Hydraulic turbine

Abstract

A hydraulic turbine includes a rotor with a runner, which is concentrically surrounded by a stator, whereby the runner comprises a plurality of runner blades arranged and distributed in a ring around a rotor axis, and each runner blade extends between a runner crown and a runner band; whereby the stator comprises a plurality of guide vanes arranged and distributed in a ring around the rotor axis, and each guide vane extends between an upper stator ring and a lower stator ring; and whereby a predetermined clearance is provided at least between the runner band and the lower stator ring. A substantial reduction of pressure pulsations in the vane-less gap between said runner blades of said runner is achieved by substantially increasing said predetermined clearance.

Claims

1. A hydraulic turbine, comprising: a runner comprising a runner crown, a runner band, and a plurality of runner blades extending between the runner crown and the runner band; a stator which surrounds the runner, the stator comprising an upper stator ring, a lower stator ring and a plurality of guide vanes extending between the upper stator ring and the lower stator ring; and a clearance between the runner and the stator, the clearance being arranged to minimize pressure pulsations developing in a vane-less gap between the plurality of runner blades and the plurality of guide vanes, whereby the clearance comprises: a first clearance between the runner crown and the upper stator ring, a first ratio being defined between the first clearance and a radius of the runner crown; a second clearance between the runner band and the lower stator ring, a second ratio being defined between the second clearance and a radius of the runner band, wherein the first ratio is less than or equal to 0.02 and the second ratio is greater than or equal to 0.02.

2. The hydraulic turbine of claim 1, wherein the first ratio substantially equals the second ratio.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is to be explained by means of different embodiments and with reference to the attached drawings:

(2) FIG. 1 shows a typical hydraulic turbine of the Francis type;

(3) FIG. 2 shows as a magnified detail the gap between runner and stator of the turbine according to FIG. 1;

(4) FIG. 3 shows the pressure fields and pressure pulsations in the gap of FIG. 2 for a prior art design;

(5) FIG. 4 shows—in contrast to FIG. 3—the pressure fields and pressure pulsations in the gap of FIG. 2 for an increased gap in accordance with an embodiment of the invention;

(6) FIG. 5 shows various geometrical parameters used for specifying the clearance increase according to an embodiment of the invention; and

(7) FIG. 6 shows an exemplary reduction in pressure pulsation amplitudes due to the proposed clearance increase.

DETAILED DESCRIPTION OF THE INVENTION

(8) The known solutions to reduce the pressure fluctuations are essentially focused on the design and the shape of components close to the vane-less gap (VG) area.

(9) Among the most known and identified methods of reducing pressure fluctuations are: changing the blade number and/or the guide vanes number; changing the runner blade diameter, the inner diameter of the guide vane and/or the pitch diameter; changing the design of the runner at the turbine inlet, for example the shape of the leading edge (parabolic shape or linear), the thickness of the blade and/or the curvature of blades.

(10) Some of these solutions are identified in the following publication: Zhigang et al. Pressure fluctuations in the vane-less space of high-head-pump-turbines—A review. Renewable and Sustainable Energy Reviews. 41 (2015) 965-974.

(11) The above ways have counter effects like hydraulic efficiency decrease, hydraulic instabilities increase and thus a trade-off shall be found to reach minimum pressure pulsation levels.

(12) An embodiment of the present invention significantly increases the mechanical clearance between rotor and stator (runner and ring) compared to prior art standards. Tests performed during two projects for two very different runners have shown impressive results when the clearance is increased with respect to prior art systems. The increased clearance acts as a damper for pressure pulsation, reducing the overall level of pressure pulsation.

(13) One of the major advantages is the ease to set-up this solution, and also that it has only few drawbacks.

(14) The damping effect of the substantial clearance increase is illustrated in FIGS. 3 and 4.

(15) In FIG. 3, the clearance C1 between runner (runner band 18) and stator (lower stator ring 20) is small in accordance with prior art practice. The relative motion between guide vanes 15 and runner blades 16 generates a pressure field with pressure pulsations 22 coming from guide vanes 15 and a respective pressure field coming from the runner blades 16 (arrows in FIG. 3(a)). The interaction of both pressure fields increases the pressure (P(t) in FIG. 3(b)), which can be measured by suitable pressure sensors 21.

(16) Increasing significantly the radial clearance C1.fwdarw.C2 (FIG. 4(a)) has a damping effect as the pulsations can easily escape from the vane-less gap VG through widened clearance C2. The measured pressure pulsations are then drastically reduced (P(t) in FIG. 4(b).

(17) A respective clearance increase has been performed during the development tests of an actual project (nominal head of 305 m) and has shown impressive results on pressure fluctuations level on the whole range of head (see FIG. 6), especially at part load (0.5 on x-axis) where the level decreased from 20% to 10%.

(18) It can be seen also on a hill chart of pressure fluctuations that the effect of the proposed greater clearance C appears not only on the whole range of head but also especially for medium (50%=130 MW) to low output. This is particularly interesting for applications that specify very low values from partial load to speed no load.

(19) FIG. 5 shows crown clearance CC between upper stator ring 19 and runner crown 17 and band clearance CB between lower stator ring 20 and runner band 18. Although shown as equal in FIG. 5, CC and CB may be different. Related to crown clearance CC is the crown radius RC. Related to band clearance CB is the band radius RB.

(20) In prior art machines a typical CC/RC ratio=CB/RB ratio is about 0.007 (<0.012). According to embodiments of the present invention the increase in clearance may be specified to be CC/RC ratio=CB/RB ratio >0.02 (tested 0.0224 and 0.0298).

(21) However, both ratios CC/RC and CB/RB need not be equal, but may differ, while both ratios are higher than 0.02 (CC/RC CB/RB and CC/RC >0.02 and CB/RB >0.02).

(22) Furthermore, the ratio CC/RC between said first clearance CC and said first radius RC may be ≤0.02, while the ratio CB/RB between said second clearance CB and said second radius RB may be ≥0.02.

(23) Alternatively, the ratio CC/RC between said first clearance CC and said first radius RC may be ≥0.02, while the ratio CB/RB between said second clearance CB and said second radius RB may be ≤0.02.

(24) Advantages of embodiments of the present invention include: The proposed solution can be generally applied to hydraulic pumps/turbines. It can also be applied to Francis turbines (mainly for high head machines). It gives an immediate gain without additional costs. It is easy to set up. There is no particular constraint as it is directly taken into account in the mechanical design. For an installed base there is only to increase the clearance gap if this is enough (fast and cheaper). For new projects: There is an additional and positive effect for a design dedicated on the decrease of pressure fluctuation level in the vane-less gap.

(25) This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.