October1999
or Care and Feeding
of Pilot Lights
youhave never experiencedanin-flightflameout,youhave
missed a bit of excitement that most of us would rather avoid.A
proper understandingof your pilotlightandburner systemcan
help youto avoid this type of excitement, or at least handle the
situationwithoutendangeringyourself and upsettingyour pas-
sengers.
The modern hot air balloon burner, or inFAA-speak, “Air-
borne Heater” varies in some details between manufacturers.All
burners share following commonelements:
1.An output control.Currently an on-off valve, known as
theblastvalve,allowsfueltoflowtoprovideheatoutput,or
blocks fuel flow to turn off the flame. The pilot controls the heat
outputby simplyturning the flame onandoff.
2.Afuel/airmixingmechanism.Modernburnersuse
arrays ofsmall jets or nozzles or a venturi system to provide quiet
combustion.
3.A vaporizingsystem.A systemof openorenclosed
coils is used to transfer heat from the flame to the liquid propane.
Thisheatchangesthe liquidpropane intovaporthatisforced
through the jets. The vapor mixes with the air, allowing it to burn.
4.An ignition source. Current systems use a standing pilot
light,orconstantignitionsource.Intheeventof apilotlight
failure, a flint andsteelweldingstriker or piezo-electric igniter
canprovide anignitionsource.A long-nosebutane lighteris
somewhat less reliable than a striker, so it should not be the only
ignition source.
A hot air balloon requires considerable heat for hot inflation
and sustained flight.Typical early burners produced outputs in
the range of 4millionBTU/hour, comparedwith more than15
million BTU/hour for modern burners.The actual output varies
with fuel pressure. At typical fuel pressures in the 100 to 125 psi
(pounds per square inch) range, output is proportional to the fuel
flow. Commercially available LP gas, typically amixture of92%
propanewithother refinerybyproducts,hasaheatcontentof
95,500 BTU/gallon.At 15 million BTU/hour input, the fuel flow
throughthe burneris157gallonsper hour, or2.6gallonsper
minute.
The proportionof propane toair mustbe intherangeof 2
percent to 10 percent to support combustion. The ignition source
musthaveatemperatureabove914degreesFtoignitethe
mixture.A pilot light provides a hot enough flame, and the pilot
light is located where it will encounter an ignitable mixture soon
after the blast valve isopened.
Pilot lights are supplied in two configurations – vapor feed,
and liquid feed.Two or more fuel lines connecting to one of the
fuel cylinders can identify a system with a vapor feed pilot light.
Asystem with a liquid feed pilot light will have only one fuel line
per cylinder.
Both systems are normally quite reliable, but things cango
wrong.Several checks are performed duringanannualinspec-
tion.They include testing the pilot light vaporpressure, checking
the flame size, shape, and resistance to wind, and checking how
fastthepilotflamegoesoutafterturningoffthepilotvalve.
Failure of any of these tests can indicate a problem that must be
corrected before the next flight.
Avapor feed pilot light draws propane vapor from the vapor
space in the topof a propane cylinder using a separate fuel line.
Apressureregulatororameteringvalvecontrolsthevapor
pressure tothe pilotlight.Pilot lights operate like a laboratory
Bunsenburner.Thepropaneisforcedthroughamicroscopic
orifice or jet intoa tube withair inlets in its sides.It thenmixes
withair, and maintainsa flame atthe topof the tube.
With avapor feed pilot system, the pressureregulator and the
orificecausemostproblems.Contaminatedfuelcancloga
pressureregulator,oritcanwearoutor stickinternally.The
normalresultistoosmallaflame,ornoflameatall.An
adjustable regulator may be adjusted, but a fixed regulator must
be replaced if it fails.A partially clogged orifice can sometimes
be cleaned using a mild acid solution and compressed air.If that
doesnotwork,theorificemustbereplaced.(Insomeolder
burners, itcan be drilledwitha fine machinist’sdrill).
Aliquid feed pilot system is more complex than a vaporfeed
system.It uses either a vaporizing chamber feeding an adjustable
