Fluid valve assembly and a process valve positioner

1. A fluid valve assembly of a process valve positioner for connection to a supply of fluid under pressure for providing a hydraulic or pneumatic actuator with an actuator fluid pressure, the process valve positioner including an electronic unit having an electrical control output, and a pneumatic or hydraulic actuator having a prestage and an output stage, where the prestage converts the electrical control output into a pilot fluid pressure to control the output stage, the output stage including the fluid valve assembly, the fluid valve assembly comprising:

a valve body having a central bore with (i) at least one supply port configured to receive the supply of fluid under pressure, (ii) at least one actuator port configured to provide the actuator fluid pressure to the hydraulic or pneumatic actuator, and (iii) at least one exhaust port;

a stem movable within the central bore in an axial direction by an axial pilot force;

a pilot diaphragm and a piston arranged at one end of the stem to provide the axial pilot force according the pilot fluid pressure that is applied to the pilot diaphragm in a pilot pressure chamber;

a counter diaphragm and a counter piston arranged at an opposite end of the stem as the pilot diaphragm and the piston, the counter diaphragm and the counter piston being configured to provide an axial counter force according to a counter pressure applied to the counter diaphragm in a counter pressure chamber;

at least one pair of counter-acting metering edges operationally linked by the stem, each metering edge of each counter-acting pair including (i) a mating seat surface on the valve body or the stem, and (ii) a poppet ring supported by a flexible element to the valve body or the stem in a manner allowing a relative axial movement of the poppet ring and the supporting valve body or stem in a closed state of the respective metering edge, each poppet ring being pressure-balanced to compensate fluid pressure forces exerted on the respective poppet ring so as to provide a very small or zero resultant fluid pressure force applied to the respective metering edge;

a first restricted flow path from a supply pressure inlet of the valve assembly to the prestage, which controls the pilot pressure in the pilot pressure chamber and the axial pilot force; and

a second restricted flow path from the supply pressure inlet of the valve assembly to the counter pressure chamber.

2. The fluid valve assembly as claimed in claim 1 , wherein:

the at least one pair of counter-acting metering edges are mechanically linked by the stem such that both metering edges of each counter-acting pair are closed in an intermediate position of the stem, and

one metering edge is closed and the other metering edge of each counter-acting pair is opened based on movement of the stem to a first axial position, and the one metering edge is opened and the other metering edge of each counter-acting pair is closed based on the movement of the stem to an opposite second axial position.

3. The fluid valve assembly as claimed in claim 1 , wherein one metering edge of each pair of counter-acting metering edges is configured to control fluid flow between a respective actuator port and the supply of fluid, and another metering edge of each pair of counter-acting metering edges is configured to control fluid flow between the respective actuator port and an exhaust port.

4. The fluid valve assembly as claimed in claim 1 , wherein:

one metering edge of each counter-acting metering edges includes the poppet ring supported by the flexible element to the stem and the respective mating seat surface on the valve body, and

another metering edge of each pair of counter-acting metering edges includes the poppet ring supported by the flexible element to the valve body and the respective mating seat surface on the stem.

5. The fluid valve assembly as claimed in claim 1 , wherein:

each poppet ring is arranged coaxially with the stem, and

the flexible element of each poppet ring includes a respective annular sealing element.

6. The fluid valve assembly as claimed in claim 5 , wherein:

in one metering edge of each pair of counter-acting metering edges, the poppet ring is supported at an inner circle of the poppet ring by the respective annular flexible sealing element to an outer circle of the stem, and

in another metering edge of each pair of counter-acting metering edges, the poppet ring is supported at an outer circle of the poppet ring by the respective annular flexible sealing element to the valve body.

7. The fluid valve assembly as claimed in claim 1 , wherein the at least one pair of counter-acting metering edges includes one pair of counter-acting metering edges at each actuator port of the fluid valve assembly, the actuator port being located between the counter-acting metering edges of the respective pair of counter-acting metering edges.

8. A double-acting actuator including the fluid valve assembly as claimed in claim 1 , wherein:

the at least one actuator port includes a first actuator port and a second actuator port, and

the at least one pair of counter-acting metering edges includes a first pair of counter-acting first and second metering edges set to the first actuator port, and a second pair of counter-acting third and fourth metering edges set to the second actuator.

9. The fluid valve assembly as claimed in claim 8 , wherein:

the first actuator port is located between the first metering edge and the second metering edge of the first pair of counter-acting first and second metering edges, and

the second actuator port is located between the third metering edge and the fourth metering edge of the second pair of counter-acting third and fourth metering edges.

10. The fluid valve assembly as claimed in claim 8 , wherein:

the first metering edge includes a first poppet ring arranged coaxially around the stem within the central bore and fixed to the valve body by a first flexible sealing element member allowing movement of the first poppet ring in the axial direction, the first poppet ring cooperating with a first mating seat surface on the stem to control fluid flow between the first actuator port and one of the supply port and the exhaust port;

the second metering edge includes a second poppet ring arranged coaxially around the stem within the central bore and connected to the stem by a second flexible sealing member allowing movement of the second poppet ring in the axial direction, the second poppet ring cooperating with a second mating seat surface on the valve body to control fluid flow between the second actuator port and the other of the supply port and the exhaust port;

the third metering edge includes a third poppet ring arranged coaxially around the stem within the central bore and fixed to the stem by a third flexible sealing element member allowing movement of the third poppet ring in the axial direction, the third poppet ring cooperating with a third mating seat surface on the valve body to control fluid flow between the second actuator port and one of the supply port and the exhaust port; and

the fourth metering edge includes a fourth poppet ring arranged coaxially around the stem within the central bore and connected to the valve body by a fourth flexible sealing member allowing movement of the fourth poppet ring in the axial direction, the fourth poppet ring cooperating with a fourth mating seat surface on the stem to control fluid flow between the second actuator port and the other of the supply port and the exhaust port.

11. The fluid valve assembly according to claim 10 , wherein:

the at least one supply port includes a common supply port located at a middle section of the central bore defined between the second pair of counter-acting third and fourth metering edges, and

the at least one exhaust port includes a first exhaust port located at a first end section of the central bore defined between the first metering edge and a first end of the central bore, and a second exhaust port located at an opposite second end section of the central bore defined between the fourth metering edge and an opposite second end of the central bore.

12. The fluid valve assembly as claimed in claim 1 , wherein the first restricted flow path and the second restricted flow path have dimensions such that a rate of change of the axial counter force and a rate of change of the pilot force due to a change in the supply pressure in the supply pressure inlet are equal.

13. The fluid valve assembly as claimed in claim 1 , wherein the first restricted flow path or the second restricted flow path includes a flow orifice restrictor.

14. The fluid valve assembly as claimed in claim 13 , wherein the flow orifice restrictor includes an orifice diameter of from 0.1 mm to 0.5 mm.

15. The fluid valve assembly as claimed in claim 13 , wherein the flow orifice restrictor includes an orifice diameter of from about 0.2 mm to 0.3 mm.

16. The fluid valve assembly as claimed in claim 1 , wherein all metering edges are aligned in the axial direction.

17. The fluid valve assembly as claimed in claim 1 , wherein the counter diaphragm is arranged to scale the axial counter force from the supply fluid force applied to the counter diaphragm.