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November/December 2018 | VehicleServicePros.com 37
appliances (i.e. fans) may need to be explosion
proof. It can be costly to relocate and/or
upgrade these appliances. (More on this later under
“Electrical, lighting and space heaters.”)
Regardless of which ventilation strategy is
employed, a certain number of “air changes per
hour” is desired, typically four or fi ve. Th at is reliant
upon airfl ow of at least 1 cfm for every 12 cu.
ft . of structural space. Some fl eets will not be able
to achieve that kind of continuous
ventilation. But for those
that can, some codes say that
the 18” area from the ceiling is
no longer an electrical hazard.
A good natural gas consultant
will help a fl eet determine
what is practical, compliant
and cost-eff ective.
It’s critically important
to get the ventilation piece
right. Proper ventilation is
also necessary to dilute any
released natural gas to a level
that is no longer combustible,
and then extract it. According
to Manley, extraction means
speeding up the migration
path by bringing in fresh
make-up air as low as possible,
and then exhausting it as
high up as possible.
“Th ere are diff erent sources
of make-up air you can use,”
Manley says. “It’s common to
use bay doors with an electrical
interlock alarm system. If a
leak is detected, the doors automatically
open up. We’ll also
oft en see fans and louvers near
the fl oor to enhance airfl ow.”
Another key consideration
is how the fl eet prevents any
released gas from entering
unprotected areas of the
facility. NGVi’s Th omason says
fl eets could put in two-hour
fi rewalls from fl oor to ceiling
to help contain gas to certain
areas (i.e. NGV bays) that have
proper ventilation. He has
also seen instances where the
AHJ approved a drift curtain.
Additionally, fl eets must look
for openings such as windows
between rooms, unsealed
conduit pipes, structural
members passing through
walls and gaps between walls
and ceiling panels.
Leak detection
Some fl eets opt to have the
facility’s ventilation system
tied to a leak detection
system. Th is allows fl eets to
ensure safety while minimizing
operating costs.
“Th e fl eet has a choice of
whether or not its ventilation
system runs all the time, is tied to the lighting
system or is tied to a detection system,” Th omason
explains. “Th is decision is a purely economic one.
For a fl eet operating in a colder climate, for example,
a lot of heated air could be allowed to escape if
the ventilation is running non-stop all day long. We
help fl eets conduct a cost-benefi t analysis on this.”
Regardless, a leak detection system’s job is
twofold: alert personnel and disable potential
electrical ignition sources. GTI’s Manley says it
is common for fl eets to install detection devices
somewhere above the area in which a vehicle is
parked, and occasionally in adjacent spaces.
Leak detection systems are mandatory
for fl eets running LNG (liquid natural gas)
because LNG is odorless. For fl eets running
CNG (compressed natural gas), on the other
hand, a detection system is considered optional
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