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Sealed System
Combustibility of HFC-134a
HFC-134a is nonflammable at ambient temperatures
and atmospheric pressure. However, tests have shown
HFC-134a to be combustible at pressures as low as 5.5
psi (139.3 kPa absolute) at 177°C (350°F) when mixed
with air at concentrations generally greater than 60%
volume air. At lower temperatures, higher pressures
are required for combustibility. (HCFC-22 is also
combustible at pressures above atmospheric in the
presence of high air concentrations). Test results and
calculations have shown:
•
At ambient temperature, all concentrations of
HFC-134a in air are nonflammable at pressures
below 15 psi (205 kPa absolute).
•
Combustible mixtures of air and HFC-134a will not
form when liquid HFC-134a is pumped into closed
vessel if initial air pressure in vessel is limited to one
atmosphere absolute and final pressure is limited to
300 psi (2,170 kPa absolute). If initial air pressure is
greater than one atmosphere, combustible
mixtures may form as tank is filled.
Based on above information, the following operating
practices are recommended:
Bulk Delivery and Storage
•
Tanks should normally be evacuated at start of
filling, and should never be filled while under
positive air pressure.
•
Tank pressure should never be allowed to exceed
300 psi (2,170 kPa) when filling with HFC-134a.
Relief devices on either tanks or HFC-134a supply
system usually prevent this.
•
Tank pressures should be monitored routinely.
•
Air lines should never be connected to storage
tanks.
Filling and Charging Operations
•
Before evacuating cylinders or refrigeration
equipment, any remaining refrigerant should be
removed by recovery system.
•
Vacuum pump discharge lines should be free of
restrictions that could increase discharge pressures
above 15 psi (205 kPa) and result in formation of
combustible mixtures.
•
Cylinders or refrigeration equipment should normally
be evacuated at start of filling, and should never be
filled while under positive air pressure.
•
Final pressures should not exceed 300 psi
(2,170 kPa).
•
Filled cylinders should periodically be analyzed
for air (nonabsorbable gas or NAG).
Refrigerant Recovery Systems
Efficient recovery of refrigerant from equipment or
containers requires evacuation at the end of the
recovery cycle. Suction lines to a recovery compressor
should be periodically checked for leaks to prevent
compressing air into the recovery cylinder during
evacuation. In addition, the recovery cylinder pressure
should be monitored, and evacuation stopped in the
event of a rapid pressure rise indicating the presence of
noncondensable air. The recovery cylinder contents
should then be analyzed for NAG, and the recovery
system leak checked if air is present. DO NOT continue
to evacuate a refrigeration system that has a major leak.
Thermal Decomposition
HFC-134a vapors will decompose when exposed to
high temperatures from flames or electric resistance
heaters. Decomposition may produce toxic and irritating
compounds, such as hydrogen fluoride. The pungent
odors released will irritate the nose and throat and
generally force people to evacuate the area. Therefore,
it is important to prevent decomposition by avoiding
exposure to high temperatures.
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