:: Fluids --- ID :: 16437
Troubleshooting Heat-Transfer Fluid Systems - Part 2
Trouble shoot pressure fluctuation problems

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). 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. 

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