Potential For Explosions In CA Storage Facilities
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 are propane and natural gas. Both fuels can be ignited by a spark or flame if they are allowed to accumulate to certain concentrations and if sufficient oxygen is present.
Explosive LimitsThe limit of gas to air ratios between which explosions 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 requited to create an explosion with propane.
Open Flame BurnersIn 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 combustion occurs, resulting in production of carbon monoxide and ethylene gas, which are detrimental to fruit quality. Also, carbon monoxide may leak through walls, accumulate in work areas and create a health hazard to workers. Fortunately, there are few of these burners left.
Inert Gas GeneratorsInert 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.
RecirculatingIn 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 de creased. 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 1,100°F to 1,300°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 completely oxidized down to an operating temperature of 1,000°F. If a minimum operating temperature of 1,000°F cannot be maintained at the recommended fuel and air ratio, then the catalyst may be defective and may need to be replaced.
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 is usually 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 burners should not be set below the manufacturer's recommendations, which is usually 1,000°F.
Check all safety devices at the start of equipment operation, including low and high cutoff thermostats, solenoid 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 the COB and set it on preheat. Run the temperature up to 600°F and recirculate the CA room atmosphere through the generator. The temperature of the catalyst may rise, and it will remain hot until the combustible gas (propane or natural gas) is down to a safe level.
Some storage operators routinely use a COB to scrub out any combustible gas which may have entered a CA room during pulldown.
It is possible to reduce risks to a minimum. By understanding the principles of CA generator operation, maintaining the equipment and safety devices, and using the available instrumentation, managers can maintain a clean and safe storage atmosphere.
Dr. Henry Waelti, WSU Agricultural Engineer(1) and Dr. Eugene M. Kupferman, Postharvest Specialist(2)
(1)Washington State University, Pullman, WA
(2)WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 6(1): 4-5