US3498728A

US3498728A - Steam turbines

Info

Publication number: US3498728A
Application number: US748474A
Authority: US
Inventors: Michael Roger Wood
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1967-12-08
Filing date: 1968-07-29
Publication date: 1970-03-03
Anticipated expiration: 1987-03-03
Also published as: GB1173929A

Description

M. R. WOOD swmm TURBINES March 3, 1970 2 Sheets-Sheet 1 Filed July 29, 1968 FIG March 3, 1970 M. R. WOOD 3,

STEAM TURBINES Filed July 29, 1968 2 Sheets-Sheet 2 United States Patent 3,498,728 STEAM TURBINES Michael Roger Wood, Whetstone, England, assignor to The English Electric Company, Limited, London, England, a British company Filed July 29, 1968, Ser. No. 748,474 Claims priority, application Great Britain, Dec. 8, 1967,

6,040/67 Int. Cl. F01d 1/02, /28

US. Cl. 415-168 7 Claims ABSTRACT OF THE DISCLOSURE To reduce erosion damage in steam turbines the fixed guide blade ring of at least one stage of one of the turbine cylinders includes a hollow outer annulus, each guide blade of the ring is provided with a slot in its trailing edge or concave surface, which slot communicates with a shaped passage opening into said hollow annulus, said hollow annulus communicating with a region which is at a lower pressure than the pressure of the steam passing over said guide blades, and the arrangement being such that steam flows through said slots in said guide blades into said hollow annulus.

This invention relates to steam turbines, arid is concerned with the problems of erosion damage in the cylinders, particularly the LP. cylinder, of such turbines. This problem arises as a result of minute water droplets forming within the steam flow, due to the steam stage point crossing the saturation line, which adhere to the surfaces of the blades of both the fixed guide blade rings and the rotor blade rings within said cylinders. On the fixed guide blades, the droplets accumulate into thin water films which run to the trailing edges of these blades. These films detach from the trailing edges, break up into drops within the wake behind the fixed blades, and impact upon the leading edges of the following rotor blades With a high relative velocity causing erosion damage. This problem has become more acute with the ever-increasing size, and tip velocities, of modern steam turbines.

An object of this invention is to provide a steam turbine in which this erosion damage is at least reduced.

In order that the invention may be readily understood, a steam turbine in which the fixed guide blade ring of the last stage of the LP. cylinder is constructed in accordance with the invention, will now be described, by Way of example, with reference to the accompanying drawings, in which:

'FIGURE 1 is a diagrammatic part cross-sectional view of said last stage, and

FIGURE 2 is a fragmentary sectional view on the line II-II of FIGURE 1.

Referring to the drawings, the fixed guide blade ring 1 of the last L.P. stage of the turbine comprises an inner solid annulus 2, an outer hollow annulus 3 and a set of equispaced radially extending guide blades 4 connected between said annuli. Each guide blade 4 is solid over most of its length and is provided near its outer end with a trailing edge slot 5 which communicates with a shaped passage 6 in the blade. If the blades are initially manufactured as solid blades, these passages may be formed by a chemical erosion process, or, alternatively, the blades may be made by an investment casting process, in which case the passages are formed during casting. The passages 6 open into the chamber 7 of the hollow annulus 3, and the shape of each passage is designed to maintain as high a steam velocity as possible therethrough in order that a maximum drag effect may be imposed upon water droplets entering through the slot 5. In the construction 3,498,728 Patented Mar. 3, 1970 shown, this shape is such that the ratio, at any select radius of the cross-sectional area of the passage 6 to the cross-section area of the slot 5 from its minimum radius to said selected radius is constant. The value of this constant could vary depending upon the pressure conditions of the cylinder in which the guide blade ring is located in the HR, LP. or LP. cylinder but would be within the range 1.2 to 2.0. In the embodiment the ring is located in an LP. cylinder having normal pressure conditions and the value of the constant would be approximately 1.4.

The chamber 7 in the hollow annulus 3 has a sloped inner surface 8 which runs into an annular channel 9 for a purpose to be hereinafter described. The hollow annulus 3 is connected to a region which is at a lower pressure than the pressure of the steam passing over said guide blades, and in this construction this region comprises the condenser (not shown) for the steam turbine which may conveniently be a radial condenser. The connection to the condenser is provided by a series of holes 11 extending through the wall of the hollow annulus and the wall 12 of the turbine, these holes being spaced around said annulus such that each hole 11 aligns with a guide blade 4 of the blade ring 1 (see FIGURE 1).

In this particular construction, strengthening lugs 13 are provided for the rigidity of the trailing edge slot 5 and are radiused at the inlet of the slot to increase the extraction efficiency.

In operation, steam flowing to the last L.P. stage of the turbine passes over the fixed guide blades 4 to the last stage rotor blade ring 15. Some of this steam is sucked through the trailing edge slots 5 of the guide blades 4 due to the pressure differential, and carries with it any water droplets formed on the concave or convex surfaces of the blades. Because of the shape of the pas sages 6, the steam fiow therethrough is maintained at a high velocity and thus a significant drag effect is imposed upon the water droplets. This is important particularly for the guide blades 4 of the ring 1 which are situated in a near vertical position in the top half of the machine where a maximum lift effect is required to extract the water droplets into the hollow annulus 3. Thus, in the upper half of the cylinder, the steam issues from the passages 6 into the chamber 7 of the hollow annulus 3, and thence through the holes 11 to the condenser and any water droplets carried by the steam into the chamber 7 fall onto the sloping floor 8 and into the channel 9, or directly into the channel 9. This extracted water then runs down the channel 9 under gravity into the lower half of the cylinder, where it accumulates with the water extracted in the lower half of the cylinder, and drains through the holes 11 in said lower half.

At present there is some uncertainty as to the respective quantities of water collected on the concave and convex surfaces of the last stage guide blades in a steam turbine and therefore the provision of trailing edge slots 5 as described hereinbefore has the advantage that water can be collected from both surfaces. In any case, if it is established that significant amounts of water form on both surfaces then the provision of these trailing edge slots is particularly advantageous, since otherwise slots would be needed in both surfaces, thus requiring more steam flow therethrough which would lead to a reduction in the efliciency of the turbine.

However, it is thought possible that the majority of the water is collected on the concave surface of the blades, and if it should be established that only a small percentage, for example, of the order of 10% or less of the total water formed on the blades, is formed on the convex surfaces, then the slots may be positioned in the concave faces rather than the trailing edges.

The arrangement of the present invention may be used in conjunction with other means for reducing the amount of water formed within the turbine. Thus, for example, it could be used in conjunction with the invention described in our pending British application No. 56,041/ 67, in which the blade tips of the rotor blade ring of at least the penultimate L.P. stage protrude into an annular cavity provided by the turbine casing, said cavity having a wall downstream of said ring which curves towards the blades, and a passage also downstream of said ring which extends through the turbine casing to a region which is normally at a lower pressure than the pressure of the steam passing over said rotor blades.

It will be appreciated that the shaped passage and S10 arrangement described above may be applied to the fixed guide blade rings of any number or all of the LP. stages. Additionally the arrangement may be used on the fixed guide blade rings of the HP. and/ or LP. cylinders of a turbine, particularly where the steam fed to these latter cylinders is initially wet, and hence erosion damage could be caused in said cylinders.

I claim:

1. A steam turbine of the kind in which the fixed guide blade ring of at least one stage of one cylinder of the turbine includes a hollow outer annulus connected to the tips of the guide blades and communicating with a region of lower pressure than the pressure of the steam passing over said guide blades, wherein the improvement comprises each guide blade of at least the upper half of said guide blade ring defining a slot and an inner shaped passage for comunicating with the interior of said hollow outer annulus to permit steam to flow, due to the pressure differential, through to the annulus via said slot and said shaped passage, each passage being shaped such that the ratio, at any selected radius, of the cross-sectional area of the passage, to the cross-sectional area of its corresponding slot from its minimum radius to said selected radius is a constant within the range 1.2 to 2.0 whereby to maintain a high steam velocity through the passage for imposing a significant drag effect on any water droplets carried by said steam.

2. A steam turbine according to claim 1, wherein each said guide blade is solid over most of its length and said slot and shaped passage are provided in the bladeadjacent the outer radius and trailing edge thereof.

3. A steam turbine according to claim 1, wherein at least the fixed guide blade ring of the last stage of an LP. cylinder of the turbine is provided with said shaped passages.

4. A steam turbine according to claim 3, wherein the area ratio constant of the shaped passages is approximately 1.4.

5. A steam turbine according to claim 1, wherein all of said guide blades of the guide blade ring are provided with said shaped passages for comunicating with said hollow outer annulus which has a multiplicity of openings formed in its wall for comunication with said region of lower pressure, the number of openings being substantially equal to the number of guide blades of the guide blade ring and each opening being in alignment with a said guide blade.

6. A steam turbine according to claiml, wherein that part of each guide blade wall having said slot formed therein is the trailing edge of said blade, and strengthening lugs are provided over the length of said slots.

7. A steam turbine according to claim 1, wherein the hollow outer annulus has a sloped inner surface into which the shaped passages of the guide blades open, and an annular chanel whereby any water droplets carried by steam issuing through said shaped passages of the guide blades into the upper half of the annulus are collected in said annular channel where they run down to the lower half of the annulus and are drained.

References Cited UNITED STATES PATENTS 741,776 10/1903 Dodge 25376 x 2,362,831 11/1944 Kraft 253-76 3,301,529 1/1967 Wood 25376 FOREIGN PATENTS 316,381 8/1929 Great Britain.

343,407 2/l 931 Great Britain.

EVERETTE A. POWELL, JR., Primary Examiner