seat upstream and seat downstream

[bao2ye]

For double offset soft seat high performance butterfly valve, the seating/unseating torque is different. The torque at the seat upstream positioned is lower than the seat is downsteam positioned. Is this due to the off set of the shaft centerline to the valve?

Are most valve used in seat down stream position?

Thanks

 

[bcd]

For double-offset butterfly valves, there is a difference in unseating torque depending upon pressure direction. Some of it is due to the offset from the shaft centerline, and some is due to different contact force between the seat and disc. To build the valve, there are small amounts of bearing clearance and pressure causes the shaft to bend. With pressure in one direction, it pushes the disc away from the seat causing less contcat force. In the opposite direction, the disc is pushed into the seat, causing higher contact force. The disc movement is a small amount, but the cone shape at the disc OD causes a wedging action with the seat, so when the disc moves into the seat, the contact force goes-up significantly. So there are two reasons for the difference in torque.

bcd

 

[JimCasey]

bcd is right. And, as you said, there is the offset that helps the valve open when in the shaft-down direction. Everybody's double-offset valve claims bidirectional shutoff, but shaft-down is the preferred flow direction because it tales pressure off the packing when the valve is closed.

 

[bao2ye]

Thanks all very much. JimCasey, can you give me more information to explain why the shaft down flow direction can cause the pressure off in the valve packing?

 

[gerhardl]

Hello Guys,

To avoid confusion. Typical sample:

Flow from direction A to B is preferred flow direction, (best sealing properties) when valve is closed and:

- Stem is on the A side, and the disc- further from the A inlet than the stem - (with seat sealing mounted on disc if not metal seated). The disc is pressed into the slightly constricted valve body inner walls, the wall cone forming the seat. The fluid pressure is adding force to push the disc and seat sealing firmer onto the seat.


Opposite: flow from B to A is typically given as 'non preferred' but with full sealig properties, when the valve is closed and:

the disc is on the B side, stem on the opposite side, and fluid is pushing in direction from B trying to lift the disc out of the seat (against the pressing torque given from stem rotation on the disc).

For solid constructed and tested double eccentric valves with correctly adjusted soft seals, for instance for water distribution and lower pressures, this is normally not significant, and both A-B and B-A can be found, or is even necessary for bi-directional flow.

For pressures nearer to pressure class limits or for maximum life expectancy preferred direction should be used.

Note: some quality-inclined end users have preferred direction as a requirement for flow direction.

 

[JLSeagull]

Also, even if the process is designed for uni-directional flow, during maintenance many valves may have high-pressure on the downstream side with the upstream side at low-pressure.

 

[JimCasey]

Gerhardl and JLSeagull are right. My comment was directed more toward normal day-to-day operation. When the valve is closed, one side has pressure and the other does not, or at least has significantly less presure. If the shaft is downstream from the seat, then it is in the unpressurized (or low pressure) zone, so there is little to force fluid through the packing. There are plenty of exceptions, but this would be the normal expectation.