A shop supervisor determined that a second shift would be necessary to complete some priority work on the spare hydrogen mitigation pump. The work scope for the shift would be dedicated to continued fabrication of designed tubing runs, repairs to existing tubing with known leaks and pressure testing of other various tubing runs. The shift craft complement would include three pipe fitters, one welder, one QC inspector and a shift supervisor.

The shift remained under normal operations prior to the event. There had been no existing problem up until the point that craft personnel implemented some hydrostatic pressure testing on some tubing runs on the spare hydrogen mitigation pump. Work activities associated with the hydrostatic testing were to be in accordance with the Hydrostatic Test Procedure. This activity was initiated at approximately 9:00 p.m.. After initial pressurization of a tubing run, an indicated pressure of about 5200-5300 P.S.I., the pipe fitters realized that they were unable to maintain the required test pressure of 5000 P.S.I.+. Immediate trouble-shooting of the 3/8" stainless steel tubing (.065" wall thickness) run revealed a leak near the far end of the run at the Swagelok fitting and cap. At this point, the time was estimated to be approximate (9:40 p.m.)

At approximately 9:50 p.m., the pipe fitters commenced removing the cap from the Swagelok fitting at the far end of the run. At this time, it was apparent (as later stated by several of the pipe fitters) that they believed the balance of the tubing run to have been bled off and not to be under pressure. They had not recognized that the portion of the tubing run down stream of the check valves and the Swagelok fitting had remained isolated, therefore, remaining under high pressure. When the pipe fitter loosened the cap off of the tubing run Swagelok, the Swagelok, ferrules, and cap still under extreme pressure, blew off the end of the tubing run, becoming an airborne projectile. The pipe fitter that was performing the work stated that the force from the tubing run was strong enough that it blew his hands back away from the area near the Swagelok fitting. Information to date is inconclusive as to "how far" the Swagelok did or did not travel or at what velocity it was traveling, but subsequent inquiries directly related to this event have classified this as a near miss. No one was standing in front of the tube run and no one was injured.

Swagelok™ fittings, should not be tampered with, loosened or removed while under extreme pressure for any reason. In this case, the fitting was loosened to relieve pressure in lieu of a non-existent vent valve.

Incident Date
Apr 17, 1996
  • Piping/Fittings/Valves
  • Piping
Damage and Injuries
Probable Cause
Contributing Factors
When Incident Discovered
Lessons Learned

In this case, failure to recognize a run of tubing still maintaining pressure could have been avoided if such information was provided in a safety briefing. Knowledge of any job is the utmost importance in promoting and maintaining a safe working environment.

Facilities should Issue instructions to appropriate personnel to establish the standard practice for hydrostatic & pneumatic pressure testing.

The set up configuration of a tubing/piping run for a hydrostatic pressure test must be carefully reviewed to ensure that means are provided to immediately shutoff pressure and safely and completely relieve pressure. Under no circumstances should a Swagelok fitting or any other fitting be used as a vent valve. Equipment should be utilized for its intended purpose only.