scope:

Description of Driving Event:

A vacuum pump isolation valve burst at the Pad B high pressure gas storage battery. The cause of the incident has been attributed to the seepage of pressure from the wheel valves after they were turned off as part of a safing operation. The wheel valve o-ring does not always seat properly. Normally this would not affect the remainder of the system, the seepage pressure would be confined. However, when the vacuum pump was connected to the system, the seepage pressure (1500 psi) was greater than the design capacity of the vacuum pump isolation valve and thus overpressurization occurred.