Nitrogen Generation System check valve broken. 737-6/7/8/900

Once we had a nitrogen generation system that was degraded (blue light on).
Bite checks revealed a 47-30023 message stating: NGSSOV, OTSOV, HFV CLOSED shows when the NGS is on, but a no-flow condition is sensed by the NGS controller.

In this case we replaced all 3 mentioned valves, but the error remained.

Troubleshooting revealed that when we removed a cap from the nitrogen enriched air line to the center tank, the error message dissapeared and the green light came on again. So a blockage in the nitrogen enriched air supply tube was suspected.
A fueltank entry into the center tank was made, and the primary backflow prevention checkvalve was removed for investigation. It appeared that the checkvalve flapper was broken off and fallen into the lower part of the assembly, blocking the flow into the center tank, causing the “no flow” which was sensed by the NGS controller.
This gave the 47-30023 error during the bite checks.
See the pictures of the broken checkvalve in the assembly p/n 2040021-101.
The checkvalve was replaced iaw amm 47-21-04-000-801 and 47-21-04-400-801.

Note: The aircraft with this complaint was under 3 years old.


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