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 ACETYLENE FLAME.

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 MAPP gas.

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 419F and 285 psig do not decompose when subjected to an energy source in the absence of oxygen. The vapor is also stable up to 600F and 1,100 psig when exposed to an 825F 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 air.