An engineer investigating a flaky industrial control system exploits a family connection
A few years back, I consulted on the construction of a
Plant in New Jersey. My job was to provide filtered, dry, oil-less air
for control valve actuation, as well as status instrumentation. The
plant took in the refuse stream, separated recyclables, and then burned
the rest. The plant was also instrumented to monitor the efficiency of
the burner and flue gas scrubbers, as well as to directly tax its
operation by levying fines for release of certain toxins from its
effluent gases. Once the site of a major landfill, this plant was
slated to be “The Future” of refuse disposal.
The filter and desiccant-dryer system for supplying control air was my
"baby.” Using the local controls, I supervised construction and the
initial checkout--all went well. Later, when we tested the remote
controls from the power plant control room, I was quite confused as to
why they wouldn’t work. We checked the switchboard wiring, “buzzed out”
the circuits to the equipment, and everything checked out “good.”
One day, after many trips from Baltimore to New Jersey, I was amazed to
discover that the indicator lights on the local controls showed that
the system was performing normally. While just a short walk
away in the power plant control room, the lights were flickering
randomly. Odd. This was not a “failure mode” that I
was familiar with. What made it most unusual was the fact that
the control system was designed for an industrial
environment: no low voltage signals here, these signals were set up
for standard 110 VAC!
The indicator assemblies employed standard NE-2 neon bulbs with a
dropping resistor, so that they would fire at about 85V and then back
off to about 70V to provide extended life. I recall that these bulbs
had about a 30,000 hour life--about 10 years of intermittent
operation. The other advantage of the neon bulb was its low power
consumption as compared to an incandescent bulb, thus making the driver
in the control unit less expensive, and more reliable to boot.
More weeks of testing verified my initial suspicions: Something was
getting into our signal wires, but how?
Out of ideas, I looked for inspiration elsewhere: Using one of my
"ask the expert" lifelines, as it were, I contacted my
recently-graduated nephew (EE from
Georgia Tech), and he provided the suggestion that solved the puzzle.
He asked how far the signal wires were from the generator room. I told
him that they ran right through the generator room up to the power
plant control room. He postulated that the
high magnetic flux from the generator (~1 MW) was coupling into the
signal wires, inducing just enough voltage to fire the neon bulbs.
Since an incandescent bulb is highly resistive, it is likely that we
would not have seen a problem if we had used them. His solution was
simple: replace the signal wires with coaxial cable and ground the
shield (only on one end to prevent ground loops) to our switchboard
signal ground. We did, and the problem was solved. Maybe in retrospect
we should have brought him in as a paid consultant!
As a PE, I have learned that in order to be successful you must take
advantage of all the available resources.
Charles R. Picek, PE (retired), graduated from Johns Hopkins
University with a BE in Chemical Engineering. His 50+ year career
includes involvement in the Petrochemical, Fertilizer, Pharmaceutical
and Environmental industries. He
is currently enjoying retirement in Florida, including his first loves:
Boating and fishing. His story was told to and ghost written by his
nephew Dwight Bues, Systems Engineer (featured in this case).