Rotary kinetic fluid motors or pumps – Working fluid passage or distributing means associated with... – Pump outlet or casing portion expands in downstream direction
Reexamination Certificate
2000-03-20
2002-07-16
Verdier, Christopher (Department: 3745)
Rotary kinetic fluid motors or pumps
Working fluid passage or distributing means associated with...
Pump outlet or casing portion expands in downstream direction
C415S211200, C415S220000, C415S226000, C415S914000, C060S692000, C060S694000, C060S697000
Reexamination Certificate
active
06419448
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to steam turbines in general and more particularly to downwardly discharging exhaust hoods for such turbines and more specifically to effecting a decrease of pressure and energy loss in the top portion of such exhaust hoods.
2. Discussion of Prior Art
The steam leaving the last row of blades of steam turbines used in power generation generally flows through an annular passage between the turbine enclosure or casing and the bearing cone into a collector called an “exhaust hood”, from which it discharges into a condenser. The most prevalent type of the exhaust hood is one of “downward-discharging” design in which the condenser is located below the exhaust hood. This arrangement saves floor space in a power station, but has disadvantages so far as efficient flow in the exhaust hood itself is concerned.
At the top of a downward-discharging exhaust hood, steam leaving the last row of blades of a turbine and after passing through the usual annular flow diffuser follows a tortuous path on its way to the condenser. It is forced by the end and outer walls of the exhaust hood to turn back, that is, to turn in a direction which is essentially opposite to that in which it leaves the turbine. Subsequently, it is further turned downwardly and directed around the turbine casing so that it exits the exhaust hood in a predominantly downward direction as it enters the condenser. A considerable amount of the steam passing through the upper portion of a downwardly discharging exhaust hood therefore changes direction from the time it leaves the annular flow diffuser to the time it reaches the condenser by as much as 270 degrees or more. In contrast, in the bottom portion of the exhaust hood steam exhausting from the annular flow diffuser changes its flow direction only from mainly horizontal to vertically downward or essentially 90 degrees. Consequently, while at the top of the exhaust hood most of the flowing steam changes its direction by approximately 270 degrees before it becomes oriented toward the condenser, followed by additional changes in flow direction caused by the necessity to flow around the turbine casing before it actually enters the condenser, its direction is changed by only about 90 degrees in the bottom of the exhaust hood before entering the condenser. Every turn of a stream of steam, or change of its flow direction, entails a change of its linear momentum, which requires forces to be exerted by the containing walls on the flowing steam causing an increase of steam pressure. As a result, the pressure in the top or upper portion of a downward discharging exhaust hood is appreciably higher than in the bottom portion. This pressure rise in the top portion of the exhaust hood increases the back pressure of the steam turbine and thus decreases the energy available to the turbine to generate power and consequently lowers such turbine's overall efficiency.
It should also be noted that the turn, or change of direction, of the steam between the end wall and the outer wall of an exhaust hood takes place mainly in the corner region which exists between these two walls. In that corner region the flow separates from the end wall and forms what is known as a “separated-flow region” in which significant kinetic energy loss occurs as a result of friction. A tightly spiraling separated flow, or vortex, tends to form, which vortex extends in the outer corner region from the top and on both sides of the exhaust hood toward the condenser flange in the general shape of a horseshoe. In addition, the steam flowing initially along the bearing cone separates in the vicinity of the corner which usually exists between the bearing cone outside surface and the exhaust hood end wall forming another “separated-flow region” filled by another vortex of steam. Energy loss occurs in both of these “separated-flow regions” as a result of friction of the vortex steam with the walls of the exhaust hood as well as with the passing steam.
Attempts have been made to eliminate the “separated-flow regions” in the corners of exhaust hoods by placing curved walls or plates over such corners to more smoothly direct the steam flow past them and thus to improve turbine efficiency.
The flow by-pass system of this invention not only eliminates the “separated-flow regions” and associated energy losses, but, by allowing a significant fraction of the steam which enters the top quadrant of the exhaust hood to by-pass the top and back portions of the exhaust hood there is a resultant decrease of the pressure rise there as a result of decreasing the amount of the turning flow stream as well as its velocity, further significantly improving turbine efficiency. As was stated earlier, lowering of pressure at the exit of a turbine results in an increase of the energy available to the turbine. Lowering of the flow velocity results in lowering of friction losses which are proportional to the square of the flow velocity. There has been a need, therefore, not only for elimination of the “separated-flow regions” in the corners adjacent to the exhaust hood end wall but also a need to lower the steam pressure and the flow velocity in the upper or top portion of the exhaust hood.
Attempts have been made in the past to decrease energy losses in steam turbine exhaust hoods. For example, U.S. Pat. No. 1,269,998 issued Jun. 18, 1918 to K. Baumann, assigned to Westinghouse, U.S.A., entitled “Steam Turbine” discloses a steam turbine having a downwardly curved exhaust at the end of the turbine. In order to better control the flow of steam to the condenser and avoid backup of steam due to vortices and the like, the steam flow is divided up into at least upper and lower streams by partitions or baffle plates. This avoids, it is said, the steam from various portions of the turbine and particularly the top portion and bottom portion from meeting each other at different angles, or from different directions, causing eddies and the like which would interfere with rapid exhaust of steam from the turbine. The uniformity of the travel passage of the steam from the turbine when it enters the condenser or exhaust is thus enhanced. This disclosure does not show an exhaust hood installation directly over a condenser, but is an early example of the widespread continuing practice of using guide vanes to aid in directing turbine exhaust flow.
U.S. Pat. No. 3,791,759 issued Feb. 12, 1974 to J. A. Tetrault, assignor to the U.S. Government, entitled “Turbine Pressure Attenuation Plenum Chambers” discloses in a gas turbine the use of pressure attenuation chambers adjacent the exit from the turbine blades which are followed by stationary vanes. Excess gas pressure causes leakage of flow through orifices into such chambers when pressure rises excessively. The reference broadly illustrates the temporary withdrawal of gas from the exhaust to equalize pressure with the intent of trying to reduce circumferential pressure distortion in a turbine which is not provided with a downwardly discharging exhaust hood.
U.S. Pat. No. 3,149,470 issued Sep. 22, 1964 to J. Herzog, assignor to General Electric Company entitled “Low Pressure Turbine Exhaust Hood” discloses a hollow, substantially frusto-conical, flow dividing member disposed inside an exhaust hood co-axial with the turbine rotor which divides the flow from the turbine casing outlet into radially inner and radially outer annular portions which are further sub-divided by additional substantially radial flow guiding walls which form a number of parallel passages leading toward the exhaust hood outlet. One of the two flow annuli is formed between the circular opening of the flow-dividing member and the outer flow guide extending from turbine casing. The other annulus is formed between the circular opening of the flow dividing member and the bearing cone. This prior invention has little if any relation to the present invention in which only a small fraction of the total turbine exhaust flow is by-passed from the top quadrant of the exhaust hood to the bottom p
Verdier Christopher
Wilkinson Charles A.
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