Patent Application
20140110617
The present invention relates to plug valves and, more particularly, to plug valves suitable for high temperature and/or high pressure applications. The invention is also directed to a plug valve having a seat formed directly on the valve body.
Prior art plug valves conventionally utilize an inserted cone with a seat that is machined outside the valve body, and then inserted and glued into a pre-machined cone shaped pocket with tight tolerance in the valve body. The metal cone generally has a raised lip around its interior frustoconical shape. This combines with a tight tolerance on the insert outer surface, to provide a leak free seal once the stem and cone are engaged and forced down into the inserted seat cone.
The prior art has significant problems when a minimum diameter seat size is utilized. The intricate machining and small size of the seat means that the tolerance is extremely difficult to hold and therefore a seal is very difficult to maintain. The raised interior lip is difficult if not impossible to replicate once the valve bore size becomes smaller than 0.250″. A hard face seat insert is disclosed in U.S. Pat. No. 4,776,566. Moreover, this prior art assembly is time consuming and difficult to manufacture, involving the use of special glues and heat treatment to ensure proper cone placement and a seal with the plug. If these steps are not performed accurately, the chances of a leaking valve are inherently increased.
The disadvantages of the prior art are overcome by the present invention, an improved plug valve is hereinafter disclosed.
In one embodiment, a plug valve comprises a valve body having a flow line connecting a fluid inlet and a fluid outlet. A valve stem having a plug supported on an end thereof controls flow through the valve body. A seat formed on the valve body includes a seating surface above and below the flow line, and provides the seating surface for sealing engagement with the plug.
These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.
This invention provides a method that utilizes a raised hard face seat formed on the valve body of a plug type valve in order to successfully seal with the plug. In preferred embodiments, a bubble tight seal may be formed using a metal-to-metal sealing surface. The seal utilizes the shape of a frustoconical seat surface to provide a surface area of such size such that the interaction between the frustoconical plug and seat will result in sufficient contact pressure to initiate and hold the seal at high pressures.
As disclosed herein, the plug valve is able to maintain sealing capabilities at high temperatures, while also providing a roddable design and retaining an instrument plug/needle valve configuration. Currently, there is no known small bore valve that still retains the ability to seal at high temperatures while maintaining roddability. Roddability is the ability to use a tool of some sort to unclog or clean out a blocked valve though a bore that is free of obstructions. This feature results from a flow line through the valve body which has a substantially straight line axis. High temperatures are defined as those outside the capabilities of soft and composite seat offerings, and typically will be over 700° F. The prior art generally maintains a common configuration of a steel ball bearing being pushed into a seat pocket. An abrupt turn is necessary in the flow bore of these valves, so they are not truly roddable.
A further feature of the valve is that if there was some form of leakage occurring during field use, a lapping of the seat may be performed in situ to remove striations or damage that may occur due to extreme use from aggressive media stream. This extends the life of the valve and makes it possible to field dress the seat and provide a temporary fix until a replacement valve could be scheduled for insertion.
A method of machining is provided that forms a seat that is vertical to the valve body. This removes the need for the usual required seat inserts. The techniques disclosed herein are particularly well suited for dealing with very small bore and small seat valves, but may also be used on larger bore and large seat valves. The technique reduces the number of moving parts and provides a solution that allows a hard face raised seat to be machined into a valve body, resulting in a metal-to-metal seal that is capable of high temperature bubble tight leakage.
The shape of the seat on the valve body is frustoconical, and is raised from its surrounding area. The raised structure results in a downward force produced during the closing of the valve pushing on the cone, which is translated to contact pressure at the seat seal area producing the required force for effective sealing.
The present invention reduces the number of parts required to make a seal and provides a configuration that is far easier to machine. No specialized tooling and CNC machining is necessary. By making the raised lips of the seat integral with the valve body, it is possible to produce a reliable seat in four steps using only a vertical mill. Moreover, the seat should have a greater longevity since material is left whole behind the seat, providing a seat which is field serviceable via a lapping process if the seat becomes damaged.
A four step machining process is disclosed, with each step removing a specific volume of metal that helps keep the shape and raised face profile to the seat. An initial drill of specific size is plunged into the center of the seat. An end mill may then be utilized to remove a swath of material on either side of the central plunged hole. A tapered end mill of specific angular and end dimension is then plunged down the previous center hole, creating the needed frustoconical shape. The top of the seat pocket is then finished with a double edge chamfer tool to finish the raised seat profile and ensure the desired surface will provide the needed seating stress. The surface finish of the machined seat face is important in this process and all burrs and chips may be removed after machining. To obtain an initial seal, or after use of the valve, the seat may need to be lapped using a lapping compound and lapping devices. This guarantees a surface finish and ensures the angle of the cone and the seat is matched as close as possible.
The valve as described herein uses a plug to seal with the seat formed on the valve body. The term “plug” is used herein for a valve includes needle valves with slender plug members.
Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.
