29.06.2016
DOUBLE-SEAT VALVE (Type 491)
BA 049100.06 EN 11 / 34
4 Description of Function
Double-seat valves are installed at critical connection points of two process pipings. The
different liquid media flowing through the pipings are sealed twice (double) against each
other in the valve’s seat area through the upper and lower valve disk.
The leakage chamber is formed between both valve disks. Should a leakage occur at one
of the two valve disks, then the respective medium can flow out through the leakage
chamber to the outside.
The leakage-free switching function is characteristic for the double-seat valve:
1. The lower valve disk moves in the valve seat’s sealing area against the upper
valve disk. At the same time, the leakage chamber is completely closed.
2. The valve disks now lying against each other move from the upper valve seat
area into the open valve position.
3. When the valve is closed, the adjacent valve disks now move into the upper
valve seat area.
The leakage chamber is still closed.
4. The lower valve disk moves in the valve seat’s sealing area to its final
position and at the same time slowly opens the leakage chamber.
Due to the piston-like valve rods, the double-seat valve is insensitive to pressure impacts.
The lower balancer is also at the same time the actuator piston i.e. the valve does not
possess a separate actuator cylinder.
The upper and lower valve disk are integrated into the respective balancer. During the
switching function, the balancers move between product chamber and the upper or lower
flushing chamber.
The sealing between the product chamber and the flushing chamber is effected through a
gap-free tensioned lip sealing.
4.1 Valve Types
The current valve type 481 has been updated. The new valve type 491 is in the product
area identical to the EHEDG tested valve type 580.
The valve type 491 has, however, no seat lifting but is therefore equipped with leakage-
chamber cleaning.
The follow-on type 491 is characterized by:
• Separate upper and lower valve disks, easier sealing change
• Form seals (EPDM) above and below in the seat area
• Updated seat design with controlled cleaning gap
• One-piece housing execution, hygienically formed product chamber
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