Restoring Performance in Fouled Heat Transfer Systems

Thermal fluid systems are among the most reliable and safe heating designs available for manufacturing operations. The lifespan of a fluid is limited by degradation and contamination, which causes a reduction in performance and the need for replacement.

In cases of severe degradation, merely draining and refilling the system may not restore performance. Instead, a more thorough cleaning of the system might be necessary.

Performance can be restored through the identification of the root cause of degradation and the application of appropriate remedies.

In this article, best practices for cleaning, draining, flushing, and recharging a thermal fluid system and recommended practices to sustain performance are discussed.

Thermal Degradation

Thermal degradation, or overheating, occurs when fluid molecules absorb more thermal energy than they can effectively release. Excessive thermal stress severs molecular bonds, leading to irreversible damage to the fluid and changes to physical properties.

Lower molecular weight species (low boilers) are formed when a fluid fragments. These low boilers can recombine to form higher molecular weight species (high boilers).

Eventually, if solubility limits are exceeded, high boilers may precipitate as sludges and tars. With this comes the possibility of fouling heat exchange surfaces or plugging lines, and in extreme cases, heavy coking and potential failure of heater tubes.

Thermal degradation rates can be minimized by never exceeding the manufacturer's recommended maximum temperatures, selecting the proper fluid based on the temperature requirements, and ensuring adequate fluid flow through the heater at all times.

In addition, a routine preventative maintenance program should be followed in accordance with original equipment manufacturer (OEM) recommendations.

Oxidative Degradation

Oxidative degradation occurs when hot fluid reacts with atmospheric oxygen in vented expansion tanks and reservoirs and can have significant impacts on fluid performance and quality.

As oxidation proceeds, sludge, and carbon soot can accumulate, resulting in significant increases in fluid viscosity and precipitation of sludge, ultimately leading to fouled surfaces and plugged lines.

In most systems, the risks of oxidation can be removed by blanketing the expansion tank with an inert gas, such as nitrogen. In cases where blanketing is not possible, thermal buffer tanks or cold seal pots should be considered.

Despite these easy measures to address oxidation, this form of degradation accounts for > 90 % of all premature fluid degradation cases.

Contamination

Contamination is usually the result of process leaks or operational errors. The latter can include sharing process equipment, adding the wrong fluid to the system, and inadequate cleanout procedures.

Rust, dirt, and pipe scale are common contaminants that can be introduced during construction or maintenance of the system. The effects of contamination depend on the contaminant.

Water contamination often manifests quickly, resulting in unexpected venting, pump cavitation, and mechanical knocking.

Other contaminants can degrade quickly, resulting in issues such as the formation of low and high boilers, increased acidity, carbon generation, and fouling of surfaces.

Contamination can lead to fouling, reduced heat transfer rates, equipment failure, and reduced operational safety.

Replacing the Fluid

A replacement plan should be determined as a thermal fluid reaches the end of its life. Process downtime is important to consider, including cooling, draining and filling. Other important factors include labor and safety provisions, third-party contractors, disposal costs, and new fluid costs.

Occasionally, a simple drain and refill with continuous filtration is sufficient to restore performance. However, often, a thorough offline cleaning procedure is necessary.

The disposal protocol must be followed for all fluid and cleaning agents used during the replacement process. Some recyclers will haul non-hazardous waste oil free of charge, but some fluids and chemical cleaning agents will require dedicated waste streams. Suppliers and local regulatory authorities can provide information on these special requirements.

System Cleaning and Restoration

System cleaning can be costly, as completely plugged lines may require replacement or mechanical cleaning with high-pressure water jets, abrasive blasting, or steam lance.

In no-flow scenarios, isolation or removal of equipment may be required for cleaning. In cases where there is some flow, organic and aqueous cleaners are often effective for restoring performance.

For cleaning, organic, non-aqueous additives are added to the existing fluid, and the system is run under normal operating conditions. This results in the loosening of sludgy deposits from heat exchange surfaces and the dispersion of solids for easier removal from the system.

After all lines have restored thermal performance, the system is drained and recharged. Additive cleaning is often the most efficient and effective option for a fast turnaround.

A suitable flushing fluid or solvent can be used to effectively thin the incumbent oil and solvate tacky residues if the fluid is very viscous or a gel at room temperature.

When used at full strength, solvent-like cleaning agents may require a sacrificial flush volume to restore safe start-up conditions with fresh thermal fluid.

Solvents are usually circulated at ambient temperature, and flow can be maximized by isolating loops where possible. Extreme care must be taken when circulating flammable solvents.

All effluent from the cleaning procedure must be handled, stored, and disposed of in accordance with the applicable policies and regulations.

Other ‘offline’ cleaning methods include the use of aqueous solutions or other specialty full-strength cleaners. Caution should be exercised when considering a water-based cleaning agent, as residual water may be difficult to remove, prolonging downtime.

Additionally, the rapid volumetric expansion of water to steam can lead to hot oil ‘geysers’ or explosive discharges. Scavenging residual water with a polar organic solvent and blow drying it with nitrogen or air can remove this risk.

During start-up, residual moisture must be boiled out of the system through the expansion tank vent or other high-point vents. For optimal efficiency, it is necessary to keep the expansion tank temperature over 212 °F and minimize condensation of the steam inside the tank.

Warm-up or vent lines that run from the heater outlet to the expansion tank are the most effective setup. De-aerators are excellent at separating air and other non-condensing gases from fluid but are less effective at venting a condensable like steam.

Figure 1. Sludge is formed over time as oxidation byproducts accumulate. Sludges are a significant detriment to performance and often can shut systems down. Oxidation is eliminated with inert gas blanketing or otherwise thwarted by filtration or reducing expansion tank temperatures.

Image Credit: Paratherm

Figure 2. Water will quickly manifest itself in high-temperature thermal fluid systems. If water does get into the system in an appreciable concentration, it can only be removed with a controlled boil-out procedure.

Image Credit: Paratherm

Figure 3. The expansion tank plays several key roles in the performance of the heat transfer system. When properly designed/configured, expansion tanks allow systems to run at low pressures, separate volatiles, and ensure the system is fully flooded.

Image Credit: Paratherm

Removal of Fluid

For best results, effective cleaning and draining cycles are necessary. This can be achieved by installing additional low-point drains and opening all high-point vents for gravity draining. During draining, it is important to keep the fluid turbulent as long as possible.

In the absence of dedicated drain provisions, the user must be prepared to open pressure taps, break flanges, and remove flex hoses, valves, and other components from the piping to ensure complete removal of fluid.

Forcing fluid through one end of an open loop by blowing out lines with compressed air or nitrogen is often effective. Draining the system while warm or hot (as applicable) will leave fewer solids and less fluid in the lines compared with cold draining.

The heater should be shut down with the pump running until the fluid has cooled to below 82 – 93 °C (180 – 200 °F) and then drained as quickly as possible. It is important to exercise caution and wear appropriate PPE when working with hot fluids and surfaces.

Charging and Initial Run

Before recharging and while the system is in this docile state, any advantageous piping, design or maintenance modifications should be devised and implemented. These could include the addition of new block valves, inert gas blanketing, heater inspection, and drainage improvements.

All pump strainers and filter cartridges should be replaced, and the filtration loop opened. New heat transfer fluid is introduced most effectively by employing an external positive displacement pump and braided transfer lines connected as close as possible to the main pump suction to fill from low to high.

All control and block valves and high-point vents should be opened, and the system vent points should be monitored to catch fluid running out of vents. Fluid should be added until the expansion tank is half full, and if the level of the expansion tank is difficult to determine, watch for overflow from the expansion tank vent.

To aid priming and initiate flow, the pump discharge valve can be throttled. Further, fluid can be added as needed if the pump or low-level switch trips begin to cavitate. Once steady circulation of the fluid through all loops is achieved, the discharge valve should be opened by a small increment, with fluid added continuously as needed.

The system is full when the pump runs steadily with the block valve fully opened. At this point, with all lines flooded, fluid can be added to reach the proper level in the expansion tank, usually two-thirds to three-quarters full at maximum operating temperature.

Once complete, it is important to ensure all safety, control, and bypass valves are restored to their ‘normal’ configuration.

Maintaining Performance

To maximize the performance of a heat transfer system, proactive maintenance and management is crucial. Following OEM preventative maintenance schedules and installing in-house proactive maintenance for the system can help achieve this goal.

Many fluid suppliers offer an analysis program for customers. Unused fluid should be used as a baseline for the tests provided for proper interpretation of results.

To optimize the ongoing performance of a system and fluid, a user should reduce pressure drops, monitor cycle times, reduce thermal cycling, and routinely monitor for anomalies.

Proper maintenance practices and routine condition monitoring can help prevent costly system restoration in the future.

Start-Up Recommendations

In cases where the expansion tank vent discharges into a catch tank, it is important that the end of the pipe is visible and that the system catch tank is completely empty.

The manual valve on the expansion tank vent line should be opened for start-up. In cases where a nitrogen blanket is installed on the tank, the nitrogen inlet pressure control valve should be set as low as possible to provide a continuous purge through the vent. This practice will prevent oxidation of the fluid and speed up water removal.

Welding blankets should be laid on top of the expansion tank to reduce condensation. The heater should then be started, and the set point increased slowly to 100 – 104 °C (212 – 220 °F). Pump noise, pressure fluctuations, crackling or popping noises, and sudden level changes in the expansion tank are all signs of water in the fluid.

The pump discharge should be throttled to establish flow through all loops of the system.

The heater should remain in low fire until pump discharge pressures are stable at > 121 °C (250 °F).

Shut-Down Recommendations

By closely adhering to proper system shutdown procedures, the performance and lifespan of heat transfer fluid can be maximized. Neglecting these procedures can lead to overheating and 'bruising' of the fluid, ultimately causing damage to the system itself.

During operation, the heater's refractory and structural metal become nearly as hot as the flame itself. If the system is shut down abruptly—stopping both the firing of the heater and the circulation of the fluid—the intense heat stored in the refractory and metal remains trapped in the firebox. This retained heat cannot quickly dissipate through the stack, leading to scorched heater tubing and overheated thermal fluid.

When shutting down the system, it is crucial to turn off the heater while keeping the circulating pump running. Once the heater outlet temperature drops to 104 °C (220 °F) or lower, it is safe to turn off the pump. If feasible, continue running the heater blower to force cool air into the firebox, which helps expel heat more quickly through the stack.

Acknowledgments

Produced from materials originally authored by Edward Cass from Paratherm.

This information has been sourced, reviewed and adapted from materials provided by Paratherm.

For more information on this source, please visit Paratherm.