Much effort is given to researching the best design features to build into a high-temperature heat transfer fluid system. But is that enough? The following situations provide insight into opportunities beyond system design that can help to avoid unplanned downtime, extra costs and ensure safe and optimal system performance. Each insight includes a situation and the corrective actions suggested.
Case 1: Adequate Heat Transfer Fluid Training
Situation. A heat transfer fluid system in Unit A was running very well with Fluid A. Employees from Unit B were reassigned to Unit A so they could remain productive while repairs to Unit B were made. Unfortunately, the relocated employees were unfamiliar with the Unit A system and were not trained adequately for their temporary assignment.
When told to add top-up fluid into the Unit A heat transfer system, the employees from Unit B mistakenly added the wrong fluid (a fuel). The potential risks caused in this situation included flashpoint depression, chemical interaction and/or excessive thermal degradation, and potentially having to drain and replace the entire volume of Fluid A.
Corrective Action. Following discussions with the heat transfer supplier’s fluid specialist, it was determined that venting the bulk of the fuel from the heat transfer fluid was possible. This corrected the error to a degree and allowed the plant to continue using the fluid. Experienced fluid specialists were able to closely monitor the fluid for changes over time to check for any unusual degradation.
Takeaway. This situation can be avoided by ensuring proper training for those operating the system (even for those temporarily assigned) and by having good technical support available from the fluid supplier. Supplemental training may also come from the supplier of heat transfer fluid or equipment.
Case 2: Ineffective Communications Prior to Fluid Fill
Situation. One process owner was installing a piping and equipment network to achieve an aggressive commissioning deadline. The system included a bulk storage tank for holding the heat transfer fluid that could later be distributed to the piping network after its completion. This allowed the delivery of the new heat transfer fluid to the customer while work on the piping continued. The fluid was unloaded into bulk storage. After the first day of filling the bulk tank, crews secured the area for the night.
On the second day, prior to unloading the next shipment of fluid, workers noticed a gasket had been removed from the unloading line and was lying on top of the piping. Follow-up determined that some of the mechanics were not informed that the unloading piping and bulk storage tank had been commissioned prior to the rest of the system. Had the removed gasket gone unnoticed, fluid would have easily poured from the ungasketed flange pair.
Corrective Action. There is no substitute for good communication, which involves clear information shared and adequate attention by those needing to know.
Takeaway. Communications may at times require verbal, written (including email) and posted signage in areas to ensure effectiveness. Regular reinforcement of the procedures and policies can provide added assurance.
Case 3: Use Technology to Monitor Key Process Parameters
Situation. A process owner required a medium-temperature organic heat transfer fluid to maintain a proper amount of heating of asphalt tanks and piping. The system was one that provided a non-flow-through expansion tank; that is, one in which the return (cooled) heat transfer fluid stream continuously bypassed the expansion tank in its normal circulation pattern. This routinely kept the fluid within the expansion tank at a relatively cool temperature. This also permitted the tank to be open to the atmosphere and to avoid blanket gas inerting system costs because the relatively low fluid temperature within the tank did not support significant oxidation.
Occasionally, the operators would temporarily divert the return-stream flow through the expansion tank to permit venting from the system of accumulated low-boiling thermal degradation products. On one occasion, the diversion valve was accidentally not returned to its normal position after venting. This permitted the fluid temperature in the expansion tank to remain elevated for many weeks, during which time it experienced significant oxidation.
Corrective Action. While the design was adequate to avoid oxidation, it was not foolproof, in that human error or inadequate training left the system fluid vulnerable to oxidation because of improper valve alignment. Because oxidation is known to create organic acids, increased insoluble solids content and viscosity, and increased corrosion potential, this circumstance represented a potentially large impact to plant costs and mechanical integrity of the system.
Takeaway. Monitoring key indicators, and the ability to set trend and deviation alarms of key parameters in modern distributed control systems (DCS), are tools process owners have at their disposal to protect their investments in fluid and equipment.
Case 4: Monitor the Right Performance Indicators and Know When to Flush the System
Situation. A process owner was using a mineral oil to provide process heating, but its quality had deteriorated with use over time. When mineral oils age, it is not uncommon for them to form sludge and fouling deposits. However, the sludge can separate fairly well from the fluid, making the analysis of a fluid sample a poor indicator of the amount of sludge deposits in the system. Because the fluid sample showed the fluid was not laden with solids, the process owner in this example decided not to flush the system and refilled with a heat transfer fluid, a synthetic aromatic chemistry.
Unfortunately, once the fluid was circulating in the system, the fresh fluid very effectively stripped the sludge deposits from the system and swept them downstream into the intake of the circulation pump, choking flow.
Corrective Action. In a situation such as this, key process indicators may better reflect the actual conditions present and can be used to determine whether flushing of the system should be performed. Key process indicators can include batch heating/cooling times or cycle times, small diameter heat tracers becoming relatively cool, and increased pressure drop through the system, among others. For this process owner, the system required additional downtime and cost to fully drain and properly flush the system before refilling with new heat transfer fluid.
Takeaway. While cost savings are important, the prudent approach is to closely look at key performance indicators. The process owner also can physically inspect areas of the heat transfer fluid circuit such as the expansion tank bottom for a sludge layer to make an informed decision on the possible need to flush. By taking the time to look at these key indicators, system flushing prior to fluid replacement may be the smart move that ultimately saves both time and money by avoiding unknown pitfalls.
Case 5: The (Potential) Issues with Hydrostatic Testing
Situation. A large percentage of high-temperature heat transfer system operators have experienced this problem more than once. A process owner dutifully conducted a hydrostatic pressure test on the system piping to ensure it would be leak free. The test was performed using water per the ANSI/ASME B31.3 Chemical Piping Code requirements. After confirming the piping installation was sound, the test water was drained from the piping network, but a few gallons remained. When the heat transfer fluid was filled, circulated and heated to approximately 240°F (120°C), the water began to flash into vapor phase, creating cavitation at the pump that resulted in erratic flow rates. As a result, the temperature had to be reduced for the symptoms to subside.
Corrective Action. For the process owner, the temperature had to be reduced for the symptoms to subside. Management of the small amount of residual water in the system typically involves circulating the heat transfer fluid through the expansion tank at a temperature just above 212°F (100°C) while venting to a safe location until all signs of excess moisture subside (figure 1).
A second method commonly used is to purge the piping network after it is drained of water by blowing dry nitrogen gas through the system. The process owner allows the nitrogen and moisture to exit at the opposite end of piping and discharge to a safe location until measurement of an acceptably low dewpoint of the exiting gas indicates sufficient piping dryness. Risks of this method include not purging each piping branch effectively and ending the process while pockets of water may remain pooled within heat exchanger shells, static piping legs, etc. This residual water can pose problems after startup if swept into very high temperature streams where steam can suddenly be evolved. These first two methods are enhanced by providing sufficient low-point drains in the system piping.
A third approach to resolving this problem is to use an alternate medium — typically the heat transfer fluid itself or air — to perform the pressure test. These options may be possible while following the allowances provided in the piping code.
Takeaway. Pressure testing the piping is a responsible practice, but plans should be in place to address the residual water left in the system.
Case 6: Use the Correct Flushing Fluid to Clean a System
Situation. A major process technology was using the DP:DPO eutectic fluid to remove the heat of reaction given off by boiling, using its heat of vaporization to convey the exothermic process heat from the system. This process can form carbon deposits on the surface of heat exchange tubes over time due to excessive film temperatures resulting from low/no circulation of the heat transfer fluid.
The process owner wanted to take advantage of system downtime to flush the equipment with an organic flushing fluid. If any flushing fluid was not purged from the system, there was a risk that residual flushing fluid would be unable to withstand the extreme temperatures of the system during operation. The process owner proceeded with flushing the system with the organic fluid. Unfortunately, residual flushing fluid in the system fouled the exchange surface.
Corrective Action. A few weeks after startup, the system was shut down and the entire tube bundle was replaced due to fouling that resulted from the flush fluid residues left in the system. Careful consideration must be made in using an organic system flushing fluid due to the potential consequences should residues be left behind in the system.
Takeaway. The supplier of the heat transfer fluids and flushing fluids should be the first resource for guidance on the proper use of these products for safe and effective cleaning and for avoiding potential chemical compatibility problems or thermal degradation problems with residues after startup.
The preceding insights are drawn from actual experiences and demonstrate that costly consequences can — and do — occur even when the system design is not a contributing factor. Adequate training and clear communication, utilizing the expertise of the heat transfer fluid supplier or equipment supplier, and proper assessment of the key performance indicators in the process can minimize human error and achieve the goals of safe operations, controlling costs and avoiding unexpected process downtimes.