Eccentric butterfly valves

This type of butterfly valve has been manufactured since 1960. The main advantage compared to concentric butterfly valves is the different sealing design that leads to significantly less wear, along with the advantage of being able to with stand higher temperature and pressure conditions. This is why these butterfly valves are normally designated as high-pressure or HP-type valves, In contrast to concentric butterfly valves, the body of eccentric butterfly valves is not pro tected by a plastic liner. Shaft, disc and the body are directly in contact with the media.

Single eccentric butterfly valves

The sealing edge of the disc, which does not match the centre line of the shaft but is displaced in parallel direction to the flow, describes a single eccentric butterfly valve operation. While closing, the disc turns tangentially into the seat and the sealing area on the lateral surface of the disc makes contact at the outside with the seat. The permanent adjustment of the seal demands precision and specific surface properties, elasti city defining the quality of this valve type.

Single eccentric butterfly valves are widely used for low-temperature and low-pressure applications in the field of ventilation, filter and water technology ranging up to 160" pipe diameters.

Double eccentric butterfly valves

Almost frictionles swingout

In the case of double eccentric butterfly valves, moving the shaft's centre line from a single offset by 90" slightly out of the valve centre line to an additional offset describes a double eccentric situation. This second offset is only a few millimetres and difficult to physically see. Even at a very small opening angle, the disc is able to move out of its seat smoothly. This feature significantly reduces wear and keeps the breakaway torque at a minimum level. The coordination of both offsets is the basis for good operation and lifetime service of the valve.
Double eccentric valves cover a broader range of applications because the metals used allow significantly higher temperatures and pressures compared with concentric or single eccentric butterfly valves. Peaks of up to 800°C (1472°F) in temperature and up to 100 bar (1450 PSI) pressure are feasible, dependent on each other. Cryogenic applications down to -200°C (-328°F) are also possible. Shaft leakages in vacuum ap plications can be widely eliminated. A stuffing box sealing system between shaft and housing achieves high tightness, also for media such as helium or hydrogen.

No Liner

Double eccentric butterfly valves are usually not lined and are typically available in at least two different metal combinations (stainless steel 1.4408 (ASTM CF8M) and cast steel 1.0619 (ASTM 216 WCB)). Applications with specific requirements such as resistance against sea water or different corrosive media use other types of materials (e.g. Duplex, Monel, alumin ium bronze, titanium, etc.).

Gas-tight shut off

The sealing systems for double eccentric butterfly valves are available in either soft-seated or metal-seated versions. Soft-seated seals are based upon PTFE compounds, permitting a gas tightness according to EN 12266, leakage rate A (no visible leakage). Pressure and temperatureparameters must comply with the pressure/tem perature diagram according to the manufactur er's specification. Butterfly valves with metal seal ing elements are suitable for temperatures up to 800°C (1472°F). A metal-seated butterfly valve is significantly less sensitive to media containing solids. And when closing against liquids, metal seated butterfly valves can reliably achieve the leakage rate A (no visible/measured leakage).

Zero-clearance design

Every closing operation for double eccentric butterfly valves depends on the precise movement of the disc path. The disc requires an excellent repeatability to move back into the tight closing position and to provide high levels of tightness, even after many operation cycles. This requires that all components of both actuator and valve are designed without any gaps and the closing position of the disc itself provides an adjustable stop limit (usually inside the actuator), so that the precise closing position is always repeated. Because the disc positions from 20° to 70° display almost a linear control property to the media flow, double eccentric valves are suitable for on-off applications as well as for control applications.

Triple eccentric butterfly valves

Asymmetrical finished sealing surfaces

Similar to double eccentric butterfly valves, the centre line for the shaft of triple eccentric butterfly valves maintains the same position, i.e. it is lo cated in the valve with a small offset from the centre line of the pipe. An additional an gular offset of the seat cone axis provides a third eccentricity. This effect causes an asymmetrical design for the conical sealing surface of the disc. It is an intended modification, enabling an en hanced closing characteristic, which overlaps the disc into the seat with an action completely lack ing in friction. Like the double eccentric butterfly valves, the torque requirement for closing the valve is proportional to the pressure difference of the application. In practice, a triple eccentric valve closes without any friction between disc and seat, eliminating wear and extending the valve life.

A major difference between double and triple eccentric butterfly valves is the closing principle. In double eccentric butterfly valves, the disc is driven into a precisely determined position to shut off the media. In triple eccentric valves, the disc is driven into the seat with maximum torque, no matter what the precise position of the disc is. This closing principle can be com pared with a globe valve.

Triple eccentric valves are usually supplied with metal sealing systems made from either solid steel rings or are of laminated design (metal graphite sandwich design). Valves with lamin ated sealing systems are rated up to 550°C (1022°F), whereas solid metal seats are suitable for temperatures up to 800°C (1472°F) and pressures up to 40 MPa (400 bar/5801 PSI).

Quadruple eccentric butterfly valves

Specific maching of the seat

This valve design type has been available for a couple of years. The fourth eccentricity is achieved by a special machining of the seat con tour. According to the manufacturer, the flow characteristics are better than with triple eccen tric butterfly valves. Similarly, the closing prin ciple of a quadruple eccentric butterfly valve is identical to the triple eccentric valve: its disc is closed by a defined maximum torque, not by positioning.

The application areas for triple and quad ruple eccentric valves do not differ in principle.