Seal for a joint or juncture – Seal between relatively movable parts – Relatively rotatable radially extending sealing face member
Reexamination Certificate
1999-12-17
2002-04-16
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Relatively rotatable radially extending sealing face member
C277S358000, C277S370000
Reexamination Certificate
active
06371488
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to sealing systems for effecting a seal between a shaft operating in a closed high pressure system and a housing, and is more particularly concerned with shaft seals employed in extremely high pressure systems such as in a main coolant pump used in a nuclear power plant.
2. Description of the Prior Art
In pressurized water nuclear power plants a reactor coolant is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator and produce electricity. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump. Systems of this type normally operate at pressures in excess of 1000 psi; in a pressurized water nuclear power plant, the system pressure during operation is significantly in excess of 1000 psi.
A reactor coolant pump typically is a vertical, single stage, centrifugal pump, designed to move large volumes of reactor coolant at high temperatures and pressures—for example, 550° F. and up to 2500 psi. The pump basically includes an intermediate hydraulic shaft seal section located between a lower hydraulic impeller section and an upper motor section. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within a pump casing to pump reactor coolant around the respective loop. The upper motor section includes a motor which is coupled to and drives the pump shaft.
Above the hydraulic section are an internal thermal barrier and external cooling coils which serve to cool system liquid passing therealong. Above the thermal barrier there may be additional clean liquid injected into the housing which is at lower temperature and therefore isolated from the system temperature by the thermal barrier and cooling coils. Sleeve bearings are also provided for the motor and pump shafts, as well as appropriate thrust bearings for the latter, none of which form a part of this invention.
In accordance with the prior art, the intermediate or shaft seal section includes a plurality of vertically separated tandem sealing assemblies, more particularly a primary sealing assembly located at the lower end of the shaft adjacent to and above the pump casing; above the latter is the secondary (back-up) sealing assembly; and above the latter is an upper or tertiary sealing assembly. The sealing assemblies are located concentric to and near the top end of the pump shaft and in a housing which is positioned above the pump impeller. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor cooling system to prevent leakage along the pump shaft to the reactor containment during both normal and abnormal operating conditions.
The lower, primary sealing assembly is the main seal of the pump. It is typically a hydrostatic, radially tapered, “film-riding,” controlled-leakage seal, whose primary components are an annular runner which rotates with the pump shaft and a non-rotating annular seal ring which is sealingly mounted to the housing of the lowest seal assembly. The initial design of such a primary seal is described in the afore-mentioned cross-referenced U.S. Pat. No. 347,552 of E. Frisch and has been subsequently modified in its details by, for example, U.S. Pat. No. 3,522,948 of A. N. MacCrum, also referred to above. The primary (or No. 1) seal causes a pressure drop of coolant water from about 2250 psi to 30-50 psi across its face. It allows a flow-rate of 1-3 gallons per minute therethrough. The liquid coolant leaking through the No. 1 seal, now normally at a much lower pressure, flows up the upwardly extending shaft and within the seal housing to a region of the middle, or back-up, sealing assembly. The latter sealing assembly (or No. 2 seal), in accordance with the prior art, has been a rubbing face-type seal. Its primary components have been a rotating runner having an upwardly facing sealing surface and a non-rotating, axially mounted ring located above the runner. During normal operation, this ring and runner provide a rubbing seal. In the unlikely event of No. 1 seal failure, however, the distribution of pressure on the No. 2 seal ring and runner causes them to act as springs and to deflect around their respective centroids, the ring deflecting in a counterclockwise direction and the seal runner deflecting in a clockwise direction, in such a way as to create the converging gap of a hydrostatic, film-riding, face-type seal. In accordance with the prior art (see U.S. Pat. No. 4,961,678, Column 2), as a film-riding face-type seal, the No. 2 seal has the entire high system pressure across it during this emergency condition. During normal operation, however, much of the No. 1 seal leak-off is diverted to a leak-off system. The remaining portion of the coolant passes through the No. 2 seal, leaking at a flow rate of approximately 2 gallons per hour at a pressure across the No. 2 seal of about 30 psi on the higher pressure (inlet) end, which is reduced by the No. 2 seal to 3-7 psi on the lower pressure (outlet) end. The still lower-pressure coolant water leaking through the No. 2 seal flows farther up the shaft and through a region of the upper tertiary (No. 3) sealing assembly.
In accordance with the prior art, the upper or tertiary sealing assembly (or No. 3 seal) has been a rubbing face-type seal, its primary components also being a rotating runner and an axially movable, non-rotating ring. Most of the flow leaking from the No. 2 seal is diverted by the No. 3 seal out through the No. 2 seal leak-off. The rubbing face-type No. 3 seal has been in one of two forms: either it has a double dam seal with two concentric sealing faces, or it has a single dam. The normal, minimal leakage from the No. 3 seal is designed to pass through a No. 3 leak-off system to the containment atmosphere, a situation that reactor systems designers would like to avoid, if possible.
In many nuclear power plants, a material which suppresses neutrons is dissolved into the reactor coolant water. This is normally boric acid enriched with B
10
isotope, which acts as the neutron suppressor. The amount of boric acid that can be retained in the reactor system coolant is pressure and temperature dependent. Thus, when the pressure in a reactor system region drops such as occurs across the No. 1, No. 2, and No. 3 seals, the amount of dissolved boric acid in the coolant frequently may not be totally retained in the coolant; thus it may precipitate out into the seal gap to interfere with the effective closing of the seal. To prevent this occurrence, the additional liquid injected into the seal region is intentionally boron-free; because the area of contact between the system liquid and this additional liquid is so small, only a minimal amount of dissolved boron in the system liquid will mix with the added clean liquid.
While it is highly unlikely that there will be a failure of the No. 1 seal, a proposed failure is postulated for safety review purposes by the regulators of nuclear power plants. Thus it is important that back-up systems be provided for this unlikely event. Similarly, it is unlikely that the No. 2 seal could fail either by itself or concurrently with the No. 1 seal; that event, however, is also a postulated safety event and the No. 3 seal is provided to accommodate a breakdown of the No. 2 seal alone. In the event both the No. 1 and No. 2 seals fail, either concurrently or in sequence, while the plant is still under pressure, the No. 3 seal will be required to accommodate full system pressure thereacross. As will be discussed, it is an objective of this invention to provide for a back-up sealing arrangement which can accommodate full system pressure in the highly unlikely event that Nos. 1 and 2 seals fail at the same time, which event may be postulated, nonetheless, by certain regulatory bodies.
Another significant requirement of reactor coolant sys
Jones Richard A.
Szymborski George E.
Dermer Zigmund L.
Knight Anthony
Peavey E.
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