Milestone Thermal Processing
Explosion Protection / Heat Transfer Explosion Protection in Heat Transfer Systems: An Overlooked Risk?
The following statements focus on the question if the use of organic heat transfer fluids creates the necessity to classify an explosion zone around the heat transfer system. They are based on a long experience in designing, manufacturing and doing maintenance, repair and technical service work on a huge variety of different HTF systems as well as on our knowledge of explosion protection regulations.
The basic considerations for the explosion hazard of a heat transfer system depend on its design: the well-known and worldwide recognized German Standard DIN 4754 describes the design and safety requirements of such a system. The crucial criterion there is the concept of “technical tightness” of a heat transfer system that is designed and built according to that standard. Such systems shall be permanently technically tight due to the design.
Potential hazard areas in a heat transfer system are seals (gaskets on flanges as well as pump seals), safety relief valves, vent lines, sample valves and tanks (drain tank and expansion tank). Generally the number of flange gaskets in a heat transfer system should be minimized by design.
The use of weld-in valves and fittings is a possible way to achieve this. Where unavoidable or at least not recommendable due to maintenance and repair work (for example pumps, control valves, safety relief valves) only approved flange gaskets should be used and the installation should strictly follow the installation guidelines of the manufacturer.
Still most of the heat transfer fluid pumps in operation worldwide are designed with mechanical shaft seals and therefore very small leakages are possible at this point. Due to the special design of these pumps, the temperature of the leaking fluid is generally below the flash point. Nevertheless, mechanically sealed pumps require a high attention of the operating personnel. The use of hermetically sealed pumps as pumps using canned motor pumps or a magnetic coupling (see picture) is therefore recommended.
Safety Relief Valves
The potential of a hazardous explosive atmosphere around safety relevant points has to be evaluated and a zone classification around those areas may be required depending on location, construction and operating procedures. It is common practice and recommended by DIN 4754 to lead such “emission points” to a safe place at the outside of buildings by welded piping. As a consequence, the classification of the rest of heat transfer plant is not influenced.
In a properly designed heat transfer plant, special oil sample drafting devices should be installed. The representative oil sample is cooled down to temperatures below 50 °C before drafted.
For storage tanks that might contain organic heat transfer fluids above their flash point, a zone classification may be required for the interior of the tank. This can be avoided when the tank is covered by an inert gas like nitrogen.
Don’t Take Further Risks
For the emergency shutdown of all electrical equipment, an emergency switch has to be installed in the power supply line. This switch has to be mounted in a safe area, usually outside the heat transfer systems room. Equipment, which has to be operational after emergency shut-down, like escape route lighting, shall be safeguarded against explosion hazards.46182195
Finally, it can be concluded that if all the above mentioned standards and design criteria are considered and state-of-the-art equipment is used, it is good practice to operate heat transfer fluid systems without any classification of explosion areas deriving from the presence of the organic heat transfer fluid itself. We hope that these statements contribute to find a technically feasible solution and get a clear and objective picture based on technical arguments.
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