Heat transfer fluids can provide years of trouble-free service, but eventually the fluid may need to be replaced as a result of contamination or degradation. Failure to address contaminated or degraded fluid can result in decreased production, reduced fuel efficiency, plugged lines, fouling of heat transfer surfaces, equipment failure and potential safety issues.
Contamination & Degradation of the Fluid
Common insoluble contaminants in thermal oil systems are rust, dirt and pipe scale. These are generally introduced during machining, construction or maintenance of the system components. Process contamination is less common, but can lead to accumulation of solids, sludge and other decomposition or reaction by-products.
Fluid oxidation is the primary cause of solids formation and fouling in thermal oil systems. Oxidation most commonly occurs in atmospherically vented systems. The rate of oxidation is dependent on temperature and oxygen availability. Mild oxidation can be tolerated, but excessive oxidation can create solids, sludges and high viscosity by-products, all of which impact the performance of the system.
Thermal degradation (overheating) occurs when temperatures exceed the thermal decomposition limit for a given fluid. The resulting decomposition by-products can result in surface coking, decreased fluid viscosity or accumulation of solids. Thermal stability is a function of fluid chemistry, operating conditions and time. Overheating of the fluid can be exacerbated by oxidation by-products.
Systems with high solids content can often be restored with side-stream filtration. Fiberglass and metal mesh are the most common filter media, but bag filters or other high surface-area designs may be utilized for heavy solids loading. Cartridge-style filters can generally be employed starting at 100-micron, and gradually reduced thereafter. Ensure that all filtration system components are rated for the temperatures and pressures of the system. To assure continued trouble-free operation, the source(s) of contamination must be eliminated as well. Clean systems can run for many months without plugging a low micron filter.
Replacing the Fluid
Eventually the fluid may reach the end of its usable life and will need replacing. However, simply replacing the fluid may not resolve performance issues. In addition to the cost of replacement fluid, be sure to consider process down time (including cooling, draining and filling), labor (including safety precautions) and disposal costs. Many fluids are compatible enough that a simple drain and fill is all that is necessary. Check with the new fluid supplier to be sure.
Cleaning the System
You should consider using a system cleaner if there are cold spots in the system or any other obvious performance decreases. Completely plugged lines will probably have to be replaced or purged using very high pressure. If there is any flow through the affected areas, the preferred option is to clean with a non-aqueous additive cleaner. Additive cleaners are blended into the existing fluid and clean while the system continues to operate. Once all the lines are hot again, the system is drained and recharged. Additive cleaners are the easiest and most efficient way to clean a fouled system. Ensure the cleaner is compatible with the fluid.
In more severe cases, other “off-line” cleanouts may be necessary. In such cases, the thermal fluid should be drained as thoroughly as possible from all parts of the system. Install additional low point drains if necessary. Use of “off-line” cleaning agents generally involves an additional flushing step to help remove any residual.
Organic solvents can be used to scavenge residual thermal fluid and to help dissolve system deposits. Solvents are typically circulated at ambient temperature, and flow should be maximized by isolating loops where possible. Extreme care must be taken since solvents are often flammable. Acidic and caustic water solutions can also be effective for removing stubborn system deposits. If a water-based cleaner is used, consider scavenging residual water with a solvent flush. Care should be taken upon start-up to account for residual water. In the most extreme cases with plugging and heavy coking, mechanical cleaning may be necessary and may require removal of system components. It is best to outsource this method to professional cleaning services.
Well-designed systems will have drains located at all low points of the piping so that the fluid can be easily drained or pumped out. Otherwise, it may be necessary to break flanges, open pressure taps or remove valves, flex hoses, or other components from the piping to ensure complete fluid removal. Using compressed nitrogen to force the fluid out one end of an open loop can be very effective.
Draining the system hot will leave less fluid and solids in the lines vs. draining it cold. Shut the heater down and continue to run the pump until the fluid has cooled to 180°F. Drain as quickly as possible, running the pump to the brink of cavitation to keep solids suspended.
Flushing fluids are designed to remove degradation residues or to dilute existing fluid that is too viscous to completely drain from the system. Unlike additive cleaners, the efficacy of flushing fluids is limited by their saturation points, and these can leave much behind. Flushing can also create additional steps because the flushing fluid may need to be flushed out of the system to prevent premature degradation of the new heat transfer fluid. Your new fluid supplier should be able to tell you whether you need to flush. Flushing fluids can usually be avoided if an additive cleaner is used.
Recharging the System
Replace all filter cartridges and open the filtration loop. Do not use the main circulating pump to charge new fluid since this can damage the seals. Use a stainless braided Teflon hose to connect a small positive displacement pump as close as possible to the main pump suction. Open all control and block valves and high point vents to begin filling the system from the bottom up. If the system does not have a de-aerator, make sure the warm-up valve to the expansion tank is open. Add fluid until the expansion tank is about half-full. If the expansion tank level is hard to determine, look for overflow from the expansion tank vent. Next, throttle the main pump discharge block valve so that it is almost closed. Start the pump and open the block valve to 1⁄4 of full flow. Add more fluid as needed when the low-level switch trips or the pump starts to cavitate. Once the fluid is circulating steadily thru the entire system, open the discharge valve another small increment, adding fluid as necessary. The system is full when the pump runs steadily with the block valve fully opened. Check all the high-level vents to make sure that the lines are filled. Add fluid to reach the proper level in the expansion tank.
Removing Water (Boil Out)
Residual moisture can rapidly vaporize leading to hot-oil “geysers”, so every cold start-up (for new or existing systems) should be approached with the conviction that there is water somewhere in the system. While draining the low points on the piping can remove gross amounts of water, the only method that will completely remove all the water is to flash it out through the expansion-tank vent (AKA boil out). For this to occur as quickly as possible:
- The expansion tank temperature must be maintained over 212°F.
- Condensation of the steam inside the tank must be minimized.
Warm-up/vent lines (which run from the heater outlet to the expansion tank) are the most effective setup. De-aerators do an excellent job of separating air and other non-condensing gases from fluid but are less effective for venting gases that condense (like steam).
To protect the fluid, it is crucial to allow heat stored in refractory and structural metal to exit the firebox when shutting the system down. Keep the circulating pump running until the heater outlet temperature is 250°F (121°C) or lower. If possible, keep the blower running to force air through the firebox and out the stack.
Typical Boil Out Procedure
The below procedure is generalized, and all elements may not apply to all systems. Paratherm recommends contacting your system engineer and/or heater OEM for more specific recommendations for completing a boil-out procedure optimized for your system.
- Open the valve on the warmup line (if applicable) and open the manual vent valve on the tank. Also open all control valves and block valves on every loop so entire system is open. Make sure overflow vessel is empty.
- Set the nitrogen supply regulator as low as it will go to keep a low volume purge blowing through the tank.
- Bring heater temp up SLOWLY to 220°F (104°C); check vent nitrogen stream for steam/humidity and listen for pump noise.
- When the heater outlet temperature has stabilized (burner on low fire or cycling on and off) increase set point by 5°F (3°C). Check again for steam and allow to stabilize.
- Reduce setpoint by 3-5°F immediately if the pump begins to cavitate. If cavitation continues reduce another 3-5°F. If the pump suction line temperature reaches 220°F too quickly you run the risk of vapor locking the pump. If this occurs, you may be forced to wait until the system cools enough to re-start the pump.
- If system is outfitted with a thermal buffer tank, open buffer tank drain for 15 seconds to drain any settled water.
- Repeat steps 4 and 5 until the pump pressure is stable with a heater inlet temperature of 250°F (122°C). Allow 15-20 minutes between increases.
- When pump suction temperature is 250°F (122°C) and pressure is stable, drain the thermal buffer tank, close the expansion tank vent, reset the nitrogen supply regulator and increase the setpoint in 18°F (10°C) intervals until operating temperature is reached. If the pump begins to cavitate, reduce the setpoint and open the expansion tank vent valve until exhaust gas feels dry.
- Once it appears all moisture has been removed and the system is stable, drain buffer tank one more time, and reset valves as needed to normal operating position.
Note: The information and recommendations in this literature are made in good faith and are believed to be correct as of the below date. You, the user or specifier, should independently determine the suitability and fitness of Paratherm heat transfer fluids for use in your specific application. We warrant that the fluids conform to the specifications in Paratherm literature. Because our assistance is furnished without charge, and because we have no control over the fluid’s end use or the conditions under which it will be used, we make no other warranties—expressed or implied, including the warranties of merchantability or fitness for a particular use or purpose (recommendations in this bulletin are not intended nor should be construed as approval to infringe on any existing patent). The user’s exclusive remedy, and Paratherm’s sole liability is limited to refund of the purchase price or replacement of any product proven to be otherwise than as warranted. Paratherm will not be liable for incidental or consequential damages of any kind. Some product names of companies found in Paratherm literature are registered trademarks. This statement is in lieu of individually noting each. ⓒ Paratherm 2020
Edward Cass is a chemicals and engineering professional with more than a decade of experience in specialty chemical processing and applications. Edward holds a B.Sc. in chemistry from Bloomsburg University of Pennsylvania and is currently Technology Manager at Paratherm Heat Transfer Fluids. Edward’s experience includes reactive and hazardous chemistries, pyrophoric gas processing, and formulation of synthetic lubricants. Edward has authored several articles and webinars on heat transfer fluid maintenance and performance and holds a patent for a specialized lubricant composition.