Why Triple Offset?

Due to low operating pressures and non critical applications, butterfly valves historically have been adopted in processes where bubble tight shut-off was not a necessity. Attempts to defy this resulted in the passing of valves and failure of systems ensuing in a long held belief by Engineers that butterfly valves as a type should not be used for more rigorous activities.

Concentric and double offset valves present during the mid twentieth century could only operate uni directionally with a maximum operating pressure of 10 barg at lower temperatures. Today, these valves are most commonly used in non critical utility services and can operate up to 52 barg and temperatures up to 200 degrees C along with the availability of bi-directional sealing if required.

However, in some applications this design of valve was found to suffer from problems concerning wear erosion of the soft sealing materials and failure at high temperatures, leading to the withdrawal of the fire test Lloyds Rule DOT 54 obscuring its future use in high intensity applications such as Offshore Fire Mains service.

As higher intensity applications started to become the norm valve manufacturers began to develop a more robust design where metal to metal sealing could be used in an attempt to enhance the capability of the butterfly valve. To have any chance of success the new design would need to eradicate the common sealing method of deformation and friction. It was this technical query that eventually pioneered the third offset design with the change to the geometry of the sealing components introducing the revolutionary, cost, weight and space saving Triple Offset Butterfly valve.

Zero Offset

Concentric valve (zero offset). Disc rotates around the centre axis allowing for a potential 360º rotation. Sealing is achieved by the Disc deforming the soft seal resulting in full friction through the full operating cycle.

Double Offset

To allow displacement of the seat the shaft is offset from the centre line of the disc seat and body seal (offset one), and the centre line of the bore (offset two). This creates a cam action during operation to lift the seat out of the seal resulting in friction during the first 10 degrees of opening and final 10 degrees of closing.

Triple Offset

The third offset is the geometry design of the sealing components not the shaft position. The sealing components are each machined into an offset conical profile resulting in a right angled cone (see Fig 1).

This ensures friction free stroking throughout its operating cycle. Contact is only made at the final point of closure with the 90° angle acting as a mechanical stop; resulting in no over-travel of the disc seat.

Fig 1

With deformation and friction being such a prevalent issue in the sealing capabilities of concentric and double offset butterfly valves, the life expectancy is substantially minimised after its first operation and can commonly be expected to pass media instantly.

Through the introduction of the revolutionary third offset, a solution has been engineered to eliminate deformation and friction leaving the advantages of ‘Triple Offset’ clear to be seen.

• The ‘cam action’ and ‘right angled conical sealing’ design ensures that the metal sealing components are never in contact until its final degree of closing, resulting in repeatable sealing and a vastly extended valve life.
• Butterfly valves provide significant cost savings over conventional valves such as gate, globe and ball due to an inherent reduction in materials and weight.
• Low torques result in low cost of automation.
• Minimal maintenance due to the robust design with the added advantage of field replaceable seat and seal components in the TVT Range.
• Low fugitive emissions due to quarter turn technology.
• Cavities do not exist between sealing components in comparison to some conventional, concentric and double offset valves therefore eliminating the ingress of particles.
• Metal to metal sealing allowing for higher pressure and temperature applications whilst still providing bubble-tight shut off.
• Ideal for use in controlling or throttling applications.
• Torque seated resulting in bi-directional, non-pressure aided frictionless seating.