MIL-HDBK-1022A
b) Use a venturi-type proportioner where the
variation in demand flow rate does not exceed the limited
range of the venturi proportioner. Where the demand flow rate
varies excessively, use a venturi-type proportioner in
conjunction with a downstream storage tank, if economically
justified. The storage tank will permit a varying rate of
flow to the system while being filled continually or
intermittently at a constant rate of flow through the
proportioner.
10.2.6.3 Specific Gravity Indication. Provide a specific
gravity indicator and recorder with high and low-limit
switches to sound an alarm if the variation of specific
gravity of an air-gas mixture exceeds acceptable limits of the
system. For air mixing systems using LPG with a propane
content of 90 percent and above, the specific gravity of the
air-gas mixture is a sufficiently accurate index of its Btu or
joule (J) content, so calorimetric controls and indication are
not required.
10.2.6.4 Calorimetric Controls. Where economically
justified, provide an automatic calorimeter to indicate and
record the Btu or J content of the air-gas mixture. Provide
high- and low-limit switches to calorimeter to sound an alarm
if the variation of Btu content exceeds acceptable limits of
the system.
10.3
COMPRESSED NATURAL GAS (CNG). Design CNG storage
and dispensing facilities to comply with NFPA 52 and
appropriate sections of NFPA 55.
10.3.1
Uses. CNG is primarily used as an alternative fuel
in light duty vehicles although it and its cryogenic
counterpart liquid natural gas are gaining acceptance in heavy
duty applications. Energy policy has mandated with certain
reservations that by fiscal year 2000, 75 percent of the
light-duty vehicles purchased by the government will use
alternative fuels. Therefore, there will be a significant
increase in alternative fuel consumption.
10.3.2
General Design Considerations
10.3.2.1 System Sizing. To size the system, determine the
total daily fuel consumption of base liquid natural gas
vehicles.
Based on daily miles driven, determine the number
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