Valves and valve actuation – Rotary valves – Ball valve
Reexamination Certificate
2000-02-28
2001-09-25
Shaver, Kevin (Department: 3754)
Valves and valve actuation
Rotary valves
Ball valve
C166S330000, C251S314000, C251S317000
Reexamination Certificate
active
06293517
ABSTRACT:
This invention relates to a ball valve and, more particularly, to a ball valve having a convex seat providing a metal-to-metal seal with a valve ball.
BACKGROUND OF THE INVENTION
Metal-to-metal seals are very desirable in high pressure valves which are subjected to erosion, such as in kelly valves used to control blow outs in a drill string. A typical design for a high pressure valve of this type uses a spherical valve ball sealing against a valve seat which is concave in the sealing area. As can be imagined, these type valves must be made to close tolerances because they will not otherwise seal.
There are several factors that are pertinent in a valve ball—concave seat arrangement: (1) the roundness of the ball, (2) the radius matching between the ball and seat and (3) the nature of the surface finish. The roundness of the ball is also related to the problem of rotating the ball on its centerline. It will be appreciated that if the ball is perfectly round, it has an infinite number of centerlines and can be rotated about any of them. If the ball is quite out of round, it must be rotated substantially only on one of its centerlines, e.g. if the valve ball is football shaped, it can only be rotated on a small number of axes, probably only two.
With valve ball—concave seat arrangements having acceptable metal-to-metal seals, the ball roundness must be in the range of + or −0.0005″ and the radius matching must be nearly perfect. Tolerances in this range are beyond the capability of state of the art metal cutting machinery such as computer controlled machining centers. Thus, these type valves are presently made by machining the ball and valve seat to as close a tolerance as possible and the parts are then lapped.
Lapping is a machining operation in which the parts are placed in fixtures and abutted together so they can be moved in the same manner as in use. An abrasive paste is applied to the parts and the parts are then rubbed together. The parts are rubbed together until the metal is worn off one or both parts to an extent where the parts meet the desired tolerance. This is a lengthy and expensive process so high pressure ball valves with metal-to-metal seals are expensive. When the time comes to rebuild a valve, one must replace the valve ball and the seat because each valve ball is made to fit its seat. Thus, modern high pressure metal-to-metal sealed ball valves are not mass produced if that term means that parts of identical devices are interchangeable.
Whether a valve ball has been lapped can be determined by examining it under magnification. Machining produces arcuate striations that are visible under magnification. When a valve ball is lapped, the visible striations are smaller and are typically cross-hatched. Interestingly, coating a valve ball does not disguise imperfections in the underlying surface, it magnifies them. Thus, inspection of a coated or uncoated valve ball under magnification shows whether it is lapped or untapped.
A disclosure of a ball valve having a metal convex valve seat is found in U.S. Pat. No. 3,888,460. Other disclosures of interest are found in U.S. Pat. Nos. 2,932,311; 2,988,109; 3,077,895; 3,185,857; 3,211,421; 3,272,472; 3,380,709; 3,386,461; 3,556,471; 3,610,575; 3,705,707; 4,813,649 and 5,632,294.
SUMMARY OF THE INVENTION
In this invention, a high pressure ball valve includes a metal convex valve seat. The valve ball and valve seat are made by conventional machining techniques which provide tolerances that are close enough to avoid having to lap the valve ball and seat. This allows the manufacture of low cost valve balls and valve seats that are capable of making satisfactory metal-to-metal seals.
Because of the small contact area between a valve ball and a convex seat, the calculated contact pressure between the ball and seat is very large compared to the calculated contact pressure concave seats. If the calculated contact pressures were actually achieved in practice, large valves subjected to high pressures would show plastic deformation of the valve ball or valve seat. Prototypes of this invention fail to show such plastic deformation. Much lower calculated contact pressures in valve ball—concave seats induces pressure welding between the ball and seat which is first noticed because of galling of the ball and/or seat. Galling occurs when very small areas of the ball and seat are pressure welded together and then torn apart when the ball is moved. Prototypes of this invention fail to show galling under calculated contact pressures much higher than what causes galling in valve ball—concave seats.
While not being bound by any particular theory, it is believed that the calculated contact pressures between the valve ball and seat are much higher than the real contact pressures because the valve ball and/or seat elastically deform under load thereby increasing the contact area between the ball and seat. Because contact pressure equals the force on the ball divided by the contact area between the ball and seat, increasing the contact area causes a reduction in the contact pressures. The contact area evidently increases substantially because valves made of relatively modest materials do not show plastic deformation when subjected to substantial pressures and thus substantial loads, loads which should cause plastic deformation unless some countervailing event was occurring.
Thus, in this invention high pressure ball valves with high performance metal-to-metal seals can be made of relatively modest materials without lapping. This provides the happy confluence of better performance and lower costs.
It is an object of this invention to provide an improved ball valve having a convex seat providing a metal-to-metal seal with the valve ball.
Another object of this invention is to provide an improved ball valve having a convex seat in which the seat and the valve ball are capable of performing under very high loads without galling.
A further object of this invention to provide a low cost valve ball and untapped convex valve seat which provide a suitable metal-to-metal seal.
These and other objects and advantages of this invention will become more fully apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.
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Bonderer D A
McKnight John D.
Moller G. Turer
Shaver Kevin
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