GB2116678A

GB2116678A - Ball valves

Info

Publication number: GB2116678A
Application number: GB08302756A
Authority: GB
Inventors: Seiji Fukuda
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 1982-02-08
Filing date: 1983-02-01
Publication date: 1983-09-28
Also published as: DE3303813A1; GB2116678B; GB8302756D0

Abstract

A ball valve has a valve body (1) a ball (3) rotatably mounted in the valve body and provided with a fluid passage therein and ring-shaped seat rings (4) interposed between the inner surface of the valve body (1) and the outer surface of the ball (3), wherein the improvement resides in the construction constituted such that the sealings pressure on the sealing surface (4a) of the seat ring contacting the inner surface of the valve body (1) and the sealing pressure on the sealing surface (4b) of the seat ring contacting the outer surface of the ball (3) are not increased when the pressure of a fluid confirmed in the valve body cavity (8) is increased. This occuring due to flexing of the seat when subjected to valve body pressure allowing such pressure to dissipate. <IMAGE>

Description

SPECIFICATION Ball valve Background of the invention The present invention relates to a ball valve mounted in pipings or various kinds of apparatus and adapted to control the flow of fluid in such pipings or apparatus. More particularly, the invention is concerned with a ball valve improved to prevent excessive pressure rise therein.

Among various types of valves used hitherto, the ball valve is rather modern one and is now finding a spreading use in general industrial fields thanks to various advantages such as small pressure drop, easy operation and automation, superior sealing characteristics, compactness of the construction, easy maintenance and excellent endurance and also from the view point of saving of energy and safety.

Brief description of the drawings Figure 1 is a schematic sectional view of a conventional ball valve; Figure 2 is an enlarged sectional view of an essential part of a seat incorporated in the conventional ball valve; Figure 3 is an enlarged sectional view similar two that in Figure 2 but illustrating the effect of the fluid pressure acting on the seat; Figure 4 is a diagram showing the relationships existing among the specific volume, pressure and temperature of water; Figure 5 is a diagram showing the result of an experiment as to how the pressure of water confined in the ball valve is changed in relation to lapse of time when the water is heated by external heat; Figures 6A and 6B are enlarged sectional views of the valve seat in the states before and after the experiment mentioned above;; Figure 7is an illustration of a simulation analysis of the process of development of plastic deformation in the seat ring shown in Figure 6B; Figures 8A, 8B and 8C are sectional views of ball valves illustrating conventional technics for preventing excessive pressure rise in the ball valves; Figures 9A to 9Eshow an example of the prior art in which: Figure 9A is an enlarged sectional view showing an essential part of a seat ring of the prior art; Figure 9B is a front elevational view of an essential part of the seat ring; Figure 9C is a rear elevational view of an essential part of the seat ring; and Figures 9D and 9E are enlarged sectional views of the seat ring in the state before the excessive rise of the pressure and in the state in which the excessive pressure rise is taking place; Figures lOA to lOC show another prior art in which: Figure lOA is a sectional view of the essential part of this prior art; Figure lOB is an enlarged sectional view of the essential part showing particularly a valve seat; and Figure l OC is a front elevational view showing specifically a recess formed in the seat ring; Figures 77to 15show in section and art a larger scale the essential parts of the preferred embodi ments of the invention in which: Figures 11A and 11B in combination show a ball valve in accordance with a first embodiment of the invention; Figures 12A and 12B show a ball valve in accord ance with a second embodiment of the invention; Figure 12C illustrates a modification in which the first and second embodiments are combined; Figures 13A and 13B show a ball valve in accord ance with a third embodiment of the invention; Figures 14A and 14B show a ball valve in accordance with a fourth embodiment; and Figures 15A and 15B show a ball valve in accordance with a fifth embodiment of the invention.

Description of the prior art Referring first to Figure 1, a typical conventional ball valve has a valve body cavity 8 defined by body parts 1,2, a ball 3, a stem 5 and seat rings 4 and 4'. In the full opening or full shuting state of the ball valve, the fluid confined in this valve body cavity 8 is completely shut-off and sealed from the fluid in the pipes connected to the ball valve, by the cooperation of the gland packing 6, gasket 7, seat rings 4, 4', body parts 1,2, valve stem 5 and the ball 3.

As will be clearly understood from Figure 2, the portion of the seat ring 4 or 4' between the sealing surface 4a contacted by the body part 1 or 2 and the sealing surface 4b contacted by the outer surface of the ball 3 has a wedge-like cross-section which converged towards the center of the seat ring 4 or 4'.

Therefore, when there is a pressure rise in the body cavity 8, this pressure acts to pressurize the seat ring from the outer periphery towards the center thereof as shown by arrows in Figure 3 so that the seat ring 4 or 4' is pushed by a wedging action into the gap between the body part 1 or 2 and the bali 3 to produce a greater sealing effect.

When a fluid of a temperature higher than that of the fluid confined in the body cavity 8 is supplied to either one of the pipes connected to the ball valve, or when the valve is heated by an external heat, the temperature of the confined fluid is increased so that the pressure of the confined fluid is increased due to thermal expansion ofthefluid.

Namely, the excessive pressure rise in the ball valve takes place when an incompressible fluid confined in the valve body cavity is heated in the full opening or full shuting state of the valve.

For instance, representing the volume of the water at 1 Kg/cm2 and 4"C by 1.0, the specific volume, temperature and the pressure are changed in a manner shown in Figure 4. As will be seen from this Figure, as the temperature is raised to 1830C, the specific volume of water is as large as 1.13, i.e. a volumetric expansion of 13% is caused, even under the pressure of 500 Kgf/cm2.

If such an excessive pressure is established in the valve, the valve has to expand in a manner like a rubber ball to absorb the increment of the volume, even though the valve is constructed as a pressure vessel which can withstand internal pressure of 500 Kgf/cm2. Thus, the excessive pressure rise may lead to a leak through the gland packing 6 or a breakdown of the gasket 7, not to mention to the breakdown of the seat ring 4 or 4'. In the worst case, the body parts 1,2 and the ball 3 are deformed or even destroyed when the sealing performance and the strength of the gland packing 6 or the gasket 7 so high as not to permit leak of the fluid therethrough.

In orderto actually confirm this phenomenon, a test was conducted with an actually usable ball valve in which the pressure of water confined in the valve body cavity was measured while gradually heating the water, the result of which is shown in Figure 5. In this case, the seat ring was destroyed as a result of buckling due to insufficient strength of the seat ring.

Anyway, this valve was broken to lose its ordinary function. The maximum level of the excessive pressure well reaches about 200 Kgf/cm2 dangerously. This is quite inconvenient considering that the valve used in the test has a rating pressure of 10 Kg/cm2. If the body parts are made of a fragile or brittle material, it is quite considerable that the body parts of the valve are completely ruptured.

Figures 6A and 6B show the cross-sectional shapes of the valve seat in the state before and after the excessive pressure rise test mentioned above. As will be seen from Figure 6A, in the normal state before the test, the portion of the seat ring between the sealing surface contacting the inner surface of the body part and the sealing surface contacting the ball exhibits a wedge-shaped cross-section which converged from the radially outer side towards the pipe connected to the ball valve. In contrast, in the state after the excessive pressure rise test as shown in Figure 6B, the seat ring has been plastically deformed largely and is driven into the gap between the body part and the ball in a manner like a wedge.

In order to quantitatively examine the state of deformation of the seat ring, a simulation was made by the finite element method to obtain a result as shown in Figure 7. In this simulation, the excessive fluid pressure established in the body cavity is 100 Kgf/cm2. It will be seen that this test result exhibits a reasonable coincidence with the test result explained in connection with Figures 6A and 6B.

It will be understood that the excessive pressure rise in a ball valve, which is a fatal catastrophic phenomena in the ball valve functioning as a pressure vessel, is attributable to the fact that the seat ring has such a construction that the sealing force at the sealing surface contacting the valve body part and the sealing surface contacting the ball are increased as the raised fluid pressure established in the valve body cavity is applied to the seat ring.

This is the mechanism of excessive pressure rise in the ball valve.

Thus, the excessive pressure rise occurs when an incompressible fluid such as water confined in the valve body cavity is heated to raise its pressure by thermal expansion in the full opening or shutting state of the valve.

Therefore, a key to eliminate the extra-ordinary pressure rise is to eliminate at least one of these occurrence conditions. As an alternative, it is possible to provide a relieving mechanism which can relieve the excessive pressure, i.e. the increment of the volume due to expansion when the pressure is increased to exceed a predetermined level, thereby to avert from the danger.

Under these circumstances, various methods have been proposed to avoid the excessive pressure rise in the ball valve, in accordance with the aboveexplained preventing theory, and some of them have been put into practical use.

Among the occurrence conditions mentioned before, the facts that the fluid is incompressible and that the temperature is raised are unavoidable. As a measure for getting rid of the rest of the occurrence conditions, a method so-called "relieving hole method" has been proposed in which, as shown in Figure 8A, a pass-through hole is formed in the ball to provide a communication between the valve body cavity and the pipe connected to the ball valve, as well as a method called "relieving slit method" in which a pass-through slit is formed in the seat ring so as to provide a communication between either one of the seat rings and the valve body cavity as shown in Figure 8B.These methods, however, cannot be carried out in such a form that the pass-through hole or the pass-through slit is disposed at the downstream side of the valve, in the floating type ball valve the sealing function of which relies upon the pressure differential of the fluid between the upstream side and downstream side.

Namely, the ball valves adopting such methods have demerits that it can be used only unidirectionally, while they can prevent the excessive pressure rise therein.

In some of the ball valves called "floating seal type valve", the sealing power is not imparted initially to the ball and the seat rings but the sealing power is produced only when the differential pressure of the fluid is applied. In this type of ball valves, no excessive pressure rise takes place because the incompressible fluid is not at all confined in the valve body cavity. In addition, there is no restriction in the flowing direction of the fluid. However, this type of ball valve suffers from a disadvantage that it tends to allow a leak of the fluid because of insufficient sealing force, particularly when the differential pressure of the fluid is small.

Figure 8C shows an example of the aforementioned relieving mechanism. Namely, in this case, the valve body cavity is communicated directly with a pipe or the like connected to the ball valve through a by-pass passage, and a self-relieving check valve is disposed in the by-pass passage. The pressure in the valve body cavity can rise up to a predetermined relief pressure of the self-relieving check valve but, as this relief pressure is exceeded, the selF-relieving check valve is automatically opened to permit the increment of the volume of the fluid to be relieved to the fluid passage. This arrangement can prevent the danger caused by excessive pressure rise almost completely, without failing ordinary function of the ball valve. However, this type of ball valve is generally costly because of the employment of the by-pass passage and the relieving check valve both of which are generally expensive and, hence, is not suited for use as general-purpose valve. Another solution called "air pot method", which employs an air pot connected to the valve body cavity, is also impractical for the same reason as the explained above.

In recent years, therefore, a ball valve of so-called "self-relieving structure" has been proposed in which the seat ring itself has a function to automatically relieve the pressure in the valve body cavity when the pressure has reached a predetermined level. An example of the ball valves having the self-relieving function, developed in Japan and disclosed in detail in Japanese Utility Model Laid Open No. 82360/1981, is shown in Figures 9A,9B,9C,9D and 9E. This ball valve has a seat ring 4 having a ring-shaped base 4a of a substantially rectangular cross-section and a lip portion 4b which project somewhat axially outwardly and radially inwardly from the mid portion of the inner surface of the base 4a and having a predetermined thickness.

The seat ring 4 has tapered surfaces 10 and 9 of different gradients in which formed is a recess 4d which provides a communication between the body cavity 8 and a space 4C when the seat ring 4 is contacted by the ball 3. In addition, a recess 4e is formed in a portion of the inner edge of the side surface of the lip portion 4b opposite to the tapered surface 10 so as to provide, in a part of the circumference of the lip portion 4b, a reducedthickness portion 4b2 of a thickness smaller than that of the other portions of the lip portion 4b.When the pressure of the fluid confined in the valve body cavity is increased excessively due to thermal expansion caused by a temperature rise as explained before, the reduced-thickness portion 4b2 of the seat ring 4 is elastically deformed towards the passage hole to relieve the excessive pressure into the passage through the recess 4d, space 4c and the reduced-thickness portion 4b2.

Figures 10A, lOB and 1 OC show another ball valve having self-relief structure. This ball valve is disclosed in the specification of United States Patent No. 3,488,033. This ball valve is substantially identical to that mentioned above although there is a slight structural difference in that this ball valve lacks the notch 4e on the rear side of the seal ring. Namely, in this ball valve, a recess is formed in the ball contacting portion of the seal ring so that a part of the seat ring is elastically deformed by the excessive pressure introduced into this recess to permit the relief of the excessive pressure.

Two types of known ball valves of effective self-relieving construction have been described. In these known structures, however, the touched area is increased inconveniently because of the necessity for the inner and outer seals formed by the inner and outer projections for the formation of the recess.

This results not only in an increase in the size of the seat ring but also in an increase of the opening and shuting operation torque for attaining a suitable surface pressure. In addition, the adoption of the double projection, i.e. the inner and outer projections, seriously deteriorates the flexibility of the outer seal portion, which in turn makes the assembling of the valve difficult. For these reasons, it is not easy to mass-produce this ball valve with a high uniformity of quality. In general, the excessive pressure rise in ball valves takes place at comparatively high temperatures. At such high temperature, the material of the seat ring, e.g. tetrafluoroethylene (P.T.F.E.) exhibits an impractically large plastic deformation such as the creep and, hence, tends to lose the elasticity force at the inner sealing portion for pressing the ball.In ordinary ball valve, the sealing pressure on the inner sealing surface 4b2 is increased by making an efficient use of the differential pressure of the fluid thereby to shut-off the fluid.

The differential pressure is applied to the outer sealing portion 4b1 which does not constitute the sealing surface but is not applied to the inner sealing portion 4b2 serving as the sealing surface. Therefore, this type of known ball valve suffers a fatal problem that the fluid cannot be shut-off at all if the inner seal portion 4b2 is deformed largely.

Summary ofthe invention Accordingly, an object of the invention is to provide a self-relieving type ball valve which is improved, in spite of a rather simplified construction, to eliminate the problems such as leak of the fluid to the outside of the piping system or breakdown of the valve body part attributable to excessive pressure rise in the valve body cavity.

A description will be made as to some preferred embodiments of the invention. These preferred embodiments have a common basic feature that the sealing pressures on the sealing surfaces of the seat ring contacting the body part and the outer surface of the ball are not increased even under the excessive pressure established in the valve body cavity.

No excessive pressure rise takes place in ball valves which are designed and constructed to meet this technical idea.

Description of the preferred embodiments Preferred embodiments of the invention will now be described with reference to Figures 11 to 15 which are enlarged sectional views of seat rings of different embodiments in the normal opening state of the valves after the assembling. Same reference numerals are used throughout these Figures to denote same parts or members.

Figures 1 1A and 11 B in combination show a first embodiment of the invention. As explained before, the excessive pressure rise in a ball valve takes place as a result of thermal expansion of a fluid confined in the valve body cavity of the ball valve when the confined fluid is heated. This can be attributed to the facts that (1) the sealing portion which is to relieve the excessive pressure is so constructed as to increase the sealing force as the pressure in the valve body cavity is increased and that (2) the seat ring as a whole is elastically deformable.In this first embodiment of the invention, taking into account the elastic deformation of the seat ring as a whole, the strength of the seat ring is increased along the outer periphery thereof to suppress the elastic deformation of the seat ring and, in addition, a difference in the strength is afforded between the sealing portion and the seat ring to provide a self-relieving structure of so-called "contractionprevention reinforcement type".

More specifically, Figure 1 1A is an enlarged sectional view of the seat ring portion in the normal opening state of the ball valve after the assembling.

The seat ring 4 is made of a comparatively hard but elastically deformable material to have a form as shown in Figure 11A. The seat ring 4 has a sealing surface 4b for preventing the leak of the fluid along the outer surface of the ball 3 and a sealing surface 4a for preventing the leak along the stepped wall of the valve body part. A reference numeral 9 designates a retainer which is made of a material having a high rigidity. In this case, a stainless steel is used as the material of the retainer 9. A gap 11 is preserved for absorbing the elastic deformation of the seat ing during relieving. Thus, the seat ring 4 is clamped between the stepped wall 10 of the body part and the ball 3.

An explanation will be made as to how this ball valve operates, with specific reference to Figure 11 B.

As an excessive pressure rise takes place in the body cavity 8, pressures are exerted on the seat ring 4 as indicated by arrows P. However, no substantial elastic deformation takes place on the outer peripheral portion of the seat ring 4 because the reinforcement of the outer peripheral portion by the retainer 9. On the other hand, the inner peripheral surface portion of the seat ring 4 is elastically deformed by the surface pressure P because of insufficiency of the strength. As a result, the sealing surface 4b is moved away from the outer surface 3a of the ball and the pressurized fluid in the valve body cavity 8 relieved to the fluid passage through the gap formed between the sealing surface 4b and the outer surface 3a of the ball 3.As the fluid pressure in the valve body cavity comes down below the relief pressure as a result of the relief, the sealing surface 4b resumes the initial position due to its elasticity so that the seat ring recovers the sealing function. This automatic pressure relieving operation is repeated to maintain the fluid pressure in the valve body cavity below a predetermined level.

Figures 12A and 12B in combination show a second embodiment of the invention. More specifically, Figure 12A shows the seat ring of the ball valve of the second embodiment in the normal opening state of the valve as in the case of Figure 1 1A. The material of the seat ring 4, forms of the sealing surfaces 4a, 4b and the gap 11 are materially identical to those of the first embodiment. In this second embodiment, however, there is no reinforcement by the retainer 9 but, insteadly, a shoulder 1b is formed on the stepped surface of the valve body part while a cooperating step is formed on the sealing surface 4a so as to fit and rest on the shoulder 1 b. In operation, as will be seen from Figure 1 2B, any excessive pressure rise in the valve body cavity 8 applies pressure forces P1, P2, P3 and P4.However, the pressure forces P2 and P4 are negated by each other and, hence, are negligible materially. Consequently, a composite force of the pressure forces P1 and P3 act in the directions of the arrows. In this state, the seat ring 4 is supported mainly by the cooperation between the shoulder 1 b and the step so that the seat ring 4 is elastically deformed by a twisting action around a fulcrum constituted by the area of contact between the shoulder 1 b and the step. As a result, the sealing surface 4b is swung away from the outer surface 3a of the ball so that the pressurized fluid is discharged to the fluid passage past this passage. After the relief, the seat ring resumes the initial position to recoverthe initial fluid sealing function.In this embodiment, the relief pressure at which the excessive pressure is relieved can be set freely and easily by suitably selecting the amount of projection of the shoulder 1 b.

Figure 1 2C shows a modification in which the structural features of the first and second embodiments are combined. No explanation will be needed for this modification, because the features and advantages of these embodiments equally apply to this modification. The preceding two embodiments are characterized by such features that the region of elastic deformation of the seat ring is restricted as much as possible to the inner peripheral portion of the seat ring.

As stated before, the excessive pressure rise in the ball valve is attributable also to the fact that the seat ring is constructed to increase the sealing force as the fluid pressure in the valve body cavity is increased. Figures 13A and 13B show a third embodiment of the invention which has been accomplished on consideration of the above-mentioned fact. More specifically, in this embodiment, the sealing surface of the seat ring is moved away from the outer surface of the ball by the differential pressure between the high pressure established in the valve body cavity and the pressure in the fluid pipe connected to the ball valve.To this end, in the ball valve of this embodiment, the sealing surface 4a on the stepped inner surface of the valve body part is tapered art a gradient a which is smaller than or at least equal to the gradient of the taper of the sealing portion 4b. The gradients a and in this case represent the angle of inclination to the axis of the flow passage.

In operation, pressure forces Pa, P2, P3 and P4 act on the seat ring 4 as in the case of the seat ring shown in Figure 12B. Materially, however, only the pressure forces P1 and P2 are effective so that the seat ring 4 is compressed in the direction indicated by an arrow. In consequence, the seat ring 4 moves in the direction of the arrow with its tapered surfaces 4a and 4b sandwiched and guided by the cooperating surfaces. Because of the aforementioned relationship of a < , the tightening margin 12 of the sealing surface 4b is never increased but is decreased or unchanged during the movement of the seat ring. Then, as the fluid pressure is further increased, the sealing surface 4b is moved away from the outer surface 3a of the ball 3 due to elastic deformation or deformation by a rational force so as to permit the pressurized fluid to be relieved to the fluid passage. The relieving pressure can be freely and easily set by suitably selectin the gradient of the tapered surface 4a and the tightening margin 12.

Thus, in the ball valve of this embodiment, the tightness of the seal of the valve body cavity is not increased but rather decreased.

Figures 14A and 14B show a fourth embodiment of the invention which is adapted to relieve the pressu rized fluid by making an efficient use of the differential pressure as in the case of the third embodiment explained in connection with Figures 13A and 13B.

The ball valve of the fourth embodiment has a metallic retainer 9'. A space 16 is formed between the seat ring 15 and the retainer 9'. Another seat ring is denoted by a reference numeral 14. The seat ring 15 has a tapered sealing surface 1 sub and is shaped to be deflectable downwardly when the pressure is applied thereto. A reference numeral 1 spa denotes a sealing surface formed on the stepped surface of the valve body part. These two seat rings are fitted in the retainer 9' in such a manner as to perform sealing function.In operation, as an excessive pressure rise takes place in the valve body cavity, the fluid P of high pressure is introduced through a threaded portion 17to depress the sealing surface 15b downwardly to create a gap through which the pressurized fluid is relieved to the fluid passage via the space 16 and the gap 11.

Figures 15A and 15B show a ball valve in accordance with a fifth embodiment of the invention in which the vertical component of the force produced by the high fluid pressure transmitted from the valve body cavity is eliminated and the relieving operation is caused only by the horizontal component of the same. Thus, this ball valve may be referred to as "inward contraction differential pressure removal type". To this end, the sealing surfaces 4b and 4a are aligned in the vertical direction substantially on the same plane as illustrated. In this embodiment, only the pressure force P1 is effective and the relieving operation is made as a result of the rotation of the seat ring around a fulcrum constituted by the sealing surface 4a.

Preferred embodiments of the invention have been described. Preferably, in each of the described embodiments, the self-relieving function is combined with the conventional relief hole type method mentioned before in such a manner that the selfrelieving function is performed when the ball valve is in the shuting state, whereas the relief hole functions when the ball valve is in the opening state. By so doing, it is possible to attain a remarkable improvement in the endurance of the seat ring.

As has been described, according to the invention, the causes of excessive pressure rise in the ball valve are eliminated by employing (1) contractionprevention reinforcement type structure in which the seat ring is reinforced at a suitable portion thereof to exhibit a contraction when subjected to the excessive pressure rise thereby to relief the excessive pressure or (2) contraction differential pressure type in which the differential pressure produced as a result of the excessive pressure rise is utilized in such a manner as to decrease the tightness of the seal of the valve body cavity or (3) radially inward contraction differential pressure removal type structure in which radially inward contraction force on the seat ring caused by the excessive pressure rise is eliminated.

The ball valve of the invention can be produced at lower cost than the known self-relieving type ball valves, without being accompanied by an increase in the operation torque and deterioration of the sealing performance. Needless to say, the self-relieving type ball valve of the invention can function as an ordinary ball valve for controlling the flow of the fluid, thus contributing to an enhancement of the safety and reliability of the system employing the ball valve.

Claims

1. A ball valve having a valve body, a ball rotatably mounted in said valve body and provided with a fluid passage therein and ring-shaped seat rings interposed between the inner surface of said valve body and the outer surface of said ball, wherein the construction is such that the sealing pressure on the sealing surface of said seat ring contacting said inner surface of said valve body and the sealing pressure on the sealing surface of said seat ring contacting said outer surface of said ball are not increased when the pressure of a fluid confined in the valve body cavity is increased.

2. A ball valve according to claim 1, characterized by comprising a relief hold providing a communication between said outer surface of said ball and a fluid passage connected to said ball valve.

3. A ball valve according to either one of claims 1 and 2, wherein said seat ring is of a contractionprevention reinforcement type in which a part thereof is reinforced to prevent contraction.

4. A ball valve according to claim 3, wherein, in the touching area where said seat ring contacts said inner surface of said valve body, said inner surface of said valve body is provided with an annular shoulder while said sealing surface of said seat ring contacting said valve body is provided with a step for fiting in said shoulder, and wherein a gap is formed between said inner surface of said valve body and the side surface of said seat ring at a portion of said seat ring spaced radially inwardly from the portion where said step fits in said shoulder.

5. A ball valve according to either one of claims 3 and 4, characterized by comprising an annular retainer made of a metal and sursounding and retaining the outer peripheral surface of said seat ring, and a gap formed between said inner surface of said valve body and the side surface of said seat ring at a portion of said seat ring spaced radially inwardly from the portion where said inner surface of said valve body and said sealing surface of said seat ring fit each other.

6. A ball valve according to either one of claims 1 and 2, wherein said seat ring is of contraction differential pressure type which makes use of the contraction of said seat ring caused by a rise of the fluid pressure in said valve cavity.

7. A ball valve according to claim 6, wherein the gradient of surfaces of said seat ring and said valve body in said touching area to the axis line of fluid passage is selected to be equal to or smaller than the gradient of the sealing surface of said seat ring contacting said outer surface of said ball.

8. A ball valve according to claim 6, wherein said seat ring is divided into a first part adjacent to said inner surface of said valve body and a second part adjacent to said outer surface of said ball, characterized in that a retainer of a material having a higher rigidity than said seat ring is interposed between said first part and said second part of said seat ring, the portion of said first part formed between the sealing surface thereof facing said valve body and the sealing surface thereof facing said retainer has a wedge-shaped cross-section which converges from the side adjacent to said fluid passage towards said valve body cavity while a space is formed at radially inner side of said first part of said seat ring.

9. A ball valve according to either one of claims 1 and 2, wherein said seat ring is of radially inward contraction differential pressure removal type in which the radially inward contraction of said seat ring caused by a rise of fluid pressure in said valve body cavity is removed or reduced and the seal between said outer surface of said ball and said sealing surface of said seat ring is dismissed by the component of fluid pressure in said valve cavity directed to said fluid passage.

10. A ball valve according to claim 9, wherein said sealing surface of said seat ring contacting the said inner surface of said valve body and said sealing surface of said seat ring contacting said outer surface of said ball are formed substantially in the same plane which intersects the axis of fluid passage at a right angle, and wherein a gap is formed between said inner surface of said valve body and the side surface of said seat ring at a portion of the latter spaced radially inwardly from said sealing surface contacting said inner surface of said valve body.