Addressing three areas -- component selection and orientation, insulation and instrumentation -- can minimize the potential for fires in thermal fluid systems.

Component Selection and Orientation. For valve stems, packing sets consisting of end rings of braided carbon or graphite fiber, and middle rings of pre-formed (pressed) graphite foil are suggested or specify bellows-type valves. Valves should be mounted stem sideward so that leakage from the valve stem or bonnet is less likely to enter insulation. Flanges gaskets should be spiral-wound carbon or graphite-filled. Flanges should not be insulated -- leave them bare or use drip covers if personnel protection is necessary. If flexible hoses are used as expansion joints, they should be installed so they always compress -- they should never move side to side. Pump couplings and shafts should have shrouds installed to prevent fluid from spraying if a seal fails.

Insulation. The type of insulation can have a significant effect on the potential for fire. Porous insulation such as fiberglass or calcium silicate allows leaking fluid to spread out inside the hot, enclosed space, coating the material. If a limited amount of air leaks in, the decomposing fluid will begin to smolder. If no air leaks in, the fluid will slowly crack to more flammable molecules. In either case, a sudden increase in the air supply will result in spontaneous ignition of the insulation.

Using closed-cell insulation is the most effective way to prevent insulation fires. It is also the most costly. If porous insulation is chosen, the cladding should be metal to prevent it from melting due to smoldering fluid. The metal cladding should be held in place with wire ties or fasteners. Plastic ties can also melt from smoldering fluid. Maintenance personnel should be cautious when working on or near any insulation that appears to be “wet” from leaking oil since removing the cladding can admit fresh air.

Instrumentation. After insulation fires, superheated fluid is the most common. This requires multiple failures. First, flow stop in the heater -- due to broken pump coupling, incorrect valve closed, etc. If the high temperature limit switch does not react, the heater will continue to put energy into the stagnant fluid and can a tube or housing failure. Secondary sensor and controls should be installed. High/low pressure switches can detect pump failure or a dead head condition. Orifice plate flowmeters provide acceptable flow alarms. High stack temperature thermocouples can also be set to warn of flow loss.

How to Checklist

Careful component selection can help prevent leaks into your insulation.

  • Minimize the use of threaded fittings. With the large degree of thermal expansion and contraction in high temperature systems, and the limited temperature range of many pipe sealants, leakage is almost guaranteed.

  • Valves should be mounted stem sideward so that leakage from the valve stem or bonnet is less likely to enter insulation.

  • Pump couplings and shafts should have shrouds installed to prevent fluid from spraying if a seal fails.

  • Porous insulation such as fiberglass or calcium silicate allows leaking fluid to spread out inside the hot, enclosed space, coating the material.

  • Use metal cladding held in place with wire ties or fasteners.


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