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Explosion Avoidance
ZONE 2
ZONE 22
ZONE 21
ZONE 1
ZONE 0
ZONE 20
TANK
FLAMMABLE LIQUID
FLAMMABLE DUST
Localisation of gas and dust ex-zones
Flash point
• Lowest temperature at which a volatile substance can vaporize to form an ignitable mixture in air
• An additional source of ignition is required to start burning . When the source of ignition is removed the burning normally ceases
• Most commonly organic heat transfer fluids are operated at temperatures above their flash point ( i . e . flash point Marlotherm SH : 200 ° C acc . DIN 22719 )
Auto ignition point
• Lowest temperature at which a volatile substance will spontaneously ignite in normal atmosphere without the necessity of an ignition source . Most commonly , organic heat transfer fluids are operated at temperatures below their auto ignition point ( i . e . auto iginition point Marlotherm SH : 450 ° C acc . DIN 51794 )
Avoidance of explosive atmospheres Basic considerations According to the regulations TRBS 2152 ( Technical Rules for Operational Safety ) and TRGS 720 ( Technical Rules for Hazardous Substances ) published by the responsible German committees , the occurrence of hazardous explosive atmospheres outside of systems can be avoided or effectively restricted by the tightness of the system . Basically , it will be distinguished between permanently technical tightness , technical tightness and not-guaranteed tightness .
• Permanently technical tight : ( either by design and / or by inspection and maintenance ): no or only negligible release of combustible substance and hence no explosion hazardous areas to be classified
• Technically tight : ( no release in normal operation , only in case of rare malfunctions ): only secondary release in areas ( zone 2 )
• Tightness not guaranteed : primary release ( zone 1 )
According to the explosion protection guidelines published by BG Chemie ( ZH 1 / 10 published June 1988 , now changed to BGR 104 ), there is no requirement for explosion protected design of electrical equipment used in heat transfer systems that are technically tight ( by construction ). At areas where a hazardous quantity of an explosive atmosphere can occur due to leakages , a classification of zone 2 in a 3m radius around such release area should be defined . The basic considerations for the explosion hazard of a heat transfer system depend on its design . The DIN 4754 describes the design and safety requirements of a heat transfer system . The crucial criterion is the concept of “ technical tightness ” of a heat transfer system , designed and built according to that standard . Such heat transfer systems shall be permanently technically tight due to the design .
Potential hazard areas 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 ).
Seals and gaskets 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 .
Pump seals 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 a magnetic coupling or canned motor pumps is therefore recommended .
48 Heat Exchanger World November 2022 www . heat-exchanger-world . com