Butterfly valves are a common type of industrial valve used to control the flow of liquids, gases, and slurries. Unlike gate, globe, and ball valves which use linear motion to open and close, butterfly valves use a disc that rotates 90 degrees to either allow or prevent flow. The “butterfly” term comes from the shape of the disc which resembles a butterfly’s wings.
Butterfly valves provide many advantages over other valve types including fast opening and closing, low torque requirements, low cost, and lightweight and compact design. They are an excellent choice for large diameter applications where linear motion valves would be impractical. Butterfly valves are available in a wide array of materials from bronze, cast iron, and stainless steel to more exotic alloys for specialized applications. Valve discs are also available in various materials with different seal types.
While standard butterfly valves work well for basic on/off flow control, high performance butterfly valves are engineered for demanding applications that require tight shutoff, precise flow control, and reliability. Two designs that offer enhanced performance are double eccentric and double offset butterfly valve.
Double Eccentric Butterfly Valves
A double eccentric butterfly valve utilizes two offset shafts or trunnions. Unlike a conventional butterfly valve which has two shafts coinciding with the centerline of the valve body, the shafts on a double eccentric valve are positioned slightly off the centerline.
This double offset configuration enables the disc to seat tightly against the downstream seat. The eccentric shaft mounting causes the upper and lower halves of the disc to contact the seat sequentially as the valve is being closed. This flexing action compensates for wear and allows the valve to maintain a tight seal even after years of service.
The double offset design also enables the disc to be larger than the inside diameter of the pipe. This reduces flow velocity around the disc which reduces pressure loss compared to a standard butterfly valve. Double eccentric butterfly valves are ideal for applications that require bi-directional tight shutoff.
Common applications of double eccentric butterfly valves include:
- As an economical substitute for ball, gate or plug valves when bi-directional shutoff is critical
- Severe service applications with fluctuating pressures and temperatures
- Process lines handling slurries or corrosive media
- Throttling services requiring precise flow control
- Larger diameter water and wastewater transport lines
Double Offset Butterfly Valves
While similar in appearance to double eccentric valves, double offset butterfly valves have a slightly different design. Instead of offset shaft mounting, double offset valves utilize offset seats and seal surfaces. The disc centerline aligns with the centerline of the valve body but the upstream and downstream seats are offset.
This configuration seals the same way as a double eccentric valve – the disc contacts the downstream seat sequentially from the upper to lower edge as it closes. Again, the flexing action maintains a tight seal even after extensive wear. A key benefit of the double offset design is reduced torque requirements compared to a double eccentric valve.
Double offset butterfly valves offer superior bi-directional shutoff capability. Their uncompromising sealing ability makes them a top choice for the following challenging services:
- Critical isolation applications in the oil & gas, chemical, and power industries
- Flow regulation of abrasive slurries in mining processes
- Block valves on LNG carriers and storage tanks
- Severe pressure, temperature, and cycling services
- Any application requiring zero leakage
Both double eccentric and double offset high performance butterfly valves provide substantial benefits compared to single offset, conventional butterfly designs:
- Bubble-tight bi-directional shutoff rated up to ANSI Class VI
- Lower pressure drop – higher CV ratings than conventional valves
- Quicker opening and closing with reduced operating torque
- Tight shutoff that withstands thermal distortion and wear
- More durable seats that resist erosion and leakage
- Compact, lightweight design compared to other shutoff valves
- Lower total cost of ownership over the long run
While the double offset configuration generally requires less torque and is more compact, double eccentric valves can handle higher pressure differentials. Both designs are vastly superior to standard butterfly valves when zero leakage and lasting tight shutoff are critical requirements. The advanced sealing capability of high performance double offset and double eccentric butterfly valves makes them a top choice for severe service applications across all process industries.