Flow transmitter damping

TheRomanov wrote:

Many transmitters (especially electronic digital ones) have an
adjustable low pass filter function included.  This is referred to as
a damping filter.  Many control systems, especially DCS systems , also
have an input filter that performs this function, but it works only on
the samples taken by the system, so it is often not as good at
rejecting (averaging) high frequency noise as the one in the
transmitter is.  Adjusting either is just a setting choice.

So John.... If a flow reading oscillates with 6 peaks for a minute,
does it call for filter adjustment?

John Popelish wrote:

TheRomanov wrote:

I hope John won't mind me leaping in here; but:
IMHO no! It calls for an investigation to find out and eliminate the root
cause of the oscillation.
Any competent Instrument Engineer, or Control Engineer, should be able to
do that. At that frequency: it sounds like a valve problem, or loop-tuning
problem, or possibly a process fluctuation.

Just as a matter of further interest: any significant degree of low-pass
filtering on the signal from a dp-type flow transmitter; e.g. one related
to an orifice or venturi-type meter, upstream of the square-root
extraction, will introduce a bias on the signal mean in the presence of a
significantly fluctuating flow - because of the square-root relationship
between flow and dP.

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

TheRomanov wrote:

A damping filter is usually used to decide what frequencies contained
in the input signal are too fast for the loop to deal with, and so,
can be safely filtered out.

That depends on how fast the control device and controller are.  But
my guess is, probably not.  A 10 second cycle sounds like a stick-slip
problem with the valve positioner that is preventing the valve from
being set to precisely the desired opening.  Or it could be wrong
tuning of the controller.  What shape is the 10 second wave?  More
sinusoidal, triangular, spikey?

Almost sinusoidal. For your information, I did an open loop step test
and it is also producing sinusoidal type oscillation before settling
out. Kinda 2nd order dynamic for a flow.

John Popelish wrote:

TheRomanov wrote:

I suspect either an under damped valve positioner or an interacting
pressure or flow control loop in the system.  Were all other control
loops involved in this flow either put in manual mode or under tuned
during your step test, to make sure the only response involved was one
valve?

John,

Your question whether other loops involved in this flow were put in
manual is very much appropriate. For your information, the flow is one
of 4 passes crude unit furnace with each having their own control
valve. I did a step change on the problem flow yesterday together with
the control valve supplier while maintaining the remaining passes flow
control loops in manual mode. Note that this control valve actuator is
a failed open type.

We stood at the valve positioner and found that as we step down the
output from DCS, the positioner output air gauge increases for a while,
decreases back, increases again and stop to a point where it increased
earlier. We recorded the flow reading and it shows that the flow went
down, up, down and settle to a new value.

What can we make out from this test? The valve supplier mentioned about
positioner dampling adjustment.

John Popelish wrote:

TheRomanov wrote:

First guess:  That the valve is sticking and sometimes overshooting the
desired position.  Sometimes it overshoots, unsticks and overshoots the
reverse correction.  If you increase the positioner damping, the
position accuracy will probably just get sloppier.  Is there a valve
stem packing pressure adjustment that you might loosen a bit or a way
to add some lubricant to the stem to reduce the friction?

What sort of piston or diaphragm arrangement does this valve have?  I
understand that the return spring is positioned to open the valve on
loss of air pressure.  But is the actuator a pressure balance type, or
strictly an air to close system?  If a pressure balance type, you might
be able to raise the positioner supply pressure and make it more stiff.

It does make one wonder whether the valve and actuator were properly 'bench set'
in the
instrument workshop before being installed. Things like packing tightness and
problems of
stiction and positioner instability should all have been visible and adjusted
out in the workshop
before the valve was installed.
Given that an old actuator has been put onto a new valve body, I can't help
wondering whether
this wasn't done.

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

Just a thought.. you say the reading is almost sinusoidal.  Flow
transmitters need non-turbulent flow to work properly.  Has the transmitter
been installed in a clear section of line (no curves or other obstructions
nearby)??

Cameron:-)

I agree about the straight run requirement. In this case, it complies
to that. After all, it has been working fine all this while until
someone changed the valve.

Cameron Dorrough wrote:

Okay..  Is the new valve fully ported?  It wouldn't be obstructing the flow
would it?