Fluid valve assembly, process valve positioner and use of a fluid valve assembly in control of a process valve
A fluid valve assembly having a central bore and a stem axially-movable within the central bore and actuated by an axial pilot force and an axial counter force. An inner supply pressure chamber is provided to retain a stabilized supply pressure providing the axial counter force affecting on the stem. A seal member is arranged coaxially with the stem between the inner supply pressure chamber and an outer supply pressure chamber which is connected to a supply pressure input line. A metering edge is arranged coaxially with the stem to control fluid flow from the outer supply pressure chamber to an actuator chamber. Means are provided to stabilize the supply pressure in the inner chamber against sudden pressure drops in the outer supply pressure chamber.
1. A fluid valve assembly for connection to a supply of fluid under pressure for providing an actuator with an actuator fluid pressure, the fluid valve assembly comprising:
a valve body with a central bore, the central bore comprising: a supply pressure chamber connected to a supply pressure input line for receiving a supply of fluid under pressure; an actuator chamber connected to an actuator pressure line for providing a control fluid pressure to an actuator; and an exhaust chamber connected to a pressure exhaust line for venting the control fluid pressure from the actuator chamber;
a stem axially-movable within said central bore and actuated by an axial pilot force and an axial counter force affecting on the stem;
a metering edge and a counteracting metering edge arranged coaxially with and controlled by the stem and arranged to control fluid flow from the supply pressure chamber to the actuator chamber and from the actuator chamber to the exhaust chamber, respectively;
a seal member arranged coaxially with the stem to divide the supply pressure chamber into an outer supply pressure chamber and an inner supply pressure chamber,
the inner supply pressure chamber being arranged to actuate the stem by the supply pressure providing the axial counter force affecting on the stem within the inner supply pressure chamber, and the outer supply pressure chamber being connected to the supply pressure input line and arranged to feed the actuator chamber; and
a flow restrictor element arranged in a supply pressure flow path between the inner supply pressure chamber and the outer supply pressure chamber and configured to restrict rapid changes in the supply pressure in the inner chamber, the flow restrictor element being configured to prevent propagation of rapid changes of the supply pressure from the outer supply pressure chamber through the supply pressure flow path to the inner supply pressure chamber and to thereby prevent uncontrolled axial movements of the stem.
2. A fluid valve assembly as claimed in claim 1 , wherein the restrictor element configured to restrict rapid changes in the supply pressure in the inner chamber comprise a restricted flow path between the inner supply pressure chamber and the outer supply pressure chamber.
3. A fluid valve assembly as claimed in claim 2 , wherein the restricted flow path is arranged to lower the stabilized supply pressure retained in the inner supply pressure chamber to the supply pressure in the outer supply pressure chamber with a predetermined rate.
4. A fluid valve assembly as claimed in claim 2 , wherein the restricted flow path is arranged to raise the stabilized supply pressure retained in the inner supply pressure chamber at a predetermined rate, if the supply pressure in the outer supply pressure chamber is higher than the stabilized supply pressure retained in the inner supply pressure chamber.
5. A fluid valve assembly as claimed in claim 2 , wherein the restricted flow path comprises a flow restrictor.
6. A fluid valve assembly as claimed in claim 5 , wherein the flow restrictor is a flow orifice restrictor.
7. A fluid valve assembly as claimed in claim 2 , wherein the seal member is an annular lip seal arranged to seal the inner supply pressure chamber from the outer supply pressure chamber, if the pressure in the outer supply pressure chamber drops below the stabilized supply pressure retained in the inner supply pressure chamber, and arranged to pass the supply pressure from the outer supply pressure chamber to the inner supply pressure chamber to raise the stabilized supply pressure retained in the inner supply pressure chamber, if the supply pressure in the outer supply pressure chamber is higher than the stabilized supply pressure retained in the inner supply pressure chamber.
8. A fluid valve assembly as claimed in claim 2 , comprising a further restricted flow path from a supply pressure inlet of the outer supply pressure chamber to a prestage which controls a pilot pressure and thereby the axial pilot force.
9. A fluid valve assembly as claimed in claim 8 , wherein the restricted flow path and the further restricted flow path are dimensioned so that a rate of change of the axial counter force and the rate of change of the pilot force due to a change in the supply pressure in the outer supply pressure chamber are approximately equal.
10. A fluid valve assembly as claimed in claim 1 , wherein the restrictor element configured to restrict rapid changes in the supply pressure in the inner chamber comprises a flow control device configured to block a supply pressure flow path between the inner supply chamber and the outer supply chamber, if the pressure in the outer supply chamber drops below the stabilized supply pressure retained in the inner supply chamber, and arranged to open the supply pressure flow path otherwise.
11. A fluid valve assembly as claimed in claim 1 , wherein the seal member comprises a lip seal, a ring seal, a sealing diaphragm or sealing bellows.
12. A fluid valve assembly as claimed in claim 1 , wherein the restrictor element configured to restrict rapid changes in the supply pressure in the inner chamber comprises a flow control device configured to block a supply pressure flow path between the inner supply chamber and the outer supply chamber, if the pressure in the outer supply chamber drops below the stabilized supply pressure retained in the inner supply chamber, and arranged to open the supply pressure flow path otherwise.
13. A fluid valve assembly as claimed in claim 1 , wherein the restrictor element configured to restrict rapid changes in the supply pressure in the inner chamber comprises a flow control device configured to block a supply pressure flow path between the inner supply chamber and the outer supply chamber, if the pressure in the outer supply chamber drops below the stabilized supply pressure retained in the inner supply chamber, and arranged to open the supply pressure flow path otherwise, and comprising a restricted flow path from the inner supply pressure chamber to a pre-stage which controls a pilot pressure and thereby the axial pilot force.
14. A fluid valve assembly as claimed in claim 1 , wherein one of the metering edge and the counteracting metering edge comprises a poppet ring supported by a flexible element to the stem and a respective mating seat surface on the valve body, and another one of the metering edge and a counteracting metering edge comprises a poppet ring supported by a flexible element to the valve body and a respective mating seat surface on the stem, a support arrangement allowing a relative axial movement of the poppet ring and the valve body or the poppet ring and the stem also in a closed state of the respective metering edge.
15. A fluid valve assembly as claimed in claim 14 , wherein each flexible element comprises a respective sealing element.
16. A process valve positioner comprising:
an electronic unit with an electrical control output; and
a pneumatic or hydraulic unit arranged to convert the electrical control output to a corresponding control fluid pressure to the actuator, the pneumatic or hydraulic unit comprising:
a valve body with a central bore, the central bore comprising supply pressure chamber connected to a supply pressure input line for receiving a supply of fluid under pressure; an actuator chamber connected to an actuator pressure line for providing the control fluid pressure to the actuator; and an exhaust chamber connected to a pressure exhaust line for venting the control fluid pressure from the actuator chamber;
a stem axially-movable within said central bore and actuated by an axial pilot force and an axial counter force affecting on the stem;
a metering edge and a counteracting metering edge arranged coaxially with and controlled by the stem and arranged to control fluid flow from the supply pressure chamber to the actuator chamber and from the actuator chamber to the exhaust chamber, respectively;
a seal member arranged coaxially with the stem to divide the supply pressure chamber into an outer supply pressure chamber and an inner supply pressure chamber,
the inner supply pressure chamber being arranged to actuate the stem by the supply pressure providing the axial counter force affecting on the stem within the inner supply pressure chamber, and the outer supply pressure chamber being connected to the supply pressure input line and arranged to feed the actuator chamber; and
a flow restrictor element arranged in a supply pressure flow path between the inner supply pressure chamber and the outer supply pressure chamber and configured to restrict rapid changes in the supply pressure in the inner chamber, the flow restrictor element being configured to prevent propagation of rapid changes of the supply pressure from the outer supply pressure chamber through the supply pressure flow path to the inner supply pressure chamber and to thereby prevent uncontrolled axial movements of the stem.
17. A process valve positioner according to claim 16 , wherein the pneumatic or hydraulic unit comprises a prestage and an output stage including the fluid valve assembly, the prestage being arranged to convert the electrical control output into a pilot pressure which is sufficient to control the output stage.