July 1999
yougoingtoshipyourballoonto
Albuquerque,FranceortheNationals?
YougototheAirportandtalktothe
shipping company.“You gotta empty your
tanks and purge them.” “Why?” “Because
theF.A.A.rulesonhazardousmaterials
onU.S.flightssezso,andtheI.A.T.A.
(International Air Transport Association)
rulesforaninternationalflightsezso!
That’sWHY.” “Oh.”
The problem is that neither the F.A.A
northeI.A.T.A.definesanacceptable
purging process. Theydon’t evendefine
anacceptableresult,otherthanthatit
must be safe. I talked toan I.A.T.A. man
attheir headquartersinCanadawhode-
clined to identify himself, and he said that
they purposely madetheregulations vague
sosomeone else would be responsible.
There are companies thatwill purge
your tanksandprovide a certificate (ac-
ceptingresponsibility)fora heftyfee.I
contactedalocalpropanecompanythat
provides such service and asked for tech-
nicaldetailsof thepurgingprocessthey
use. I wanted to know where this process
camefromandwhoapprovedit.They
didn’t knowwherethe process came from,
onlythat “everybodyuses it,” and appar-
ently no one had approvedit. The certifi-
cate theyprovide is one theymade up.
This is just what the I.A.T.A. is look-
ingfor: someone else totake the respon-
sibility.After I finishedcommentingon
of course) and cooled off, I decided to
investigate how the various purging pro-
cesses really work.
Purging
Youstartwithanemptytankat“0”
pressure.Thegasinthetankis100%
propane vapor. Then air, or some inert gas
such as nitrogen or helium, is pumped into
the tank to 40 psi. Due to the turbulence in
thetankthepurgegasandthepropane
vapor are thoroughlymixed. The tankis
then vented to “0” pressure. The propane
vaporisthusdilutedto27%propane
vapor.Thisprocessisthenrepeateda
second time. That reduces the tank vapor
to 7.29% propanevapor. Afterthe process
is repeated a third time, the vapor has been
reducedto1.96% propane.
The flammability rangeof propane is
2.12% to 9.35% or approximately 2 to 10
% (Handbook of Chemistry and Physics,
ChemicalRubberCo.)Therefore,after
three cycles of purging, thepropane vapor
isless thanthe lower flammability limit.
If a pressure of less than 40 psi is used, it
would take more thanthree purge cycles
toreachthe lower flammability limit.
InertGas
If air is used as a purge gas, during the
purgecyclethegascontentofthetank
goes through the flammability range of 2
ard. If the exhausting gas were to catch
fire, it would burn in the hose and into the
tank. This is NOT GOOD. There are sev-
eral methods to prevent this from happen-
ing. One method is to use an inert gas that
prevents oxygen from being in the tank
mixture, so that internal tank burning can-
not take place. Once the tank propane
percentage is under 2% (the lower flam-
mability limit) it makes no difference
of many tanks tested with different valve
configuratio ns. Dark blue valve is liquid
withdrawal, light blue is vap or with-
drawal.
July1999
to purge the tank:
helium
nitrogen
Advantages to helium and nitrogen:compressed
inertgas.
Disadvantagestohelium and nitrogen: storedin
largeheavytanksand notreadilyavailable in
remotelocations.
air
Advantages to air: readyavailabe and can be
used withportablecompressor.
Disadvantagestoair: mustusea desiccator and
flame blockdevicewhen purgingtank.
John Terryrecommendsthat eachtank have
two valveson topofthetank(each labled to
identifythe liquid and vapor withdrawal). One
with the dip forwithdrawing liquid propanetothe
burner and one withoutthe diptubefor extract-
ingthepurged vapor.Twovalvesallow the
continuousflow methodfor purging tankswhich
iseasiest andrequireslesssophisticated
equipment.
desiccator (low or highpressure dependingon
operation)
flame blockwithhigh pressure hose
helium/nitrogen tankwith pressuregauges,
hoses,and fittings.
or air compressor withappropriate hosesand
fittings.
using
an inertgasand pro-
panetank with two
valves,one forliquid
withdrawal and onefor
vapor withdrawal. After
ventingvaporfrom the
tank until the liquid has
evaporated, connect the
fitting from the gastank
tothe liquidwithdrawal
valve.Connectthe
exhausthose to the
vapor withdrawal valve
and orientdownwind.
Open both tankvalves
and the inert gastank
valveregulating the
pressure to40 PSI.
using air,an air compressor,
and propanetank with two valves. After venting
vapor from the tankuntil the liquidhas evapo-
rated,connectthehose from the compressor to
a desiccator. Thiswill removeanymoisturefrom
the air.Connect thedesiccator to the liquid
withdrawal valve.From the vapor valve connect
a hose leadingtoa flame block andorient
downwind.Using a 1.33cu.ft./min.portable air
compressor it takesabout10 minutestobringa
10gallon tankdown to.035%propane.
with an inert gasusing a tank
with onevalve(liquid withdrawal valve).After
ventingvaporfrom the tankuntil the liquidhas
evaporated, connect inert gastankto tankvalve.
Open tankvalveand inert tankvalve,regulating
the pressure to 40 PSI.Once 40 PSI hasbeen
reached, closebothtanks. Disconnect inerttank
and connectbleed hosetotank valve. Venttank
to“0” pressure. Repeat at least three more
times.
using airandcompressor with
one valve on tank.Afterventingvapor from the
tank until the liquid hasevaporated,connectthe
compressor hose to ahigh pressure desiccator
usinghigh pressurehose.Then connect the
desiccator withhigh pressure hose to the tank
valve.Add compressed air until pressure intank
is40PSI. Turnofftankvalves, disconnectthe
hose to the propane tankandconnect exhaust
hose leading to aflame block.Repeatatleast
three moretimes.
July1999
not. It cannot burn. Another method is to
provide a flame block in the exhausting
hose. More about this later. Thereis a
psychological importance in using inert
gas. It is a non-flammable dry gas and it
sounds high-tech and jazzy. Most com-
mercial companies that I contacted use
this method with nitrogen and stop after
the propane % in the tank is under 2%
(three purges at 40 psi). If you wanted to
be more safe, whatever that is, a fourth
purge gets you to .53%, and a fifth purge
to .14 %, a sixth purge to .038% and a
seventh purge to .001% propane.
These values hold true whether you
use air, nitrogen or helium as a purge gas.
There is one other thing. When you are
using aninertpurgegas, theresultant
dumped gas ofthefirst purgeis 27%
propane,73% inertgas. ThisstillBURNS.
It is flammable in air.
Thesecond purgeresultantgasis
7.3% propane, 92.7% inert gas. Thisstill
BURNS,but in flashes, notin a sustained
flame.BEWARE, until youget below
2%. Whether you purge with inert gas or
air, it is still a hazardous operation, and
you shouldfollowthe same safety proce-
dures as when re-fueling.
The balloonpilotor operator isfaced
with the purge problem two times during
his or her trip to Southern Podunk. The
first time, when he ships hisballoonfrom
the big city. Here, he has accessto big air
compressors, nitrogen, helium, and com-
panies willing to do the job for a price.
The second time, he is 10 miles outside
Southern Podunk, beside some farmer’s
cabbage fieldwitha rentedchase vehicle.
tanks and looks around for high pressure
air, lots of helium, or nitrogen. Nothing
but cabbages.
I started lookingfor a procedure that
would solve both problems.
TheColemancampingequipment
company makes an air mattress inflator
that pumps approximately 1.4 Cu. Ft. per
minute(Inflate-AllH/VModel
#2210A718T)and willrunfor 20minutes
without getting hot. It plugs into a cigar
lighter in the rented chase vehicle, and
takes very little room in the equipment
bag. Itwon’tgenerate 40psi. pressure, but
itwillpush a lotof air through a 10 gallon
propane tank.
Thereare some safetyconcernswhen
purgingwithair. The gasexhaustingfrom
the tank being purged starts out as 100%
propane vapor and slowlychangesto10%
propane.Duringthis phase,thegas is
flammable when mixedwithatmospheric
air. Care must be taken to observe good
safety procedureswith regardto clear
space, winddirection, sourcesof ignition,
etc. Later in the purging process the gas
willbe between 10% and 2%. Under this
condition, if the gas should ignite, it will
burn in the hose and tank. This ignition
can be prevented with a flame block in-
stalled on the end of the exhausting hose.
Thisflame blockis made of a smallmani-
fold and four metal fret filters.
Balloon tanks are of different sizes,
from 10 to 20 gallons. A method of dis-
playing test results independent of tank
size was used in the test. The volume of a
tank was used to report the resultant fuel
percentage, - one tank volume - two tank
volumes -threetank volumes,andso
forth. Thus four tank volumes of air will
produce the same resultsregardlessof the
tank size. With a given air source, it will
take longer witha largertank toreachfour
tank volumes. A 10 gallon Worthington
aluminumtankhasa volumeof 1.66cubic
feet,whilea15 gallon AerostarV-15
stainless steel tank has a 2.36 cubic foot
volume.
73% helium. Sustained flame.
Above: Second purge dump 7.3%
propane - 92.7% heliu m. Intermittent
flashes of flame.
