POWER April 2021 - 27

INSTRUMENTATION & CONTROL
position will not match what the valve is
actually doing (Figure 4). If a rotary valve
is sticking, the positioner may see shaft
movement caused by torsion or slip
when the valve ball has not moved at all.
3. Diagnostics can be performed on valves to pinpoint problems. Courtesy: Emerson
fully closed, the control valve operated
normally.
Identifying the Issue
When troubleshooting a control issue,
the team should start by fully understanding
and identifying the problem.
Often, operators will fixate on a single
effect (such as erratic flow control, control
valve behaving oddly, or noisy equipment)
and complain about that. However,
by posing the right questions, investigators
can obtain a more complete view
of the problem. Does this happen all
the time or just occasionally? Are there
certain situations where this always happens
or never happens? What else was
going on during those times? Such questions
often expose contributing conditions
and may take the investigation in
an entirely different direction.
Troubleshooting Tools. Conversations
with operations, maintenance, and engineering
personnel are invaluable, but
it is wise to utilize other troubleshooting
tools, if available. Historical data can
better define the situation and indicate
when it occurs. Other process trends
from the same time period might expose
upstream or downstream conditions
that are contributing factors.
Surging pressures, swinging flows, or
other process variations could be causing
the problem.
Diagnostic tools on digital positioners
may also shed some light on how the
valve is performing (Figure 3). This can
confirm (or eliminate) issues caused by
supply air pressure, positioner tuning, or
mechanical sticking.
April 2021 | POWER
4. Worn linkage components can create
problems for a valve positioner. The worn
valve coupler on the left will mislead the positioner
into indicating the valve is moving when
it is not. Courtesy: Emerson
www.powermag.com
Analyze Data Carefully. Unfortunately,
data can be very misleading. Poor
resolution or update times on a historical
trend may mask problems or imply issues
that may not actually be occurring.
If historical data is sampled infrequently,
the data may miss a spike entirely and
show a flat line because many historical
trends " fill in " smooth curves between
the captured data points. If historical
data resolution is low, the trend may look
like stair steps when the actual process
was smooth.
Current trends often have full resolution
as they are drawn and usually provide
a much more accurate view of the
process. Using these during tests can be
much more illuminating then calling up a
historical trend after the fact.
Valve diagnostics also have their limitations.
Positioner data is only as good as
the position information it receives. If the
linkage is loose or worn, the indicated
Systematically Eliminate Causes
When armed with a full understanding of
the issue and relevant data, one can begin
to track the root cause by identifying all
possible culprits and systematically eliminating
them through further testing. It is
important to be diligent in these tests-a
sticky valve may only stick under specific
process conditions and when moved in
small amounts. Stroking a valve in 25%
increments tests virtually nothing. An
electrical short may occur on a device installed
in the field, yet it could work flawlessly
when sitting on an insulated rubber
mat in the maintenance shop.
If possible, try to isolate the problem
using the " divide and conquer " approach.
For example, if a solid 4-20 mA signal is
read from the control system while the
connected valve continues to stroke unexpectedly,
that likely eliminates the wiring,
input-output (I/O) card, and control
system as root causes.
Once the list of possible causes has
been refined as much as possible, start
attacking the problem by implementing
easy and cheap solutions. Adjusting the
tuning is easier than replacing a positioner.
Replacing a positioner is cheaper
than replacing a valve. Even if the cheap
and easy solutions seem unlikely, it often
makes sense to start with those as there
is little cost or effort required to try them.
When faced with a control problem,
go in with open eyes and an open mind.
Look beyond the obvious source of the
issue. As Sherlock Holmes said: " It is a
capital mistake to theorize before one
has data. Insensibly one begins to twist
facts to suit theories, instead of theories
to suit facts. "
A long conversation with the operating
staff and a handful of probing questions
will often expose new information
and help better identify what might be
happening. That effort, combined with
some data gathering and careful analysis,
will usually allow the investigating
team to quickly eliminate suspects and
ultimately discover the root cause. ■
-Sean Raymond is the technical
marketing manager for Fisher
Instrumentation with Emerson
Automation Solutions in Marshalltown,
Iowa. He is also the ValveLink Software
Integration marketing manager and leads
the Marshalltown Solution Center.
27
http://www.powermag.com
POWER April 2021

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