Flow transmitter damping

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.

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:

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?

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?

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:-)

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

obstructions

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 menti> I suspect either an under damped valve positioner or an interacting

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: