There are four types of butterfly valves, concentric butterfly valve, single eccentric butterfly valve, double offset butterfly valve and triple offset butterfly valve. How to choose these four? Concentric butterfly valve, single eccentric butterfly valve, double offset butterfly valve and triple offset butterfly valve reflect the fact that the opening and closing process is more and more labour-saving and the wear and tear of sealing surface is less and less. By setting the rotating shaft position of the butterfly valve plate, the sealing and opening state of the butterfly valve can be changed.
A concentric butterfly valve is characterized by the same position of stem axis, butterfly centre and valve body centre. Generally speaking, the concentric butterfly valve is used as much as possible if concentric butterfly valve can be used in principle. The concentric butterfly valve is a conventional product because of its low requirement of sealing both in structure and operation. In order to overcome extrusion, scratch and ensure sealing performance, concentric butterfly valve seats are basically made of elastic materials such as rubber or polytetrafluoroethylene, which are soft-sealed butterfly valves.
Double eccentric butterfly valve is the most widely used. The structure features that the axis of the stem deviates from both the centre of the butterfly plate and the centre of the valve body. It deviates from two central positions and is called double eccentric butterfly valve. Many double offset butterfly valve are linear seals. Friction occurs between the disc plate and valve seat when the sealing surface is closed. The sealing effect is very good. It has the characteristics of small area and strong pressure. After the valve is opened, the butterfly plate can quickly detach from the valve seat, greatly eliminating the unnecessary over-extrusion and scratch between the butterfly plate and the valve seat, reducing the opening resistance, reducing wear and tear, and improving the service life of the valve seat.
Triple eccentric butterfly valve undertook the third eccentricity on the basis of the double offset butterfly valve. The shape of the sealing pair is not a positive cone, but an oblique cone. Most of them are short distance force and surface seal. The stem shaft of a triple eccentric butterfly valve is a three-section axis structure. The two-axle segments of the three-section axle stem are concentric, and the centre line of the axis of the central segment deviates from the axis of both ends. The butterfly plate is installed on the middle axle segment. Such an eccentric structure makes the butterfly plate double eccentric in the fully open position and single eccentric in the closed position when the butterfly plate rotates. Because of the eccentric axis, when closing, the butterfly plate moves a distance into the sealing cone of the seat, and the sealing surface of the butterfly plate and the seat coincides to achieve reliable sealing performance. It makes up for the contradiction that the hard seal is poor in sealing and the soft seal is good in sealing but not high temperature resistant.
When to use concentric butterfly valve and when to choose the double eccentric butterfly valve or triple eccentric butterfly valve, it mainly depends on usage habits and budget. If it is not an extreme working condition, the concentric butterfly valve is usually chosen, whereas concentric butterfly valve is chosen as far as possible. Because concentric butterfly valve is simple in structure and relatively economical in price.
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