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Symptom:
Do some
loops have excessive deviations of the process variable from setpoint after
disturbances or they just seem to respond too slowly?
Probable cause:
Process noise is often a problem and can come
from several sources. Some measurement sensors are naturally noisy, some
measurement installation techniques cause noise, agitation mixers can cause
noise, and other parts of the plant can cause noise on interacting loops.
If the noise cannot be
reduced at the source, filters can be added to add a lag to slow down the
signal to remove the peaks and valleys of the noisy signal. This added lag,
can cause significant deviations of the process variable from setpoint
because of this induced lag in the loop.
Many devices can now have adjustable
filtering including controllers, transmitters, and positioners. Sometimes
the amount of filtering in each device is not known by most plant
personnel. This can lead to excessive filtering.
Corrective action: Several
guidelines apply to filtering:
- Keep any filtering
as small as possible.
- Add only enough lag
to remove some of the noise amplitude, but do not try to remove all of
it by filtering. A guideline is to keep the filter time constant no
greater that 10% of the process time constant. (See Loop Tip #3).
- Keep any filtering
in one loop device only (usually the controller).
Keep the transmitter
fast, meaning unfiltered, to see what the process is doing.
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