Sound Design, Good Procedures Pay Off for Thermal Fluids Systems

Sept. 1, 2000
Although thermal fluid, a petroleum-based liquid formulated for high temperature service, has proven exceptionally safe, there is no way to completely

Although thermal fluid, a petroleum-based liquid formulated for high temperature service, has proven exceptionally safe, there is no way to completely prevent fires in the systems. That is because of major components present - fuel, air, and ignition source. Typically, the heating systems are used in storage facilities where the product has to be heated, such as asphalt.

However, the potential for a serious fire can be minimized by observing sound design and operating procedures, said Jim Oetinger, Paratherm Corporation, Conshohocken, Pennsylvania.

"Mechanical design and installation should be taken into consideration where the heater is located in a building," he said. "Provisions should be made for adequate ventilation to prevent any possible buildup of vapors."

Oetinger made the comments at the Independent Liquid Terminals Association meeting June 12 in Houston, Texas.

A dike encompassing the heater and pumps can contain leaks. Other fired equipment such as steam boilers should be located in another area. Hydraulic systems that have the potential for spraying fluid across extended distances should be located well away from the heater.

Flange leaks are usually more of a nuisance than a hazard, but they create a housekeeping problem and result in smoke. Leaks can be minimized by using 300-pound flanges and gaskets made of graphite, or an appropriate fiber reinforced by Teflon.

Isolation and bleed valves should be installed in the piping to each heat user so that maintenance can be performed without draining the whole system, he said.

Globe, ball, or plug valves with appropriate stem packing are recommended for thermal fluid service. "Wherever possible, valves should be installed with stems sideways so that any leaks run down the stem and away from the piping," he said.

Expansion Joints Expansion joints should always be located so that they expand axially and not sideways.

To prevent excessive thermal cycling of the heater tube bundle, oversized heaters should be derated by the manufacturer. If cold startups are planned, a bypass valve should be installed at the heater to avoid overheating the fluid.

Another area of concern is the catch drum that collects material discharged from the relief valves, as well as overflow from the expansion tank. Catch drums can contain water that has been boiled out of the system and the more volatile components of the thermal fluid that accumulate in the expansion tank. The drums should be drained periodically. Either of these types of materials can create a vapor cloud if hot fluid is discharged into it. Catch drums should be located inside a fireproof cabinet away from the exit door of the heater room.

Another way to minimize risk involves expansion tanks. They provide room for the fluid as it expands during heatup. The tank should maintain a positive head pressure on the pump suction. Because the tank is usually located above the system, it can provide a continuous source of fuel to a fire if a leak develops.

"A properly sized expansion tank will minimize the amount of fuel available for any fires," he said. "A widely used rule of thumb is to multiply the expansion volume of the fluid by 2.25 to obtain the total expansion tank volume."

Another concern with expansion tanks is that they can cause fluid oxidation. Oxidation occurs if the tank remains hot (more than 140???F) during normal operation and is open to air. The reaction of the hot fluid and air forms tars and sludge that coat surfaces and reduce heat transfer. In the heater, these deposits create the hot spots that can ultimately cause cracks. Oxidation can be prevented by keeping the expansion tank cool (less than 140???F) and/or oxygen free, he said.

Flowrate is another risk to consider. The typical fired heaters transfer heat to the thermal fluid in two ways - by radiant and convection heat transfer. The convection section is the portion of the piping that is in contact with the high temperature combustion gases. The radiant section is the portion of the piping that directly faces the flame. Because a disproportionate amount of heat is transferred in the radiant section, the tube surface temperature can routinely reach 700???F, which can cause thermal cracking of the fluid. Thermal cracking causes the fluid molecules to break apart, forming smaller, more volatile components ("low boilers"), and also forming solid carbon.

The increase in concentration of low boilers decreases the fluid density, which in turn reduces the heat delivered by each gallon. Users compensate for this loss of heat by increasing the heater outlet temperature, thereby increasing the rate of fluid degradation. The accumulation of the low boilers will also decrease the fluid flash point, which can increase the potential problems due to leaks.

To prevent excessive cracking, the fluid flow rate must be maintained under all operating conditions. It is critical that pumps are sized to provide the required flow rate under all process conditions. Systems with multiple users should incorporate a pressure control valve connecting the fluid feed and return headers. Any filters should be installed on a side stream loop around the pump.

Insulation Fires Insulation fires almost exclusively occur when fluid leaks into open-type materials, such as fiberglass wool and calcium silicate that allow the fluid to migrate away from the source of the leak. Closed-cell materials, such as Pittsburgh Corning Foamglas, contain the fluid near the leak, reducing the fire potential of the fluid. While open-cell insulation does present more of a potential for fires, it is extremely cost effective in comparison to closed-cell materials.

"For the best of both worlds, open-cell insulation should be installed only on long piping runs where the possibility of leakage is extremely remote," he said. "Closed-cell insulation should be installed on valves, filters, or anything that has a potential for leaks."

The closed-cell material should be extended 18 inches on either side of the potential leak point, and weep holes should be drilled in the bottom to drain any leaked fluid.

"Do not use plastic ties to attach weather shields or insulation," he said. "They can melt during a fire and allow burning insulation to fall off the pipe."

Flanges should not be insulated. Install drip caps if necessary for personnel protection. Do not insulate pump seal and shaft areas, he said.

Low flow shutdown should be included in the burner safety interlock. Flow detectors that are immersed in the fluid are not recommended since they can fail in the wrong position. Pressure sensors have proven to be the most reliable in long-term service. To provide effective indication of a no-flow situation, pressure sensors can be installed across a fixed restriction such as the heater to measure pressure drop or as high and low pump discharge pressure monitors, he said.

"Excessive thermal cracking can also occur during heater start-up and shutdown," he said. "Cold fluid, less than 100???F, has high viscosity, which not only makes it difficult to pump but also reduces its ability to transfer heat."

During startup, the burner should not be operated at a high firing rate until the fluid temperature has reached 150???F. To properly shut a heater down, the fluid should continue to circulate until the temperature is below 250???F. This will insure that the residual heat from the heater refractory has been removed.

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