Allows the lining material to extend through the vessel opening to the outside of the tank, thereby providing an uniterrupted barrier between the hot water and the steel vessel. In other designs, the lining ends at the steel fitting (particularly the threads) and invites corrosion.

The PVI fitting is removable
and replaceable in the event that
the fitting threads be stripped or damaged. Other manufacturers
weld or otherwise permanently
attach the fittings to the tank, creating considerable difficulty
should the threads in the fitting
be damaged.