inch (6.2 mm) or thicker steel, set MAPP-gas pressure
at 4-5 psig and oxygen pressure at 15-20 psig.
When indoors or in an enclosed area, you should
light the torch by slightly cracking the gas valve on the
torch. Light it with an approved friction lighter and
then open the oxygen valve to obtain a neutral flame
(very blue). This is best for welding outdoors or where
there is a draft or when a smokeless flame is desired.
Crack the oxygen and gas valves slightly, light the
torch, and adjust it to a neutral flame.
Check the cone length and make sure the flame is
neutral (fig. 3-50). REMEMBER THE NEUTRAL
MAPP-GAS FLAME IS LONGER THAN THE
Flame adjustment is the most important step of
successful welding with MAPP gas. As with all other
fuel gases, there are three basic types of gas flames:
carburizing, neutral, and oxidizing (fig. 3-50).
A carburizing flame looks much the same with
MAPP as with acetylene. It has a yellow feather on the
end of the primary cone. Slightly carburizing or
reducing flames are used when you are welding alloys
that oxidize easily, such as aluminum.
As the oxygen is increased or the fuel gas
decreased, the carburizing feather pulls off and
disappears, and the inner flame becomes a deep blue.
This neutral flame (fig. 3-50) is ideal for welding.
Increasing the oxygen flow produces a lighter blue
flame, a longer inner cone, and a louder burning sound
to give you an oxidizing MAPP-gas flame.
Occasionally a harsh, bushy flame may be
required for a job; in such cases, counterboring is
needed to provide a harsh, yet stable, flame with
Bulk MAPP-gas facilities, similar to liquid oxygen
stations, are installed at some activities where large
supplies of gas are used. In a bulk installation, MAPP
gas is delivered through a piping system direct to the
points where it is used. Maximum pressure is
controlled centrally for efficiency and economy.
Cylinder filling facilities are also available from
bulk installations that allow users to fill their cylinders
on site. Filling a 70-pound MAPP cylinder takes one
person approximately 1 minute and is essentially like
pumping water from a large tank to a smaller one.
MAPP-GAS SAFETY.Liquified MAPP gas is
insensitive to shock. A MAPP-gas cylinder does not
detonate when dented, dropped, hammered, or even
incinerated. It may also be used safely up to full-
cylinder pressure. The gas vapors up to 419°F and 285
psig do not decompose when subjected to an energy
source in the absence of oxygen. The vapor is also
stable up to 600°F and 1,100 psig when exposed to an
825°F probe. The explosive limits of MAPP gas range
from 3.4 percent to 10.8 percent in air or 1.5 percent to
60 percent in oxygen. These limits are very narrow in
comparison with acetylene (fig. 3-51).
Figure 3-50.MAPP-gas flames.
Figure 3-51.Explosive limits of MAPP gas and acetylene in