Wenzhou Unique Valve Co., Ltd.
Soft Sealed Valve Seats of Ball Valves
Jun 11, 2021
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The sealing principle of the ball valve seat varies according to its structure. The commonly used structure of the valve seat can be divided into two categories: fixed seats with floating spheres and floating seats with fixed spheres. When the fixed sphere of the valve seat floats, the sphere is moved in the direction of fluid flow under the action of fluid pressure, so that it is sealed and more closely contacted with the valve seat behind the valve, that is, the specific pressure on the sealing surface increases, forming a single-sealing surface. At the same time, the specific pressure on the sealing surface of the valve seat in front of the valve is correspondingly reduced, which often does not guarantee sealing. When the floating sphere of the valve seat is fixed, the fluid pressure cannot cause the sphere to move, but the valve seat is pressed against the sphere by means of a spring or fluid thrust to obtain a sealing specific pressure. A ball valve seat is divided into soft sealed valve seats and hard sealed valve seats. Soft sealed valve seats are mainly introduced in this article.
1. Soft sealed valve seats
(1) Ordinary valve seats
The characteristic of the ordinary valve seat (Figure 1) is that the valve seat is pressed with the ball under the action of pre-tightening force or fluid pressure, and the seat generates elasticoplastic deformation to obtain sealing. The sealing effect depends on the ability of the valve seat compensating for the roundness of the sphere and the microscopic unevenness of the surface under the action of the fluid pressure or pre-tightening force. Therefore, there must be sufficiently sealing specific pressure between the valve seat and ball.
(A) Arc types (b) Cone types (c) Fireproof types (d) Inner ring types
Figure 1 Ordinary valve seats
(2) Elastic valve seats
The elastic valve seat (Figure 2) uses elastic elements such as metal spring supports and springs to generate elastic deformation under the pre tightening force or fluid pressure and compensate for the influence of changes in external conditions such as temperature difference, pressure and wear on the sealing performance of the ball.
(A) Single slope types (b) Double slope double line types (c) V-shaped linear self-sealing types (d) Special types
Figure 2 Elastic valve seats
(3) Spring combined types
The spring combined valve seat (Figure 3) is a set of compression springs or only one disc spring that is uniformly distributed along the circumference of the transverse section of the valve channel to always keep the valve seat pressed against the sphere and obtain a pre-tightened state.
(A) Compression spring types (b) Disc spring types
Figure 3 Spring combined valve seats
(4) Piston valve seats
The piston valve seat (Figure 4) relies on the pressure of the inlet medium to generate sealing force. The valve seat is pushed to compress the sphere, and the sealing ring produces elasticoplastic deformation to achieve the purpose of sealing.
(A) Cut-off and relief valve seats (b) Valve seats with double movements (c) Two-stage sealed valve seats
Figure 4 Piston valve seats
When the pressure of the cut-off and relief valve seat in the valve cavity rises abnormally to be greater than the pressure at the downstream end, the pressure difference generates an outward thrust, which overcomes the spring force to separate the valve seat from the ball and makes the pressure of the valve cavity discharge downstream. Afterward, under the action of the spring, the valve seat fits the ball well.
The valve seat with double movements is that when the pressure in the middle cavity is greater than the outlet pressure, the valve seat is pressed against the sphere, and the sealing ring produces elasticoplastic deformation to achieve the purpose of sealing. When the valve is used, the sealing fails and leaks upstream, and the downstream valve seat can obtain sealing.
The two-stage sealed valve seat adopts a rubber and plastic combined sealing structure for bearing greater sealing force. The rubber mainly plays the role of sealing, and the plastic plays the dual role of supporting and auxiliary sealing.
(5) Special structures
The valve seat with a special structure (Figure 5) has internal or external adjustment types, bellows types, grease injection sealing types, double push types and combined types.
(A) Internal and external adjustment types (b) Bellows types (c) Grease injection sealing types (d) Double push types (e) Combined types
Figure 5 The valve seat with special structure
The internal adjustment valve seat is the sealing ratio between the valve seat and ball is adjusted by pressing the inner adjusting nut, and its advantage is that there is no leakage point to the outside. The external adjustment valve seat is that the sealing ratio between the valve seat and the ball is adjusted by pressing against the outer adjusting screw plug to compress the wedge to cause the displacement of the telescopic sleeve.
The bellows type valve seat is generally connected to the valve seat and the guide sleeve by electron beam welding or argon arc welding. When designing, ensured that the sealing center of the valve seat is smaller than the middle diameter of the bellows. This structure has the characteristics of reliable sealing, wide elastic compensation ranges, and flexible three-dimensional space.
Grease injection sealed valve seats can increase the sealing performance, reduce friction and wear of the valve seat, extend the service life, and prevent the ball valve from causing accidents due to damage of the valve seat or a fire during use. A set of grease injection holes can be added to the valve seat. The grease injection seal only serves as an auxiliary seal or as a temporary emergency measure.
The double push valve seat is when the N chamber is connected to the control pressure, the valve seat and the ball are compressed and kept sealed. When the M chamber controls the pressure and the N chamber discharges the pressure to the outside, the valve seat will move back to make it not be in contact with the ball. The wear, friction and operating torque between the valve seat and the ball are smaller, which is suitable for high and medium pressure large-diameter ball valves. The combined type is composed of a spring valve seat and a piston valve seat. The former plays the main role of sealing, while the latter is connected to the control pressure and plays the role of auxiliary sealing.
1. Soft sealed valve seats
(1) Ordinary valve seats
The characteristic of the ordinary valve seat (Figure 1) is that the valve seat is pressed with the ball under the action of pre-tightening force or fluid pressure, and the seat generates elasticoplastic deformation to obtain sealing. The sealing effect depends on the ability of the valve seat compensating for the roundness of the sphere and the microscopic unevenness of the surface under the action of the fluid pressure or pre-tightening force. Therefore, there must be sufficiently sealing specific pressure between the valve seat and ball.
(A) Arc types (b) Cone types (c) Fireproof types (d) Inner ring types
Figure 1 Ordinary valve seats
(2) Elastic valve seats
The elastic valve seat (Figure 2) uses elastic elements such as metal spring supports and springs to generate elastic deformation under the pre tightening force or fluid pressure and compensate for the influence of changes in external conditions such as temperature difference, pressure and wear on the sealing performance of the ball.
(A) Single slope types (b) Double slope double line types (c) V-shaped linear self-sealing types (d) Special types
Figure 2 Elastic valve seats
(3) Spring combined types
The spring combined valve seat (Figure 3) is a set of compression springs or only one disc spring that is uniformly distributed along the circumference of the transverse section of the valve channel to always keep the valve seat pressed against the sphere and obtain a pre-tightened state.
(A) Compression spring types (b) Disc spring types
Figure 3 Spring combined valve seats
(4) Piston valve seats
The piston valve seat (Figure 4) relies on the pressure of the inlet medium to generate sealing force. The valve seat is pushed to compress the sphere, and the sealing ring produces elasticoplastic deformation to achieve the purpose of sealing.
(A) Cut-off and relief valve seats (b) Valve seats with double movements (c) Two-stage sealed valve seats
Figure 4 Piston valve seats
When the pressure of the cut-off and relief valve seat in the valve cavity rises abnormally to be greater than the pressure at the downstream end, the pressure difference generates an outward thrust, which overcomes the spring force to separate the valve seat from the ball and makes the pressure of the valve cavity discharge downstream. Afterward, under the action of the spring, the valve seat fits the ball well.
The valve seat with double movements is that when the pressure in the middle cavity is greater than the outlet pressure, the valve seat is pressed against the sphere, and the sealing ring produces elasticoplastic deformation to achieve the purpose of sealing. When the valve is used, the sealing fails and leaks upstream, and the downstream valve seat can obtain sealing.
The two-stage sealed valve seat adopts a rubber and plastic combined sealing structure for bearing greater sealing force. The rubber mainly plays the role of sealing, and the plastic plays the dual role of supporting and auxiliary sealing.
(5) Special structures
The valve seat with a special structure (Figure 5) has internal or external adjustment types, bellows types, grease injection sealing types, double push types and combined types.
(A) Internal and external adjustment types (b) Bellows types (c) Grease injection sealing types (d) Double push types (e) Combined types
Figure 5 The valve seat with special structure
The internal adjustment valve seat is the sealing ratio between the valve seat and ball is adjusted by pressing the inner adjusting nut, and its advantage is that there is no leakage point to the outside. The external adjustment valve seat is that the sealing ratio between the valve seat and the ball is adjusted by pressing against the outer adjusting screw plug to compress the wedge to cause the displacement of the telescopic sleeve.
The bellows type valve seat is generally connected to the valve seat and the guide sleeve by electron beam welding or argon arc welding. When designing, ensured that the sealing center of the valve seat is smaller than the middle diameter of the bellows. This structure has the characteristics of reliable sealing, wide elastic compensation ranges, and flexible three-dimensional space.
Grease injection sealed valve seats can increase the sealing performance, reduce friction and wear of the valve seat, extend the service life, and prevent the ball valve from causing accidents due to damage of the valve seat or a fire during use. A set of grease injection holes can be added to the valve seat. The grease injection seal only serves as an auxiliary seal or as a temporary emergency measure.
The double push valve seat is when the N chamber is connected to the control pressure, the valve seat and the ball are compressed and kept sealed. When the M chamber controls the pressure and the N chamber discharges the pressure to the outside, the valve seat will move back to make it not be in contact with the ball. The wear, friction and operating torque between the valve seat and the ball are smaller, which is suitable for high and medium pressure large-diameter ball valves. The combined type is composed of a spring valve seat and a piston valve seat. The former plays the main role of sealing, while the latter is connected to the control pressure and plays the role of auxiliary sealing.
