GF JRG 5195.025 Notice De Montage Et D'utilisation page 67

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Make sure that the sealing faces of the
JRG fitting are clean and undamaged
and that no components of the backflow
preventer core (e.g. O-ring) are left
behind in the fitting.
If necessary, replace the fitting.
Put the supplied O-ring into the respective
O-ring groove of the blind flange.
Before assembly, make sure that the
O-ring is intact. In case of damage,
replace it with a faultless one.