Butterfly valves control fluid flow in a pipe by using a disc attached to a central shaft. This disc controls the amount of fluid passing through the valve by creating a variable opening.
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Butterfly valves can be classified according to the alignment of the disc with respect to the valve body:
Majority of butterfly valves with plastic linings are zero offset. Those that are made of metal and some with rubber lining are either double or triple offset butterfly valves. If you’re really interested and want to have a solid grasp of butterfly valve types, components and major parts, you can read our ultimate guide for high performance butterfly valves.
In this article, however, we will only discuss what a double offset butterfly valve. Let’s start with the differences between the aforementioned butterfly disc alignment types.
The disc and stem of zero offset butterfly valves are situated at the center of the valve seat which is centered in the valve body. The disc rotates on the central axis which allows a 360° rotation. A sealing is done when the disc deforms the soft seat causing full friction during the whole operating cycle.
Zero offset butterfly valves are common in liquid and gas applications under 250 PSI and up to 400° Fahrenheit. These valves are able to handle food, coatings, chemicals, abrasive, and some solid applications.
A notable advantage of opting for zero offset valves is that the flowing media on the valve is not going to get into contact with the body because the valve seat is covering it.
Double offset butterfly valve is popularly known as a high-performance butterfly valve and has two offsets from the valve body’s center:
The position of the disc in a double offset butterfly valve creates a cam action when the valve operates. The seat is lifted from the seal which results to the occurence of friction on the first 10° and final 10° of the opening and closing operations.
Double offset butterfly valves are best suited for pressures higher than zero offset butterfly valves can handle.
A triple offset butterfly valve has three offsets two of which are the same as the double offset butterfly valve. The third offset is on the geometry of the valve’s seating surface instead of the shaft position.
The components of the sealing are machined into the valve to create a conical profile of the valve seat and disc. The disc only gets into contact with the seat in the first and last 1° of the opening and closing operations.
Triple offset butterfly valves are used in applications similar to double offset butterfly valves but are preferred in tighter and more demanding applications.
Double offset butterfly valves are widely known as high-performance butterfly valves. They are one of the most available eccentric-type butterfly valves in the market aside from triple-offset ones. Additionally, they are also called the elite type of butterfly valves.
Double offset butterfly valves can be found in higher pressures compared to zero offset ones. They are made from materials with ANSI Class ratings of up to 600 such as Hastelloy, stainless steel, carbon steel, and others. Their rigid Polytetrafluoroethylene (PTFE) seats (ANSI class butterfly valve), usually from Teflon, can be varied and they also come in Fire Safe designs with metal seats as back ups.
Double offset butterfly valves have the following major components:
High-performance butterfly valves are available in three basic body designs:
Wafer body types has two or more holes for easier installation. They are cheaper and lighter but cannot be installed at the end of pipes or as end-of-line services.
Lug-style double offset butterfly valves have bolt holes where pipe flanges can be connected. Their threaded bolts on the whole area makes them a better option. This is because wafer valves users can switch to a lug valve design but not the other way around.
Unlike wafer types, lug types can be used at the end of pipes or as end-of-line services. However, because of some body materials that have lower strengths, the threads of the lug are weak. This results in bolt torque ratings that are lower compared to water.
Double flanged types have a pair of flanges on the sides of the butterfly valve. This body design are common in large valve sizes. Some manufacturers use U-shaped valves which are more affordable but the flanges aren’t always parallel which causes problems when bolting on the flanges of the pipe.
High-performance double offset butterfly valves can be operated using gears, handles, or actuators. They move the disc of the valve to its optimal position to fully seal or open the valve during operation.
Moreover, operation by actuation can use either one of the three types of actuators:
Electrically-operated butterfly valve convert electrical energy into torque. An electrical motor that is mechanically connected rotates a screw and a threaded ball nut with threads matching those of the screw is kept from rotating with it. The nuts move along the threads once the screw is rotated. The direction of the nut movement depends on the direction the screw has rotated.
Pneumatically-actuated butterfly valves are air-driven or air actuated. A pneumatic linear actuator has a hollow cylinder with a piston inside. A pneumatic pump or external compressor drives the piston. The cylinder will move along the piston’s axis to create a linear force as the pressure increases. The piston will return to its default position when a fluid or other force is applied to the opposite side of the piston.
A hydraulic linear actuator operates similar to a pneumatic actuator but rather than a pressurized air, an incompressible fluid supplied by a pump drives the cylinder.
If you want to know more about the advantages and disadvantages of each actuator type, you can read this article.
The movement of a double offset butterfly valve follows a conic profile in sealing the valve. The arrangement of the offsets of the disc and the shaft generates a cam action of the disc into the seat, reducing the required torque to close or open the valve.
This offset geometry also allows the 90° rotation of the disc to get into contact with the seat on the first 1° to 3° of rotation only. Moreover, the design of double offset butterfly valves minimizes wearing at the touch points of the seat and disc which happens in conventional butterfly valves.
For a more detailed explanation on how a double offset butterfly valve works, you can go to this video:
High-performance double offset butterfly valves are popular because they can be used in a wide range of applications. They can survive extreme pressures and temperatures and are suitable for handling gas, oil, steam, and water in a light weight and cost-effective design compared to triple offset butterfly valves.
The corrosion resistance and design strength of double offset butterfly valves allow them to be used either as buried valves or as parts of plants. Below are some of the most common applications and industries that use these kinds of butterfly valves.
Industrial valves have higher requirements regarding pressure and temperature ranges compared to traditional butterfly valves. This gave to the rise of high-performance butterfly valves that can handle high pressure and high temperature, one of which is a double offset butterfly valve.
In recent years, metal-seated eccentric butterfly valves have competed with more traditional industrial valves like the globe and ball valves for critical applications. However, even though lots of people already know how a butterfly valve works, much of the confusion lies in how such eccentric valves differ and what applications are suitable for each.
If you want to learn more, please visit our website 2-Offset Butterfly Valve.
In this article, you are going to walk through the differences between these two kinds of butterfly valves to guide you when choosing a suitable industrial valve for your application.
Double-offset valves feature have two offset stems that offer especially long life due to reduced contact between valve, discs, and seats, and are known as high-performance butterfly valves. If your operations involve high pressures and particulate fluids, this type of valve can be especially useful. Apart from triple-offset butterfly valves, they are one of the most widely available eccentric-type butterfly valves on the market. They are also known as the “elite” type of butterfly valves.
When compared to zero offset butterfly valves, double offset butterfly valves can withstand higher pressures. They’re made of Hastelloy, stainless steel, carbon steel, and other materials with ANSI Class ratings of up to 600. Their rigid Polytetrafluoroethylene (PTFE) seats (ANSI class butterfly valve) are usually made of Teflon and come in Fire Safe designs with metal seats as backups.
A two-offset disc can be used up to 1,480 psi [10 MPa] in double-offset butterfly valves. Unlike the single-offset design, the stem of the double-offset butterfly valve is located behind the disc. The second offset stem is pushed to one side from the middle of the disc in the high-performance butterfly valve. Because of the offset geometry, the disc can be rubbed over the seat with only 1° to 3° of 90° rotation. The cone profile while sealing the valve is the action of the double offset butterfly valve. The configuration of the disc and shaft offsets creates the cam action of the disc in the seat, which reduces the torque required to operate the valve.
Double Offset Butterfly Valve
With the triple offset butterfly valve there are three offsets. Two are placed in a similar position to the double offset butterfly valve, while the third offset is the geometry of the seating surface, creating a type of cone shape of the disc and seat. This cone angle, along with the two eccentric shaft offsets, allows the disc to seal against the seat with no friction.
This seat design also allows for uniform sealing, and thus a tight shutoff in a metal seat design. This design is a lower cost, lower torque option (easier to automate), than alternative style metal seated valves.
Triple offsets are generally used in applications similar to gate valves, where a metal seat is required, and tight shutoff and/or quarter turn actuation is desired. Applications like high pressure steam (over 150 PSI), superheated- steam, high temperature gases and oils, high temperature applications are good for this type of valve because a metal seat is required over a soft seat.
The triple offset valve is used in similar industries as the double offset butterfly valve, but in more demanding applications.
Triple Offset Butterfly Valve
One of the main differences between these two butterfly valves is how they look. In the double offset butterfly valve, the two offsets of this kind of valve allow for easy opening yet retains its strong sealing power.
The offset of the disc can be found in two areas:
1. Slightly on the right side of the valve centerline
2. Slightly near the center of the valve body where the stem is attached to the disc.
The first offset offers continuous sealing whereas the second offset allows the valve to open with little friction, increasing its life service.
On the other hand, the triple offset butterfly valve is called as such because it has three offsets located in three different positions. These can be best understood if you check out the image below.
1. The center of the disc and seat
2. In the centerline of the bore and attached to the stem
3. The offset is in centerline of the valve where it becomes the axis of the seat cone angle.
The angled position of the disc in relation to the seats minimizes seat damage as well as preserve the integrity of the sealing surface. Additionally, it also reduces the chances of the valve sticking to the seats and it lowers operating torque.
Commonly, double offset butterfly valve seats are made of plastic or elastomeric material like PTFE. To use this kind of valve in high-temperature applications, the metal seat design is utilized but this needs to meet standard specifications of API 607.
Double offset butterfly valves are can reach pressure ratings up to ANSI Class 600. These can be made from carbon steel, stainless steel or Hastelloy with an elastomeric material such as Teflon.
The nature of media with which these valves operate dictates what materials to use for the valves. Triple offset butterfly valves deal with corrosive media so the valve often has metal seats made from high-grade materials such as stainless steel or duplex steel.
The triple offset butterfly valve can also have double seat type design, that is, an elastomeric or graphite material covers a stainless steel seat. Often, the seats of this valve are field replaceable. This should only be employed in cases where media is not abrasive or has a high temperature.
As it is eccentric, the double offset butterfly valve disc moves away from the disc as it begins to open. To have an effective bubble-tight seal, this valve utilizes line pressure that acts against the disc.
The triple offset type does not act like a typical butterfly valve which, with a turn of a lever, moves sideways to allow entry of media, like a waving of a hand. The triple offset type acts like a globe valve where the disc moves somewhat like a cone, moving upwards to downwards.
When the cone moves to become a circle, a tight seal happens. There is no drag when the seat takes contact with the disc because of the angled alignment of the disc in relation to the seat.
Double offset butterfly valves can often be found in large underground water applications. You can often see these valves with gate valves since both operate in the same. These are best used when you need to have the highest degree of tight sealing.
Often, you can find double offset butterfly valves in oil and gas applications, HVAC, waste and wastewater treatment facilities and Shipbuilding factories.
Triple offset butterfly valves are efficient in applications that have special challenges such as the abrasive material, high pressure or temperature and even those that are prone to fugitive emissions.
You would find triple offset butterfly valves in applications such as steam with over 150 psi or high-temperature pure gas or liquid. In cases like these, the metal seat types without the soft coating are more economical.
These valves are often used in industries such as oil and gas processing, refining, chemical and petrochemical applications, power generation, offshore applications, pulp and paper, and so on.
Between the double offset and the triple offset butterfly valve, if your primary concern is to have bubble-tight sealing, choose the double eccentric butterfly valve. If the application is highly corrosive, abrasive or deals with high pressure and temperature, the triple eccentric butterfly valve is worth considering.
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