Concentric vs. Double Offset Butterfly Valve

High Performance Butterfly Valve
(double eccentric)
EBRO Armaturen

Butterfly valves serve a broad range of applications throughout fluid processing industries. Among their positive attributes, quarter turn operation and compact design make this valve type a primary selection for many shutoff applications.

Within the butterfly valve family, their are several basic design subdivisions that are substantial to the point of limiting or expanding the potential application of the valve. One of those subtypes is the double offset butterfly valve.

A general purpose (concentric) butterfly valve, often referred to as a resilient seated butterfly valve, has a disc connected to a rotating shaft. The rotational movement of the shaft positions the disc to fully obstruct the flow path (closed) or provide continuing degrees of open space in the fluid path cross section (open). The rotational movement of the shaft from fully open to fully closed is 90 degrees. The shaft is centered in the pipe bore and is also centered on the disc. This shaft orientation will cause the disc to contact the sealing surface throughout a considerable portion of the rotation nearing closure. This contact tends to accentuate wear on the resilient seat and limits the pressure rating for this type of valve.

A double offset butterfly valve is also known by two other common names, "double eccentric" and "high performance". The "offsets" refer to the position of the shaft relative to the pipe bore and the centerline of the disc. In this valve design, the shaft is behind the centerline of the disc and slightly to one side of the pipe bore centerline. This results in a different travel path for the disc as it moves from the open to close positions. The cam action movement of the disc limits its contact with the seat until the final few degrees of travel. The limited wiping contact of the seat tends to extend its useful life. Additionally, this type of closure movement affords the double offset butterfly valve a higher pressure rating than the concentric design.

Valve selection can be a challenging task, with numerous types and variants from which to choose. Share your industrial valve requirements and challenges with a valve specialist, combining your process knowledge and experience with their product application expertise to develop an effective solution.

Process Control Solution for Dosing and Discharging of Bulk Goods - Demonstration Video

EBRO Amaturen Cycle Lock System

Industrial processing applications can sometimes require controlled dosing or dispensing of bulk dry material. Transfers may be injected into an ongoing continuous process, or may be coordinated with a filling operation. There are many possibilities.

One manufacturer has developed a system for controlling the dispensing and flow of bulk solids and powders, combining the necessary valves, sensors, and controllers into a pre-engineered package that can be easily integrated into a new or existing process. The EBRO Armaturen Cycle Lock is compatible with a wide range of communication protocols and can operate as a standalone unit or part of a larger system. The Cycle Lock accommodates all of the manufacturer's extensive line of control valves. The bulk material chamber can be customized in shape, size, material, and coatings to meet customer requirements.

The demonstration video illustrates the operating modes of the Cycle Lock. More information and assistance with all your process control challenges is available from product application specialists. A product data sheet is available.

Industrial Valves - When Butterfly Valve Is The Best Choice

Butterfly Valve
Courtesy Durco - Flowserve

Industrial process control valves are available in uncountable combinations of materials, types, and configurations. An initial step of the selection procedure for a valve application should be choosing the valve type, thus narrowing the selection field to a more manageable level. Valve "types" can generally be classified by the closing mechanism of the valve.

A butterfly valve has a disc that is positioned in the fluid flow path. It rotates around a central axis, the stem, through a 90 degree arc from a position parallel to the flow direction (open) to perpendicular (closed). A variety of materials are used in the valve body construction, and it is common to line the valve with another material to provide special properties accommodating particular process media.

What attributes might make a butterfly valve a beneficial selection over another valve type?

Manually Operated Butterfly Valve
Courtesy Ebro Armaturen

• The closure arrangement allows for a comparatively small size and weight. This can reduce the cost, space, and support requirements for the valve assembly.
• Generally low torque requirements for valve operation allow for manual operation, or automation with an array of electric, pneumatic, or hydraulic actuators.
• Low pressure drop associated with the closure mechanism. The disc in the flow path is generally thin. In the fully open position, the disc presents its narrow edge to the direction of flow.
• Quarter turn operation allows for fast valve operation.
• Some throttling capability is provided at partially open positions.
• Small parts count, low maintenance requirements.

What may be some reasons to consider other valve types?

• Butterfly valve throttling capability is generally limited to low pressure drop applications
• Cavitation can be a concern.
• Some sources mention the possibility of choked flow as a concern under certain conditions.

Butterfly valves, like other valve types, have applications where they outperform. Careful consideration and consultation with a valveexpert is a first step toward making a good selection. Combine your process know-how with the product application expertise of a professional sales engineer to produce the best solutions to your process control challenges.

Industrial Process Control Valve Quality Detail

Wafer Style Resilient Seated Butterfly Valve
Courtesy EBRO Armaturen

Industrial process control operations have a lot riding on every component integral to the process. The operator's challenge is to get every individual element to work as planned, every time. Most process engineers would likely agree with the premise that high quality components tend to deliver higher reliability and consistent performance. Employing properly specified, high quality, componentry throughout a process installation is a generally accepted methodology for achieving the goal of consistent process output and quality.

Any process that involves fluid is bound to have valves as primary control elements. Proper selection from among the many different types used for process control, as well as the many manufacturers, can be a challenge in its own right.

How do you determine the quality level of a valve?

In industrial valves, like many products, it's in construction details that quality resides. One manufacturer, EBRO Armaturen, gives us a detailed look inside and throughout their soft seal butterfly valve. The video is included below, and it is short and to the point.

Attention to detail is what makes your operation work at its best performance level. That same level of attention also contributes to the design and manufacture of a great product. Invest two minutes to watch the video. The construction details are educational, probably things you did not know. You can get more information, or discuss your process control challenges, through contact with an application specialist.

EBRO Resilient Seated Butterfly Valve Seat and Seal Replacement

The video below provides detailed, step-by-step training on replacing the seat and stem seals on an EBRO Armaturen "soft-seated" butterfly valve.

This style of valve is used for both on-off applications and as throttling control valves. Their design uses an "interference fit", meaning the metal disk is forced in contact with the soft rubber seat to provide a leak-tight seal. Because of this physical interaction, it is normal to replace the seats after a reasonable time in service.    

For more information on EBRO Armaturen valves, contact:

CTi Controltech
22 Beta Ct.
San Ramon, CA 94583
Phone: 925-208-4250
Fax: 925-208-4251
Email: contactus@cti-ct.com