Volume 3, Issue 4 (April 2006)
Promoted Ignition of a Cylinder Valve in NF3
Nitrogen trifluoride (NF3) is used in the manufacturing process of semiconductors and is believed to be 1.5 times more oxidizing than oxygen. Compatibility data of materials with NF3 is limited and has only recently been addressed by companies which routinely handle NF3. Even more limited is the ability to conduct configuration testing of components in NF3 to evaluate kindling chain and reaction effects. In NF3 systems, the cylinder valve is subjected to severe operating conditions, and is known to have experienced fires in service. A new test approach was developed to positively ignite the non-metallic seat of a cylinder valve pressurized with NF3. The goal of the work was to investigate whether the ignition of the seat would kindle the surrounding sub-components of the valve and ultimately whether a kindling reaction would develop that could breach the valve body. The ignition concept of the test method consists of electrically heating a thin section of wire positioned across the seat to act as a promoter. The concept required minimal modifications to the valve and the overall valve design remained consistent with normal operation. The assembled valves were tested at a pressure of 1500 psig and no burn-through was observed during any test. The post-test valve disassembly and inspection confirmed the wire fused and successfully ignited the seat but the seat retainer and surrounding sub-components did not kindle. All of the other parts of the valve were unaffected by the ignition and consumption of the non-metallic seat. The new test provides an approach for evaluating the propagation propensity of NF3 components containing nonmetallic materials.