Trouble shoot pressure fluctuation problems
Articles about thermal fluid systems often start with a variation of the statement that “thermal fluid systems typically require little ongoing maintenance for the first few years of operation” and then go on to extol the various advantages of indirect thermal-fluid process heating over competitive heating methods, such as direct heat, steam and so on. The corollary to that statement, however, is that by the time there is a problem, the operating personnel that were trained on the system have moved on, been excessed or promoted. As a result, when things do go wrong, the guessing begins. And, unless there is an obvious cause like a geyser from the expansion-tank vent or a pump that sounds like it’s moving ball bearings, someone will likely blame the thermal fluid for the problem. There are several problems that seem to occur with some frequency. This article reviews a number of real examples and describes how the symptoms can be misinterpreted. The suspected fluid properties and the testing procedures necessary to determine which of the fluid properties (if any) is responsible for the problem are examined. Finally, recommended corrective actions are proposed.
Why discharge pressure of the main circulating pump began to fluctuate?
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).
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.
uthor Jim Oetinger is the director of technology at Paratherm Corp. (4 Portland Rd., West Conshohocken, PA 19428; Phone: 800-222-3611, Fax: 610-941-9191; Website: www. paratherm.com). He has over 30 years experience in the chemical and plastics indus- tries. He has been involved with a wide range of products and processes including pig- ments, 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.
content and image courtesy:www.paratherm.com