Potential for Explosions in CA Storage Facilities 



Henry Waelti and Eugene M. Kupferman 

 Washington State University 



A certain amount of risk is involved when using 

 combustible gases to generate CA atmospheres. This risk 

 can be minimized if users understand the principles of 

 operation of CA equipment so that they can take the 

 needed precautions. 



The common gases used in CA generators arc propane 

 and natural gas. Both fuels can be ignited by a spark or 

 flame if they are allowed to accumulate to certain concen- 

 trations and if sufficient oxygen is present. 



Explosive Limits 



The limits of gas-to-air ratios between which explo- 

 sions can occur are expressed as lower explosive limit 

 (LEL) and upper explosive limit (UEL). Outside of these 

 limits there is no danger of explosion. For propane they are 

 2.2% (LEL) and 9.5% (UEL). This means that if there is 

 less than 2.2% propane in a room, there is not enough fuel 

 to explode, or if there is above 9.5% propane in a room, 

 there is not enough oxygen present to allow an explosion. 

 (For natural gas, the LEL is 5.3% and the UEL is 14%.) In 

 addition to the gas, a minimum of 11% oxygen is required 

 to create an explosion with propane. 



Open Flame Burners 



In open flame burners, without catalytic converters, it 

 is essential that the correct amount of gas is used to obtain 

 complete combustion (4.2% propane in normal air). If the 

 ratio of gas to air is lower, or higher, incomplete combus- 

 tion occurs, resulting in production of carbon monoxide 

 and ethylene gas, which are detrimental to fruit quality. 

 Also, carbon monoxide may leak through walls, accumu- 

 late in work areas, and create a health hazard to workers. 

 Fortunately, there are few of these burners left. 



Inert Gas Generators 



Inert gas generators, such as Tectrol and Isolcell, 

 produce an inert gas by direct and catalytic combustion of 

 fresh air and propane or natural gas. As long as the fuel- 

 air mixture remains at the correct ratio and the catalyst is 

 working properly, complete combustion is assured. 



Recirculating 



In recirculating systems, such as the catalytic oxygen 

 burners (COB), the fuel is oxidized on a catalytic surface 

 without a flame. As in the open flame burners, enough 



oxygen must be available to combine with ALL the fuel 

 present. Thus, as the oxygen level in a storage decreases, 

 the fuel supply must also be decreased. Although catalytic 

 oxidation of fuel may occur without a flame down to 0.5% 

 oxygen, these burners should not be operated below 3% 

 oxygen. With the proper gas-to-air ratio in the COB, the 

 operating temperature of the catalyst is 1100°F to 1300°F. 

 This temperature range will be maintained as long as the 

 fuel and air flow remain at the proper setting and the 

 catalyst remains functional. A properly designed and 

 operated catalyst allows the fuel to be oxidized completely 

 down to an operating temperature of 1000°F. If a minimum 

 operating temperature of lOOO'F cannot be maintained at 

 the recommended fuel and air ratio, then the catalyst may 

 be defective and may need to be replaced. 



Accident Prevention 



-Purchase a propane/methane monitor. They can be 

 installed on the burner to monitor the effluent stream. 

 Portable units can be used to monitor individual rooms for 

 combustible hydrocarbons and other gases toxic to workers 

 and detrimental to the fruit. It usually is NOT possible to 

 detect propane or natural gas by smell. 



-In early summer, have a competent technician check 

 out all CA equipment so that there is enough time to make 

 the necessary repairs before the storage season. These 

 tests should include the use of portable gas analyzers to 

 monitor fuel/air ratios and combustion efficiency. 



-CA generating equipment must be operated and 

 maintained according to manufacturer's instructions at all 

 times. 



-Low temperature fuel cut-offs on recirculating burn- 

 ers should not be set below the manufacturer's recommen- 

 dations, which is usually 1000°F. 



-Check all safety devices at the start of equipment 

 operation, including low and high cutoff thermostats, sole- 

 noid valves, fuel regulators, and air pressure switches. 



-Do NOT turn on the fans and open the door when 

 bringing up the oxygen in a room if there is a suspicion of 

 gas in the room. 



-A recommended procedure when opening a room is 

 to scrub out the combustible gases that may be present in 

 the room atmosphere using the COB catalyst. Bring up the 

 oxygen level in the room to 5%. Turn off the fuel supply to 



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