Chemical Engineering December 2011 - 36

Troubleshooting Pressure Fluctuations
Cover Story
the greatest number of failures. While
the fluid tests showed no significant
change in the fluid condition, there
was visible residue of a previous brand
of insoluble fluid. The most compelling
evidence of what the problem might be
was that particles were collecting on
the sidestream filter elements. To prevent
further problems from what was
suspected to be carbon (from degraded
heat-transfer fluid), the user began
to evaluate a system flush and fluid
change.
The majority of carbon particles
produced by fluid degradation are
the result of fluid oxidation (as determined
in fluid analysis by the total
acid number). These acids are formed
when hot fluid is exposed to air in the
expansion tank. They are thermally
unstable (compared to the fluid itself)
and thereby degrade into carbon
at relatively low temperatures (375
to 400°F) once the concentration has
reached an acid number of 0.3 or so.
If the expansion tank continues to run
hot, the acid number will either stabilize
or continue to increase. If the
cause of the oxidation has been corrected,
the acid number will decrease
as the acids are consumed.
The carbon that is formed is similar
to " soot " in appearance and will
remain suspended in the fluid, which
causes the fluid to appear to be black.
The particles will drop out of suspension
in stagnant fluid and form sediment
(sludge). However, individually
these particles are extremely fine
(<0.5 micron) and as such are incapable
of damaging rotating seals because
they pass between the rotating faces.
However, they will clump on 25-micron
filter elements, which can be misleading
during troubleshooting. In this
case, the solution required analysis of
the filter. The particles were analyzed
and found to contain over 90% iron.
This information was transmitted to
the user, who then shelved the plan to
flush the system and replace the fluid.
Instead, he concentrated on improving
his filtration system to eliminate the
metallic particles.
Erratic production
A food processor began experiencing
sporadic production problems
with a multiple-user heat-transfer
Fluctuations appear
at >230ºF
Fluctuations appear at
ambient to 230ºF
Heater operating normally
Add fluid
If water is <150ppm, clean strainer and
check for obstructions
If water is >150ppm,
boilout the system
FIGURE 3. When pressure fluctuations occur, this simple chart can guide a first
analysis of the problem
system that was used to heat tanks.
Once again, the pump pressures and
temperatures were all within the expected
ranges. Because the fluid had
been in service for a number of years,
the likely solution was deemed to be
fluid replacement. The shutdown was
planned and quotes were obtained.
After the costs of the fluid and lost
production were totaled, cooler heads
prevailed, and it was decreed that the
fluid should be tested by the current
fluid suppliers to be sure it really did
need to be replaced. Although the fluid
had not been tested for a number of
years and actually was a blend of several
fluids, the supplier was able to determine
that the fluid was in acceptable
condition. Now that the " easy solution "
was not applicable, the real investigation
started. Particularly confusing,
but overlooked when the fluid was the
prime suspect, was the fact that the
most significant decline in production
occurred when there was the least demand
on the heater. Fluid velocity has
even more effect on heat transfer performance
than viscosity, so whenever
there is a drop in heat transfer, it's time
to look at the flowrate.
Liquid-phase heaters require continuous
flow to prevent fluid degradation.
Hence these systems need some way
to bypass the heat users when heat is
not required. There are two ways to accomplish
this: 1) A backpressure control
valve that maintains flow when
the two-way control valves are closed;
and 2) One or more three-way control
valves (depending on the number of
users) with a manual pressure-equalization
valve on the bypass port.
Theoretically three-way valves are
superior to a backpressure valve arrangement
because they provide a
constant flow through the heater - a
concept that is favored by the pur36
CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011
ists - if the balancing is done rigorously.
This exact balance is difficult to
maintain over time due to changes in
equipment and the ever-present potential
for third-shift adjustments. In this
particular case, it was discovered that
the bypass valves on the least-used leg
of the system had been fully opened so
that when that system was not operating,
a substantial amount of fluid was
bypassing. When the unit was operating,
the bypass volume was reduced,
which in turn increased the pressure
and thus flow to the other units bringing
production rates back up. Instead of
attempting to balance all of the bypass
valves (which would have required the
installation of multiple pressure gages)
the solution was to install a backpressure
valve between the feed and return
header and then close all of the
bypass valves, effectively turning them
into two-way valves. While this control
scheme did allow the heater flow to
vary, it made the system much easier
to control since each user was independent
of the others.
■
Edited by Dorothy Lozowski
Author
Jim Oetinger is the director
of technology at Paratherm
Corp. (4 Portland Rd., West
Conshohocken,
Phone:
800-222-3611,
PA 19428;
Fax:
610-941-9191; Website: www.
paratherm.com). He has over
30 years experience in the
chemical and plastics industries.
He has been involved
with a wide range of products
and processes including pigments,
refrigerants, consumer plastic recycling,
polymer compounding, process instrumentation
and spray dried polymers. In addition, Oetinger
has over 20 years experience in sales, marketing,
and technical support of thermal fluids. He has
authored articles on thermal fluid and system
troubleshooting for this and other publications.
A member of the Delaware Valley Chapter of the
AIChE, he holds a B.S.Ch.E. from Clarkson University
and a Masters of Management degree
from Northwestern University. Oetinger and his
family reside in a suburb of Philadelphia, Pa.
http://www.paratherm.com http://WWW.CHE.COM
Chemical Engineering December 2011

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