Heat transfer fluid heating systems often are a critical aspect of many manufacturing processes. If the heat transfer fluid running through these systems is not properly operated, monitored and maintained, fluid replacement may be required before the expected fluid lifetime. Fluid replacement can be costly and result in production downtime. It is important, therefore, to ensure that the heat transfer fluid remains healthy and operational. Consider these guidelines for extending the life of your heat transfer fluid.
Operating, Monitoring and Maintaining Heat Transfer Fluids in Industrial Processes
Heat transfer fluid maintenance begins by ensuring that the system was properly designed, and the initial fluid selection was appropriate for the particular application and operating temperature range. Given that, it is expected that most heat transfer fluids will provide good performance for many years before fluid replacement is necessary. However, the manner in which a heat transfer fluid heating system is operated and maintained will impact this expected fluid lifetime. If a system is not properly operated, monitored and maintained, fluid replacement may be required much earlier than expected.
The following operational and maintenance practices play a significant role in achieving long heat transfer fluid life.
Minimize Degradation. During normal operation, heat transfer fluids can degrade due to thermal stress or oxidation. All heat transfer fluids experience degradation as a result of thermal stress or molecular cracking when subjected to high temperatures. Degradation from oxidation takes place when hot heat transfer fluid comes into contact with oxygen. Oxidation is accelerated when temperatures are above 300°F (149°C).
The extent to which a heat transfer fluid exhibits the effects of degradation while in operation depends upon the particular fluid properties and the level of degradation products in solution. When the products of degradation in solution reach the fluid’s solubility limit, the formation of solids will occur.
Excessive degradation and the resulting buildup of byproducts lead to potential mechanical and operational issues. These include sludge, fouling and coking of piping and equipment as well as decreased heat transfer efficiency.
To minimize fluid degradation, operate the fluid below the manufacturer’s recommended maximum bulk operating temperature. The maximum bulk operating temperature is the highest temperature at which the fluid can be used and still have acceptable thermal stability as recommended by the manufacturer. Longer fluid life can be achieved when the system is operated at temperatures below the maximum bulk operating temperature. It is best to operate the fluid temperature of the process near the middle of the operating range of the heat transfer fluid for minimal degradation.
Fluid thermal degradation rates are closely related to temperature. Keep in mind that continuous use above the manufacturer’s recommended maximum bulk operating temperature will increase degradation rates exponentially. Additionally, fluid can be protected by ensuring that fluid flow through the heat transfer system is sufficient to maintain turbulent flow in the bulk region. Monitor heat transfer fluid flow throughout the system, especially in the heater section.
Degradation can be minimized as well through the proper use of an expansion tank. This allows for a reservoir of fluid that is at a much lower temperature than the fluid running through the circulation loop. By using the expansion tank only for fluid expansion and not as part of the fluid flow through, minimal fluid degradation occurs for this portion of your fluid. Furthermore, keeping a clean inert-gas blanket on the expansion tank will protect fluid from oxidation.
Eliminate Contamination. Contaminated heat transfer fluid can result in unplanned downtime and can be a costly fluid changeout. Contamination from the process can cause fluid degradation and lead to operational issues as well as present a potential safety risk.
Contaminants can be introduced into the system from process fluid leaks that are a result of failed equipment or piping. Another source of contamination is through the addition of an incorrect product or fluid during routine maintenance such as top-off activities. Additionally, if the expansion tank experiences a leak due to corrosion, external contaminants could enter the system at that point.
Heat transfer fluid contamination is eliminated through proper design and maintenance. The heat transfer system should be included in a preventive maintenance program that routinely monitors and inspects equipment to ensure sources of contamination are eliminated.
Manage Heat Transfer Fluid Life Through Fluid Sample Analysis
It is important to manage the life of your heat transfer fluid through a fluid sample analysis program using a qualified laboratory. Many heat transfer fluid suppliers offer this service free of charge. Monitoring your heat transfer fluid plays a significant role in maintaining production rates, minimizing unscheduled fluid-related downtime and reducing overall heat transfer fluid costs.
Proper preventive maintenance analysis starts by taking a fluid sample at predetermined time intervals. This time interval may be determined based upon past experience with the system. In the case of no experience or a new system startup, samples should be taken quarterly. The sampling time interval then can be adjusted based on an interpretation of the data, with the first sample serving as a baseline.
Once the sample is analyzed, the data is provided to the user. Often, the heat transfer fluid supplier will interpret the sample result in a report. In some cases, the interpretation is left to the user. In cases where the results of the sample analysis are outside the used fluid limit (determined by experience), the heat transfer fluid supplier will provide heat transfer system recommendations. In other cases, where the fluid sample results are within the used fluid limits, the fluid is satisfactory for continued use.
Extending the Life of Heat Transfer Fluids in Industrial Processes
When degradation and contamination lead to a significant decrease in heat transfer fluid efficiency and production time, and costs have significantly increased, options for fluid replacement are evaluated. In many cases, a full fluid changeout may be required to restore system performance. Many times, however, the cost-effective alternatives listed below can extend fluid life and maximize system performance.
Partial Fluid Changeout. Replacement of a percentage of the initial fluid charge with new fluid can improve performance by diluting degradation byproducts or contaminates to within acceptable limits. Often, the off-spec fluid that was removed from the system can be returned to the fluid manufacturer for credit, or the fluid can be reprocessed, returned and kept on-site for future makeup requirements.
Fluid Filtration. The formation of hard carbon or coke particles can lead to the fouling of heat transfer surfaces. By using portable filtration units brought on site, a service team from a fluid manufacturer can remove these particles either while the system is in operation or during a shutdown. On-site filtration saves transportation costs and fluid drainage time and expense.
Fluid Reprocessing. Degraded or contaminated fluid is removed from the system and sent to the fluid manufacturer for reprocessing. Low boilers and high boilers are separated by distillation, and the recovered heat transfer fluid is sent back to the original user. Reprocessed fluid usually will meet new fluid specifications. Partial fluid volumes can be drained from the system and sent in for reprocessing, allowing continuous operation and eliminating a system shutdown. Reprocessing costs are based on the total quantity shipped and offer savings over the purchase of a new charge.
In conclusion, heat transfer fluid users can achieve long fluid life if steps are taken to properly operate and maintain these systems. Eliminating degradation and contamination is key to extending fluid life. It also is important to monitor and maintain heat transfer fluids through a sample analysis program. When heat transfer fluids reach a point where fluid replacement is necessary, options other than total system changeout can be evaluated. Considering these guidelines can result in improved system performance and an overall lower heat transfer fluid cost.