A butterfly valve whose valve stem axis deviates from the center of the disc and the center of the body at the same time, and the rotation axis of the valve seat has a certain angle with the channel axis of the valve body is called a triple offset butterfly valve.
To be resistant to high temperature, hard seal must be used, but the leakage is large; To achieve zero leakage, a soft seal must be used, but it is not resistant to high temperature. In order to overcome the contradiction of double offset butterfly valve, the butterfly valve is offset for the third time. The utility model is characterized in that the conical axis of the sealing surface of the butterfly plate deviates from the cylindrical axis of the body while the double offset valve rod axis position is offset, that is, after the third offsetity, the sealing section of the butterfly plate is no longer true circle, but ellipse, and the shape of the sealing surface is asymmetric, one side is inclined to the centerline of the body and the other side is parallel to the centerline of the body. The biggest feature of this third offsetity is that it fundamentally changes the sealing structure, no longer the position seal, but the torque seal, that is, it does not rely on the elastic deformation of the valve seat, but completely depends on the contact surface pressure of the valve seat to achieve the sealing effect. Therefore, the problem of zero leakage of the metal valve seat is solved in one fell swoop. Because the contact surface pressure is directly proportional to the medium pressure High pressure and high temperature resistance can also be easily solved.
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.
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.
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.
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.