Some AHJs require a "by-pass", manually operated and normally closed, to be piped around the FDC check valve. These are invaluable in back flushing otherwise contaminated FDC connections. Arrow points to the top end of the by-pass. The FDC check valve is shown on the right.
Another by-pass installation:
And another FDC-By-Pass, in a desert environment, overseas:
Another FDC by-pass: