MIL-HDBK-1022A
to be refueled each day. The number of vehicles refueled
during surges limits the capacity of most fast fill (3 to 6
minutes) operations. Scheduling vehicles to refuel through
the day will effectively increase system capacity. Use a
computer program to size the system because manual
calculations usually result in larger systems than needed.
Refer to the Institute of Gas Technology in the References
section of this handbook for ordering information of one
possible program. Use the latest version.
10.3.2.2 Future Requirements. Anticipate future requirements
when sizing the system but normally limit the project to 100
to 150 standard cubic feet per minute (scfm) (47 to 71 L/s).
If additional capability will be needed in the future, plan a
second system later or consider other options such as slow
fill systems for overnight fueling. This will provide
redundancy and reduce initial cost. Usually, surge
requirements drive machine size and can be controlled by
management actions. Additionally, boosters operating from
system pressure or special control systems may increase surge
handling capacity (e.g., 25 to 30 percent of the gas in a
cascade system is available for fast fill operations. A
booster or special control system can increase it to 60
percent.) The combination of these actions could delay
installing a second system many years at most installations.
10.3.2.3 Pressures. Most vehicle conversions use 3,000 psig
(21 000 kPa) storage systems while original equipment
manufacturers use 3,600 psig (25 000 kPa) systems. The
compressors should operate up to 5,000 psig (35 000 kPa) to
refuel at either pressure.
10.3.2.4 Connections. Design the systems to be skid-mounted
with compressor system, cascade storage, and controls. Limit
field tie-ins to connecting electricity and high and low
pressure gas.
10.3.2.5 Compressors. Use crosshead guide type compressors
for CNG service. Although more expensive, the design life of
these units is significantly longer. Another option is a
conventional style compressor designed specifically for CNG
service. Do not use modified air compressors. Choose the
type compressor after comparing maintenance and reliability
data. Test all compressors at the factory with natural gas
before shipping.
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