- 14 October 2004 -

New developments in triple-offset butterfly valves

Fluid control plays a key role in keeping the wheels of the modern day industry in motion. Hardev Singh Phull, engineering group leader at Dunlop Aerospace Fluid Dynamics discusses the contribution that a new generation of triple-offset butterfly valves can make to fluid control.

Fuelled with an increased demand for automated systems offering superior flexibility and higher levels of safety, the dependence on efficient and reliable ancillary equipment, such as valves, has never been greater. The humble ‘butterfly valve’, which essentially utilises a disc element suspended inside a tubular housing to form the desired obstruction in the fluid flow, is one such example.

Control is often achieved by adjusting the orientation of the disc in the valve housing relative to the axis of discharge by a rotary motion, actuated pneumatically, hydraulically, electrically, or indeed, manually. The flexibility offered by such valves has ensured their acceptance in a variety of applications covering a wide range of industries that include water treatment, petrochemical, oil and gas, aerospace, marine etc. and demand for them remains buoyant as ever. In view of this, Dunlop Equipment, an OEM supplier generally associated with air bleed systems for the aero-engine sector, has developed a range of butterfly valves for high performance applications including gas turbines for the industrial power generation market.

Triple offset concept

The challenge often encountered at the design phase of a typical butterfly valve is how to operate the unit with minimal torque from the actuator whilst enabling effective fluid sealing. Dunlop Aerospace Fluid Dynamics has addressed these issues by making use of the ‘triple-offset’ concept. Although the original concept dates back to the 1970s, the innovative aspects of Dunlop’s triple offset valves lie primarily within the unique design and the manufacturing processes developed by the Company.

The distinct feature of the triple-offset concept is the unique manner in which it achieves fluid sealing at the interface between the disc element and its seat within the valve housing.  Essentially, the peripheral geometry of the disc element is generated by slicing a conical ‘wedge’ at a pre-defined angle relative to the axis formed by the cone. The seat interface, on the other hand, is generated by slicing a matching conical ‘hole’ at the same angle relative to the cone axis and is incorporated into the valve body of the unit. 

This arrangement offers excellent sealing with minimal friction at the interfaces during engagement and disengagement. However, the geometry does not assist in achieving the relevant movements for effective operation. As a result the disc is suspended relative to its seat interface via three separate offsets relative to the valve axis - hence the term 'triple offset'. Each offset is selected to ensure that the desired torque levels are achieved over the specified operating pressure range and to comply with the failure case requirements.

In practice, the interrelationship between the three offsets is normally dependent on the size of the valve and can be critical to safeguard against possible seizure, or poor sealing, at the seat interface during valve operation. Although it is possible to ascertain the said offsets by trial and error methods, this approach can be rather demanding on development funds and specialist skills. Dunlop has resolved this issue by developing specific mathematical codes designed to simulate the valve characteristics for a given set of operating parameters and the selected valve geometry.

Evaluation

The task of evaluating the offsets requires an in-depth knowledge of valve characteristics and packaging the components at the design stage can be a challenge. Normally, interdependent factors have to be considered at this stage to deliver an efficient and effective design, these being:

•           Flow characteristics

•           Leakages rates

•           Operating torque

•           Envelope and Interfaces

•           Materials

In the case of butterfly valves, the ‘flow characteristics’ are best conveyed by comparing the effective area of the through unit at different rotary positions of the disc element. Traditionally, methods used to determine the flow characteristics were mainly based on established tests and tended to offer reasonable repeatability for handling fluids in liquid form. However, where the handling fluids are in the form of gas or steam, it can be rather difficult to attain good repeatability in tests, primarily due to the compressible nature of such fluids and the difficulty in accessing suitable test equipment. 

As ‘time to market’ now tends to be a critical factor, such practices can be impractical and expensive. The methodology adopted by Dunlop to resolve this issue was based on comprehensive CFD analysis which provided the flexibility to refine the profile of the disc assembly for optimum flow rates. The added benefit offered by this approach was that it also enabled the designer to determine the three-dimensional aerodynamic forces generated at the disc assembly by the flow and so predict the torque generated at the drive shaft with reasonable accuracy.

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