NASA-LLIS-0606

Abstract

<strong>Description of Driving Event:</strong> <br>A four inch-diameter cast iron strainer in a 225 psig steam line failed catastrophically, releasing high pressure steam and hurling strainer pieces and attached parts in the equipment building. The steam and projectiles weighing up to 50 lbs. posed life-threatening hazards; however, they caused only minor damage to the steam piping and surrounding equipment. Two employees opening a steam supply valve nearby were not injured. <br>The failure was caused largely by the inappropriate use of material for the strainer. The strainer was manufactured from gray cast iron to ASTM Specification A 126 Class B. A stress analysis was performed by the A&E contractor assuming cast steel construction. The principle stress at the strainer position would not exceed the maximum allowable stress for steel but would for cast iron, as specified in ASME B31.1 (1995 edition). Repeated water hammer effects during operations hastened eventual strainer failure. <br>Upon acceptance of the strainer, material criticality associated with analysis was not recognized as a determining factor in component acceptability. Operating and performance parameters (pressure and temperature) were the principle considerations. <br>Also, the hazard analysis did not address steam line rupture resulting from steam flashing or water hammer. It is common within NASA and industry to dismiss catastrophic failure potential of ASME code-compliant pressure systems as non-credible, given intrinsic safety factors. Procedural problems combined with some malfunctioning steam traps caused the part to be subjected to significant water hammer during system startup. Employees were not instructed explicitly in how to recognize water hammer symptoms.