10.3.2.6 Compressor Drives
. Use either electric or engine-
driven compressor drives. Gas engine drives are less
expensive to operate, but maintenance costs are higher. Use a
life cycle cost analysis to determine which compressor drive
is best. Although an engine-driven compressor is more
expensive than an electric motor, electrical upgrade costs may
be reduced. It also may be able to operate during power
10.3.2.7 Compressor Inlet Pressure
. Suction pressure is a
mains to reduce both initial and operating costs. Avoid
pressures less than 20 psig (140 kPa). If high and low
pressure lines are near each other and the pressure
differential is at least 150 psig (1030 kPa), 250 psig (1720
kPa) is preferred, and the low pressure line has a continuous
load, a system can be installed and powered by the
differential pressure. Such systems are extremely effective
and have low initial and operating costs. Where a high
pressure differential exists, another option is a turbine to
drive the compressor.
10.3.2.8 Storage. Install a cascade system using ASME
vessels; either tubes or spheres. Avoid banks of DOT
cylinders since they must be inspected every 5 years.
10.3.2.9 Controls. Use either pneumatic or electronic
controls depending on local practices. Electronic controls
are preferred in most areas because they provide more accurate
compensation for temperature effects. Pneumatic controls are
simpler, but do not fill tanks to their limits. Normally,
this is not a problem, since base vehicles do not normally
operate to their maximum range in one day. Since vehicle tank
pressures are rated at 70 degrees F (21 degrees C), tanks will
fill to a higher pressure when outside temperatures are warmer
and to a lower pressure when temperatures are cooler.
10.3.2.10 Dispensers. Although more expensive, provide
conventional rather than post style dispensers. To reduce
installation costs, use dispensers with the electronics
internally mounted and calibrated at the factory. A dispenser
makes CNG refueling similar to conventional refueling.
Depending on funds, a post style dispenser is an option.