CA1292950C - Apparatus and method for reducing effects of draft tube pressure fluctuations - Google Patents

Abstract

TITLE: APPARATUS AND METHOD FOR REDUCING EFFECTS OF DRAFT TUBE
PRESSURE FLUCTUATIONS
ABSTRACT OF THE DISCLOSURE
Apparatus and method for reducing the effects of draft tube pressure fluctuations acting on a runner in a hydraulic reaction turbine or the effects of suction tube pre-rotation acting on the impeller in a centrifugal pump. A closed surge suppression chamber is mounted about the exterior of the turbine draft tube or pump suction tube. A plurality of openings are provided about the exterior of the draft tube or pump suction tube communicating with both the interior of the draft tube or pump suction tube and the interior of the chamber near its bottom thereof. An air inlet is provided leading into the chamber at the top thereof. In operation, and during operation of the turbine or pump where a rotating helical vortex of water has formed in the draft tube or pump suction tube, water flows from the draft tube or pump suction tube through the openings and into the bottom of the chamber while air is injected into the top of the chamber. This forms an air bubble in the chamber and forces air through the openings into contact with the lower pressure center of the vortex as it rotates in the draft tube or pump suction tube abating the same.
This chamber also provides an air cushion to attenuate pressure surges which may be provided as the helical vortex rotates through the elbow in a hydraulic reaction turbine draft tube or pump suction tube, and it also provides an air cushion when the water vapor bubbles in the core of the vortex collapse.

Description

Z9~0 BACRGROUND OF q~ INV~TION
2FII~LD OF THE INV~TION
3The invention relates to hydraulic reaction turbines and 4centrifugal pumps; and, more particularly, to apparatus and 5method for reducing the effects of draft tube pressure 6fluctuations acting on a runner in a hydraulic reaction turbine 7or the effects of suction tube pre-rotation acting on the 8impeller in a centrifugal pump.
9D~SCRIPTION OF TH~ PRIOR ART
10Hydraulic reaction turbines have been known in the art for 11many years. In the early development of such turbines, it was 12discovered that turbines which were operated at heads and flows 13other than the design point of the turbine, experienced rough 14operation and power swings resulting from water passing out 15through the draft tube of the turbine in a whirling and 16spiralling vortex. ~his is particularly true of Francis-type 17turbines, propeller turbines and pump turbines. In April l9iO, 18W.J. Rheingans published a paper in the Transactions of the 19American Society of Mechanical Engineers in which he outlined 20the results of an extensive study of the draft tube surging 21phenomenon in turbines. In his paper, and in the comments from 22others, several schemes or suppressing draft tube surging were 23discussed; these included fins projecting from the draft tube 24walls, an extended runner cone filling the space occupied by 25the draft tube vortex, venting of the turbine head cover and 26air injection into the draft tube through the straightening 27fins and through a hole in the draft tube wall. Since that 28time there have been numerous papers on the subject and many 29suggested schemes to reduce draft tube surging. Such schemes 30have included air admission to the turbine runner, to the draft 31tube, to the lower side of the runner through the runner cone, 32or through holes in streamlined shapes placed across the draft 33tube. Air has also been injected into the penstock, into the 34space between the wicket gates and the runner, and through the 35runner band. There have also been many schemes for the use o 36fins in the draft tube; these include flow splitters and a 37coaxial hollow cylinder placed below the center of the runner .
lZ9Z~O
1 cone and supported by fins which can be rotated, and several 2 types of fins as discussed hereinabove.

3 When a hydraulic reaction turbine is operated at a head 4 and/or flow which is different from the design point, the water w:ill follow a helical path as it passes out through the draft 6 tube. If the operating conditions are sufficiently remote from 7 the design point, a vortex will form in the swirling water 8 just below the runner cone and this vortex usually will follow 9 a helical path as it passes out through the draft tube. The core of the vortex is usually filled with water vapor. The t 11 spiralling vortex causes pressure fluctuations which vary the 12 net head experienced by the runner and cause the generated 13 power to fluctuate in a similar manner. Net head is also 14 affected when the water vapor bubbles in the core of the vortex collapse.
16 A similar problem exists in centrifugal pumps. Centrifugal 17 pumps are designed to operate with a given quantity of flow and 18 against a given head. When a centrifugal pump is operated at 19 its design point, the water flowing through the machine is laminar, irrotationai flow, and the pump runs smoothly.
21 Centri~ugal pumps have been in use for many years. Early in 22 their development, it was known that prerotation occurred in 23 the suction tube~ if a pump was operated at a head andtor 24 quantity of flow different from the design values. This phenomenon,has bqen noticed especially in large pumps with 26 Francis-type impellers. It has also been suspected in some 27 large Deriaz-type machines. If the operating conditions are 28 sufficiently remote from the design point, a vortex will form 29 in the center of the water flowing in through the suction tube and usually will follow a helical path. This vortex flow 31 causes a rotating low pressure area under the pump impeller 32 which can induce destructive vibration of the pump, pump shaft, 33 motor shaft and the moto~
34 There is thus a need for reducing the effects of draft tube pressure fluctuations acting on a runner in a hydraulic 36 reaction turbine or the effects of suction tube pre-rotation 37 acting on the impeller in a centrifugal pump.

lZ9Z9~0 1 SUMMARY OF TH~ INVENTION
2 It is an object of this invention to provide improved 3 apparatus and method for reducing the effects of draft tube 4 pressure fluctuations acting on a runner in a hydraulic reaction turbine or the effects of suction tube pre-rotation 6 acting on the impeller in a centrifugal pump.
7 It is a further object of this invention to carry out the 8 foregoing object by providing an annular chamber surrounding 9 the draft tube with air and/or water from the chamber injected into the rotating low pressure area of the vortex in the draft 11 tube of a hydraulic reaction turbine or the suction tube of a 12 centrifugal pump.
13 It is still further an object of this invention to carry out 14 the foregoing objects by providing a plenum of air to cushion the pressure fluctuations as much as possible before they get 16 to the turbine runner of the turbine or pump impeller of the 17 pump and affect the operation of the same.
18 These and other objects are preferably accomplished by lg providing a closed surge suppression chamber about the exterior of the turbine draft tube or pump suction tube. A plurality 21 of openings are provided about ~he exterior of the tube 22 communication with both the interior of the tube and the bottom 23 of the chamber. An air inlet is provided leading into the 24 chamber at the top thereof. In operation, and during operation of the turbine or pump where a rotating helical vortex of water 26 has formed in the draft tube or pump suction tube, water flows 27 from the draft tube or pump suction tube through the openings 28 and into the bottom of the chamber while air is injected into 29 the chamber. This forms an air bubble in the chamber and forces air through the openings into contact with the lower 31 pressure center of the vortex as it rotates in the draft tube 32 or pump suction tube abating the same.
33 A pressure fluctuation causes the water level in the surge 34 suppression chamber to rise and fall, or fall and rise, as the air cushion compresses and expands to cushion the fluctuation 36 in the turbine draft tube pressure or pump suction tube 37 pressure.

2 FIG. 1 is an elevational view, partly schematic and partly 3 sectional, of a hydraulic reaction turbine system in accordance 4 with the teachings of the invention;
FIG. 2 is a section through the draft tube and suppression 6 chamber:
7 FIG. 3 is a schematic illustration of the helical effect 8 taking place in the draft tube of the apparatus of FIGS. 1 and 9 2; and FIG. 4 is a view similar to FIG. 2 showing a modification 11 thereof.

Z9~0 1DESCRIPTION OF THE PRFF~RRED EHBODIHENTS
2Referring now to FIG. 1, a hydraulic turbine system 10 is 3shown having a base foundation 11, an elevated source of water 412 and a lower tail water reservoir 13. Foundation 11 includes 5a generally vertical back wall 14 setting off the tail water 6in reservoir 13 with a floor 15 on base foundation 11 extending 7from wall 14.
8An intake structure 16 is provided on the foundation at the 9upper or elevated water source 12. An opening 17 is provided 10in structure 16 fluidly communicating with a downwardly slanted 11penstock 18 extending through structure 16. Penstock 18 leads 12through, and is supported in, an anchor block 19 and from said 13block 19 through the shut-off valve 20 of the turbine in system 14 10.
15From valve 20, penstock 18 extends to the hydraulic reaction 16turbine 21 of system 10 fluidly co D unicating with a draft tube 1722 which in turn communicates with the tail water reservoir 13 18through an opening 23 in wall 14.
19A turbine shaft 24 communicates with the runner (not visible 20in FIG. 1) in the interior of turbine 21, as is well known in 21the turbine art, an~ shaft 24 is coupled via coupling 25 to a 22generator shaft 26. Shaft 26 is in turn coupled to the rotor 23(not visible in FIG. 1) of generator 27 for driving the same.
24The foregoing describes conventional parts of a typical 25hydraulic reaction turbine system in a hydroelectric generating 26station and, as such, forms no part of the invention other than 27in the environment of draft tube pressure fluctuations acting 28on the runner of the turbine 21 as will now be descr-ibed.
29Thus, the turbine runner of the turbine 21 drives the turbine 30shaft 24 which in turn drives the generator shaft 26 through 31coupling 25. The shaft 26 drives the rotor of generator 27, 32which may be a synchronous generator, thereby generating 33electricity.
34Thus, as particularly contemplated in the present invention, 35draft tube pressure fluctuations reducing means 28 are provided 36for reducing the effect of draft tube pressure fluctuations 37acting on the runner of turbine 21. (See FIG. 2) lZ9Z950 1 As particularly seen in F~G. 2, such means 28 includes a 2 surge suppression chamber 29 surrounding draft tube 22 adjacent 3 turbine 21. Chamber 29 is toroidially-shaped and welded or 4 otherwise secured to the outer peripheral surface of draft tube 22. An air tank or air reservoir 30, or other source of air, 6 is provided having a pipe or conduit 31 leading to the outer 7 wall of chamber 29 and communicating with the interior 32 of 8 chamber 29 via opening 33. A conventional air pressure 9 regulator 34 coupled to tank 30 is provided for regulating the pressurized air therein. As seen in FIG. 2, a plurality of 11 spaced openings or holes 35 extend about the outer periphery 12 of draft tube 22 adjacent the floor or lower wall 36 of chamber 13 29. These holes 35 extend through the wall of draft tube 22 14 and serve to fluidly communicate the interior of draft tube 22 with the interior of chamber 29.
16 As seen schematically in FIG. 3, during operation of system 17 10, a vortex can form below the tip of the nose cone 37 of the 18 runner of the hydraulic reaction turbine 21. The center of the 19 vortex is a low pressure area which is usually below the vapor pre~sure of water at that temperature. The vortex can follow 21 a helical path in the interior 39 of draft tube 22. Air from 22 tank 30, regulated by regulator 34, is introduced into the 23 interior 32 of chamber 29 where it flows through holes 35 into 24 the interior 39 of draft tube 22 as indicated by arrow 40.
This air meets vortex 38 and abates the same. Simultaneously, 26 water 41, trapped at the bottom of chamber 29, flows with the 27 air through holes 35 back into the interior 39 of draft tube 28 22, as indicated by arrow 42 which is in essence a continuation 29 of 40.
The chambex 29 acts as a surge suppression chamber and said 31 means 28 performs two functions. First, it injects air or 32 water into the lower pressure center of the draft tube vortex 33 38 as vortex 38 rotates in itS peculiar spiral or helical path 34 inside of draft tube 22. Second, it provides an air cushion to dampen pressure pulsations travelling up through draft tube 36 22 to the hydraulic reaction turbine 21.
37 Although only a single series of holes 35 have been provided lZ9Z950 1 about the outer periphery of draft tube 22, obviously more than 2 one such row of holes or openings may be provided, such 3 plurality of rows may be of any desired configurations.
4 Further, as seen in FIG. 4 wherein like numerals refer to like parts of the apparatus of FIGS. 1 and 2, pipe 31 may be 6 replaced by a manifold 44 having a plurality of inlet pipes, 7 such as pipes 45 to 47, communicating with the interior of 8 chamber 48 which is divided into a plurality of interior 9 compartments 51 to 53 via annular partitions 49, 50. Each compartment 51 to 53 fluidly communicates with the interior of 11 draft tube 22 through spaced holes (rows 54, 55 and 56, 12 respectively), which holes open into each respective 13 compartment adjacent the bottom thereof, as shown. The 14 operation of the compartments and holes is identical to that discussed hereinabove with respect to the embodiment of FIGS.
16 1 and 2.
17 Water can flow through draft tube 22 in either vertical 18 direction depending on the operation. Thus, with regard to 19 FIGS. 1, 2 and 4, a hydraulic turbine has been discussed. In a centrifugal pump, water will flow in the opposite direction 21 up through the ~uction tube. In this case, the surge 22 suppression chamber 29 or means 48 again performs two 23 functions. The first function is to inject air or water into 24 the lower pressure center of the suction tube vortex as it rotates in its spiral path inside of the suction tube 22. The 26 second function is to provide an air cushion to dampen pressure 27 ~pulsations travelling up through the tube 22 (now a suction 28 tube) to the pump (instead of the turbine).
29 It can be seen that there is disclosed apparatus and method to suppress pressure pulsations in vertical hydraulic turbine 31 draft tubes and pump suction tubes.
32 Any suitable materials and dimensions may be used. For 33 example, chamber 29 ~idy be about 20% to 90% of the vertical 34 height of draft tube 22. The outer diameter of chamber 29 alone may be about twice the outer cross-sectional diameter of 36 tube 22. For example, the outer cross-sectional diameter of 37 tube 22 may be about 4 feet with an overall height of 20 feet.

1~92~SO

1 In that case, the outer diameter of chamber 29 may be about 7 2 feet with an overall height of about 6 feet. The holes may 3 then be about 2 inches to 4 and 1/2 inches in diameter and 4 spaced about 1/2 inch apart.
The regulator 34 is used to control the pressure of air 6 irljected into each chamber 29 or 48. Preferably, the air 7 pressure therein is maintained a few psi above the dra~t tube 8 pressure at its point of connection to the surge suppression 9 chamber. This will feed air into the vortex spiral as it rotates within the tube and reduce the pressure disturbance.
11 The tail water elevation at reservoir 13 determines the 12 pressure at the point of connection 35 of tube 22 to the surge 13 suppression chamber 29. If the tail water elevation is low 14 enough, the pressure will be below atmospheric pressure and the surge suppression chamber needs only to be left open to the 16 atmosphere to aspirate air through a pipe of sufficient size.
17 If tail water is high, the chamber pressure is preferably set 18 a few psi above the draft tube pressure at the interconnecting 19 point to the turbine. The lowest chamber pressure which will suppress the effects of the draft tube surge is generally the 21 mo~t desirable. This pressure will rarely be more than 10 psi 22 above draft tube pressure.
23 If the tail water is high enough that air must be injected, 24 it is most economical to limit air injection through the surge suppression chamber to those operating regimes where it is 26 really needed to stabilize operation. The control device can 27 be anything from a valve which is opened and closed as the unit 28 servomotor passes certain locations in its stroke, to a 29 computerized control system which would use sensors to measure the disturbance in the generator output which was being caused 31 by the draft tube surge and regulating the volumé and pressure 32 of the air being sent to the surge suppression cha~ber.
33 Although .he foregoing discusses a turbine, similar effects 34 take place in a pumping operation. However, most problems of this nature as discussed herein occur in a turbine or 36 generating operation.
37 The size and number of air entry points or holes depends on 12~Z9~0 1 the diameter of the draft tube at the interconnecting point to 2 the turbine draft tube, the rotating speed of the machine, the 3 magnitude of the draft tube surge which is expected and the 4 size of the surge suppression chamber. The Rheingans formula for frequency of draft tube surges is still considered a good 6 means for estimating surge frequency. It is frequency = ~
7 ~n~, For the surge suppression chamber to attenuate the 8 pressure pulsations caused by draft tube surging, the total g aréa of the ring of holes must be sufficiently large to accommodate water flow into the surge chamber and out again 11 within the time allowed by the draft tube surge frequency. For 12 example a 200 RPM machine could experience a draft tube surge 13 at frequency = ~ = 55.6 cycles/minute.
14 Air can be injected in any desired quantity, such as between about 0.05 to 2% of the unit flow. With peripheral air 16 injection as disclosed herein, the volume of air at standard 17 temperature and pressure injected may be as low as 0.05~ of the 18 unit flow. The least quantity of air injection required to 19 suppress the draft tube surges is desirable and the operator can observe the conventional wattmeter on the unit control 21 board to determine such (or even by audible discernment, as 22 such surges are quite noisy). By placing the holes as close 23 as possible to the bottom of the surge suppression chambers, 24 the largest possible air bubble is created which cushions pressure pulsations in the draft tube. The air bubble or 26 bubbles act as a cushion or shock absorber and abate the 27 pressure wave before it reaches the turbine runner.
28 Preferably, air is injected into the holes continually when the 29 apparatus is running. Air pressure pulsations come up the draft tube and reaches the holes, the water carried thereby 31 sloshes through the holes into the surge suppression chamber 32 and the air cushion dampens the pressure pulsations.
33 It can be seen that there is disclosed a simple and 34 inexpensive process for abating the vortex created in a hydraulic reaction turbine or centrifugal pump. Although there 36 is disclosed a preferred embodiment of the invention, 37 variations thereof may occur to an artisan and the scope of the lZ9Z950 in~ion is to be limited only by the scope of the claims.

Claims (13)

1. Apparatus for reducing the effects of pressure fluctuations acting within a tube wherein a vortex has formed naturally during fluid flow through the tube comprising:
a closed surge suppression chamber having a bottom wall and a top wall mounted about the exterior of the tube having an opening leading into the upper end thereof adjacent the top wall;
a plurality of spaced apertures formed about the exterior of said tube fluidly communicating the interior of said tube with the interior of the surge suppression chamber adjacent to the bottom wall of said chamber; and air injection means communicating with said opening for selectively injecting air therein from any angle.

2. In the apparatus of claim 1 wherein said tube is a draft tube in a hydraulic reaction turbine having a runner therein, said tube being located adjacent said runner.

3. In the apparatus of claim 1 wherein said tube is a suction tube and located adjacent an impeller in a centrifugal pump.

4. In the apparatus of claim 1 wherein a plurality of said chambers are provided, each of said chambers surrounding said tube and having common walls interconnecting said chambers, each of said chambers having air injection means communicating with its respective opening and said tube having a plurality of spaced rows of said apertures fluidly communicating the interior of said tube with the bottom wall of each of said chambers.

5. In the apparatus of claim 1 wherein said air injection means includes regulating means for regulating the injection of air.

6. In the apparatus of claim 1 wherein said chamber is about twice the outer diameter of said tube and between about 20% to 90% of the overall height of said tube.

7. In the apparatus of claim 1 wherein the outer diameter of said tube is about 4 feet, the overall height of said tube is about 20 feet, the outer diameter of said chamber is about 7 feet and the overall height of said chamber is about 6 feet.

8. In the apparatus of claim 7 wherein said apertures are each about 2 inches to 4-1/2 inches in diameter and spaced about 1/2 inches apart.

9. A method for reducing the effects of pressure fluctuations within a tube in a turbine or pump system wherein a spiraling vortex of fluid is formed during fluid flow through the tube comprising the steps of:
forming a closed surge suppression chamber about the exterior of said tube;
fluidly communicating the interior of said tube with the bottom interior of said chamber; and injecting air into the top of said chamber thereby forming an air bubble in said chamber above a water reservoir formed in said chamber at the bottom thereof thereby abating said vortex.

10. In the method of claim 9 wherein the step of injecting air includes the step of injecting air at a pressure a few psi above the pressure within said tube adjacent its connection to the turbine or pump of the system.

11. In the method of claim 9 wherein the step of injecting air includes the step of injecting air in a quantity of between about 0.05% to 2% of the unit flow of fluid through said tube.

12. In the method of claim 9 wherein a pressure fluctuation in the hydraulic turbine draft tube or pump suction tube will cause the water level to rise and fall, or fall and rise, in the surge suppression chamber as the air in the chamber compresses and expands to cushion the pressure fluctuation.

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