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July 2003: Heat Transfer Systems

Heat transfer systems are used widely throughout various industries. Their most common use is in chemical processes where temperature control is needed for large quantities of materials. The greatest concern is where the heat transfer fluid (HTF) is an organic or synthetic fluid. Organic and synthetic HTF's have high flash points but are typically heated up to or above their flash points. Heating of the HTF is done using fuel-fired heater/vaporizers; oil- and gas-fired heater/vaporizers are normally used, but wood dust suspension-fired heater/vaporizers are also utilized.

Incidents involving HTF systems include operating beyond the safe operating range of the fluid, improper location of a discharge outlet from a safety valve or system breather vent, and leakage from connection points on piping. The causes for these incidents revolve around the design and operation and maintenance of the systems. Most of these incidents resulted in fires, though in some cases explosions have occurred.

Prevention of the above incidents first starts with the proper design of the HTF system. The operating temperature range for the process should not exceed the suggested maximum operating temperature for the HTF. You should also look at using a HTF that has a flash point above the process operating temperature range. Using an HTF near or above its boiling point should be avoided; leaks on the system will cause the HTF to vaporize and create an explosion potential.

The heater/vaporizer should be cut off from the process being heated; if the HTF is heated near or above its boiling point then damage-limiting construction is needed. Construction of the cut-off room should conform to what is needed for flammable liquid operations. Impact resistant walls are needed if the HTF system operates above 15 psig.

The process equipment and piping should be arranged so it will not be exposed to temperatures below the pour point of the HTF. If this cannot be suitably arranged insulation and/or steam heat tracing may be needed.

Piping for the HTF system needs to be protected from damage and have adequate clearance from combustible materials. Combustible deposits on piping and process equipment should be removed on a periodic basis. Shielding around seals, gaskets, flanges, valve packing and other potential leak points will help prevent spray release which can lead to a fire or mist explosion. Pressure relief valves that do not discharge back into the HTF system should be piped to a safe outdoor location keeping in mind that the HTF may be burning.

Adequate combustion safeguards should be provided on the heater/vaporizer, using the appropriate standard for the type of heater/vaporizer fuel used.

Proper operation and maintenance is essential to preventing a malfunction of the HTF system. The HTF system should be operated well within the operating temperature limit for the HTF. Visual examinations are needed of the heater/vaporizer for leaks, broken refractory and hot spots on tubes. The HTF should be sampled and tested on a regular basis for impurities and/or degradation.

Maintenance and housekeeping starts with promptly and permanently correcting all leaks on the HTF system. Any HTF spilled should be cleaned up immediately. All safety valves should be disconnected on an annual basis, inspected, repaired or replaced as needed, and tested to ensure they operate below the rated pressure of the HTF system (this is a requirement of all pressure vessels subject to the National Board Inspection Code). The heater/vaporizer should also be inspected on a regular basis per National Board standards.

Training of operators should include normal and emergency procedures. The operators should understand the hazards relating to the misoperation of the HTF system and to any leaks. The site Emergency Organization should be aware of the location of HTF piping, shutoff valves, proper fire fighting methods and freeze-up potentials.

Automatic sprinklers are needed where they are subject to an HTF spill fire, including the heater/vaporizer room, process areas/rooms and HTF piping. If the heater/vaporizer provides a major source of process heat for production, the interior of the firebox and portions of the heat exchanger subject to an internal HTF spray or spill fire should be protected with a fixed inerting system. The inerting system should be automatically actuated. Portable extinguishers should be provided throughout all areas containing HTF equipment or piping.

If you would like further information regarding heat transfer systems, please contact Risk Logic Inc.