August1999
lenges in balloon operations, most notably reduced fuel pressure.
Hot weather operations provide a number of other challenges,
some related to high fuel pressure, and some related to preventing
damage to the balloon system.
Theballoonisquiteoftenrefueledinthemorningafter
flying, or in the eveningbefore a morningflight. If the cylinder
isfueledusingthefixedliquidlevelgaugeor“spitvalve”to
determinethefuelquantity,morecoldfuelwillfitintothe
cylinder because cold fuel is denser (a given weight of fuel takes
up less space). The fixed liquid level gauge has atube that extends
fromthe top of the fuelcylinder to the maximum level (80% of
thecylinder’svolume)thatthecylindercanbesafelyfilled.
When the fuel is warmed during the day, it expands, and will take
up more than 80% ofthe cylinder’s volume. The 20%vaporspace
isrequiredtoallow for expansionofliquidpropane,but if the
cylinder has been overfilled with very cold fuel and the cylinder
isleftoutsideinthesun, the fuelcanexpandenoughtocause
liquid propane to vent through the pressure relief valve. If you are
fuelingaballoonsystemwithcoldpropaneandexpecthot
weather, be extremely careful that you turn off the cylinder valve
whenyou see the firstsignof liquid. If you have anyreasonto
believe thecylinderisoverfilled,openthespitvalveandvent
liquidpropane(inasafelocation)untilyounolongerhavea
steadystreamof liquid.
At high temperatures, propane vapor pressure will increase,
andliquidpropane will expand. Liquidpropanedoesnot com-
press, soanyexpansioncan cause hydraulic pressure wherever
liquidpropane iscontained.Primaryareasof concernare fuel
manifolds, burners, and fuel cylinder service valves. If you hook
up your fuel system, pressurize it, then decide not to fly, or if you
land and pack up the system without burning off residual fuel, the
fuel left in the lines can cause serious damageto yourfuel system.
The expansioncanrupture fuellines, damageblastvalve “O”
rings,damageothervalveseals,ordamageself-sealingquick
disconnect fittings. Such damage can be costly, and may prevent
your next planned flight.
Beforeyour systemis exposedtoheat,be sureyousafely
depressurize your fuel system, preferably by burning remaining
fuel from the lines after closingthe cylinder valves. If youhave
quickdisconnectfittings,makesurethatnofuelistrapped
between the cylinder valve and the fitting. Trapped fuel will blow
adapter todepressurize cylinder valves.
Fuel cylinders are equipped with a pressure relief valve. The
pressure relief valve may be a separate unit, usually with a small
elbow and rubber cap, or it may be part of a service valve. If it is
part of a service valve, a plastic cap may cover the opening. The
pressure relief valve is designed to open and vent propane vapor
if the pressure inside the cylinder exceeds 375 pounds per square
inch. Ventingvapor reducesthe cylinder pressure. As the pres-
sureis reduced, someof theliquid propane turns to vapor, cooling
the liquid. Releasing a very small quantity of propane vapor will
rapidlyreducethepressure,allowingaproperlyfunctioning
relief valve toreclose.
It is extremelyimportant thatthe cylinder istransported or
storedwiththe pressure relief valve upsoit isnotcoveredby
liquidpropane. If the cylinder islaidonitssidewith the relief
valvedownintheliquidpropane,andthepressure buildsup,
liquid propane will vent. Instead of releasing a small amount of
vapor, it will release a large amount of liquid, creatinga serious
hazard. Every time a pressure relief valve operates, it reduces the
strength of its spring by a small amount. Later operations may be
at a lower pressure.
In someregions butane is mixed with propane during warmer
seasons. The butane is added to reduce fuel pressure. If you have
fueled with a butane mix and attenda rally in a cooler area, you
may find that your fuel pressure is too low to fly. Make sure that
youknowwhatisinthefuelyouareusing,oryoucouldbe
unpleasantlysurprisedon a cool morning.
Storage
Some pilots store their balloon system in a garage or hangar,
while others use an enclosed trailer that is stored outside. A trailer
thatgetshotcancausepropane tovent.Itcanalsoaccelerate
envelope fabric aging. The envelope should be kept in a cool, dry
place. If you are unable to keep the trailer cool, you should store
the envelope somewhere else.
Anenclosedtrailer shouldbepaintedalightcolor,andit
should be parked in a shaded area. A carport or a portable shade
over the trailer will significantlyreduce the temperature inside
the trailer. The trailer should also have vents in the roof to allow
hot air to get out, and vents near the floor to allow cool air to get
in, or propane to get out. If your trailer does not have roof vents,
you may wanttoinstall oneor twosquarerecreationalvehicle
style vents, available at a recreational vehicle supply store. Vent
