Valve assembly for a gas chromatograph

1. A valve assembly for a gas chromatograph, the valve assembly including:

a main valve body defining a first inner cylindrical cavity coaxially disposed with reference to a central axis;

a support plate positioned above the main valve body and including a planar upper surface that is generally normal to the central axis, the planar upper surface having a surface groove circumferentially-disposed about the central axis and an array of holes extending generally parallel to the central axis across the support plate;

an upper valve head positioned above the support plate, the upper valve head including a bottom planar bottom surface and a plurality of discrete conduits, each conduit extending between an outer end and an inner end;

a sealing diaphragm interposed between the planar upper surface of the support plate and the planar bottom surface of the upper valve head;

a first piston positioned inside the first inner cavity to reciprocate, along the central axis and relative to the sealing diaphragm, between at least a bottom position and an upper position, the first piston having a body defining a second inner cylindrical cavity;

a second piston positioned inside the second inner cavity of the first piston to reciprocate, along the central axis and relative to the sealing diaphragm, between at least a bottom position and an upper position;

a set of normally-closed plungers and a set of normally-opened plungers that are generally extending parallel to the central axis and disposed in a staggered arrangement, the normally-closed plungers being carried by the first piston and the normally-opened plungers being carried by the second piston;

an actuating gas circuit extending through the body of the first piston to establish a fluid communication with a space located between a bottom surface of the second inner cavity and the second piston;

a first force-generating mechanism to urge the first piston towards its upper position; and

a second force-generating mechanism to urge the second piston towards its bottom position.

2. The valve assembly as defined in claim 1 , wherein the holes of the support plate are axisymmetric and are in registry with the surface groove.

3. The valve assembly as defined in claim 2 , wherein the inner end of each conduit is located on the planar bottom surface and is in registry with the surface groove at a corresponding intermediate position that is substantially halfway between two adjacent ones of the holes on the support plate.

4. The valve assembly as defined in claim 1 , wherein the second inner cavity is coaxially disposed with reference to the central axis.

5. The valve assembly as defined in claim 1 , wherein each normally-closed plunger has an upper portion, slidably mounted inside a corresponding one of the holes of the support plate, with an upper end engaging the sealing diaphragm when the first piston is in the upper position and that is out of engagement with the sealing diaphragm when the first piston is in the bottom position, each normally-opened plunger having an upper portion, slidably mounted inside a corresponding one of the holes of the support plate, with an upper end engaging the sealing diaphragm when the second piston is in the upper position and that is out of engagement with the sealing diaphragm when the second piston is in the bottom position.

6. The valve assembly as defined in claim 1 , further including a mechanical fastener disposed to removably affix the support plate underneath the upper valve head, the fastener being generally coaxial with the central axis.

7. The valve assembly as defined in claim 6 , wherein the mechanical fastener of the support plate includes a screw engaged into a threaded hole provided underneath the upper valve head.

8. The valve assembly as defined in claim 1 , wherein the first force-generating mechanism includes a spring pack unit positioned between a bottom side of the first piston and the main valve body.

9. The valve assembly as defined in claim 8 , wherein the spring pack unit is adjustable in position along the central axis.

10. The valve assembly as defined in claim 1 , wherein the first force-generating mechanism includes at least one Belleville spring.

11. The valve assembly as defined in claim 1 , wherein the second force-generating mechanism is located inside the second inner cavity.

12. The valve assembly as defined in claim 11 , wherein the second force-generating mechanism is located between a bottom side of a perforated plate that is affixed to an upper side of the first piston, and an upper side of the second piston.

13. The valve assembly as defined in claim 1 , wherein the second force-generating mechanism includes a wave disc.

14. The valve assembly as defined in claim 1 , wherein the first piston engages a sidewall of the first inner cavity using a pair of vertically spaced-apart gaskets mounted in corresponding outer grooves around the first piston.

15. The valve assembly as defined in claim 14 , wherein the actuating gas circuit passes through the main valve body and is in fluid communication with an intervening space located between the sidewall of the first inner cavity, a sidewall of the body of the first piston and the two spaced-apart gaskets, at least one channel extending across the sidewall of the body of the first piston to establish a fluid communication between the second inner cavity and the actuating gas circuit.

16. The valve assembly as defined in claim 15 , wherein the valve assembly includes at least one of the following features:

at least one channel leads to the space located between the bottom surface of the second inner cavity and the bottom side of the second piston;

the space is annular and is surrounded by a generally annular section projecting upwardly inside the second inner cavity.

17. The valve assembly as defined in claim 1 , wherein the valve assembly includes at least one of the following features:

the support plate has a circular cross section, the surface groove extending uninterruptedly on the support plate;

at least some of the plungers include a central cavity at an upper tip end;

the main valve body includes a housing to which is affixed a bottom cover, the first inner cavity being provided inside the housing.

18. The valve assembly as defined in claim 1 , wherein the normally-closed plungers are mounted on a perforated plate affixed to the first piston and encasing the second piston inside the second inner cavity.

19. The valve assembly as defined in claim 1 , further including a purge gas circuit passing at least around the support plate.

20. The valve assembly as defined in claim 19 , wherein the support plate includes at least two spaced-apart and radially-extending channels, at least a portion of the purge gas circuit passing inside the channels of the support plate to vent a central area of the sealing diaphragm.