Air or Pressure Spray

T.O. 33B-1-1 2-38 2.4.5.2.2 Air or Pressure Spray. WARNING Paint type respirators may be required when spraying penetrant as determined by the Base Bioenvironmental Engineer. Atomized penetrant is very flammable. Penetrants can be applied from most types of spray equipment using liquid pressure only, air aspiration only, or a combination. The equipment used is similar to that used in spraying paint. It consists of a supply tank, hoses, and a spray gun or nozzle. The supply tank is pressurized to force the penetrant through the fluid hose to the gun. The gun, which may be hand held or mounted in a fixture for automated spraying, is connected to an air line. The air applied to the gun converts the stream of penetrant into a spray. The air pressure, usually between 10 and 90 psig, controls the size of the spray droplets. Too low a pressure may produce a solid stream of penetrant. This would cover only a narrow area requiring many passes to coat the surface, and it also splatters the penetrant on adjacent surfaces. Too high a pressure can atomize the penetrant into a fine fog with poor covering ability and which drifts away from the part. Spray gun application, other than isolated cases, requires a spray booth and exhaust system for confining and removing the overspray. 2.4.5.2.3 Electrostatic Spray. The equipment required for electrostatic spraying is similar to that used in air spraying. In addition, a high voltage power supply is connected to the gun. This puts a positive electrical charge on the penetrant particles as they leave the gun. The part is electrically grounded and attracts the charged penetrant particles. The attraction is strong enough to pull the particles to surfaces not in front of or perpendicular to the spray. This ability makes electrostatic spray a preferred method for automated lines where complex shaped parts are to be coated. However, coverage inside cavities is limited. An advantage of the electrostatic spray method is the large savings resulting from reduced material requirements. Electrostatic spraying deposits a thinner layer of penetrant on the part than air spraying and greatly reduces penetrant loss due to overspray. Savings of over 80% compared to immersion application have been claimed. 2.4.5.2.4 Aerosol Spray. Penetrant packaged in aerosol containers provides a convenient method of application when portability is required. Packaging in sealed containers also eliminates contamination and evaporation of penetrant. Another advantage is that special exhaust equipment, such as used in pressure spray booths, and is not normally required as the amount of penetrant involved is small. There are disadvantages: a. Material cost is increased by the special packaging. b. Should not be used on large areas due to small spray pattern and high material cost. c. Overspray coats adjacent surfaces and complicates penetrant removal. d. Cans lose propellant and remaining penetrant must be discarded. 2.4.5.2.4.1 Penetrants, unlike nonaqueous developers, do not settle out of solution. Therefore, a mixing ball in the container is not essential. However some manufacturers buy only a single type aerosol can which is then used to package penetrant, solvent remover, or nonaqueous developer. Whether the can does or does not contain a mixing ball, it is good practice to shake the can thoroughly before spraying. This ensures an even distribution of penetrant and propellant. The propellant pressure is directly proportional to the ambient temperature. At temperatures below 60°F (15.6°C), the pressure may be too low for proper spraying. On the high temperature side, the pressure becomes excessive and may burst the container if the temperature reaches 120°F (49°C). When applying penetrant from an aerosol container, the nozzle should be held 3 to 6 inches from the part surface and the can moved in a line to completely cover the area to be