Safety Alert! - Nitrogen Test Rig

What happened?


During the testing of a valve on a nitrogen test rig in a Valve Repair Workshop, the technician discovered that the hydraulic clamps could be released whilst there was still pressure underneath the valve being tested. This resulted in the nitrogen being released uncontrollably from beneath the valve when the clamps were released.

On further investigation it was found that pressure could be applied under the test valve before the correct clamp pressure for the size of valve being tested had been reached, meaning the safety interlock system on the test unit had failed.
 Preliminary Investigation Findings
The rig was taken out of service so that the fault with the interlock system could be determined and resolved.

 

3 Way Valve

 

 



On investigation by the technician it was found that the 3-way valve had stuck and required freeing off; to achieve this, the valve was stripped, cleaned, repaired, refitted and tested OK.

 

Initial Actions to Prevent Reoccurrence

1. Do a practical test on the interlock device by fitting the clamps at a low pressure then slowly apply nitrogen to the rig; this action should not be possible as the interlock valve should prevent the nitrogen entering the rig without the clamps being in place and at the correct pressure first. If it does allow ingress of nitrogen then the interlock is not working.

2. Review documentation, line drawings etc, to determine where the interlock valves are fitted and the safest way for removal, if required.

 

Immediate Actions Required

Check test rigs at each location to ensure that the interlock valves relied upon to cut out pressure before release of the clamps are working correctly (as above).

Remind technicians that if they have any doubts about the integrity of the rig controls to raise it as an issue to enable the appropriate tests and maintenance to be implemented.