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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 safety, trouble-free operation and long life. Even so, unless you have a rigorous maintenance program, important maintenance needs are apt 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 from Heatec check heaters that have been in operation for a while they regularly 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 startup and adequately heat the thermal fluid flowing through coils in the heater. So, let’s discuss some of the things needed to keep your thermal fluid heaters going and safe.
Fuel valve leak tests
You know full well 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, entitled Recommended Practice for Fluid Heaters (paragraph 3.3.37) now requires the implementation of a valve proving system. 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 of one valve manufacturer may differ from those of another valve manufacturer. Heatec thermal fluid heaters use fuel valves from a variety of manufacturers, usually Honeywell, Maxon and ASCO. Plant operators should regard their 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. But Heatec recommends replacing fuel valves with any detectable leakage, regardless of the amount. 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. But 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. (Fuel trains on Heatec thermal fluid heaters have these provisions.) So 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 will then 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. And when a new heater is initially commissioned, all fuel line pipe connections should be leak tested to make sure that none have loosened from vibrations during shipment. Leak testing of the connections should be done after fuel source piping has been connected to the heater’s fuel train. The heater should actually be running on the supply fuel when the testing is done.
Thermal fluid
Using a suitable thermal fluid in your heating system is extremely important. The task begins with your 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 fluid needs to be analyzed periodically to ensure that it has not deteriorated and lost its ability to transfer heat. It should be analyzed at least once a year by an experienced specialist. Fluid suppliers may do free chemical analysis in their laboratory and provide you with a report along with their recommendations.
Always choose a fluid product designed specifically for use in thermal fluid heating systems. You would do well to consult with a qualified representative from a reputable supplier in your area when choosing a fluid for your system.
Primary concerns in choosing a fluid are startup temperature, operating temperature and film temperature. You also need to be aware of temperature limitations that valves and components in your thermal fluid system may have. Temperatures higher than 450 degrees F may damage plug valves and cast iron strainers. Steel valves and strainers are suitable for temperatures up to 550 degrees F.
Expansion tanks
Don’t overlook the need to keep a close watch on the expansion tank in your 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. But you need to regularly check the tank’s sight glass to make sure the level is not approaching its low limit. Otherwise, your heater may shutdown at an inopportune time, and cause unwanted disruption of your heating system.
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Another concern with expansion tanks is the accumulation of sludge inside the tank. You can get an indication of the sludge by examining the appearance of the fluid in the tank’s sight glass. The development of heavy sludge in your thermal fluid system may be a sign that it’s time to get the thermal fluid analyzed. It may be time to replace the fluid.
It’s also important to check the fluid level switch to make sure it is working properly. Go by instructions provided with your heater. You also need to keep the sight glass clean. To clean the sight glass, remove it from the tank and flush it out with a solvent.
If your expansion tank employs a nitrogen system (or other gas) to minimize oxidation you need to do additional maintenance tasks. The pressure imposed by the gas system should not exceed the recommendation by the tank manufacturer. The system normally has a pressure gauge at the tank that indicates pressure. 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 you can do. Accordingly, you need to replace the old valve with a spare and send the old one to a lab for testing.
Burner
The burner on your heater may need a tune-up 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 1/3 the length of the coil in a helical coil heater. The flame diameter should be approximately 2/3 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.
Other issues
The foregoing discussion mainly covers issues we frequently encounter. There are a number of other issues worthy of discussion. For example, you may want to explore the need to use sealants in plug valves and the lubrication requirements for pumps, motors and valve stems. Be sure to examine maintenance recommendations published by the manufacturer of your heater. You will find numerous Tec-Notes on the website heatec.com for Heatec heaters.
Don't overlook the possibilities for training on operation and maintenance for your heater. Heatec offers annual training classes on thermal fluid heaters used in industrial applications. We can also send our instructors to your location to train your plant operators. Call us to learn more about training possibilities.
For more information on maintenance and products, visit www.heatec.com or contact us at 423-821-5200.
