Choosing the right type of butterfly valve is key to every application. The available range of butterfly valves from equipment manufacturers includes concentric, single (1x) offset, double (2x) offset, and triple (3x) offset butterfly valves. In this article, you will learn the benefits of a triple offset butterfly valve, see a sample drawing, and realize its benefits vs other types of butterfly valves.
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Triple offset, as the name implies, uses three separate offsets to achieve a bubble-tight shut-off. One of the offsets applies to the location of the valve shaft relative to the centerline of the bore. Another offset is between the centerline of the disc/seat sealing surface and the shaft. Then, the third offset is the geometry of the seating surface, which creates a cone shape of the disc and seat. The combination of these three offsets gives this valve its key benefits which are as follows:
A sample drawing of a triple offset butterfly valve above clearly highlights its three offsets. The first offset (offset 1) is between the axle center of the valve stem and the valve body centerline or sealing surface. This design helps to minimize the squeezing complications between the disc and seat that are found in concentric butterfly valves. However, friction between the seat and disc remains a challenge. For this reason, the double offset design sees seldom use.
The second offset (offset 2) is between the axle center of the valve stem and the centerline of the bore/pipe. This leads to minimal friction as the valve opens and closes. In combination with the first offset, the valve&#;s operational life extends.
The third offset is that of the cone axis that deviates from the axle centerline and thus eliminates friction completely. Users can expect longer seat life in the triple offset butterfly valve because the sealing mechanism relies on contact pressure rather than elastic deformation of the valve seat.
This section compares the features of the triple offset butterfly valve to other types of butterfly valves.
The single offset butterfly valve is not common in the market because it shares a lot of similarities with the concentric/zero offset valve. Below is a table highlighting the differences between the single and triple offset valves.
FeatureSingle Offset/Concentric ValveTriple Offset ValveSeal typeThe valve seat is usually made from polymers or other soft materials to enable a friction seal with the metal disc.The three offsets allow for metal-to-metal seals using alloys for both the seat and disc.Seat frictionHigh, because friction is the basic sealing mechanism. Thus, it is susceptible to wear and results in a shorter seat lifespan.Seat friction is insignificant because sealing is by contact pressure. As a result, the valve seat enjoys much longer operational life.ANSI pressure class ratingAlthough this valve can deliver bubble-tight shut-off, its inferior sealing mechanism, and susceptibility to wear limit this to class 150.Delivers dependable sealing which does not wear easily. Models such as the Durco triple offset can achieve a -pound rating.Fluid servicesBecause of its friction sealing mechanism, it is only effective with clean fluid.The triple offset sealing mechanism is more tolerable to dirt, so works with clean or dirty fluids.ApplicationsCommon in utilities and domestic applications.Can function effectively in severe service and high cyclic operations.Single Offset vs. Triple Offset Butterfly ValveThe double offset valve is an improvement of the concentric/single offset valve and provides more versatility than the single offset. Feature comparisons are noted between the double and triple below:
FeatureDouble Offset ValveTriple Offset ValveSeal type and frictionWhen compared to the single offset, the double offset greatly reduces friction. However, it still relies on plastic deformation for sealing. As a result, soft materials are still used for the valve seat.Maintains metal-to-metal seal with contact pressure.Fluid serviceSuitable for handling clean fluids only, due to friction in its sealing mechanism.Maintains effectiveness, even in dirty fluids.ApplicationsCan serve in several industrial applications including water treatment, chemicals, oil and gas, and HVAC.Also serves in industrial applications, but suits scenarios with higher operational demand.CostComes at a moderate cost i.e. more expensive than single offset valves but less than triple offset valves.The specialized machining required to achieve superior sealing makes it more expensive.Double Offset vs. Triple Offset Butterfly ValveButterfly valves have developed over the past 50 years into one of the leading valve solutions for large volumes of flow. They perform exceptionally well in high pressure and high-temperature applications. Performing a similar task to a gate valve, the butterfly provides an excellent solution where space is limited as they offer the facility to regulate or throttle flow. However, there are a range of butterfly valve types in the market.
This article looks at the differences between double offset and triple offset butterfly valves and why you might select one over the other. We also look at the advantages of the triple offset butterfly valve compared to the double offset valve.
A double offset butterfly valve is named so because of its double offset, which allows the disc to be displaced from its seat via a cam action. The cam action is created through the offset of the shaft from the disc seat&#;s centreline and body seal, which is also offset from the bore&#;s centreline. The cam action results in friction between the disc and the seat only during the first and last 10% of its opening and closing action.
To account for this friction and prolong the metalwork&#;s life, the disc is made from metal. However, around the edge of the seat, many designs incorporate a plastic or elastomeric material such as PTFE to prevent metal-to-metal wear during operation. Fully metal options are available for a bubble-tight seal but will wear at a faster rate.
The triple offset butterfly valve incorporates the two offset points of the double, but it also has a third point of offsets. Relating this time to the sealing points rather than the shaft, the point of sealing on the disc and seat are both machined into an offset conical profile. This allows it to operate fully without any contact until the point of full closure as a mechanical stop. Therefore both the sealing components can be made of metal without the need for an elastomeric element.
Having seen in mechanical terms the difference between double offset and triple offset butterfly valves, what does this mean for their uses and applications? Use our quick guide below:
Double offset
Triple offset
Seat material
Soft seat &#; elastomer
Metal seat
Seat friction level
Low
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Very low
Seat lifetime
Long
Extremely long
Fluid type
Clean
Clean and/or dirty
Shut-off type
Bubble-tight
Bubble-tight
Applications
Process on/off and throttling
Extreme service and high cycling demands
Temperature sensitivity
Moderate
Low
Pressure sensitivity
Moderate
Low
Cost
Moderate
High
The key advantages of the triple offset butterfly valve lie in their ability to seal metal-on-metal without wear through friction. This means that not only do they last longer, but there is little sensitivity to temperature. In contrast, double-offset valves with an elastomeric seat can only cope with temperatures that the chosen material will tolerate.
Some triple offset valves do use a double-seat design, with an additional cover of a different material on the seat itself, but these are replaceable in the field. The metal seats ensure that they are fire-safe even if they include a cover. Triple offset valves will also cope with much higher pressures and require a lower torque, so larger valves are not required.
Another triple offset butterfly valve advantage relates to the types of fluid that they will cope with. Suitable for highly corrosive or abrasive materials and steam or high-temperature gases and liquids, they can often be found in industries such as oil and gas, petrochemical processing, pulp and paper and power generation applications.
As we have demonstrated, although there is only one point of design difference between double offset and triple offset butterfly valves, the result of this single difference significantly changes the performance of the valve. The triple offset butterfly valve advantages far outweigh those of the double offset valve if used in an environment where elements such as temperature, pressure or media are critical to the application. If not, then the double-offset valve will perform perfectly well. For more information on which butterfly valve is right for your application, or to discuss whether the triple offset butterfly valve advantages are relevant to your needs, why not speak to one of our engineers on 762 or .
Contact us to discuss your requirements of "which type of valve is best suited for fine flow control and throttling . Our experienced sales team can help you identify the options that best suit your needs.