Sealing arrangement in turbine machinery

Abstract

A sealing arrangement in a steam turbine is deployed in a position where it can restrict flow of steam along a flowpath in a gap between two bodies of the turbine rotatable relative each other, the flowpath proving communication between a space of relatively high pressure and another of relatively low pressure. The sealing arrangement comprises a leaf seal mounted on one of the bodies and sealingly engaged with the other. Two or more leaf seals can be juxtaposed with each other, without need for labyrinth seals, in a steam turbine application, without deleterious effects experienced in use of brush seals in similar circumstances. The sealing arrangement can include segments displaceable from a sealing position to increase clearance during start-up of a turbomachine, thereby taking account of potential vibration problems as the turbine rotor passes through natural frequencies.

Description

The present invention relates to a sealing arrangement in turbine machinery, particularly in steam turbine machinery.

Seals are employed in steam turbine machinery at points where a surface of a rotating body of a machine (the rotor) opposes a surface of a stationary part of the machine (the stator). Seals are used to restrict the passage of fluid from a space of relatively high pressure to a space of relatively low pressure.

Seals for steam turbine machinery have historically been in the form of so-called “labyrinth” seals. A labyrinth seal, formed in a flow pathway between a rotor and a stator comprises one or more formations on surfaces of the rotor and/or stator, to present obstructions to the flow of fluid in the flow pathway. These obstructions constrict the cross-sectional area of the flow pathway so that the fluid is forced to accelerate and thus become of relatively low pressure, and then to decelerate very rapidly as it passes through the labyrinth seal. This rapid deceleration of the fluid as it exits the constriction causes uncontrolled expansion of the fluid, which induces energy losses which are effective at presenting upstream obstruction to fluid flow. However, it is appreciated widely that labyrinth seals are only partially effective, and improvements thereto are desired.

To provide improved performance, brush seals have been introduced to act instead of or alongside labyrinth seals. A brush seal comprises a backing plate into which are embedded a very large plurality of pliable wire or fibre filaments. A brush seal also operates to provide a tortuous path to the progress of gas or other fluid through the seals. In operation, a brush seal is about 90% more effective than a labyrinth seal in holding a pressure drop between two spaces in a turbo machine.

In some circumstances, therefore, brush seals provide an effective seal in turbo machines where the gap between the rotor and stator is in the order of 1 mm. However, in gas turbines, the gap between the rotor and stator components can vary widely. In particular, thermal, gyroscopic and other mechanical effects can cause variation in the gap between the rotor and stator, and brush seals have been found to be effective at dealing with this.

One observation has been made that the maximum pressure drop that can be carried by the seal is inversely proportional to the square of the gap. Thus, by increasing the gap to, say, 3 mm, a seal with the same bristles as the seal described above would only be able to accommodate a maximum pressure drop one ninth of that which can be accommodated by the seal described above with the 1 mm gap.

It has been found through experience that the construction of a brush seal capable of withstanding these gaps would require fibres so thick and strong that their compliance would be severely limited. This would raise the risk of damage to the brush, and to wear within the turbo machine. Thus, the need has been identified to devise a seal capable of overcoming this problem.

However, in steam turbines different technical problems are presented that are of greater significance than the above problems relating to the operation of gas turbines. Indeed, the above problems may not be exhibited at all in steam turbines. Steam turbines are generally much larger than gas turbines and they operate at relatively lower temperatures. Further, rotational speeds of rotors in steam turbines can be lower than in a gas turbine.

For these reasons steam turbines are perceived to operate in more controlled conditions than those under which a gas turbine operates. Because of these less extreme conditions, rotor-stator gap is not highly variable—temperature expansion of moving parts in a steam turbine is likely to be less significant and gyroscopic effects are likely to be greatly reduced. In these circumstances, a 1 mm clearance between the rotor and stator is usually sufficient and can usually be relied upon to remain relatively constant, at least to the extent that ‘hard on hard’ contact (contact of the rotor and stator surfaces themselves) can be realistically assumed not to be a risk.

On the other hand, pressure drops experienced in steam turbines are often significantly higher than in gas turbine technology. For example, a typical pressure in a high-pressure cylinder of a steam turbine could be as much as 200 bar, while exhaust pressure might be in the region of 80 bar. In the past, labyrinth seals have been used to control this substantial pressure drop. However, it is appreciated that the length of flow path presented by such a labyrinth must be significant to provide significant constriction on the flow path, and this will add significantly to the axial length of a turbo machine. This could significantly increase the size of the machine in relation to its purpose and could impact on the positioning of other machines to which the turbo machine should be connected in installation.

Brush seals have been presented as possible solutions to this problem, in that they can control such a pressure drop over a shorter axial length than a labyrinth seal. However, in practice, brush seals have been found to be inappropriate in steam turbine applications. In particular, it has been found that brush seals are inappropriate in situations where very high-pressure steam and high swirl are encountered. The swirl kinetic energy can cause significant disruption of the brush elements of the brush seal, which can lead to failure of the seal. Further, this swirling of the inlet steam can, over time, lead to fatigue related damage to the filaments of the brush.

In addition, high-pressure steam, such as is encountered in steam turbines, can carry significant particulate matter, such as detritus from the interior of boiler apparatus used for generating the steam. Such particulate matter can impact with the brush seal so as to cause substantial damage to the brush filaments.

Thus, it would be desirable to be able to provide a seal for a steam turbine machine capable of operating in conditions of swirling intake and in which the risk of damage to the seal by steam borne particulates is reduced.

In accordance with one aspect of the invention, a steam turbo machine comprises a stator and a rotor rotatable with respect to the stator, opposing circumferential surfaces of the stator and rotor defining between them a plurality of gaps capable of fluid communication, the gaps being sealed by sealing arrangements, wherein at least one of the sealing arrangements is a leaf seal.

By using a leaf seal in a steam turbine in preference to a labyrinth seal or a brush seal, problems previously exhibited through the use of a labyrinth seal (such as the extreme length of such a labyrinth seal) or a brush seal (susceptibility to damage through swirl and steam borne particles) can be reduced.

In a balance piston seal environment, pressure differences between the high-pressure area and the low-pressure area can be extreme. Even in the case of a leaf seal, it can be difficult to arrange for a sealing means to support the pressure drop required for effective operation of a steam turbine. Therefore, it has long been understood that placing two or more seals in series may be an effective way of producing an appropriate seal.

However, it has been found that brush seals, placed in series are less effective than anticipated, as the flow of steam through the first seal produces unpredictable flow effects in the space between the two seals, affecting the sealing effectiveness of the second seal. This can have a deleterious impact on the efficiency and operation of brush seals in series.

In the past, this problem has been solved by inclusion of labyrinth seals between brush seals in series; as noted above, labyrinth seals can compromise the compactness of a sealing arrangement, which can have a negative impact on the effective arrangement of turbine machinery in a plant. The inclusion of a labyrinth seal can also sometimes compromise the assembly of a sealing system. This has led to compromise in the level of compactness of a sealing arrangement comprising two or more brush seals in series. Thus it is desirable to provide a sealing arrangement directed at addressing these problems, such that sealing effectiveness can be improved without compromising machine compactness.

According to a further aspect of the invention, a turbomachine powered with a flow of steam defines a region of relatively high pressure and a region of relatively low pressure, separated by a flow path between two relatively rotatable members of the turbomachine, and a sealing arrangement operable to restrict the passage of steam in said flowpath, wherein said sealing arrangement comprises a series of leaf seals.

A surprising consequence of providing leaf seals in series is that the need to provide labyrinth seals to stabilise flow patterns to maintain seal effectiveness is obviated.

Further aspects of the invention, advantages thereof, and problems solved thereby will become apparent from the following description of specific embodiments of the invention accompanied by the appended drawings in which:

FIG. 1 is an axial cross-section through a steam turbine in accordance with a specific embodiment of the invention;

FIG. 2 is an axial cross-section through a sealing arrangement in the steam turbine of FIG. 1;

FIG. 3 is a cross-section through a line marked A-A in FIG. 2; and

FIG. 4 is a cross-section, in a similar orientation to that illustrated in FIG. 2, of a sealing arrangement according to a second embodiment of the invention.

FIG. 1 illustrates a steam turbine of substantially known construction, but fitted with seals in accordance with a preferred embodiment of the invention. Thus, the turbine 10 comprises a cylindrical casing 12 and a rotor 14 which includes a balance piston 16 and trunnions 18, 20 at upstream and downstream ends respectively.

An outer cylindrical surface 22 of the rotor 14 and an inner surface 24 of the casing 12 define a turbine chamber 26. The turbine chamber is defined such that it is radially narrower at the upstream end than the downstream end, in accordance with usual turbine design practices.

Projecting from the rotor surface 22 and the casing surface 24 are series of rotor blades 30 and stator blades 32 respectively.

An inlet manifold 34 is defined in the casing 12, in fluid communication with the upstream end of the turbine chamber 26. As indicated in FIG. 1, in operation the turbine 10 receives a flow of steam through this inlet manifold 34 at a pressure of up to 200 bar. An exhaust manifold 36 is defined in the casing 12 and is in fluid communication with the downstream end of the turbine chamber 26; in operation exhaust steam exits the turbine 10 through the exhaust manifold 36, as indicated in FIG. 1.

Upstream end glands 40 are located circumferentially around the upstream end trunnion 18, to prevent escape of inlet gas from the gap formed between the trunnion 18 and the casing 12. Balance piston seals 42 are placed circumferentially around the balance piston 16, to restrict flow of gas through the gap defined between the balance piston 16 and the casing 12. The balance piston seals 42 can experience a pressure drop across them of up to 120 bar in operation. This is because the input high pressure steam introduced by inlet 34 can be input at a pressure of up to 200 bar, whereas the pressure in the exhaust manifold 36 (with which the balance piston is in communication in order to provide balancing thrust to the rotor) may be at a pressure of the order of magnitude of 80 bar. Rotor blade seals 44 are provided in the gaps between the tips of the rotor blades 30 and the inner surface 24 of the casing 12. Corresponding stator blade seals 46 are provided between the tips of the stator blade 32 and the outer surface 22 of the rotor 14. Provision of rotor blade seals 44 and stator blade seals 46 restrict flow of gas at the tips of the blades 30, 32, thereby increasing the tendency of steam in the turbine chamber 26 to be forced past the reaction surfaces of the turbine blades and thereby provide torque to the rotor 14.

FIG. 2 illustrates a balance piston seal 42, which comprises a support casing 50 of substantially annular construction, mounted on the interior surface 24 of the casing 12.

The casing 50 defines a channel in which are loaded a plurality of leaves 52. The leaves 52 are generally rectangular and extend longitudinally of the channel and thus radially of the balance piston 16. Laterally extending lugs 54 correspond with opposing surfaces 56 of the channel 50, so as to cause the leaves 52 to be retained in the channel. The ends of the leaves 52 directed away from the retaining lugs 54 extend longitudinally from the casing 50 and protrude beyond the radial extent of casing 50.

FIG. 3 shows in further detail the leaves 52 in relation to the direction of rotation of the piston 16. The leaves 52 are shingled so as to engage against the surface of the balance piston 16 as it moves past. It is anticipated that the compliance of the leaves 52 will provide a return force to provide the desired sealing effect, as the seal leaves 52 engage the surface of the balance piston 16.

It has been found that a leaf seal in this form is capable of supporting a pressure drop of 120 bar, such as will be experienced in the position of a balance piston in a steam turbine. Further, it will be appreciated that similar leaf seals can be placed at the point of inlet gland seal 40, exhaust gland seal 48, and for rotor blade and stator blade seals 44, 46. Further, the structural rigidity of leaves 52 is superior to a brush seal of similar size, and is thus able to withstand the impact of a swirling inlet gas. In addition, leaves 52 will be less susceptible to damage as a result of impact from particles borne on the inlet steam.

FIG. 4 illustrates a sealing arrangement according to a second embodiment of the invention. This sealing arrangement is shown as, for the purposes of this example, a balance piston seal 42.

Two axially spaced, parallel, bearing piston leaf seals 42 are arranged on the casing 12 and directed inwardly of the turbine casing 12, to engage sealingly with the balance piston 16. No labyrinth features are interposed between the two leaf seals 42. The two leaf seals act in tandem to provide adequate separation between an upstream zone and a downstream zone in the turbine, thereby affecting the seal required for effective operation of the turbine 10.

By arranging leaf seals in this manner, previous problems related to efficiency of brush seals operated in series, together with the additional axial length that would be required of the machine in which two brush seals were placed in series, together with supposed labyrinth seal, can be avoided.

In either of the above-described embodiments, it would be desirable to arrange that the leaf seals 42 are detachable from the casing 12, to allow worn leaves 52 of the leaf seals to be replaced from time to time.

It would be appreciated that the present invention is not limited to its application to steam turbines. Other machines, in which similar conditions of temperature and pressure arise, could be susceptible to the implementation of the present invention. The present invention essentially addresses problems related to sealing a flow path defined between two relatively rotatable parts of a machine, running at speeds comparable to those encountered in a steam turbine, wherein seal limits flow of steam between a region of high pressure (a pressure that might be experienced at an inlet of a steam turbine) and a region of low pressure (a pressure that might be experienced at an exhaust outlet of a steam turbine).

It will be appreciated further that other modifications and adaptations, which provide technical effects complementary to the technical effect of the present invention, can be provided in a steam turbo machine. For example, as per the disclosure of U.S. Pat. No. 4,436,311A, the sealing arrangement may be mounted on radially movable segments about the rotor, the segments being urged apart by circumferentially aligned springs to provide for a larger rotor/stator gap at low rotational speeds and low pressure to avoid ‘hard on hard’ contact occurring during a start up phase of a turbine. As described in that document, the turbine is particularly susceptible to hard on hard contact during start up as turbine rotor may, as it accelerates rotationally, pass through speeds at which a natural frequency of the rotor is encountered. This can lead to vibration of the rotor and consequent risk of fouling against a stator component that is positioned too close to the rotor.

Once the rotor has reached a driving speed, high-pressure steam is then introduced to the turbine. The segments of the sealing arrangement are arranged such that the reaction force applied by the steam to the segments of the sealing arrangements is operable to urge the segments towards a sealing position against the rotor. Radial movement of the segments towards the rotor may be limited by a captive formation with respect to the turbine casing, to prevent the sealing arrangement segments from fouling the rotor.

Claims

1. A steam turbo machine comprising a stator and a rotor rotatable with respect to the stator, for receiving an inlet flow of steam for driving the rotor, wherein opposing circumferential surfaces of the stator and rotor define between them a gap capable of enabling fluid communication between a space of said machine of relatively high pressure in use to a space of relatively low pressure in use, the gap being sealed by a sealing arrangement, wherein the sealing arrangement is a leaf seal.

2. A machine according to claim 1 wherein said rotor includes a balance piston, for maintaining a compensating force with respect to the reaction of the rotor against the inlet steam in operation, said balance piston having a circumferential surface which defines, with an opposing circumferential surface of the stator, said gap between a space which, on operation of the turbine, is at a relatively high pressure and a space which, on operation of the turbine is at a relatively low pressure, a sealing arrangement being interposed in said gap and wherein said sealing arrangement is a leaf seal.

3. A machine according to claim 1 wherein said relatively high pressure is a steam inlet pressure at a steam inlet of the steam turbine machine.

4. A machine according to claim 1 wherein said relatively low pressure is an exhaust pressure at a steam exhaust of said steam turbine machine.

5. A machine according to claim 1 wherein said rotor has an external circumferential surface, and said stator has an internal circumferential surface, developed around the axis of rotation of said rotor, and a turbine chamber is defined between said surfaces, said rotor having a plurality of substantially radially extending turbine blades, configured to present a surface against which said inlet flow of steam reacts in use to generate a torque acting on said rotor relative said stator, each of said turbine blades extending radially to adjacent said inner surface of said stator, so as to define a gap in which a sealing arrangement is interposed, said sealing arrangement being a leaf seal.

6. A machine according to claim 5 wherein said stator has a plurality of inwardly substantially radially extending turbine blades configured to direct, in operation of said turbine, said inlet flow onto said turbine blades of said rotor to cause said torque to be developed in use, wherein each of said stator turbine blades extend radially to adjacent said external surface of said rotor and thereby defining a gap in which a sealing arrangement is interposed, said sealing arrangement comprising a leaf seal.

7. A machine according to claim 1 wherein the rotor comprises opposing and coaxial mounting means for cooperation with corresponding formations of said stator, said mounting means and formations each having opposing adjacent circumferential surfaces defining a gap in which a sealing arrangement is interposed, wherein said sealing arrangement is a leaf seal.

8. A machine according to claim 1 wherein said sealing arrangement comprises two leaf seals, interposed in said gap.

9. A machine according to claim 1 wherein the or each leaf seal comprises a plurality of leaves extending from one of the rotor and stator across said gap towards the other of the rotor and stator.

10. A machine according to claim 1 wherein said sealing arrangement comprises a plurality of support segments, circumferentially arranged about the gap, each support segment presenting a leaf seal in said gap, each segment being radially displaceable in said gap, wherein said sealing arrangement further comprises resilient means for urging said segments into an initial position displaced away from a sealing position, said segments are arranged to be urged into said sealing position by a reaction force developed in use through high pressure in said high pressure region of said turbine machine.

11. A method of sealing a gap in a steam turbomachine between a region of relatively high pressure and an area of relatively low pressure, between relatively rotatable members of said turbomachine, said method comprising interposing a leaf seal in said gap, mounting said seal on one of said members and directed sealingly at a surface defined on the other of said members.

12. A steam turbine machine substantially as described herein with reference to FIGS. 1 to 3 of the accompanying drawings.

13. A steam turbine machine substantially as described herein with reference to FIG. 4 of the accompanying drawings.

14. A method of sealing a gap in a steam turbomachine substantially as described herein with reference to FIGS. 1 to 3 of the accompanying drawings.

15. A method of sealing a gap in a steam turbomachine substantially as described herein with reference to FIG. 4 of the accompanying drawings.