PetroStreet

We have been facing issues with NRVs getting choked at the inlet and outlet of the pilot operated PSVs, installed on the pilot lines.
What could be the maintenance best practices for these NRVs?
Other than diaphragm servicing, nothing seems to be applicable.

Workshop PSV technicians reported some damages to NRV but not the choking.
Reported damages were on the aluminium body asking for replacement. Those particular NRVs in aluminium, as they reported, are not available as spares however available in SS material.
When having functional issues, passing etc. they perform in-house (not part-to-part) replacement of diaphragm when condition is bad.
I could not get any inputs on choking though. No such case found for the team working now.

The only thing which is required to be checked is the diaphragm.
Practically, there is nothing else which is required to be maintained looking at the design of most of these backflow preventers.
What is the material of these at your site?
Choking can be caused by a lot of factors as the holes within these NRVs for flow are very small and the kind of design if you carefully see is so narrow for continuous operation that any process carry over will result in choking practically failing NRV function and eventual opening of PSV as it will loose all its force over the main body diaphragm.
Another thing, clean service, deposits?
Choking before as well?