NASA-LLIS-0935

Abstract

<strong>Description of Driving Event:</strong> <br>During routine starting of the HVOF thermal spray gun, an incident occurred that resulted in the unintended mixing and ignition of liquid fuel (kerosene) and oxygen in the HVOF system. By some mechanism, the kerosene fuel was allowed to mix with gaseous oxygen upstream of the oxygen check valve in the combustion chamber of the gun. Upon starting the HVOF spray gun, this fuel ignited outside of the combustion chamber, in which it normally mixes and is burned. The flame front traveled up stream in the oxygen supply line and into the controller console, burning or melting several plastic tubes and lines in the cabinet, and contaminating the oxygen-regulating portion of the system. No personnel were injured and the equipment sustained approximately $7,800 worth of damage. <br>As the manufacturer (TAFA) repaired the equipment, they offered the most likely cause of the mishap and suggested some corrective actions. If the HVOF gun is not mounted in! a slightly (at least 5 degrees) downhill orientation (barrel is lower than the rear of the gun), it is possible for a small amount of residual fuel to seep into the oxygen supply line. This can lead to "hard" starts or, as in this extreme case, damage the equipment. It is suspected that the HVOF gun was mounted level (parallel to the floor), or even slightly uphill, though not enough to be perceived by the operator. While the equipment manufacturer recommends this practice, it was not described or included in the documentation that originally came with the HVOF system. Consequently, the operator had no idea of the potential consequences related to gun orientation. This equipment had been in operation for over 6 years witout incident before this mishap.