Good things come in sets of three

Apart from centrically seated shut-off valves and double eccentric valves, Gefa Processtechnik now also offers a triple eccentric version. The HGT shut-off valve was developed in order to offer a tightly sealing valve solution for shut-off processes under conditions of high temperature and pressure.

In centrically seated shut-off valves, the sealing surface towards the valve disc and its position inside the housing are concentric in relation to the shaft fulcrum. Gefa has been using this form of design for the past 35 years, as an elastically lined shut-off and control valve in nominal widths from DN 25 to DN 1200/PN 10 to 16. The seat ring (elastomer) that forms the housing lining is the essential heart of the valve. The optimisation of the various rubber recipe options for media properties and the remaining pressure deformation ensures a long service life, while providing a safe sealing function.

The design and construction of this important sealing element/seat ring are therefore decisive. The transition to the dovetailed guide (clamping zone near the flanges) has rounded edges and the stable, thick walls in the sealing area and towards the sides result in the safe arrest of the seat ring inside the housing, which has a high tearing strength.

High flow speeds, crust formation and high-wearing media are thus optimally controlled. The advantages of this well-considered design are particularly apparent under extreme loads.

Another variant are the PTFE-lined butterfly valves in nominal widths from DN 50 to DN 800/PN 10, which are also based on the principle of centricity. Owing to the material properties of the PTFE plastomer, however, its internal design is completely different from that of the elastically lined shut-off valves. This is in keeping with the basic chemical requirements. The PTFE lining in the housing is at least 3 mm thick and is continuously pressed down by stainless steel pressure pads, the main seal and plate springs at the shaft apertures in a defined relation to the main seal of the valve disc. This means that the sealing function based on the VDI 2440 clean air guidelines is fulfilled directly at the main seal in the form of a dual sealing system, with an additional gas barrier as the third seal over the entire temperature range up to +180 °C. The shaft does not come into contact with the me-dium. This means that no subsequent sealing in a no longer chemically resistant part of the material will be required.

The double eccentric principle can be explained by the double displacement of the sealing surface on the valve disc and the seat inside the housing in relation to the shaft fulcrum. The sealing surface on the valve disc is offset forwards in relation to the fulcrum and laterally on the pipe axis.

The high-performance valves with their double eccentric bearings have been produced for the past 15 years as Gefa’s in-house developments in the nominal width range from DN 50 to DN 1200/PN 10 to PN 40. This has enabled the company to broaden the applications of shut-off valves to supersede high-priced, heavy gates and valves, which are difficult to install. Ball valves can also be replaced with this high-performance valve.

With the larger nominal width, the advantages in comparison with conventional valves have become increasingly apparent. The installation of the seat ring inside the housing protects the ring against the direct flow of the medium if the flow is in the recommended direction, and thus against wear and tear caused by abrasive solids contained in the medium. In this flow and pressure direction, the seat ring is subjected to pressure as a result of the differential pressure in the closed switch position, with such pressure being proportional to the sealing surface of the valve disc. When the valve is opened, the valve disc is lifted from its seat at an early stage, thus taking all load off the seat ring. This ensures the absolutely gas-tight function (for seat rings made of PTFE compounds) under low-torque conditions during the switching process. The metallic seal design (seat ring made of 1.4571) is additionally surface-hardened on both sides of the pair, i.e. the sealing surface of the valve disc and the metal seat ring, using a tried and tested process, so that friction between the metal surfaces is achieved to guarantee long-term, low-wearing operation. Although these high-quality valves with metal seats always leave the factory as tightly sealing valves, this version is deemed to fall into Sealing Group B according to DIN EN 12266-1, as only this can be considered serious.

Triple eccentric shut-off and control valves constitute a further development of the double eccentric technology. In addition to the previously described, double displacement of the sealing surface in relation to the shaft fulcrum, the third eccentricity is achieved by shifting the axial symmetry of the sealing surfaces (the seat axis is shifted away from the pipe axis). As a cubic body, the cone is the function’s point of departure. The cone is not cut in the straight centric plane but, for example, at right angles to its outer body contour. This means that the valve disc only engages the seat at the last moment. The two sealing surfaces make contact without friction or tension. This design principle ensures a low switching momentum, even under conditions of high temperature and pressure.

When developing this valve technology, Gefa spent a lot of time optimising and testing its invention. After an intensive test phase, the triple eccentric HGT shut-off valve will be first presented at the Achema 2009 fair and can already be ordered.

The development of the HGT shut-off valve was based on the consistent implementation of the triple eccentric design principle with a kinematic investigation of the mode of action. Tight sealing in both pressure directions is ensured. The engagement of the seat takes place without friction. The tension-free function is guaranteed by defining the offset angle to the sealing surfaces.

Other advantages are :

The radial elastic deformation results in contact with the plates all around the circumference. This in turn provides absolute leak-tightness according to DIN EN 122661 Part 1, Leakage Rate A, while at the same time ensuring a low switching momentum without tension.

The pressure is safely absorbed by the shaft bearings, which have a high load capacity. Continuous, stable shaft guidance over the entire area of the single-piece shaft made of high-strength material is guaranteed.

Due to the seat design, the triple eccentric valves engage the seat as a function of their path. The drive momentum is consequently always used to press the seat surfaces down – a precondition of reliable sealing.

Halle 8.0, Booth G15

Online-Info www.cpp-net.com/2209427

Gefa Processtechnik GmbH has been supplying high-quality industrial valves for the past 45 years. The company was one of the first in the German market to manufacture three-piece ball valves, soft-seated butterfly valves and pneumatic part-turn valve actuators. Today, shut-off valves, ball valves, check valves and knife gate valves are developed and produced at its head office in Dortmund.

All Gefa shut-off valves are modular and have several important factors in common, helping users design an efficient system and facilitating its maintenance. The most important shared advantages are as follows:

Apart from valve technology, Gefa covers two other business sectors: filtration technology, with solutions for filtering solids out of liquids, as well as a division for high-quality measuring and control technology solutions. At this year’s Achema trade fair, Gefa will be represented with two stands (valves in Hall 8.0, Booth G15 and filters in Hall 6, Booth J12).