Table of Contents

1. section-1″>What Is a Double Offset Butterfly Valve?
2. section-2″>Double Offset vs Single Offset vs Triple Offset
3. section-3″>Key Design Features of Double Offset Butterfly Valves
4. section-4″>Working Principle of a Double Offset Butterfly Valve
5. section-5″>Typical Materials of Construction
6. section-6″>Common Seat Designs and Sealing Concepts
7. section-7″>Pressure Ratings, Sizes, and End Connections
8. section-8″>Standards and Design Codes
9. section-9″>Advantages of Double Offset Butterfly Valves
10. section-10″>Limitations and Application Considerations
11. section-11″>Typical Industrial Applications
12. section-12″>Selection Guidelines and Sizing Considerations
13. section-13″>Maintenance, Inspection, and Service Life
14. section-14″>Frequently Asked Questions (FAQ)

1. What Is a Double Offset Butterfly Valve?

A double offset butterfly valve is a quarter‑turn shut‑off and control valve in which the

disc is offset in two distinct ways from the centerlines of the valve body and the pipe.

This double eccentric geometry reduces friction between the disc and seat, improves

sealing performance, and extends service life compared with concentric butterfly valves.

The double offset butterfly valve belongs to the family of high performance butterfly

valves. It is designed to handle higher pressures, elevated temperatures, and more

demanding service conditions than traditional resilient‑seated concentric butterfly

valves, while still maintaining a relatively compact and lightweight construction

compared with globe valves or gate valves.

In many industry documents and specifications, the terms double offset butterfly

valve, double eccentric butterfly valve, and

high‑performance butterfly valve are used interchangeably.

2. Double Offset vs Single Offset vs Triple Offset

The term “offset” in butterfly valve design refers to the displacement of the disc

rotation axis relative to the geometric centerlines of the valve bore. Understanding the

difference between single, double, and triple offset butterfly valves is essential when

choosing a valve for a specific application.

2.1 Concentric (Zero Offset) Butterfly Valves

In a concentric butterfly valve, the disc is centered in the middle of the valve body and

the stem passes through the center of the disc. The sealing is typically provided by an

elastomeric (soft) seat. Concentric butterfly valves are economical and suitable for low

pressure and low temperature applications, such as HVAC, water distribution, and

non‑corrosive services.

2.2 Single Offset Butterfly Valves

A single offset butterfly valve has the shaft offset from the centerline of the disc but

still aligned with the centerline of the pipe. This reduces seat wear compared with

concentric designs but does not provide the same level of performance as double or triple

offset configurations. Single offset designs are less common in high‑performance

industrial applications and are often seen as an intermediate step between concentric and

double offset valves.

2.3 Double Offset Butterfly Valves

In a double offset butterfly valve, there are two offsets:

• First offset – The shaft is offset from the centerline of the valve

  seat, moving the axis of rotation behind the disc.

• Second offset – The shaft is further offset from the centerline of

  the pipe or valve bore, shifting it to one side.

This double eccentricity causes the disc to move away from the seat quickly during

opening and closing, minimizing friction and achieving a more uniform compression on the

seat when fully closed.

2.4 Triple Offset Butterfly Valves

Triple offset butterfly valves add a third geometric offset: the sealing surface is

machined into a conical profile, which results in a “cam‑like” motion during closing. The

disc contacts the seat only at the final degree of closure, allowing for zero‑leakage

metal‑to‑metal seating suitable for very high‑temperature and critical isolation

services.

2.5 Comparison Summary Table

3. Key Design Features of Double Offset Butterfly Valves

Double offset butterfly valve design features are specifically developed to achieve

improved sealing performance, higher reliability, longer service life, and reduced

operating torque. The most important design aspects include the offsets, disc and body

configuration, seat arrangement, bearings, and shaft sealing.

3.1 Double Eccentric Geometry

The hallmark design feature of a double offset butterfly valve is the double eccentric

geometry:

• First offset: The stem is placed behind the plane of the disc, moving

  the axis of rotation away from the sealing surface.

• Second offset: The stem is offset laterally from the centerline of

  the pipe, which causes the disc to move off‑center relative to the seat as it opens

  and closes.

This geometry ensures that the disc does not rub heavily against the seat during the

majority of its travel, significantly reducing wear and required operating torque.

3.2 Disc Design

The disc of a double offset butterfly valve is often streamlined to reduce flow

resistance and avoid cavitation. Common disc design features include:

• Precision‑machined disc profile to match the seat contour.
• Optimized thickness for strength with minimal obstruction.
• Balanced design to reduce dynamic torque and vibration.
• Optional hardfacing or overlay on the sealing edge for wear resistance.

3.3 Body Configuration

Double offset butterfly valve bodies are available in several configurations:

• Wafer type – Compact body clamped between flanges using through‑bolts.
• Lug type – Body with threaded or through lugs for bolt‑up to flanges.
• Double flanged – Body flanged on both ends for secure installation on

  large diameters or high‑pressure lines.

• Butt‑welded ends – For permanent installation and critical services.

3.4 Shaft and Trunnion Arrangement

High performance double offset butterfly valves frequently use a one‑piece shaft (stem)

for greater rigidity and reduced deflection. Some designs incorporate a trunnion‑mounted

disc with bearings at both top and bottom to support large diameters and higher

differential pressures.

3.5 Bearings and Bushings

Bearings or bushings in a double offset butterfly valve support the shaft and reduce

friction. Typical design features include:

• Self‑lubricating bearing materials such as PTFE‑lined or composite bushings.
• Corrosion‑resistant bearing surfaces compatible with process media.
• Oversized bearing areas to handle high dynamic and static loads.

3.6 Shaft Sealing and Packing

The shaft sealing system prevents leakage to the atmosphere and ensures environmental and

operator safety. Common features are:

• Adjustable gland packing using PTFE, graphite, or combination packing sets.
• O‑ring seals for low‑torque operation in specific designs.
• Live‑loading of packing for stable sealing under thermal cycling and pressure

  variations.

4. Working Principle of a Double Offset Butterfly Valve

The working principle of a double offset butterfly valve is based on quarter‑turn motion:

the disc rotates 90 degrees between fully open and fully closed positions. The double

offset geometry determines how the disc engages and disengages with the seat.

4.1 Opening Sequence

1. The actuator or hand lever applies torque to the shaft, initiating rotation of the

   disc.

2. Due to the double offset, the disc immediately lifts away from the seat as it starts

   to open, reducing friction and preventing abrasion of the seat surface.

3. As the disc approaches 90 degrees open position, it aligns more parallel to the flow,

   offering a larger flow area with minimal obstruction.

4.2 Closing Sequence

1. The actuator or handle rotates the disc back towards the closed position.
2. The disc moves toward the seat following an eccentric path, making light contact

   only at the final part of the stroke.

3. The disc compresses the seat uniformly around the circumference, creating a tight

   seal capable of meeting stringent leakage classes, depending on seat design.

4.3 Flow Control Characteristics

Double offset butterfly valves provide a nearly linear flow characteristic over a

significant portion of travel, making them suitable for both isolation and throttling. At

small openings, the change in flow is more pronounced; at larger openings, the increase

in flow is more gradual and predictable.

While they can be used for control service, they are not typically as precise as

dedicated control valves with specialized trim. However, the balance of cost, compact

size, and controllability often makes double offset butterfly valves attractive for many

general control applications.

5. Typical Materials of Construction

Selecting the right materials of construction for a double offset butterfly valve is

critical to ensure compatibility with process media, pressure, temperature, and

environmental conditions. Below is an overview of common materials used for the main

components.

5.1 Valve Body Materials

5.2 Disc Materials

Disc materials are often selected to match or complement the body material, but may

include hardfacing or overlay on the sealing edge. Common choices are:

• Stainless steel discs (e.g., 316) for corrosion resistance.
• Duplex stainless steel for seawater and chloride‑rich environments.
• Carbon steel or alloy steel discs with stellite or hardfaced sealing edges for high

  temperature and erosive service.

5.3 Shaft (Stem) Materials

The shaft must maintain high strength and rigidity under torque and pressure loads:

• Stainless steels (e.g., 17‑4 PH, 410, 420) for strength and corrosion resistance.
• Alloy steels for high‑temperature or special duty applications.
• Duplex steels where required by media conditions.

5.4 Seat Materials

Seat selection is fundamental to the performance of double offset butterfly valves. See

also Section 6 for more on seat designs. Main categories:

• Soft seats: EPDM, NBR, FKM (Viton), PTFE and reinforced PTFE.
• Metal seats: Stainless steel, hardfacing alloys such as stellite.
• Composite seats: Metal backing with soft sealing inserts.

6. Common Seat Designs and Sealing Concepts

Double offset butterfly valve design features include a variety of seat configurations to

meet different sealing requirements, pressure ratings, and temperature ranges.

6.1 Soft‑Seated Double Offset Butterfly Valves

Soft‑seated double offset butterfly valves typically use elastomer or PTFE seats. Key

characteristics:

• Excellent bubble‑tight sealing (zero leakage) in many cases.
• Relatively low operating torque compared with metal seats.
• Limited maximum temperature based on elastomer selection.

6.2 PTFE‑Seated High Performance Butterfly Valves

PTFE‑seated double offset butterfly valves provide:

• Wide chemical resistance to corrosive media.
• Extended temperature range compared with elastomer seats.
• Favorable friction characteristics for lower torque operation.

6.3 Metal‑Seated Double Offset Butterfly Valves

For higher temperature, higher pressure, or mildly abrasive services, metal‑seated double

offset butterfly valves are used:

• Metallic seat rings or integral body seats with hardfacing.
• Disc sealing edge often hardfaced or overlaid for wear resistance.
• Leakage rate typically meets industrial standards such as ANSI/FCI 70‑2 Class IV or

  V, depending on design.

6.4 Bidirectional vs Unidirectional Sealing

Some double offset butterfly valves are designed for unidirectional sealing,

where full rated differential pressure is allowed only from one direction. Others are

designed for bidirectional sealing, which allows sealing against

differential pressure from either side.

When specifying a double offset butterfly valve, it is important to confirm whether the

seat design is unidirectional or bidirectional, especially in systems where flow

direction may be reversed or where back‑pressure can occur.

7. Pressure Ratings, Sizes, and End Connections

Double offset butterfly valves are manufactured in a wide range of sizes, pressure

classes, and end connections. The following subsections provide typical ranges and

considerations.

7.1 Typical Size Range

Double offset butterfly valves are commonly available in sizes from DN 50 (2 inch) up to

DN 2000 (80 inch) and beyond, depending on application and manufacturer capabilities.

Larger diameters often make use of double flanged or butt‑weld connections for enhanced

structural integrity.

7.2 Pressure Ratings

Typical pressure ratings for double offset butterfly valves include:

• PN 10, PN 16, PN 25, PN 40 in metric systems (per EN standards).
• Class 150, Class 300, occasionally Class 600 (per ASME/ANSI B16.34).

The actual allowable working pressure at a given temperature depends on the valve

material, design standard, and seat type. Always consult the applicable pressure‑

temperature rating tables for the chosen material and standard.

7.3 End Connection Types

8. Standards and Design Codes

Double offset butterfly valves are designed and manufactured according to a variety of

international standards. Key standards influencing design, dimensions, sealing

performance, and testing include:

8.1 Design and Face‑to‑Face Dimensions

• EN 593 – Industrial valves – Metallic butterfly valves.
• API 609 – Butterfly valves: double flanged, lug, and wafer type.
• ISO 5752 – Metal valves for use in flanged pipe systems – Face‑to‑face dimensions.

8.2 Pressure–Temperature Ratings

• ASME B16.34 – Valves, flanged, threaded, and welding end.
• EN 12516 – Industrial valves – Shell design strength.

8.3 Flange Connections

• ASME B16.5 – Pipe flanges and flanged fittings.
• EN 1092‑1 – Flanges and their joints.

8.4 Testing and Leakage Classes

• API 598 – Valve inspection and testing.
• ISO 5208 – Industrial valves – Pressure testing of metallic valves.
• ANSI/FCI 70‑2 – Control valve seat leakage (for classification of leakage rates).

9. Advantages of Double Offset Butterfly Valves

The design features of double offset butterfly valves translate into several distinct

advantages compared with concentric butterfly valves and other valve types.

9.1 Reduced Seat Wear and Longer Service Life

Because the double eccentric disc moves away from the seat quickly during opening and

closing, there is less rubbing and friction. This lowers seat wear and prolongs the

lifetime of both soft and metal seats, which is especially important in high‑cycle or

throttling applications.

9.2 Lower Operating Torque

The reduced friction resulting from the offset geometry also decreases the torque

required to operate the valve. This enables the use of smaller, more cost‑effective

actuators and can improve responsiveness in automated systems.

9.3 Compact and Lightweight

Compared with globe valves, gate valves, or ball valves of equivalent size and pressure

class, double offset butterfly valves offer a smaller footprint and lower weight, which

helps to reduce construction costs, support requirements, and installation complexity.

9.4 High Performance Sealing

Double offset butterfly valve designs can achieve very low leakage rates and, with

appropriate seat selection, can meet or exceed industry sealing standards. Soft‑seated

designs can offer bubble‑tight shut‑off, whereas metal‑seated versions can provide good

sealing at higher temperatures.

9.5 Versatility in Applications

The combination of high performance sealing, moderate throttling capability, and a wide

range of materials and seat options allows double offset butterfly valves to be used

across numerous industries and fluid types, from clean water to corrosive chemicals and

steam.

10. Limitations and Application Considerations

Despite their many advantages, double offset butterfly valves are not suitable for every

scenario. Design and application engineers should be aware of the following limitations

and considerations.

10.1 High Differential Pressure Throttling

In severe throttling applications with very high differential pressure, the disc may be

subject to vibration, noise, and potential cavitation damage. While double offset

butterfly valves can handle moderate control duties, extremely severe service often

calls for dedicated control valves with anti‑cavitation trim.

10.2 Abrasive or Slurry Service

Media with high solids content or abrasive particles can wear the disc and seat, even in

metal‑seated designs. Special materials, protective linings, or alternative valve types

may be more appropriate when dealing with highly erosive slurries.

10.3 Temperature and Seat Compatibility

Soft seats have temperature limits defined by the elastomer or polymer used. When

process temperatures exceed these limits, metal‑seated double offset butterfly valves or

other valve types must be considered. In addition, rapid temperature cycling may affect

seat performance and sealing stability.

10.4 Flow Direction and Unidirectional Designs

Unidirectional double offset butterfly valves must be installed according to the marked

preferred flow direction to ensure rated shut‑off. Improper installation can result in

leakage or seat damage.

11. Typical Industrial Applications

Due to their high performance, compact design, and relatively low cost, double offset

butterfly valves are used in a broad range of industries.

11.1 Water and Wastewater Treatment

• Isolation of pumps, filters, and pipelines.
• Control of flow in distribution networks.
• Use in desalination plants for seawater and brine services (with suitable materials).

11.2 Power Generation

• Cooling water systems.
• Auxiliary steam systems (with high‑temperature materials and seats).
• Fuel oil and gas handling in auxiliary circuits.

11.3 Oil and Gas, Petrochemical, and Chemical Processing

• Hydrocarbon liquid and gas isolation.
• Corrosive chemical lines where PTFE or special alloys are used.
• Onshore and offshore installations where weight and space savings are beneficial.

11.4 HVAC and Building Services

• Chilled water and hot water circuits.
• District heating and cooling systems.
• Fire protection and sprinkler systems (subject to applicable approvals).

11.5 Pulp and Paper, Mining, and Other Industries

• Pulp stock lines (with suitable seat and disc materials).
• Process water and auxiliary services in mining and mineral processing plants.
• General utility and process piping across many industrial facilities.

12. Selection Guidelines and Sizing Considerations

Correct selection of a double offset butterfly valve involves not only choosing the right

size and pressure rating, but also carefully evaluating media characteristics,

temperature, seat type, and actuation method.

12.1 Key Selection Parameters

• Nominal size (DN / NPS) and line pressure rating (PN / Class).
• Media type: clean liquid, gas, steam, corrosive fluids, or slurries.
• Maximum and minimum operating temperature.
• Maximum differential pressure across the valve.
• Required shut‑off tightness or leakage class.
• Need for bidirectional or unidirectional sealing.
• Frequency of operation and whether throttling is required.
• Available space and preferred end connection type.

12.2 Cv / Kv and Flow Capacity

Double offset butterfly valves generally offer high flow capacity due to their

streamlined disc design and relatively unobstructed flow path when fully open. Valve

sizing is usually based on required flow coefficient (Cv or Kv) at given pressure drop

and process conditions.

12.3 Seat Type Selection

12.4 Actuation Options

Double offset butterfly valves can be operated manually or automated:

• Manual actuation: Hand lever or gear operator for small to medium

  sizes or where infrequent operation is expected.

• Pneumatic actuation: Common in process plants for quick operation and

  integration with control systems.

• Electric actuation: Suitable where compressed air is not available or

  for remote, precise control.

• Hydraulic actuation: Used in specialized applications requiring very

  high torque or fail‑safe positions.

12.5 Installation Considerations

To realize the full benefits of double offset butterfly valve design features, correct

installation is essential:

• Ensure the valve is installed with the disc able to fully open and close without

  interference with adjacent piping or equipment.

• Observe the indicated flow direction for unidirectional seat designs.
• Align flanges properly to avoid excessive stress on the body and shaft.
• Avoid welding near soft‑seated valves without appropriate heat protection.

13. Maintenance, Inspection, and Service Life

Double offset butterfly valves are generally low‑maintenance devices when properly

selected and installed. However, regular inspection and preventive maintenance help to

ensure reliable operation over the long term.

13.1 Routine Inspection

• Visual inspection for external leakage around the shaft packing or body joint.
• Functional checks to ensure smooth operation and full travel of the disc.
• Monitoring of actuator performance and limit switch settings if automated.

13.2 Seat and Disc Wear

Over time, seats and disc sealing edges may experience wear due to mechanical contact,

corrosion, or erosion:

• Soft seats are typically replaceable without full valve replacement.
• Metal‑seated designs may allow for lapping or refurbishment of sealing surfaces.
• In high‑cycle or severe service, more frequent inspection intervals are recommended.

13.3 Packing Adjustment and Replacement

Gland packing around the shaft may need periodic adjustment to maintain a tight seal

without over‑tightening, which would increase operating torque and wear. When packing

becomes worn, it should be replaced following appropriate procedures.

13.4 Expected Service Life

The service life of a double offset butterfly valve depends on:

• Correct sizing and material selection.
• Operating conditions (pressure, temperature, media).
• Frequency of operation and whether throttling is used.
• Quality of installation and maintenance practices.

With proper design and maintenance, double offset butterfly valves can deliver long

service life and dependable performance in a wide range of industrial environments.

14. Frequently Asked Questions (FAQ)

14.1 What Makes a Butterfly Valve “Double Offset”?

A butterfly valve is termed “double offset” when the shaft is offset in two ways: first,

from the centerline of the disc, and second, from the centerline of the valve bore. This

double eccentricity allows the disc to move away from the seat early in its travel,

reducing friction and improving sealing performance.

14.2 Is a Double Offset Butterfly Valve the Same as a High Performance Butterfly Valve?

In many industrial contexts, “high performance butterfly valve” refers to a double

offset or double eccentric butterfly valve. These valves are designed to handle higher

pressures and temperatures than concentric butterfly valves and therefore are considered

“high performance.”

14.3 Can Double Offset Butterfly Valves Be Used for Throttling?

Yes, double offset butterfly valves can be used for throttling and flow control, in

addition to on‑off isolation. However, for very critical or severe control applications, a

specialized control valve with appropriate trim design may be more suitable.

14.4 What Is the Difference Between Double Offset and Triple Offset Butterfly Valves?

While both designs use double eccentric stem geometry, triple offset butterfly valves

have a conical sealing surface that introduces a third offset. This results in a

“cam‑like” closing motion and pure metal‑to‑metal sealing, suitable for higher

temperatures and zero‑leakage critical isolation. Double offset valves can use soft or

metal seats and are generally more economical for many high‑performance applications.

14.5 When Should I Choose a Metal‑Seated Double Offset Butterfly Valve?

Metal‑seated double offset butterfly valves are recommended when:

• Process temperatures exceed the limits of soft seats.
• Some level of erosion, abrasion, or particulate is expected.
• High‑pressure steam or hot gases are involved.
• Fire‑safe or high integrity metallic sealing is required by specification.

14.6 What Leakage Class Can I Expect from a Double Offset Butterfly Valve?

Soft‑seated double offset butterfly valves often provide bubble‑tight shut‑off

(comparable to ANSI/FCI 70‑2 Class VI) under test conditions. Metal‑seated designs

typically achieve Class IV or V leakage rates, depending on seat and disc design, as well

as manufacturing tolerances.

14.7 Are Double Offset Butterfly Valves Bidirectional?

Some double offset butterfly valves are bidirectional, while others are unidirectional.

The design of the seat and disc geometry, as well as the differential pressure rating,

determine bidirectionality. Always check the specification sheet and markings on the

valve body to confirm the intended flow direction and pressure limits.

14.8 How Does a Double Offset Butterfly Valve Compare with a Ball Valve?

Compared with ball valves, double offset butterfly valves typically offer:

• Lower weight and more compact face‑to‑face dimensions, especially in larger sizes.
• Good shut‑off performance and moderate throttling capability.
• Potentially lower cost in large diameters.

Ball valves often provide bubble‑tight shut‑off and can be better suited for certain

high‑pressure and high‑cycle services, but the choice depends on specific project

requirements.