Valves and valve actuation – Valve – Seats
Reexamination Certificate
2003-02-18
2004-08-10
Hirsch, Paul J. (Department: 3754)
Valves and valve actuation
Valve
Seats
C251S333000
Reexamination Certificate
active
06772993
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to valves and, more particularly, to a uniquely configured plug and seal assembly which is specifically configured to mitigate the erosive and cavitating effects of flashing water and steam flowing within a fluid control valve.
In power plants, boiler water is typically circulated within a closed loop by a feedpump. The feedpump takes the water from a deaerator and increases the pressure from ambient up to 6000 psi for delivery to a boiler. Because the feedpump operates at a relatively constant rotational speed, a minimum amount of flow through the feedpump is required in order to avoid overheating and cavitation of the feedpump components. However, the boiler feed flow requirements fluctuate regularly throughout the day in proportion to changing electricity production demands placed upon the power plant. When the boiler feed flow requirements are reduced to a level below the minimum required flow through the feedpump necessary to avoid overheating and cavitation of the feedpump, a recirculation system is engaged to direct a portion of the high pressure flow back to the deaerator or to a condenser and then to the feedpump. Ideally, the recirculation system will meter the flow in response to the feedpump requirements such that an optimal flow level is circulated through the feedpump in order to prevent excessively high feedpump operating temperatures and to prevent cavitation of the internal feedpump components.
A recirculation valve is typically utilized in the recirculation system to selectively block and unblock the flow of high pressure water to the deaerator or condenser. The recirculation valve may be configured as a linear displacement valve. Such recirculation valves include plugs that are linearly displaced during normal operation of the valve. Within these valves, the plug is axially slidable within a valve cage. The valve cage defines a multiplicity of tortuous or non-tortuous fluid passageways. Certain linear displacement valves are configured for “over plug flow” wherein water flows radially inward into the interior of the valve cage from the exterior thereof, with the water undergoing a pressure drop as a result of the flow through the valve cage. To open the valve, the plug is lifted off of a seat ring which then allows fluid, such as boiler water, to flow from the interior of the valve cage and out of the valve via the unblocked seat ring. Conversely, the movement of the seating surface of the plug into sealed engagement with the complementary seating surface of the seat ring facilitates a closed or shut-off condition for the recirculation valve.
The recirculation valve must be capable of handling water at extreme thermodynamic conditions. For example, within the interior chamber of the valve, the water pressure may range from 2500 to 6000 psig and the temperature may be at 300° F. to 500° F. At the outlet of the valve, the pressure of the water may range from only −12 to 10 psig with corresponding temperatures of only 110° F. to 240° F. Thus, in the open position, the plug and seal of the recirculation valve must be capable of withstanding an extreme pressure drop under flashing conditions while simultaneously minimizing erosion and cavitation of the seating surfaces of the plug and seat ring. Cavitation causes “pitting” of the metallic surface of the valve plug and seal that may occur during the sudden extreme pressure drop of the passing boiler water. The “pitting” of the plug and seal may result in leak paths that may increase in size over time due to erosion. As distinguished from normal evaporation, flashing is the sudden vaporization of the water caused by an instantaneous temperature and/or pressure drop.
In recirculation valves such as those that are utilized in power plants, the fluid flowing from valve inlet to outlet may undergo a pressure drop of up to 5500 psig at a temperature of up to 500° F. Such a pressure drop of the boiler water may be characterized as a violently explosive event occurring within the valve. In addition, in the closed position, the valve seat and plug must be capable of blocking flow with no leakage against pressures as high as 6000 psig. Should even a slight amount of leakage occur, flashing steam and water will cut and erode the seating surface of the metallic valve seat and plug with the effectiveness of a saw blade, quickly increasing the leakage path and rendering the valve useless. The constant leakage of the flashing steam and water will further accelerate the erosion damage of the seat ring and the plug. As mentioned above, during periods of high electricity production demands upon the power plant, the boiler feed flow requirements are at a maximum. Excessive leakage in the recirculation valve may prevent the required flow of water from reaching the boiler resulting in the failure of the power plant to meet the electricity demands that may be placed upon it. In extreme cases, excessive leakage of a recirculation valve may necessitate that the power plant be taken off line in order to replace the leaking valve.
The present invention specifically addresses the above-described erosion and cavitation damage problem by providing a plug and seal assembly with unique, complementary configurations specifically adapted to prevent the water exiting the valve cage from directly impinging the seating surface of the seat ring. The present invention alternatively provides a plug and seal assembly having a redundancy of sealing surfaces in order to reduce the risk of leakage. In this regard, the present invention provides a plug and seal assembly capable of neutralizing the erosion of the seating surfaces due to flashing and cavitation. These, and other features of the present invention, will be described below.
BRIEF SUMMARY OF THE INVENTION
The plug and seal assembly of the present invention is adapted for use in a fluid control valve. The fluid control valve includes a valve housing defining an interior chamber and a flow opening configured to fluidly communicate with the interior chamber. The interior chamber receives the fluid therein and the flow opening allows the fluid to escape the interior chamber. In a first embodiment, the plug and seal assembly comprises an annular seat ring and a generally cylindrical plug. The seat ring is at least partially engaged to the valve housing at the flow opening.
The valve housing may also include a sleeve and a valve cage concentrically disposed within the interior chamber. The valve cage is captured between the sleeve and the seat ring. The valve cage defines annular flow passages configured for reducing the pressure of the fluid flowing therethrough from the interior chamber prior to exiting the flow opening. The sleeve has an elongate bore extending axially therethrough. The plug and seal assembly includes the plug. The plug is comprised of a plug body and a plug head. Extending axially from one end of the plug body is a rod which is advanced through the bore within the sleeve of the valve housing. The rod is coupled to an actuator which reciprocally moves the valve plug between a closed position and an open position. The engagement of the plug to the seating surface defined by the seat ring effectively blocks the flow of fluid out of the interior of the valve cage. Fluid flows into the interior chamber and thereafter radially through the valve cage from the exterior to the interior thereof. When the valve plug is moved from its closed position towards its open position, fluid is able to flow downwardly through the seat ring and out of the fluid control valve. The plug head may comprise a plug head of a first embodiment or a plug head of a second embodiment. Likewise, the seat ring may comprise a seat ring of the first embodiment or a seat ring of the second embodiment. The plug head and seat ring of the first embodiment are configured for use in conjunction with each other as are the plug head and seat ring of the second embodiment.
The seat ring of the first embodiment includes first, second, and third sealing d
Miller Stanley F.
Sterud Curtis George
Control Components Inc.
Hirsch Paul J.
Stetina Brunda Garred & Brucker
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