Chemical Engineering December 2011 - 35

Troubleshooting for a Production Decrease
Viscosity
Normal
If the viscosity is above 300 cSt at 40ºC,
check the acid number
Is carbon present?
If the acid number
<0.2, then there is
contamination
Yes: Clean strainer,
clean system
No
Change fluid
Heater operating normally?
If the acid number
>0.2, then the
expansion tank is
too hot
Yes: Check flow-control valves,
process sensors, obstructions in
heat exchanger, gas pockets
No: Clean out pressure and
temperature sensors on heater
FIGURE 2. A simple flowchart can help analyze the symptom of production decreases
the vacuum system, undetected by a
faulty sensor, which resulted in an
increased heat load required for distilling
the product.
Example 3: A poultry processor was
experiencing reduced throughput in
a continuous convection oven. The
heater and pumps were checked for
problems, and all temperature and
pressure sensors were replaced. Someone
suggested cleaning the heat-transfer
fluid system. Since the fluid had
been in service for a number of years,
it was assumed to have degraded and
formed blockages in the coils because
the temperature drop across the heat
exchanger was much lower than when
the unit was new. The fact that the
fluid had been tested routinely and
found to be in good condition was totally
ignored in the evaluation. Management
personnel wanted to clean
the system and then change the heat
transfer fluid. A lube-oil additivetype
cleaner was added to the system
with the expectation that the problem
would be solved. When there was no
progress, a thermal-fluid sample kit
was requested along with a request
to estimate the cost of replacement
fluid. Once again the viscosity of the
sample was found to be well within
the normal range. The plant manager
was very disappointed with the results
showing that the fluid was not
the problem, because he had to send
the maintenance staff back in to keep
looking for the real culprit. Eventually
it was discovered that an air damper
inside the oven had a broken weld
that allowed it to flip up into the air
stream, effectively blocking the coils.
Throughput was reduced because insufficient
heat was getting to that section
of the oven.
Pressure fluctuations
In one chemical plant, personnel noticed
that the discharge pressure of
the main circulating pump began to
fluctuate as the fluid temperature
approached 350°F after an extended
shutdown. Thermal fluid was added
to the system through the expansion
tank, which made the situation worse
for a period of time. Since the system
had been kept under a nitrogen blanket
during the downtime, water absorption
through the expansion-tank
vent was ruled out. Convinced that the
fluid had degraded during the shutdown,
personnel made plans to take
another outage and replace the fluid.
To pacify management, a fluid sample
was taken and tested. The test results
indicated high water levels (greater
than 150 ppm, versus the normal level
of less than 50 ppm).
Pump-discharge pressure
fluctuations in a closed-loop
heat-transfer system are always
the result of entrained gas. Aeration
of the fluid is often blamed
for such fluctuations, particularly
if fluid is added through
the expansion tank. However,
entrained air doesn't abruptly
become gaseous, but instead it
causes problems from the start.
While it is true that overheating
a fluid can produce more volatile
molecules that will theoretically
vaporize, in practice the
relatively low liquid-to-vapor
expansion rate (which is about
20) pretty much rules this out
as the source of gas.
The real culprit is most often
water, which has an expansion
rate of 1,000. Until water is either
drained from the system
or flashed off through the vent,
it remains in the bottom of the
thermal buffer tank or the expansion
tank. In fact, tanks have been known
to rust through at the bottom because
water has been in the same place for
many years. When the heat-transfer
fluid flows out of the tank as the system
cools, the water is carried into the
system piping, and then is dispersed
into the circulating fluid when the
pump starts. As the system temperature
reaches about 220°F, the water
droplets become steam bubbles and
the pressure fluctuations begin. What
causes confusion is that the pressure
problems don't appear at the expected
212°F. Depending on the system pressure
and design as well as the amount
of water present, symptoms may not
begin until the heater temperature
reaches 280-300°F. If the pump is operating
at a slightly negative suction
head, even lower water concentrations
can result in pressure fluctuations.
Figure 3 shows a simple chart that
can be used to help troubleshoot pressure
fluctuation problems.
Pump seals
A hot-roll calendering operation was
experiencing repeated rotating jointseal
failures. The seal faces were
being scored severely enough from the
inside out to create fluid leakage. Two
of the oldest seals were experiencing
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