POWER April 2021 - 26

INSTRUMENTATION & CONTROL
Troubleshooting Issues
and Eliminating Headaches
Related to Control Valves
An oscillating control valve may appear to be the source of control instability and
repair efforts are usually focused only there. When this fails to solve the issue, further
investigation often proves the valve behavior was merely the symptom of some
other condition. This article discusses troubleshooting techniques to help plant
personnel get past the obvious and discover the true cause of control problems.
Sean Raymond
"
That new control valve is acting up
again! " Similar words have been uttered
by thousands of control room
operators all over the world. The plant
is not running well, and operators are
quick to identify the culprit-a recently
installed, misbehaving control valve. It
might be cycling, it might be squealing,
it might sound like it has rocks going
through it, but it is definitely the cause.
Or is it? When troubleshooting control
issues, it is important to keep an
open mind and look beyond the obvious.
It is human nature to blame the
" last thing changed " for any new problem
that occurs. While erratic control
valve behavior might be the apparent
source of concern, the true cause is
usually located elsewhere.
Thorough Investigations
Find the True Problems
The following application examples illustrate
this point.
Screaming Control Valve. A highpressure
spray valve was squealing after
a few months of service. The valve
was pulled, checked, and appeared to
be functioning normally. When returned
to service, the squealing resumed, and
the plant demanded the " defective
valve " be replaced.
The vendor was called to investigate.
A little checking indicated the valve was
being cycled by the control system between
0% and 10% open at a rate of
250,000 times a year. The very high cycle
rate at such low flows and high-pressure
drop was creating the problem. Adjustment
of the loop tuning and applying a
little backpressure on the valve stopped
the cycling and eliminated the squeals.
Jumpy Valve Response. A boiler
26
1. A boiler feedwater pump recycle valve would jump 15% to 20% (shown by the green line)
when it first opened. This created major control issues due to surging flows. Courtesy: Emerson
feedwater pump recycle valve was sticking
in the seat at startup. When the valve
would first come off the seat, it would
jump open, creating control upsets due
to uncontrolled flow (Figure 1).
The valve vendor was called to diagnose
the valve. Diagnostics were run and the air
supply pressure was found to be set well
above specification and four times higher
than was required for adequate seating.
When the valve was pulled for inspection,
the technicians discovered damage on the
seat and seat rings due to the excessive
actuator force, which caused the valve
plug to hang. Those components were
replaced, the air supply pressure lowered,
and the valve was returned to service
where it performed as expected.
Unhappy Boiler Level. A boiler control
system had been recently upgraded.
After a short time in service, the level
control became increasingly erratic (Figure
2). The system integrator was called
to re-tune the boiler level controller.
The repetitive oscillation of the feedwater
flow strongly suggested the feedwater
valve was sticking, so the engineer
suggested the plant check the feedwater
control valve. The valve was put on bypass,
stroked, and tested, but no sticking
was found. The automation engineer
and technician went back out and retested
the valve, stroking it very slowly.
The valve stem hung for a few percent
and then suddenly jumped. Ultimately,
the cause of the cycle was found to be
a worn component in the positioner. The
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original check failed to find the cause
because the valve was moved in increments
of 10%.
Strange
Control
Valve
Behavior.
A new control valve would act erratically
while in operation, with flows
surging and the valve swinging wildly
in response. Online diagnostics were
run on the valve and adjustments were
made to the positioner tuning, supply air
pressure, and pneumatic relays. These
corrections helped, but still the valve
swings remained.
Ultimately, the source of the behavior
was found to be a leaking
bypass valve, which would pass excessive
flow when the differential
pressure across the valve reached a
certain level. When the bypass was
2. The boiler level/feedwater control valve
was oscillating constantly. In this screenshot,
the orange line represents boiler level,
magenta shows feedwater flow, green is the
feedwater setpoint, and blue indicates the
feedwater valve signal. Courtesy: Emerson
POWER | April 2021

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POWER April 2021

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