Spring loaded steel balls are provided on stem and ball which gives continuity, thereby reducing the possibility of static electricity that builds during operation when the ball rubs continuously on seat. This can be potential fire hazard.
EFVT design gives stem entry from inside while doing assembly which gives effective sealing just starting at the stem & at the same time collar prevents blowout of stem due to cavity pressure.
In the design secondary metal to metal seat is provided, when the soft seat is burnt out or damaged during the fire. Ball is forced against the metal leap provided and hence forms a metal to metal seat.
No leakage for soft seat. (Class VI) / Metal Seat (Class IV)