Double Offset Butterfly Valves

What are double offset butterfly valves?

Based on the single eccentric butterfly valves, the double offset butterfly valves are further improved and formed, which are the most widely used at present. The utility model is characterized in that the axis of the valve rod deviates from both the center of the butterfly plate and the center of the body. The second eccentricity, that is, the valve shaft separates the butterfly plate into two parts: more on one side and less on the other. This eccentricity can make the butterfly plate quickly leave the valve seat during opening and closing, greatly eliminate the unnecessary excessive extrusion and scraping between the butterfly plate and the valve seat, reduce the opening resistance, reduce the wear and improve the service life of the valve seat. The significant reduction of scratch also makes the double offset butterfly valve adopt metal seat, which improves the application of butterfly valve in high temperature field.

The sealing principle of double offset butterfly valve is similar to that of single eccentric butterfly valve. It belongs to position sealing structure, that is, the sealing surface of the butterfly plate and the valve seat is in line contact, and the sealing effect is produced by the elastic deformation caused by the extrusion of the valve seat by the butterfly plate. Therefore, it has high requirements for the closing position (especially the metal valve seat), low pressure bearing capacity, and one-way use is recommended.

Application of double offset butterfly valve

Double eccentric butterfly valve, also known as high-performance butterfly valve, is mainly used for drainage of water plants, power plants, steel mills, smelting, chemical industry, water source spring engineering, environmental facilities construction and other systems, especially for waterway pipelines and used as regulation and closure equipment. Compared with the center line butterfly valve, double offset butterfly valve is more resistant to high pressure, longer service life and good stability. Compared with other valves, the larger the diameter, the lighter the material, and the lower the cost. However, due to the butterfly plate in the middle and large flow resistance, the butterfly valve smaller than DN200 is of no great significance.

Working principle of double offset butterfly valve

Double eccentric butterfly valve is further improved on the basis of single eccentric butterfly valve. The utility model is characterized in that the axis of the valve rod deviates from both the center of the butterfly plate and the center of the body. The double eccentric effect enables the butterfly plate to quickly separate from the valve seat after the valve is opened, greatly eliminates the unnecessary excessive extrusion and scraping between the butterfly plate and the valve seat, reduces the opening resistance, reduces the wear and improves the service life of the valve seat. At the same time, the double offset butterfly valve can also adopt metal seat, which improves the application of butterfly valve in high temperature field.

Structural features of double offset butterfly valve

1. Reasonable design, compact structure, easy assembly and disassembly and easy maintenance.
2. The eccentric structure is adopted to reduce the friction of the sealing ring and prolong the service life of the valve.
3. Completely sealed with zero leakage. It can be used in ultra-high vacuum conditions.
4. Replace valve plate, sealing ring, butterfly plate, rotating shaft and other materials, which can be applicable to a variety of media and different temperatures.

Double eccentric butterfly valve vs triple eccentric butterfly valve

What is a double offset butterfly valve

Double eccentric offset butterfly valve is also known as high-performance butterfly valve or double offset butterfly valve. This valve is designed to avoid excessive contact between the valve disc and the valve seat, so it is suitable for viscous media with high pressure.

Advantages of double offset butterfly valve

The balance point of this valve is located on the stem behind the circular disc, which can automatically adjust the disc to the center position, so as to maintain a strict seal. Even if the valve is fully opened, it will not contact the soft seat, so it can ensure a longer service life.

What is a triple eccentric butterfly valve

Triple eccentric butterfly valve, also known as triple offset butterfly valve (TOV), has been on the market for more than 50 years. TOV was originally designed for water applications and has developed into the preferred valve in many key process applications.

In the field of industrial valves, especially Chinese butterfly valve manufacturers, are studying how to give full play to the flexibility of these valves to replace expensive and bulky valves on the market. Recently, TOV has been developed to deal with fugitive emission control.

Advantages of triple eccentric butterfly valve

As a kind of butterfly valve, three eccentric butterfly valve also belongs to one of the four series of the family. It is light, simple to operate and easy to install. Because it does not need too much pipe support, it is easy to replace the valve seat ring. It belongs to an up entry valve.
The torque of the triple eccentric butterfly valve is very low and there is no need to use a larger valve, which means that it can significantly save cost and zero leakage at both ends.
The introduction of soft seat improves the value of the valve. In addition to the fire safety provided by the metal seat under the soft seat, it can also be automatically centered due to the use of elastic seat material.

Difference between double offset butterfly valve and three eccentric butterfly valve

One of the main differences between the two butterfly valves is their appearance. In the double offset butterfly valve, double eccentricity makes it easy to open and maintain strong sealing capacity at the same time.

Structural differences

The offset of the disc can be found in the following two areas:

• 1. Slightly deviate to the right side of the valve centerline;
• 2. Slightly close to the center of the valve body where the valve stem is connected with the valve disc.

The first offset provides a continuous seal, while the second offset allows the valve to open with little friction and prolong its service life.
On the other hand, it is so called because there are three offsets in three different positions of the triple eccentric butterfly valve. Look at the figure below for a better understanding.

• 1. Center of disc and seat
• 2. On the centerline of the hole and connect to the valve stem
• 3. The offset is located at the centerline of the valve and becomes the axis of the cone angle of the valve seat.
• First offset: located at the centerline of the disc / seat sealing surface to provide an uninterrupted sealing surface.

Second offset: it refers to the offset of the shaft relative to the centerline of the hole to make the valve disc cam to minimize the friction and wear between the valve seat and the seal.
The third offset: refers to the deviation of the vertebral axis of the valve seat from the center line of the valve shaft, which can eliminate the friction during closing and opening, and achieve uniform compression sealing effect around the whole valve seat.
The inclined setting of the valve disc relative to the valve seat can minimize the damage of the valve seat and maintain the integrity of the sealing surface. In addition, it can reduce the possibility of the valve sticking on the seat and reduce the operating torque.

Differences in working principle

Due to the eccentric design, the disc of double offset butterfly valve will leave the disc when it starts to open. The valve uses the pipeline pressure acting on the valve disc to achieve effective air tightness sealing.
The three eccentric valve is different from the typical butterfly valve. It moves laterally by rotating the lever to make the medium enter, just like waving a hand. Its function is similar to the stop valve, in which the valve disc moves up and down a bit like a cone.
When the cone moves into a circle, it is tightly sealed. When the valve seat is in contact with the valve disc, there will be no resistance due to the inclination of the valve disc and the valve seat.

Material differences

Usually, the double offset butterfly valve seat is made of plastic or elastic material, such as PTFE. In order to use this valve in high temperature applications, metal seat design can be adopted, but this needs to meet API 607 Standard Specification.
The pressure grade of double offset butterfly valve can reach ansi600. It can be made of carbon steel, stainless steel or Hastelloy and an elastic material such as Teflon.
The nature of the medium handled by the valve determines its manufacturing material. Three eccentric butterfly valves deal with corrosive media, so they usually use metal seats made of advanced materials such as stainless steel or dual phase steel.
The triple eccentric butterfly valve can also adopt the double seat design, that is, the elastomer or graphite material covers the stainless steel seat. The valve seat of this valve can usually be replaced on site. This design can only be used when the medium is not abrasive or the temperature is high.

Application differences

Double eccentric butterfly valve is often used in large groundwater applications. It can often be seen that this kind of valve is equipped with gate valve, and the two operate together. This is best used when the tightest seal is required. Double eccentric butterfly valves are commonly found in oil and gas applications, HVAC, waste and wastewater treatment facilities, and shipyards.
Triple eccentric butterfly valves are effective in applications with special challenges, such as abrasive materials, high pressure or high temperature, and even fugitive emissions. In applications where steam pressure exceeds 150psi or high temperature pure gas or liquid, you will find three eccentric butterfly valves. In this case, the metal seat type without soft coating is more economical. These valves are commonly used in oil and gas processing, oil refining, chemical and petrochemical, power generation, offshore applications, pulp and paper industries.
Between double offset butterfly valve and triple eccentric butterfly valve, if the main concern is to achieve air tightness seal, please select double offset butterfly valve. If dealing with highly corrosive and abrasive media or high pressure and high temperature applications, the three eccentric butterfly valve is worth considering.

China double offset butterfly valve manufacturer www.pilgrimpipeline.com supplies double offset butterfly valves for industrial use, in kinds of materials such as carbon steel, stainless steel, alloy steel, etc.