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The 'Jump' being discussed in the soft seated valve is the characteristic of the material rather than design, PTFE based seats are prone to 'cold flow' i.e. the small movement of the material at high load points, so at the lip or sealing point of a valve there is typically a higher load which enables the material to provide the seal against the line pressure (approx 2000psi for rtfe) this load deforms the material and causes a sticking point. If a valve is moved within a short time then the this is less of a problem, if there exists either long dwell times before movement or elevated temperatures (because PTFE and like materials will move more quickly the warmer they are and ultimately the reason for their failure at higher temperatures) then there will be a rise in torque greater than normal to enable the seat to be released, ultimately once movement starts then the high torque will cause overtravel and the controller will have to compensate, consequently there are a lot more 'ripples' in the medias flow as the controller brings the valve to the correct position.I don't know why you assume soft seats cause an intial jump in valve position. That is very much a property of the valve design. Maybe double-offset butterfly valves with TFE seats, but that's about it. I have seen a LOT more metal seated valves with poor positioner calibration that had a jump at initial opening.
HmmmI've got to disagree with the previous statement. Jump is a function of the design. Not the material
Unseating and breakout of metal or soft is another word for 'jump' sorry if the terminology was a little simplified.You need to build up enough pressure in the actuator of overcome the unseating torque. Once the disc breaks contact with the seat, the air prressure inside the cylinder is greater than needed to continue rotation of the disc so the air rapidly expands causing the disc to rotate past the desired control position.