Double Offset Butterfly Valves: Design, Benefits and Industrial Applications
Butterfly valves are popular because they are compact, light and quick to operate. But the simplest concentric design has a weakness: the disc rubs against the seat through a large part of its travel, which wears the seat and raises operating torque. The double offset butterfly valve, also called a double eccentric or high-performance butterfly valve, was developed to solve that problem. By moving the shaft away from the centre of the disc in two directions, it lets the disc swing clear of the seat almost as soon as it starts to open.
This guide explains how the double offset design works, the benefits it brings, the seats and materials available, typical applications and how to specify the right valve for your process.
What Is a Double Offset Butterfly Valve?
A double offset butterfly valve is a quarter-turn valve in which the stem (shaft) is offset from the disc in two ways. These offsets change how the disc moves into and out of the seat, so contact happens only in the last few degrees of closing. The result is a valve that seals tightly, needs less torque and lasts longer than a concentric (zero offset) butterfly valve, while still being far smaller and lighter than a gate, globe or ball valve of the same size.
Understanding the Two Offsets
First Offset: Shaft Behind the Disc
In the first offset, the shaft is positioned behind the sealing face of the disc rather than passing through its centre. This allows the seal to form a complete, uninterrupted ring around the disc edge, eliminating the two weak points where the shaft passes through the seat in a concentric valve.
Second Offset: Shaft Off the Pipe Centreline
In the second offset, the shaft is also moved slightly to one side of the pipe and valve centreline. Because the pivot point is now eccentric, the disc follows an arc that lifts it away from the seat as soon as rotation starts. During closing, the disc only touches the seat in the final few degrees of travel, typically around the last 10 to 15 degrees.
Together, these two offsets reduce rubbing, which is the main source of seat wear and high torque in conventional butterfly valves.
Key Benefits of Double Offset Butterfly Valves
- Lower operating torque: Less friction between disc and seat means less force to open and close the valve. This allows smaller, less expensive gearboxes and actuators, especially on larger sizes.
- Longer seat life: Because the disc does not drag across the seat throughout its stroke, seat wear is greatly reduced and maintenance intervals are longer.
- Tight shut-off: Soft-seated designs can achieve bubble-tight shut-off, while metal-seated versions provide good sealing at higher temperatures.
- Higher pressure and temperature capability: Double offset valves handle higher pressures than rubber-lined concentric valves, commonly up to ASME Class 300 and, in some designs, Class 600.
- Compact and lightweight: A wafer or lug body takes up a fraction of the space and weight of a gate or ball valve, which simplifies piping supports and installation.
- Good flow characteristics: The streamlined disc offers a relatively high flow coefficient (Cv) and moderate throttling ability.
- Cost-effective: For mid-pressure isolation duties, they offer much of the performance of more expensive valve types at a lower total installed cost.
Seat Options and Materials
Soft Seats
Soft seats such as PTFE, reinforced PTFE (RPTFE), EPDM and NBR provide excellent sealing at low torque. Elastomers are generally limited to around 120-150 °C, while PTFE seats cover roughly -40 °C to +200 °C and offer wide chemical resistance.
Metal and Fire-Safe Seats
Metal seats, often stainless steel or hard-faced, are used for higher temperatures, abrasive media or fire-safe service. Many designs combine a soft primary seal with a metal secondary seal so the valve continues to seal if the soft seat is damaged in a fire.
Body and Disc Materials
Common body materials include cast iron, ductile iron, carbon steel (WCB), stainless steel (CF8, CF8M), low-temperature carbon steel (LCB), duplex and super duplex stainless steel, Alloy 20 and Hastelloy. Discs are offered in matching or upgraded metallurgy to suit corrosive service such as seawater and desalination.
Body Styles and End Connections
- Wafer: Clamped between two flanges by the flange bolts. Lightest and most economical.
- Lug: Threaded lugs allow the valve to be used for dead-end service and allow removal of downstream piping.
- Double flanged: Preferred for large diameters and for higher pressure pipelines.
Valves are typically designed to API 609, AWWA C504 or EN 593, tested to API 598, with ISO 5211 actuator mounting pads for easy automation.
Applications of Double Offset Butterfly Valves
- Water treatment, distribution and desalination plants
- Power plant cooling water, condenser and auxiliary steam systems
- Oil and gas utilities, tank farms and terminals
- Chemical and petrochemical process lines with moderate pressure and temperature
- HVAC chilled water and district cooling systems
- Pulp and paper, mining and general industrial utilities
- Compressed air, inert gas and low-pressure steam
Double Offset vs Concentric vs Triple Offset
A concentric butterfly valve has the shaft through the disc centre and relies on a rubber-lined body. It is the most economical but suits only low pressures and temperatures. A double offset valve adds two eccentric offsets to reduce seat contact, making it the workhorse for medium-pressure isolation and moderate throttling. A triple offset valve adds a third, conical offset that creates a metal-to-metal, friction-free seal. It is the choice for high pressure, high temperature, fire-safe and zero-leakage duty. For severe service, Bliss Flow also supplies the triple eccentric butterfly valve.
Limitations to Consider
- Severe throttling at high differential pressure can cause vibration, noise and cavitation; a dedicated control valve may be more suitable.
- Abrasive slurries can still wear the disc and seat edge.
- Soft seats have upper temperature limits and may be affected by rapid thermal cycling.
- Some designs are unidirectional, so the marked preferred flow direction must be respected during installation.
How to Select a Double Offset Butterfly Valve
- Service: Isolation or throttling, flow direction and required shut-off class.
- Pressure and temperature: Confirm the pressure-temperature rating of the body and seat at the maximum operating condition.
- Media: Check chemical compatibility of the seat, disc and body, and whether the fluid carries solids.
- Size and body style: Wafer, lug or double flanged, with the correct flange standard.
- Operation: Lever, gear operator, or pneumatic or electric actuator. Size the actuator on the maximum seating and unseating torque with a suitable safety margin.
- Standards and testing: API 609, API 598, fire-safe certification and any project-specific requirements.
For automated duty, valves are commonly fitted with a pneumatic actuator and limit switch box for remote open/close or modulating control.
Installation and Maintenance Tips
- Install with the disc partly open so it does not hit adjacent pipe or flange bores, and check disc clearance before tightening bolts.
- Respect the preferred flow direction arrow on unidirectional valves.
- Leave adequate straight pipe upstream of the valve where possible to avoid turbulent flow onto the disc.
- Periodically cycle valves that remain in one position for long periods.
- Inspect seat and stem seals during scheduled shutdowns, and replace soft seats before leakage becomes excessive.
Butterfly Valves from Bliss Flow Systems
Bliss Flow Systems supplies butterfly valves in centric, single eccentric and double eccentric designs, sizes from 80 mm to 3000 mm, ratings PN 10, PN 16 and PN 20, and designs to AWWA C504 and API 609 with testing to API 598. Bodies are available in cast iron, WCB, CF8, CF8M, LCB, super duplex, Alloy 20 and Hastelloy, with EPDM, nitrile, PTFE or metal seats and manual, gear, pneumatic or electric operation. We supply projects across India, Singapore, Malaysia and the Middle East.
Frequently Asked Questions
What is a double offset butterfly valve?
It is a butterfly valve where the shaft is offset from the disc centre in two directions, behind the disc and to one side of the pipe centreline. This makes the disc lift clear of the seat early in its stroke, reducing friction, torque and seat wear.
What is the difference between double offset and triple offset butterfly valves?
A double offset valve uses two offsets and usually a soft or resilient seat. A triple offset valve adds a third conical offset that gives a friction-free, metal-to-metal seal suited to high pressure, high temperature and zero-leakage service.
Can a double offset butterfly valve be used for throttling?
Yes, it can be used for moderate throttling, typically between about 20 and 80 percent open. For high differential pressure or cavitating service, a dedicated control valve is recommended.
What pressure rating do double offset butterfly valves have?
They are commonly available in ASME Class 150 and 300, PN 10 to PN 40, and in some designs Class 600. The actual rating depends on body material, seat and temperature.
Are double offset butterfly valves bidirectional?
Many are bidirectional, but some designs have a preferred flow direction for best sealing. Always check the manufacturer’s marking and datasheet before installation.
Conclusion
The double offset butterfly valve offers an excellent balance of sealing performance, low torque, long seat life and compact size. For most medium-pressure isolation and control duties in water, power, oil and gas and chemical plants, it delivers reliable performance at a lower installed cost than heavier valve types. Matching seat material, pressure class and actuator to the duty ensures years of trouble-free service.
Contact Bliss Flow Systems
For product selection, technical datasheets or a quotation, reach our engineering sales team at the office nearest to you, or send an enquiry through our contact page.