Pumps – Motor driven – With means to prevent heat transfer between pump and motor
Reexamination Certificate
2000-03-07
2001-12-11
Walberg, Teresa (Department: 3742)
Pumps
Motor driven
With means to prevent heat transfer between pump and motor
C417S366000, C415S180000, C376S404000
Reexamination Certificate
active
06328541
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
This invention relates to pumps utilized to circulate coolant water in nuclear reactors. More particularly, it relates to a thermal barrier which protects pump seals and bearings from the hot reactor coolant water and to a pump incorporating such a thermal barrier.
2. Background Information
The pumps which circulate coolant water through a nuclear reactor are subjected to harsh conditions. The reactor coolant water in a pressurized water reactor (PWR) is typically at a pressure of about 2,250 psi and a temperature in excess of 500 degrees Fahrenheit. The bearings and seals for the pump shaft are protected from these conditions by a thermal barrier. A common type of thermal barrier includes a cylindrical cover which seats in a recess in the pump housing where the pump shaft extends into the pump chamber. This cover has an end wall through which the pump shaft extends into the pump chamber. Coolant water is injected through a flange on the opposite end of the cover seated in the pump housing and flows outward into the pump chamber through a clearance between the pump shaft and the opening in the end wall of the cover. As a back-up to the cooling provided by this injected water, a stack of pancake cooling coils encircle the shaft under the cover. Inlet and outlet sections of the pancake cooling coils extend axially from the periphery of the coil stack and through the cover flange. A separate supply of cooling water can be circulated through this closed loop system. Additional thermal protection is provided by an annular insulator disposed against the inner surface of the cover sidewall. Such thermal barriers maintain the temperature of the water inside the cover well below the 550° Fahrenheit of the reactor coolant water being pumped and also below the 220° Fahrenheit maximum temperature for the seals and bearings.
However, after many years of service some cracks have developed at the intersection between the end and side walls of the cover, in the weld between the cover side wall and flange, and in the flange adjacent to the pancake cooling coil inlet and outlet penetrations and the penetration for the injection water.
There is a need therefore for an improved thermal barrier for reactor coolant pumps and a reactor coolant pump incorporating such an improved thermal barrier.
SUMMARY OF THE INVENTION
This need and others are satisfied by the invention which is based on the recognition that the current configuration of thermal barriers for reactor coolant pumps results in poor mixing of the relatively cool injection water that enters the cover at about 130° Fahrenheit and at a flow rate of about 8 gallons per minute, and the hotter (about 180° Fahrenheit) water within the thermal barrier. The flow stratification created as a result, exposes the inside walls of the thermal barrier cover to fluctuating water temperatures. The higher the steady state wall temperature of the thermal barrier, the worse are the effects of the water temperature fluctuations in inducing cyclic thermal stresses in the barrier. Vortices caused by the high-speed rotation of the pump shaft contribute to the uneven temperature distribution across the end wall of the thermal barrier cover. Finally, it has been determined that gaps open between the internal can insulator and the inner surface of the cover wall thereby aggravating the thermal fluctuation effects.
Accordingly, in the thermal barrier of the invention a generally cylindrical cover has an inner surface which is complimentary to the irregular peripheral surface of the pancake cooling coil stack resulting from the axially extending peripheral inlet and outlet tubing of the pancake cooling coils. This minimizes the free flowing water volume in the annulus between the stack of pancake cooling coils and the inside surface of the cylindrical cover to reduce the tendency for flow stratification and to increase flow turbulence which produces better mixing of the hot and cold streams.
As another aspect of the invention, a collar extends along the pump shaft from the end wall of the generally cylindrical body to prevent vortices from developing between the end wall and the stack of pancake cooling coils. This collar has a plurality of circumferentially distributed radially extending through holes. Preferably, this collar is combined with an annular shim disposed between the end wall of the generally cylindrical cover and the stack of pancake cooling coils to preload the coils. In this arrangement the collar assures centering of the shim.
As an additional aspect of the invention, the internal can insulator is eliminated in favor of an external insulator which extends circumferentially around and axially along at least a portion of the generally cylindrical cover. The external insulator comprises an external sleeve forming, with the generally cylindrical cover, an annular chamber containing substantially stagnant reactor coolant water. Preferably, a plurality of concentrically disposed annular cans divide the annular chamber into a plurality of concentric sections each containing reactor coolant water. The annular chamber, and therefore the concentric sections, communicate with the pump chamber sufficiently to equalize the pressure but yet maintain the coolant chamber water substantially stagnant.
The external sleeve is shrink fit onto the generally cylindrical cover and fixed in space by axially spaced shoulders on the cylindrical cover. Preferably, the sleeve has a lower coefficient of thermal expansion than the cylindrical cover.
REFERENCES:
patent: 4886430 (1989-12-01), Veronesi et al.
patent: 5118466 (1992-03-01), Raymond et al.
patent: 5604777 (1997-02-01), Raymond et al.
Drake James A.
Veronesi Luciano
Campbell Thor
Walberg Teresa
Westinghouse Electric Company LLC
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