When properly maintained and operated, thermal fluid heaters can last 20 years or more. Here’s how to get the most from yours.

Heaters, because they are designed with specific fuel inputs, thermal fluid flow rates and operating temperatures, should never operate outside of a manufacturer’s design parameters. While it is important to always consult manufacturer instruction manuals for specific system maintenance practices, the following discussion and tips will provide general guidelines applicable to all systems.

Heater manufacturers provide controls and instrumentation to ensure proper equipment operation. When commissioning the equipment, all safety interlock settings and pressure gauge readings should be recorded for future reference. The key areas of maintenance for the heater include:

• Thermal fluid flow.
  
• Burner combustion settings.
  
• Functionality of all interlocks.
  
• Thermal fluid leaks.

Most heaters have minimum thermal fluid flow rate requirements. Manufacturers typically provide a means to verify flow through the heater (pressure gauges) and often include an interlock to ensure the heater will not provide heat to the fluid in a low flow condition. When performing heater maintenance, it is critical to ensure that the flow rate (pressure drop across the heater) is based on the manufacturer’s specifications and that the setpoints of any pressure switches have not been reset or jumpered. A low flow condition will degrade the fluid and could result in pressure vessel failure.

The control panel and fuel train provided with the heater ensure a proper combustion sequence. It regulates the combustion based on fuel input and pressure, status of the fuel valves and adequate combustion air. Fuel inputs and fuel pressure readings should be verified and recorded weekly.

In addition to flow and combustion safeties, additional interlocks such as a high temperature limit or a high stack temperature limit may be included with the heater. The high limit switch shuts down the heater to prevent the fluid from overheating, and the stack limit will shut down a heater in the event of improper combustion or pressure vessel failure. Functionality of these interlocks should be tested monthly.

It is recommended that a log sheet be maintained to record the operating temperature, stack temperature, pressure gauge readings and differential pressure across the heater. The log sheet will help identify any anomalies in the heater operation and can help prevent a catastrophic equipment failure. Typically, log sheets should be completed once per shift.

Valves and piping should be inspected daily for any leaks, which should be repaired as soon as possible when the system is cool. Thermal fluid vapors may be flammable, so work areas where weld repairs will be performed on vessels or piping should be continuously purged with an inert gas. Consult the thermal fluid manufacturer for appropriate safety precautions.

Thermal Fluid Pumps. The thermal fluid pump is a critical system component. Pumps with mechanical seals should be cold aligned prior to starting the system and hot aligned within the first 24 hours of operation, per the manufacturer’s instructions. Failure to hot align the pump can result in premature pump seal failure.

Valves. If a valve is leaking through its packing, consult the heater or valve manufacturer for proper repair procedures. If the valve cannot be repaired, it should be replaced as soon as possible. Horizontal or downward orientation of the valve handle is preferred.

Heat Exchangers. Whether used to cool the process thermal fluid or generate hot water or steam, control of the water-side chemistry is imperative to prevent fouling or scaling and to minimize corrosion. Failure to do so can result in poor performance or failure of the heat exchanger.

Thermal Fluids. Nearly all manufacturers of heat transfer fluids provide after-sale service to monitor the condition of the fluid in operation and make recommendations when replacement becomes necessary. Each fluid manufacturer has procedures for regular testing and fluid analysis. These usually allow for a sample to be taken and analyzed at least once a year; actual frequency will depend on operating temperature, number of hours operated weekly and the results of tests made during the first weeks of system operation. The thermal fluid in a system should be analyzed frequently within the first two months after startup to confirm that system performance has been predicted correctly. Thermal fluid suppliers should contact new operation points within four weeks of commissioning to make certain that the “fill” is registered for routine analysis.

Suggested Maintenance Schedule

Now that you know what to service, it is important to service it according to a schedule. General guidelines include daily, weekly, monthly, semi-annually and yearly tasks. On a daily basis:

• Make a visual inspection of the entire system for leaks. Any leaks should be repaired as soon as possible while the system is cool.
  
• Note any failures on the flame programmer, including the fault number, fault code, fault annunciation, fault hour code, cycle and time. Faults are not typical and should be discussed with the manufacturer.

On a weekly basis:

• Check the gas pressure at the inlet of the gas train and last elbow pressure. These readings should be within 0.2" w.c. of the readings taken when the system was commissioned. An appropriately scaled gauge should be installed to facilitate this.

On a monthly basis:

• Check and clean any fuel filters.
  
• Check the blower fan and clean if needed.
  
• Manually check the fluid level in the expansion tank.
  
• Check the operation of all safeties. Contact the heater manufacturer for instructions specific to the instrumentation supplied with your heater.
  
• Verify operation of your flame sensor.
  
• Verify proper fluid level for pumps using barrier fluid.
  
• Check and maintain proper cooling water flow rate and temperature for water-cooled pumps.
  
• Provide a visual inspection of the pump to ensure that the pump seal is not leaking (vapor, dripping fluid and/or carbonized fluid are indicative of a seal leak).
  
• Review the log sheets and note any deviation from the norm. Contact the heater manufacturer should you notice any trends or differences in items such as pressure readings or stack temperature.

On a semi-annual basis:

• Pull the burner and inspect for heat stress or soot. Clean or replace if necessary.
  
• Inspect the pilot assembly and ignition electrode. Clean or replace if necessary.
  
• Inspect any refractory or insulation. Replace if cracked or missing pieces.
  
• Check the combustion for efficiency.

On a yearly basis:

• Replace the ignition electrode.
  
• Clean the thermal fluid strainers
  
• Have the thermal fluid tested.

These tips are standard among all manufacturers’ thermal fluid systems. Some additional requirements may be dictated by local codes and regulations. Many heater manufacturers offer customized maintenance programs and training for plant personnel.

Links

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