right process data - like Min. and Max. flowrate and pressure and temperature at that flowrate and range of opening of control valve so that it gives you better controllability.

Definitely go back and start from the beginning to see if the valve was sized properly. With operating specifications put into order with differential taken into consideration, then your safety factor. See what you come out with and what you have. Also make sure all aspects that go into sizing are true. Just because process is written one way, that doesn't necessarily mean that is what is in process always, sometimes changes are overlooked and not always changed or noted. What has changed? Why possibly were things changed? Just make sure you try to cover all bases, sometimes the obvious is the answer. Good luck...........

Julius 8,

The problem is that the valve is only open 2%. Manufacturing precision for valve trim parts are not good enough at only 2% open to assure smooth operation in that region. Can you get a valve trim set with the same flow capacity and open at least 10% of travel? I assume that you did confirm that the flow characteristic is equal percentage for the valve trim installed. Fast opening or linear would not be good for the service you are describing.

Regards,

Luther M

Good point Luther which brings me to a new question. I may have misseed it but is this rotary or linear? (6010-6020)

try calibrateing the valve opening at 3.5%-4% that might solve the hunting problem.