The facts at your fingertips: Heat Transfer Fluid System Venting
August 1, 2022 | By Kapil Bathla, Eastman Chemical Co.
Heat transfer fluids (HTFs) provide heating and cooling for process equipment, including reactors, autoclaves, distillation columns, reboilers, mixers and dryers. Designs of FTC systems must provide for effective ventilation of the system, both of residual water at start-up and of degradation products during operation.
When commissioning new FTC systems (Figure 1), a major concern should be the effects of water: new systems can be vulnerable to excessive residual water pressures. Hydrostatic pressure tests (leak checks) performed on the system during manufacturing or on site after maintenance can be a typical water source. Complete removal of water can be impeded by traps and changes in plumbing elevation. The best system designs provide piping installations with slopes to strategically placed low point drains. After the water has drained, but before refilling, the system can be further dried by purging warm, dry air (or N2) through the system circuits until the dew point of the outgoing gas reaches -34 to -40°C, indicating that the moisture has been properly dried. Careful attention to the drying process will greatly reduce the time required to reach the high operating temperatures expected at start-up.
Start-up moisture removal
Before circulation, ensure that the level of cold fluid of the HTF in the system is adequate. This is usually indicated by the expansion tank fluid level instrument (Figure 1). Then slowly heat the liquid while circulating it through all the piping circuits assuming that the water content may be excessive. Valve A is closed and valves B and C are open. The FTC circulates in the expansion vessel and heats up to just above 100°C. This temperature forces moisture into the vapor space of the expansion tank. Valve E is opened and the inert gas inlet sweeps water vapors from the downstream vapor space to a recovery tank or flare system. The process continues until symptoms of dampness – including pump cavitation, erratic flow from the discharge side of the pump, and clicking, knocking and boiling noises in the expansion tank and the pipe – disappear.
Once the FTC is deemed adequately dried, the fluid should be able to continue to heat to higher operating temperatures. Typical valve alignment during normal operation is to open valves A and B and close valves C and E. This valve placement allows for a lower temperature in the expansion tank (typically around 25% of the volume of the system), where its rate of thermal degradation is negligible.
Breakdown of degradation products
In operation, HTF deteriorates at increasing rates as operating temperatures approach the overall operating maximum for the specific HTF, altering the state and composition of the HTF. Thermal degradation leads to the formation of high boiling point compounds, which increase fluid viscosity and the potential formation of solids which increase the chances of coke deposits or fouling, and low boiling point compounds which decrease the viscosity of the fluid and which have boiling points below the boiling range of HTF. Additionally, increasing the low boiling point content may result in a lowering of the flash point by 45°C or more.
Low boilers can be managed by routine ventilation of the system. Systems are best vented only when the low boiling point concentration exceeds recommended limits based on analysis of fluid samples.
The same procedure is followed when commissioning a new system, except that higher temperatures are required. For many organic FTCs, the venting procedure is performed at fluid temperatures near 180-200°C. This temperature range supports expansion into the vapor phase and low boiling point separation of the heat transfer fluid for disposal without causing significant loss of HTF components. With valve A closed and valves B and C open, all fluid flows into the expansion tank. This process increases the temperature of the fluid in the expansion tank and increases the partial pressures of low-boiling degradation products. This allows low boiling point boilers to enter the vapor phase, where their removal is facilitated by the opening of valve E. An inert gas can be used to effectively sweep these vapors over the surface of the liquid and out of the vent line, where they can condense. and collected for disposal.
Circulation through the expansion tank also ensures that all HTFs benefit from the reduced levels of low boiling point boilers.
After the purge process, return to the typical valve alignment for normal operation, where valves A and B are open and valves C and E are closed. This arrangement allows thermal expansion and contraction of the FTC volume to and from the expansion tank with changes in temperature. Continuous venting and purging is not recommended, as this can deplete fractions of the HTF itself, creating related changes in its properties, performance, and service life.
Editor’s note: This content was written by Kapil Bathla, Product Development and Technical Customer Support Specialist at Eastman Chemical Co. (Kingsport, Tennessee; www.eastman.com).