As a plant operator, you know that equipment at your facility requires maintenance. You undoubtedly know that thermal fluid heaters need a certain amount of maintenance to ensure safe, trouble-free operation and long life. Even so, unless you have a rigorous maintenance program, important maintenance needs likely are to be overlooked. For example, when was the last time you did a leak test on block valves in the fuel train on your fired heater? When was the last time its burner was tuned? When was the last time the thermal fluid in your system was analyzed for deterioration?
When service technicians check heaters that have been in operation for a while, they frequently find several items that have not been properly maintained. In some cases, the heaters have become unreliable due to poor maintenance. When properly maintained, thermal fluid heaters are extremely reliable and long lived. They rarely fail to start up and adequately heat the thermal fluid flowing through coils in the heater.
This article will review some of the things needed to keep thermal fluid heaters going safely.
Fuel-Valve Leak Tests
It is well known that leaking fuel is a fire and explosion hazard. But, some plant operators are not aware that regular leak tests of fuel valves are needed — and are required. Over time, seals in valves can fail or deteriorate, allowing leaks to go unnoticed. Accordingly, NFPA 87, Recommended Practice for Fluid Heaters, now requires implementation of a valve-proving system (paragraph 3.3.37). Leak tests usually involve doing a bubble test to prove their safety.
Each valve manufacturer covers leak testing for their valves in the instruction materials they furnish. The test recommendations from one valve manufacturer may differ from those of another valve manufacturer. Most thermal fluid heaters use fuel valves from a variety of manufacturers. Plant operators should regard the manufacturer test recommendations as minimum requirements. Different valves have different features to facilitate testing.
The amount of allowable valve leakage also varies from one manufacturer to another. It is recommended that fuel valves with any detectable leakage — regardless of the amount — be replaced. Leaking valves tend to get worse with time. So, letting a leaking valve continue in service until the next scheduled test could be risky.
As you would expect, leak testing the fuel-train valves requires shutting down the heater. Fortunately, it only requires a few minutes to leak test the valves if the fuel-train piping already includes isolation valves and fittings downstream of the fuel valve being checked. If you have a heater lacking these provisions, you will have to install them before you can do the necessary leak tests of the main fuel valves. You then will need to initially leak test the new isolation valves and connections before you proceed to test the main fuel valves.
Block valves in the fuel system should be leak tested at least once a year. Also, when a new heater is initially commissioned, all fuel-line pipe connections should be leak tested. This will help ensure that none of them have loosened as a result of the vibrations the heater experienced during shipment. Leak testing the connections should be performed after the fuel-source piping has been connected to the heater’s fuel train. The heater should be running on the supply fuel when the testing is done.
Thermal Fluid Testing
Using a suitable thermal fluid in the heating system is extremely important. The task begins with the initial choice of an appropriate fluid. Fluids designed specifically for heat transfer have special properties. They have additives that maximize heat transfer and minimize oxidation.
All thermal fluids must be analyzed periodically to ensure that they have not deteriorated and lost the ability to transfer heat. The fluid in your heating system should be analyzed at least once a year by an experienced specialist. Fluid suppliers may offer a free chemical analysis in their laboratory and provide a report along with their recommendations.
Always choose a fluid product designed specifically for use in thermal fluid heating systems. When choosing a fluid for your system, consulting with a qualified representative from a reputable supplier in your area is strongly suggested.
Primary concerns in choosing a fluid are startup temperature, operating temperature and film temperature. It also is important to be aware of any temperature limitations the valves and components in the thermal fluid system may have. Temperatures higher than 450°F (232°C) may damage plug valves and cast-iron strainers. Steel valves and strainers are suitable for temperatures up to 550°F (287°C).
Expansion Tank Maintenance
Do not overlook the need to keep a close watch on the expansion tank in a thermal fluid system. The first concern is the fluid level in the tank. Most tanks have a switch that is activated and shuts down the heater if the fluid level in the tank becomes too low for safe operation. Still, it is necessary and important to regularly check the tank’s sight glass to make sure the level is not approaching its low limit. Otherwise, the heater may shutdown at an inopportune time and cause an unwanted disruption of the heating system.
Another concern with expansion tanks is the accumulation of sludge inside the tank. An indication of the sludge can be obtained by examining the appearance of the fluid in the tank’s sight glass. The development of heavy sludge in the thermal fluid system may be a sign that it is time to get the thermal fluid analyzed. It may be time to replace the fluid.
It also is important to check the fluid level switch to make sure it is working properly. Go by instructions provided with the heater. The sight glass must be kept clean. To clean the sight glass, remove it from the tank and flush it out with a solvent.
If the expansion tank employs a nitrogen system (or other gas) to minimize oxidation, additional maintenance is required. The pressure imposed by the gas system should not exceed that recommended by the tank manufacturer. The system normally has a pressure gauge at the tank that indicates pressure in the tank. It should be checked regularly. The tank also has a relief valve to prevent overpressure. This valve needs to be tested periodically, but this is not something that can be done in-house. Accordingly, it is necessary to replace the old valve with a spare so that the old one may be sent to a laboratory for testing.
The burner on the heater may need a tuneup from time to time. An improperly tuned burner reduces fuel efficiency and produces unwanted emissions. Under normal operating conditions with a properly tuned burner, the heater should not smoke. The burner flame can be viewed through the sight glass on the heater endplate. It should be about one-third the length of the coil in a helical coil heater. The flame diameter should be approximately two-thirds the diameter of the coil. Flame color on natural gas fuel should be bright blue. The flame should be stable and should not impinge the coil.
Only a thoroughly qualified burner technician should tune the burner. The technician will use an exhaust gas analyzer to ensure that the exhaust meets emission standards.