When correctly maintained and operated, thermal fluid (hot oil) systems can provide many years of safe, reliable and efficient process heating where moderate to high temperatures -- typically 350 to 750oF (177 to 399oC) -- are required. To ensure your thermal fluid heater has a long service life, it is essential that the system is operated and maintained by well-trained personnel. It also is necessary to ensure easy access to the supplier's equipment manuals; operators need to know whom to contact with questions and how to get help with troubleshooting. Additionally, a routine program should be in place where normal readings of temperature, pressure, flow, etc., are known and regularly compared to operating conditions. Follow these eight tips to help extend your thermal fluid system's service life:
Maintain Design Flow. One of the primary causes of reduced service life is low heat transfer fluid flow. This can lead to premature fluid breakdown and even heater damage due to hot spots, and these effects will worsen over time. Low flow also will result in less heat transfer at the heated equipment. Many systems include flow meters, and your system supplier can provide details relating actual flow to the flow meter reading. An alternative method to monitor flow is by measuring differential pressure across the heater. Check that proper system flow is present and demonstrated by your supplier's technician at startup. As quickly as possible, investigate and cure low flow during operation.
Choose the Right Heat Transfer Fluid and Check It Regularly. Literature and assistance are available from fluid suppliers to ensure that the chosen fluid is appropriate for the system operating conditions. A representative fluid sample should be taken, typically once a year, and sent to the manufacturer for analysis. (Many fluid suppliers provide sampling kits.) The results may show anomalies in the fluid properties such as oxidation, carbonization and viscosity increase that can indicate specific system problems. Also, sampling kits will indicate when the fluid should be changed. If the fluid is left in the system too long, heat transfer efficiency will be reduced and components such as pumps and control valves can be damaged by particulate buildup. If particulates are present, they can be removed by the use of a sidestream filter (figure 1). If additional fluid is required due to replacement or to makeup losses, it should be pumped into the system at a low point, not added to the expansion tank. Filling the system from the low point(s) allows air to be vented naturally.
Another problem is that in many cases, systems are oversized, leading to repeated burner cycling. Often, cycling can be avoided by derating the burner capacity, which results in continuous firing with modulating control and causes much less wear and tear on the burner, heater and heat transfer fluid. Do not let the system overheat; temperature alarms should be in place to shut off the burner if the temperature exceeds the required operating temperature.
Keep Expansion Tank Cool. Unless the tank has an inert gas blanket, which is recommended, there is an interface in the expansion tank between the heat transfer fluid and air. This is where oxidation breakdown of the fluid can occur, and the oxidation rate increases exponentially with temperature. Oxidation can cause sludge to build up in the tank that can interfere with the operation of level switches and be drawn into the system piping. Most systems have a temperature block or buffer design to prevent high expansion tank temperatures. Check the expansion tank temperature regularly to ensure it is near ambient. If the tank has an inert gas blanket, make sure the nitrogen (or other gas) supply is reliable and maintained at the correct pressure.
Ensure Sufficient Venting in Heater Room. To dissipate heat lost through the heater casing and to provide combustion air, the heater room should be vented adequately. Generally, the heater controls are in the heater room. The lower the ambient temperature is, the longer the heater controls' service life will be. Often, the heater is close to a process that produces a lot of dust, and in this case, it is better to duct in combustion air from a clean area or from outside.
In addition, pressure gauges at the pump suction and discharge are valuable for troubleshooting. A reduced inlet pressure could indicate a strainer blockage or low fluid level as usually the pump suction is connected to the system expansion tank to provide the necessary suction head to avoid cavitation. Unsteady readings may indicate cavitation that can quite quickly cause internal damage to a pump.
Check Burner. Burner combustion efficiency should be checked regularly by measuring the combustion products' carbon dioxide (CO2) content. Too little combustion air can overheat the burner, produce soot and cause damage. Too much combustion air reduces the system efficiency by increasing fuel consumption. Simply monitoring the heater flue gas temperature can indicate a problem quickly. The lower the temperature, the higher the heater efficiency. Increased flue gas temperature could indicate the burner is out of adjustment or soot buildup in the heater.
Inspect Heater. A daily external inspection can catch heater problems before they become serious. Look for signs of overheating (discoloration, deformation) and schedule an internal inspection if they appear. In many cases, a minor refractory failure can be repaired before it becomes a major problem. Also, keep an eye on the heater exhaust as smoke may indicate a poorly adjusted burner or a tube leak. Again, a small tube leak caught early will be easier to repair than a leak that has been allowed to develop over time.
Being aware of these eight points can aid in extending the life of your thermal fluid heater, but the most effective insurance is trained operators who know the system and can quickly spot anything unusual that may lead to a breakdown. In addition, keep a good relationship with your system supplier -- a great source of information, assistance and compatible spare parts. Working together will ensure optimum equipment life and operating efficiency.