US3306575A

US3306575A - Steam turbines

Info

Publication number: US3306575A
Application number: US435506A
Authority: US
Inventors: Frankel Adolf
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1964-03-05
Filing date: 1965-02-26
Publication date: 1967-02-28
Anticipated expiration: 1984-02-28
Also published as: CH434300A; FR1426543A; DE1426788A1; SE305220B

Description

United States Patent Ofiice Patented Feb. 28, 1967 3,306,575 STEAM TURBINES Adolf Frankel, Altrincham, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Feb. 26, 1965, Ser. No. 435,506 Claims priority, application Great Britain, Mar. 5, 1964, 9,389/ 64 12 Claims. (Cl. 253-76) This invention relates to improvements in steam turbines, and is directed towards the problem that modern steam turbines, in which the steam is expanding into the wet region, are liable to suffer from erosion near the tips of the blades of the low pressure rotor stages. This problem is pronounced in the last stages of most modern large turbines, in which :the blade tips run at a very high peripheral speed. This increased danger of erosion is due to the fact that erosion damage seems to rise with a fairly high power of the relative impact speed between the water drops and the blades, so that rotor blades running with higher peripheral speeds are much more liable to damage.

It has been found that the bulk of the moisture passing through the last stage of a turbine in which the expansion terminates in the wet region, is in the form of very small droplets, which are easily accelerated to a velocity approaching that of the steam, and can then pass through the rotor blades without doing any serious damage. Most of the damage seems to be caused by a small portion of the moisture, which portion collects on the surfaces of the stator blades preceding the last stage of rotor blades. This water drools off the trailing edge of these stator guide blades in an irregular manner, and as the surface of the stator guide blade provides the opportunity for coalescense into very large drops and even sheets of water, the water comes off the stator guide blade trailing edges in the form of very large drops. Because of their size, these drops do not get broken up and accelerated by the steam in the axial gap between the stator guide blades and the rotor blades. As a result, they travel towards the rotor blades with a relatively low velocity, which means that they get hit by the rotor blades with a relative velocity approaching the peripheral speed of the rotor blades, and this causes serious erosion damage. This process is particularly accentuated by the fact that moisture comes off the trailing edge of the stator blades into the wake of the blade, where the velocity and the kinetic energy of the steam are very low compared with the main steam stream, so there is a deficiency there of the energy which is required to accelerate and break up the water drops.

An object of the present invention is the provision of an improved steam turbine in which erosion in the later rotor stages is at least reduced.

According to the present invention, a steam turbine provided with circumferential rows of blades mounted on a rotor and with circumferential rows of guide blades mounted in a fixed casing and arranged respectively intermediate pairs of circumferential rows of blades on the rotor, is also provided with means by which steam can be discharged from port means in the trailing edges of at least the guide blades in the final circumferential row of guide blades and substantially in the direction of the main steam flow past the blades.

The invention will now be described, by way of example, with reference to the accompanying drawing, in which:

FIGURE 1 is a sectional side elevation of one guide blade of a steam turbine and of an adjacent rotor blade;

FIGURE 2 is a sectional plan view of the guide blade, taken on the line II-II of FIGURE 1;

- FIGURE 3 is a sectional plan view, drawn to a larger scale than FIGURE 2, of a trailing end or tip of the guide blade shown in FIGURE 2; and

FIGURE 4 is a sectional plan view, similar to FIGURE 3, but showing a modified construction.

Referring first to FIGURES 1 to 3, a steam turbine guide blade diaphragm 1 includes an inner ring 3 and an outer ring 5 between which extends a multiplicity of fixed guide blades 7. Such an arrangement is common practice in large steam turbines, and the diaphragm is divided along a horizontal plane containing the bearings of the rotor 9 of the turbine, to permit installation and removal of that rotor. The rotor 9 carries a plurality of bladed stages, each consisting of a circumferential row of rotor blades, and the drawings show one of the blades 15 of the last bladed stage. These drawings are largely diagrammatic, and irrelevant details as to the fixing of the blades will not be described.

The guide blade 7, and in fact each guide blade of this circumferential row of guide blades, is hollow, being formed from a bent strip of material, and the trailing edge 7B of the blade forms a nozzle 17. At its radially outer end the hollow interior of guide blade 7, and those of the other guide blades in the same circumferential row, are in communication with a steam manifold 19 formed in the outer ring 5. The internal cross-section of this nozzle 17 can be divergent, as shown in FIGURE 3, or convergent, as shown in FIGURE 4. It will be seen that the guide blade 7 is curved in transverse cross-section, If desired the trailing edge of the blade can be thinned down by cutting it back from the outside, as shown by the dashed line in FIGURE 4. Although this thinning is shown only on the concave surface of the blade form, it can be applied to both sides or to either side of the blade.

The steam manifold 19 is connected by passages, indicated diagrammatically by a pipe 21, to a source of steam which is at a higher pressure than the steam in the region of the guide blade 7 during normal working of the turbine. A suitable source. of this steam could be a bleed point from a higher pressure stage of the turbine.

Because of the high pressure ratio at which such guide blades normally operate, it is however possible to supply the discharge port in the blade by steam from the entry to the stage. Although, due to the fact that it will suffer additional pressure losses compared with the steam flowing through the blade passage, the velocity of the steam issuing from the slot 17 is likely to be lower in such a case than the velocity of the main stream the effect aimed at by the invention will nevertheless still be present. As such an arrangement involves a smaller thermodynamic loss, in that the steam issuing from slot 17 has not bypassed any previous expansionstages, it is possible in such a case to use larger quantities of driving steam.

In the special arrangement referred to above, the'steam can enter each blade individually, so that the-collecting manifold is not necessary. If an entrance port 8 is provided in each hollow blade at or near the leading edges, and at a radial position at which the total pressure at the leading edge of the guide blade 7 is highest, this can help to achieve a steam velocity from slot 17 very similar to the main steam velocity.

In use of the turbine, steam from the pipe 21 flows through the manifold 19 into the blade 7, and the other blades in this circumferential row of blades, and is discharged through the nozzle 17. As a result of the arrangement shown, a thin flat jet of steam issues from the trailing edge of the stator blade. The slot is shaped in such a way that the jet is directed in the same direction as is the main stream of steam issuing from the stator blade. As mentioned above, there is a tendency for water drops to impact on the surface of the guide blade, there to coalesce, and to drool off the trailing edge of the blade as a sheet or as large drops. vThe 3 steam jet injects momentum into the wake leaving the trailing edge of the blade; it entrains the water coming off the trailing edge, and helps both to break it up into relatively small drops and to accelerate it to a velocity of the same order as the velocity of the main stream of steam. If this acceleration is achieved, even approximately, the relative impact velocity of the water on the rotor blades is very considerably reduced, resulting in great reduction of erosion damage, even if the drops are still fairly large in size.

The thickness of the steam jet is of the same order of magnitude as the thickness of the side edges of the nozzle 17, and all these dimensions are small compared with the axial distance between the trailing edge of the stator guide blade 7 and the leading edge of the rotor blade, and very much smaller still compared with the distance the steam has to travel obliquely, due to its leaving angle, before it reaches the leading edge of the rotor blade. As a result, there is plenty of length available for attenuation of the wake and for momentum transfer from the central jet of steam into the wake and into the water coming off the trailing edge of the blade 7.

By the'supply of steam to the interior of the blade at a higher pressure than the steam pressure at the entry to this particular stage or circular row of stator guide blades,'for example by steam bled from a previous stage, say the entry of the preceding stage, the velocity of the jet can be 'made higher than the velocity of the main stream of steam. Since, in modern large turbines, stator blades of the type illustrated Will generally operate at about the critical pressure ratio, the pressure ratio of the expansion of steam through this nozzle 17 is likely to be higher than critical. As a result the static pressure in the throat of the nozzle 17 will be higher than the mean static pressure in the surrounding steam, and the resulting expansion of width of the steam jet after it has left the nozzle 17, which is a well known effect in super sonic jets, will aid the momentum transfer and the acceleration and the break-up of the water drops or film.

The performance of the stator guide blade can be improved by shaping the inside of the guide blade in such a way as to provide a smooth acceleration of steam flowing towards the nozzle 17. FIGURE 4 shows such an arrangement. Alternatively, it may be advantageous, particularly when the pressure ratio across the slot is higher than critical, to shape the slot passage itself as a convergent-divergent nozzle, as shown in FIGURE 3,

to allow acceleration of the driving jet to supersonic 3 speed in the nozzle. Suitable rounding off, or tapering (as shown by the dashed line in FIGURE 4) of the outside Wall of the trailing edge of the guide blade, to thin down the trailing edge at the slot to bring the water coming oh" the blade surfaces as near to the trailing edge as possible, may also be advantageous. This tapering can also be applied to the other side of the nozzle. Further, both inside and outside of the nozzle can be tapered off in this manner.

Since the steam passing through the guide blade 7 to the nozzle 17 is hotter than the steam flowing over the outside of the blade, the blade metal can be heated well above the saturation temperature of the steam surrounding it in the main steam flow passages. This will result in partial evaporation of the water collecting on theblade surfaces, and also in a temperature gradient in the main steam flow surrounding the blade, reducing the flow of microscopic water drops towards the blade surface, due to thermal diffusivity effects. This heating effeet can be improved by adding partitions, ribs or baflles inside the guide blades so that the steam passing through the blade of the nozzle 17 follows an extended path.

The discharge of steam should take place over at least the radially outer end of the trailing edge of the guide blade and can, if desired, take place over the whole length of the trailing edge.

What I claim is:

1. A steam turbine comprising:

(a) a rotor including circumferential rows of rotor blades,

(b) a fixed casing including circumferential rows of guide blades respectively arranged intermediate pairs of circumferential rows of rotor blades,

(c) port means provided in the trailing edges of at least the guide blades in the final circumferential row of guide blades, and

(d) means arranged to convey steam to the port means, so as to permit the steam to be discharged from the port means and substantially in the direction of the main steam flow past the blades whereby the steam causes water which has condensed on said guide blades to break .into small drops and accelerates the droplets to substantially the same velocity as the main stream of steam.

2. A steam turbine according to claim 1, wherein the trailing edge of each guide blade is provided with a steam discharge port extending inwardly from a radially outer end of the blade.

3. A steam turbine according to claim 2, wherein the discharge port comprises a slot-like nozzle.

4. A steam turbine according to claim 1, wherein the leading edge region of each guide blade in the circumferential row includes further inlet port means connected to the port means at the trailing edge and effective to receive steam flowing through the turbine and discharge it through the port means at the trailing edge, whereby to increase the quantity of steam discharged from the port means in the trailing edge.

5. A steam turbine according to claim 1, wherein the radially outer ends of the circumferential row of guide blades are secured to an outer ring of a diaphragm attached to the turbine'casing and the outer ring includes a steam manifold connected to a suitable steam supply source and effective simultaneously to supply steam to the port means in each of the blades in the circumferential row.

6. A steam turbine comprising;

(a) a rotor including circumferential rows of rotor blades,

(b) a fixed casing including circumferential rows of guide blades respectively arranged intermediate pairs of circumferential rows of rotor blades, each of the guide blades in at least the final circumferential row of guide blades being hollow and comprising two appropriately shaped metal sheets respectively forming opposite faces of the blade,

(c) at least a part or parts of the adjacent edges of the sheets at the trailing edge of the blade, being suitably spaced apart so as to form port means, and

(d) means arranged to convey steam to the port means, so as to permit the steam to be discharged from the port means and substantially in the direction of the main steam flow past the blades whereby the steam causes Water which has condensed on said guide blades to break into small drops and accelerates the droplets to substantially the same velocity as the main stream of steam.

7. A steam turbine according to claim 6, wherein the port means comprise a slot like nozzle extending inwardly from a radially outer end of the guide blade.

8. A steam turbine according to claim 6, wherein the regions of the sheets adjacent their spaced apart edges are suitably shaped so as to provide a convergent flow passage to steam flowing through the port means.

9. A steam turbine according to claim 6, wherein the regions of the sheets adjacent their spaced apart edges are suitably shaped so as to provide an initially convergent and subsequently divergent flow passage to steam flowing through the port means.

10. A steam turbine according to claim 6, wherein the leading edge region of each guide blade in the circumferential row includes further inlet port means connected to the port means at the trailing edge and effective to receive steam flowing through the turbine and discharge it through the port means at the trailing edge, whereby to increase the quantity of steam discharged from the port means in the trailing edge.

11. A steam turbine according to claim 6, wherein the radially outer ends of the circumferential row of guide blades are secured to an outer ring of a diaphragm attached to the turbine casing, and the outer ring includes a steam manifold connected to a suitable steam supply source and effective simultaneously to supply steam to the port means in each of the blades in the row.

12. A steam turbine according to claim 6, wherein the interior of the blade includes partition means effective to extend the path of steam flowing to the port means and through the blade, whereby to increase the rate of heat exchange between the steam and the blade.

References Cited by the Examiner FOREIGN PATENTS 12/1955 France.

EDGAR W. GEOGHEGAN, Primary Examiner.

5 EVERETTE A. POWELL, ]R., Examiner.

Claims

1. A STEAM TURBINE COMPRISING: (A) A ROTOR INCLUDING CIRCUMFERENTIAL ROWS OF ROTOR BLADES, (B) A FIXED CASING INCLUDING CIRCUMFERENTIAL ROWS OF GUIDE BLADES RESPECTIVELY ARRANGED INTERMEDIATE PAIRS OF CIRCUMFERENTIAL ROWS OF ROTOR BLADES, (C) PORT MEANS PROVIDED IN THE TRAILING EDGES OF AT LEAST THE GUIDE BLADES IN THE FINAL CIRCUMFERENTIAL ROW OF GUIDE BLADES, AND (D) MEANS ARRANGED TO CONVEY STEAM TO THE PORT MEANS, SO AS TO PERMIT THE STEAM TO BE DISCHARGED FROM THE PORT MEANS AND SUBSTANTIALLY IN THE DIRECTION OF THE MAIN STEAM FLOW PAST THE BLADES WHEREBY THE STEAM CAUSES WATER WHICH HAS CONDENSED ON SAID GUIDE BLADES TO BREAK INTO SMALL DROPS AND ACCELERATES THE DROPLETS TO SUBSTANTIALLY THE SAME VELOCITY AS THE MAIN STREAM OF STEAM.