Need help on flow control tuning

Just a guess: the valve's tight new seals cause enough hysteresis for
the valve positioner to have problems?

Regards,
Andrey Romanenko

TheRomanov wrote:

A positioner is a closed loop controller that may be unstable, all by
itself.  Before you try to tune the flow control loop, you have to
make sure the position control loop is stable.  Some positioners have
a gain or speed adjustment that can be set wrong enough to cause
position instability.

Whatever you do, your end objective should be for an open loop valve
response that is overdamped, ie. clearly no overshoot. A bit overslow is
better than wobbly in the control world on most occasions.  You should also
check for stiction, that's a common cause of flow loop instability.

Bruce,

The valve responded quite resonably fast(as indicated by change in PV)
when I did an open loop test. Therefore, I don't think it is sticky.

bruce varley wrote:

I don't understand how you can relate speed and stiction so readily.  They
are different things.

Stiction presents as a form of hysteresis.  A better open loop test for
stiction is perhaps a sinusoidal or bang-bang control signal using a variety
of amplitudes.  If there's stiction it will be evident in the response.  At
low amplitudes you may get no response at all.

On the other hand, the response to a giant step will probably overcome the
stiction rather immediately and all you will see is the expected speed of
response with not much indication of what stiction is doing.

Fred

Fred Marshall wrote:

Valve stiction is sticky valve.


1% open loop step caused a change in 8m3/hr. Sinusoidal or bang-bang
control on a furnace pass flow with low flow trip setting will get you
a boot out from the control room by the panel operator. This is not a
university work but real plant.


In open loop test with PV responding more than a minute or even half a
minute is a good indication of sticky valve. This test indicates a PV
change in 3 sec after a step on the controller output.

I guess my response didn't make any sense to you.

I remain of the belief that stiction represents a form of hysteresis no
matter what the mechanism.  Now, it may be that your sticky valve presents
another type of behavior that isn't of that sort.  But, it's not something
that I can well imagine.

I still don't understand how you can relate speed to stiction.  I surely do
understand how stiction can affect another kind of speed - as below.

Here is the model that I had in mind:

The valve will turn at some speed when it is already moving.  The speed may
be proportional to a control or it may be constant if the control is
bang-bang or on-off-on ... well, after an initial transient due to its
dynamics (e.g. mass, etc.).
So, this "speed" is a parameter of the valve and the control and is
independent of stiction.

The valve also has stiction that is characterized by the amount of control
force necessary to get it to start moving.  The force necessary to overcome
static friction / stickiness is indepedent of the speed at which the valve
will move once the stiction is overcome.

Depending on how the control signal is done, the stiction can affect
response times - which may be called "speed" but not the "speed" mentioned
above which is an inherent property of the valve.  Surely if the control is
varying (shall we say in a sinusoidal manner just for discussion?) then the
stiction can well affect the valve position.

Perhaps this model is too simple for the situation.  But, separating out
critical device parameters seems useful to me!  Both parameters can be
measured if one has the luxury to do so and both parameters can surely be
estimated.

If "1% open loop step caused a change in 8m3/hr" then 1% change in the
control is sufficient to overcome the stiction - is that right?  The
question is, what is the maximum step size that is inadequate to cause any
change at all?  That's the stiction "force" in my simple-minded model here.

Perhaps a simple stiction model is too simple.  Perhaps the globe valve has
rotating friction that is a function of speed - so it's not a classical
static/dynamic friction situation - perhaps it's a gradual changing friction
force that goes from static to ever-decreasing dynamic values as a function
of speed.

Only you know the limitations imposed by the environment you're working in.
Some things can be done on site and other things cannot.  A suggestion for
how to deal with things may not fit.  That's not a reason to label the
suggestion as "academic".

Fred

wrote:

Positioners are not recommended on flow-control valves(although they are often
fitted without
problem). Clearly, if your valve without positioner cannot close against the
process pressure at full
control signal, this is because the actuator is not providing enough driving
force. It was probably a
mistake to re-fit the old actuator. An actuator has to be matched to the valve,
and your new valve
has, presumably because of its design, a different balance of forces then the
previous valve.

Kelvin B. Hales
Kelvin Hales Associates Limited
Consulting Process Control Engineers
Web: www.khace.com

Correction......

When we change the new valve, we also fitted it with new actuator but
maintaining old I/P converter. After we failed to close the valve, we
fitted with new I/P converter.

TheRomanov wrote: