GB2247736A - Steam turbine control valve - Google Patents

Abstract

A steam turbine control valve has a spindle (9) carrying a valve head (8) and passing through one end (A) of the valve housing (1) to an external actuator (not shown). The housing (1) has an inlet (2) and an outlet (3) disposed radially of the spindle (9). The valve internal components further comprise a valve seat (7), an inlet control sleeve (12) and a strainer sleeve (13). A removable cover (6) closes an opening (5) in the valve housing (1) disposed axially of the spindle (9) at the end (B) of the housing opposite the actuator end (A). All the valve internal components may be removed through the opening (5) after disconnecting the spindle (9) from the actuator without the need to remove the actuator and its supporting arrangement. Radial projections (14) on the strainer sleeve (13) engage radial projections (15, 16) in the housing (1) to form a bayonet type fitting which holds the strainer sleeve (13), control sleeve (12) and valve seat (7) to the seat location (4) in the housing (1). Inlet holes (12B, 12C) in the control sleeve (12) are designed to promote both long seat life and quietness of the valve. <IMAGE>

Description

STEAM TURBINE CONTROL VALVE This invention relates to steam turbine control valves.

Such valves control the amount of steam passing into, and hence the power driving, a turbine between zero and a maximum. The term 'control valve' is normally used when the valve is intended for use with a turbine which drives a propulsion unit in which case the control valve determines the speed of the propulsion unit. The term 'governor valve' is normally used when the valve is intended for use with a turbine which drives an electricity generator in which case the valve maintains a constant speed of the electricity generator responsive to varying loads. In this patent specification the term 'control valve' Is used to mean a valve for use in either of these two applications.

Steam turbine control valves have a valve housing formed with a steam inlet, a steam outlet, an internal seat location, and an opening through which valve internal components may be passed when the valve is not In use, they have a removable cover on said opening, and said valve internal components comprise a valve seat on said seat location and a valve head on a valve spindle, the spindle being connectable in use to an external actuator for moving the valve head relative to the valve seat along the spindle axis. The external actuator, my include, for example, a hydraulic cylinder or an electric motor and gearing or a hand wheel and gearing, with supporting arrangements being provided for the actuator.High velocity steam passing through the valve in use erodes the valve seat and valve head contact surfaces and these internal valve components must be removed several times during the life of the valve for inspection and repair or replacement. It is desirable that the valve is out of action for inspection and repairs for as short a time as possible.

An object of the present invention is to provide a valve configuration enabling the time taken for removal and replacement of the valve internal components in an inspection operation substantially to be minimised.

According to the invention there is provided a steam turbine control valve in which a valve housing is formed with a steam inlet, a steam outlet, an internal seat location, and an opening through which valve internal components may be passed when the valve is not in use; in which a removable cover is on said opening; in which said valve internal components comprise a valve seat on said seat location and a valve head on a valve spindle, the spindle being connectable in use to an external actuator for moving the valve head relative to the valve seat along the spindle axis; in which the steam inlet and outlet formations are both disposed radially with respect to the spindle, the spindle passes through one end of the housing, and said opening is disposed axially with respect to the spindle at the other end of the housing opposite said one end, such that upon disconnection of the spindle from the actuator and removal of the cover the valve internal components may be withdrawn through said opening; and in which said valve internal components further include holding means for holding the valve seat on the seat location, said holding means being engageable with means provided on the housing as a bayonet type fitting.

This configuration allows the valve internal components to be removed in a direction away from the actuator. Thus the actuator and its support arrangement may be left in place when inspection of the valve internal components is required, which helps significantly to reduce the time for which the valve is out of action. The holding means of the bayonet type fitting will need less time to be removed and replaced than any holding means involving bolting and will further assist in keeping short the time for which the valve is out of action for Inspection of the valve internal components. A further advantage of the bayonet type fitting is that it provides a positive location of the valve internal components without putting any load on the removable cover.

A valve according to the invention as described in the penultimate paragraph may be provided in which said valve internal components include a control sleeve for providing the control function of the valve by movement of the valve head relative thereto, and in which the valve seat is held on the seat location via said control sleeve. Furthermore said valve internal components may include a strainer sleeve, with the control sleeve being held in position by the strainer sleeve. Although control sleeves and strainer sleeves are each known per se, it is advantageous for ease of assembly and disassembly of the valve internal components in a valve according to the invention to have them configured and arranged to hold the valve seat on the valve seat location.A valve with a control sleeve and a strainer sleeve so arranged may furthermore have said holding means engagable with means provided on the housing as a bayonet type fitting provided as means on the strainer sleeve, whereby the strainer sleeve, control sleeve and valve seat are detachably held in position by said bayonet type fitting. The bayonet type fitting just described may be formed by said holding means on the strainer sleeve comprising a number of outwardly extending radial projections which are circumferentially ramped in an axial direction and said means provided on the housing comprising the same number of inwardly extending radial projections which are positioned to provide slots between which said strainer sleeve projections may be axially inserted and withdrawn and which are correspondingly ramped to lock the strainer sleeve when it is rotated into engagement therewith.Three is a preferred number of said projections as this provides self centralising of the assembly.

In a valve as described in the previous paragraph, the control sleeve may be positioned across the radial steam inlet with small primary inlet holes near to the valve seat and large secondary inlet holes away from the valve seat. This arrangement promotes quietness of the valve. Thus for a total hole area required for a given flow, this flow will be quieter through a large number of small holes rather than through a small number of large holes. The highest pressure drop through the control sleeve and hence a significant noise potential occurs at the beginning of the valve head stroke, that is when steam passes only through those holes in the control sleeve near the valve seat.Quietness is thus promoted by having these holes near the valve seat comparatively small. . advantage in having the control sleeve inlet holes away from the valve seat comparatively large is that there is the possibility of arranging and shaping these larger holes to provide a desired flow/lift characteristic, that is the particular variation of steam flow over the course of the valve head stroke. The arrangement of the primary control holes may also be varied to assist in providing a desired flow-lift characteristic. The position and area of said primary inlet holes in relation to the seat area opening may be such that after approximately 1X of the valve head stroke away from the valve seat the inlet pressure drop of steam occurs mainly across said primary inlet holes rather than across the opening at the valve seat.

This arrangement minimises the time for which the main inlet steam pressure drop is across the contact areas of the valve seat and valve head, and therefore promotes long life for those contact areas. Said primary inlet holes, or primary and secondary inlet holes, may be shaped to provide a multi-stage pressure drop. This will further promote quietness of the valve.

In a valve according to the invention the valve head may have a bobbin-shaped axial extension providing an annular space within the valve housing downstream of the contact areas of the valve seat and the valve head, the valve seat may have an axial extension forming an outlet sleeve positioned across the radial steam outlet, and said outlet sleeve may divide said annular space and have radial holes via which steam flow is changed from axial to radial after passing the valve seat, the outside of said outlet sleeve being sealed against the valve housing at its downstream end, and a flange of said bobbin-shaped valve head extension at its downstream end being in sliding sealed contact with the inside of said outlet sleeve.The radial holes in the outlet sleeve thus provided promote straightening the steam flow by minimising swirl both in the axial and transverse plane, facilitating the transition from axial to radial flow and minimising absolute steam velocities in the outlet chamber and possibly promoting quietness of the valve.

In a valve according to the previous paragraph the spindle may pass through an axial bobbin hole in said bobbin-shaped valve head extension and be slidingly sealed thereto at its downstream end, a balance valve may have a balance valve seat formed at the upstream end of said axial bobbin hole in the valve head extension and a balance valve head formed at the upstream end of the spindle, and a balance space may be provided between the downstream flange of the bobbin-shaped valve head extension and the valve housing into which steam passes from the axial bobbin hole via ports at its downstream end when the balance valve is opened. Although incorporation of a balance valve in a steam turbine control valve is known per se, this is a convenient arrangement for making this provision in this particular valve.

An example of a steam turbine control valve according to the invention will now be described with reference to the accompanying drawings, in which Figure 1 shows a section through the centre of the valve, and Figure 2 shows a section through the valve housing and strainer sleeve of the valve, transverse to the section of Figure 1, in order to show clearly the bayonet type fitting incorporated in the valve.

The construction of the valve shown in the drawings will be conveniently described by way of first describing the general configuration of the valve, then the method of assembly of the valve internal components into the valve housing, then their method of disassembly, and then the operation of the valve.

General Configuration A valve housing 1 is formed with a steam inlet 2, a steam outlet 3, an internal seat location, diameter 4, and an opening 5 through which valve internal components may be passed when the valve is not in use. A removable cover 6 is provided for the opening 5.

The valve internal components include a valve seat 7 and a valve head 8 on a valve spindle 9. The spindle 9 is connectable in use to an external actuator (not shown) for moving the valve head 8 relative to the valve seat 7 along the spindle axis. The steam inlet 2 and steam outlet 3 formations are both disposed radially with respect to the spindle 9, the spindle 9 passes through one end A of the housing 1, and the opening 5 is disposed axially with respect to the spindle 9 at the other end B of the housing 1 opposite the end A, such that upon disconnection of the spindle 9 from the actuator and removal of the cover 6 the valve internal components may be withdrawn through the opening 5.

Assembly Fitting of all valve internal components takes place from end B except for a gland bush 10 for the spindle 9 which is fitted into the housing 1 from the actuator end A. This gland bush 10 does not require periodic attention or maintenance and so it is acceptable for this component to be'fitted from end A. Quick maintenance of the valve is enabled by having the valve internal components which need to be periodically inspected or maintained readily accessible from end B, because the external actuator Is at end A and so does not need to be first removed, and also by the method, described below, of fastening and locking the valve internal components without employing any bolting to the valve housing.

The valve seat 7 is first installed and located on its locating diameter 4. The valve seat 7 has an axial annular extension 11 which forms an outlet sleeve positioned across the radial steam outlet 3. An inlet control sleeve 12 is then installed onto its locating diameter 7A on the valve seat 7 where it is positioned across the radial steam inlet 2 and a strainer sleeve 13 is then fitted over it. The strainer sleeve 13 is introduced into the valve chest with three radial projections 14 on its outer diameter which form part of a bayonet type fitting entering first.

The projections 14 are circumferentially ramped in an axial direction, each from a lower end 14A to a higher end 14B as shown in Figure 2, corresponding to a three-start screw thread. These projections 14 are axially inserted in slots formed between three lugs 15 on the housing 1 which each hold a swivelling retaining pin 16, and the strainer 13 is pushed home so that it contacts the inlet control sleeve 12 at three axial flange extensions 17 forming strainer seating surfaces corresponding to the bayonet projections 14. The inwardly extending radial projections formed by the lugs 15 and pins 16, which have surfaces ramped corresponding to the ramps on the strainer projections, form a bayonet fitting together with the strainer projections 14.Thus the strainer 13 is rotated clockwise by a special tool (not shown) to lock and hold the valve seat 7, inlet control sleeve 12 and strainer sleeve 13 in position. To ensure that the three retaining pins 16 are loaded, the strainer ramps 14A, 14B, and the retaining pins 16 have contact surfaces machined at an angle which corresponds to the 60 thread angle of a modern screw thread. As the strainer 13 is tightened, the first pin 16 to contact produces a side force that moves the strainer towards the non-contacting pins. The strainer 13 will find a position in a plane perpendicular to the valve axis that causes all three retaining pins 16 to be loaded.

The three axial control sleeve flange extensions 17 mentioned above have hard facing bearing surfaces, nominally stellite. This is to prevent galling and give known, uniform sliding properties, thereby giving more uniform axial contact forces thus ensuring that the strainer 13 can be 'unscrewed' after long periods of service. A radial clearance is provided around the strainer 13 to allow it to float in order to load the pins 16. The clearance is sufficient to compensate for compounded worst case machining tolerances in the retaining assembly.

A positive retaining method Is used on the strainer 13.

After locking the valve seat 7, inlet control sleeve 12 and itself in place using the bayonet fitting the strainer is retained in position by a locking pin 18 fitted into a slot 19 in the strainer and engaging in a hole 20 in the valve housing 1 drilled on assembly. To allow for small differences in the orientation of the locking strainer 13 on subsequent re-builds, the strainer has several slots machined into its skirt as provision for the locking pin 18. A guide (not shown) is used for accurate drilling into the valve housing on assembly.

The gland bush 10 should be fitted at or prior to this point in the assembly process, and with the gland bush 10 in place, the valve head sub-assembly is now fitted. This is a compact sub-assembly comprising the valve spindle 9, valve head 8, a bobbin-shaped axial extension 21 of the valve head 8, a bobbin gland/seal arrangement 22/22A, a bobbin bush 23, a thrust plate 24, thrust plate retaining screws 25, seals 26 for the retaining screws 25 and a spindle guide-pin assembly 27. The sub-assembly is installed into the valve chest so that the spindle 9 passes through the gland bush 10 in end A of the valve housing. As the sub-assembly is inserted further into the valve housing, the sealing arrangement 22/22A on the bobbin 21 is carefully guided onto the outlet sleeve 11 of the valve seat 7. The sub-assembly can then be inserted fully home so that the seating area 8A on the valve head 8 contacts the main corresponding area 7B on the valve seat 7.

Assembly of the valve is then completed by fitting of the end cover 6. The end cover 6 also features a location diameter 6A giving support to the inlet control sleeve 12.

Disassembly Disassembly is a straightforward reversal of the above assembly procedure. Once the end cover 6 is removed by undoing bolts 28 and using jacking screws 29 provided for the purpose, the strainer locking pin 18 is extracted from the hole 20 by means of an extraction hole in the locking pin 18. The valve head sub-assembly can be pulled from the valve chest by using holes 24A in the thrust plate 24 once the actuator coupling (not shown) to the spindle 9 is disconnected.

A special tool, a spider spanner, is fitted to the strainer sleeve 13 in defined recesses 13A by screwing into holes 13B. The spanner is rotated anti-clockwise to undo the bayonet type fitting and hence release the strainer 13, inlet control sleeve 12 and valve seat 7. Tapped holes to facilitate pulling out of the inlet control sleeve and the valve seat, are provided in each of these two components.

Operation With the valve shut, steam enters the valve via the steam inlet 2, passing through holes in the strainer sleeve 13 and ports 12A in the inlet control sleeve 12 into the inlet chamber C of the valve formed between the valve housing 1 and the cover 6. The steam also passes through holes 24A in the thrust plate 24 and occupies a space between the thrust plate 24 and the upstream end 9A of the spindle 9.

The spindle 9 passes through an axial bobbin hole D in the bobbin-shaped valve head extension 21 and is slidingly sealed thereto at its downstream end via the bush 23. A balance valve has a balance valve seat 8B in the valve head 8 formed at the upstream end of the axial bobbin hole D and a balance valve head with a contact area 9B formed at the upstream end 9A of the spindle 9.

Thus with the valve shut steam is retained in the inlet chamber C with the main valve head contact area 8A and the balance valve head contact area 9B firmly seated on their respective seat contact areas 7B and 8B, and by virtue of seals 30 and 31 at the end cover 6 and valve seat 7 respectively and seals 26 at the bobbin/head retaining screws 25.

The valve is opened by the application of a thrust on the valve spindle 9 by an actuator (not shown). When the thrust on the spindle 9 becomes sufficiently large to overcome the inlet steam pressure force acting on the seat diameter of the balance valve and any friction between the spindle and its sliding surfaces, the spindle moves and steam flows through the hollow centre D of the bobbin to a balance space E at the rear of the bobbin 21, between the downstream flange of the bobbin 21 and the valve housing 1, via ports 21A in that end of the bobbin. The thrust required to further open the balance valve reduces considerably after the balance valve is first open since after this point the unbalanced area changes from the seat diameter of the balance valve to the spindle diameter.

The outside of the outlet sleeve 11 is sealed against the valve housing 1 at its downstream end by a seal 11A, and a flange of the bobbin-shaped valve head extension 21 at its downstream end is in sliding sealed contact via the gland/seal arrangement 22/22A with the inside of the outlet sleeve 11. The flow of steam out from the balance space E is controlled to a minimum by the seal 11A and the gland/seal 22/22A, thus enabling the pressure in this space E to build up to provide a balancing force for opening the main valve.

Further movement of the valve spindle 9 will result in only a slight increase in flow (if any), dependent upon the pressure-drop across the balance valve seat and the effectiveness of the seals lIA and 22A, until the head of the spindle 9A reaches the thrust plate 24. A guide pin assembly 27 is fitted to the thrust plate 24 such that the guide pin is always engaged in the spindle. This guide pin 27 reduces any vibration of the unsupported head of the valve spindle 9A, at spindle positions other than fully closed and in contact with the thrust plate 24.

Opening of the main valve can only occur by the application of an increase in thrust on the valve spindle 9 when it is in contact with the thrust plate 24. When the thrust on the valve spindle becomes slightly greater than the unbalanced pressure force on the main valve head 8 and sliding friction forces, the main valve will crack open. On cracking, steam will flow across the main valve seat 7B/8A via a first set of small holes, primary inlet control holes, 128 in the inlet sleeve 12 near the main valve seat owing to the pressure drop across the seat of the valve from the inlet chamber C to a downstream chamber F which is at a downstream pressure corresponding to turbine inlet conditions.The bobbin-shaped extension 21 of the valve head 8 provides an annular space within the valve housing downstream of the contact areas 7B/8A of the valve seat and the valve head, and the outlet sleeve 11 divides this annular space into chamber F inside the sleeve 11 and chamber G outside the sleeve 11.

As the valve is opened further and the valve head contact area 8A is moved further away from the valve seat contact area 7B the valve head contact area 8A successively uncovers more and more primary control holes 12B in the inlet sleeve 12, thereby permitting a higher flow-rate of steam. At initial cracking of the main valve, the most significant pressure-drop occurs across the opening at the valve seat 7B/8A since the annular area formed at the throat of the seat is the limiting area in the flow-path of the steam.However the diameter and arrangement of the primary control holes 128 in the inlet sleeve 12 is such that at very small openings where the valve head lift corresponds to only approximately 1X of the valve head stroke away from the valve seat, the throat area at the seat exceeds the total area of the control holes that are uncovered by the valve head seat at that lift. Hence, from openings greater than this point, the main inlet steam pressure-drop occurs across the control holes in the inlet sleeve 12 rather than across the opening at the main valve seat 7B and 8A. This feature reduces erosion damage of the contact areas 78 and 8A and hence improves seal life of the valve seat 7 and valve head 8.As the valve head 8 is opened further, large secondary inlet holes 12C in the inlet control sleeve away from the valve seat 78 are uncovered by the valve head. The size, shape and arrangement of these secondary holes 12C are such as to give a desired overall flow/lift characteristic for the valve. Furthermore, the shape of the holes 12C enables a much greater flow area to be uncovered for a given lift compared with simple close pitched circular holes, thus the overall lift of the valve can be reduced.

The size and arrangement of the holes 12B and 12C in the inlet sleeve can be modified (not shown) to produce a given flow rate/lift characteristic by providing a given opening area against lift for the valve. The arrangement of small inlet holes 128 near the valve seat 7B and large inlet holes 12C away from the valve seat 7A promotes quietness of the valve. This quietness could be further promoted by having the primary inlet holes 12B each shaped to provide a multS-stage pressure drop.

After passing through the orifice at the valve seat 78, the steam flows generally in the direction of the spindle 9 axis and enters the annular space F surrounding the concave external surface of the bobbin 21, from which it flows radially outward through radial holes 118 In the outlet sleeve 11. The concave surface will give a good flow regime downstream of the seat and assist in the transition from axial flow from the seat opening to radial flow outward through the radial holes in the outlet sleeve. The holes 11B in the outlet sleeve 11 promote straightening the flow of the steam from axial to radial and hence quietness of the valve. The radial outflow of steam passes into the annular space G and thereby out of the valve to the turbine via the valve outlet pipe 3.

To allow for differential thermal expansion between the valve head and the inlet sleeve, the valve head incorporates flexible members formed by machining axial slots (not shown) in a sleeve-like upstream axial extension of the valve head 8, and these slots form petal-like extensions to the valve head 8. Raised contacts strips 8C are incorporated at the ends of these petal extensions to ensure contact only occurs at the ends of the petals, thus giving adequate flexibility with acceptable stress levels at the root of the petals.

The flexibility of these strips 8C serves to minimise friction between the valve head 8 and the inlet sleeve 12. They are tipped with stellite hard facing to give good erosion/wear resistance.

Alternatively, surface hardened or through hardened rubbing strips of a suitable material can be used (not shown).

Claims (12)

1. A steam turbine control valve in which a valve housing is formed with a steam inlet, a steam outlet, an internal seat location, and an opening through which valve internal components may be passed when the valve is not in use, in which a removable cover is on said opening; in which said valve internal components comprise a valve seat on said seat location and a valve head on a valve spindle, the spindle being connectable in use to an external actuator for moving the valve head relative to the valve seat along the spindle axis; in which the steam inlet and outlet formations are both disposed radially with respect to the spindle, the spindle passes through one end of the housing, and said opening is disposed axially with respect to the spindle at the other end of the housing opposite said one end, such that upon disconnection of the spindle from the actuator and removal of the cover the valve internal components may be withdrawn through said opening; and in which said valve internal components further include holding means for holding the valve seat on the seat location, said holding means being engagable with means provided on the housing as a bayonet type fitting.

2. A valve as claimed in Claim 1, in which said valve internal components include a control sleeve for providing the control function of the valve by movement of the valve head relative thereto, and in which the valve seat is held on the seat location via said control sleeve.

3. A valve as claimed in Claim 2, in which said valve internal components include a strainer sleeve, and in which the control sleeve is held in position by the strainer sleeve.

4. A valve as claimed in Claim 3, in which said holding means engagable with means provided on the housing as a bayonet type fitting are provided as means on the strainer sleeve, whereby the strainer sleeve, control sleeve and valve seat are detachably held in position by said bayonet type fitting.

5. A valve as claimed in Claim 4, in which said bayonet type fitting is formed by said holding means on the strainer sleeve comprising a number of outwardly extending radial projections which are circumferentially ramped in an axial direction and said means provided on the housing comprising the same number of inwardly extending radial projections which are positioned to provide slots between which said strainer sleeve projections may be axially inserted and withdrawn and which are correspondingly ramped to lock the strainer sleeve when it is rotated into engagement therewith.

6. A valve as claimed in Claim 5, in which said number of radial projections is three.

7. A valve as claimed in any one of Claims 2 to 6, in which the control sleeve is positioned across the radial steam inlet with small primary inlet holes near to the valve seat and large secondary inlet holes away from the valve seat.

8. A valve as claimed in Claim 7, in which the position and area of said primary inlet holes in relation to the seat area opening is such that after approximately 1X of the valve head stroke away from the valve seat the inlet pressure drop of steam occurs mainly across said primary inlet holes rather than across the opening at the valve seat.

9. A valve as claimed in Claim 7 or Claim 8, in which said primary inlet holes, or said primary and said secondary inlet holes, are shaped to provide a multi-stage pressure drop.

10. A valve as claimed in any preceding claim in which the valve head has a bobbin-shaped axial extension providing an annular space within the valve housing downstream of the contact areas of the valve seat and the valve head, in which the valve seat has an axial extension forming an outlet sleeve positioned across the radial steam outlet, and in which said outlet sleeve divides said annular space and has radial holes via which steam flow is changed from axial to radial after passing the valve seat, the outside of said outlet sleeve being sealed against the valve housing at its downstream end, and a flange of said bobbin-shaped valve head extension at its downstream end being in sliding sealed contact with the inside of said outlet sleeve.

11. A valve as claimed in Claim 10, in which the spindle passes through an axial bobbin hole in said bobbin-shaped valve head extension and is slidingly sealed thereto at its downstream end, in which a balance valve has a balance valve seat formed at the upstream end of said axial bobbin hole in the valve head extension and a balance valve head formed at the upstream end of the spindle, and in which a balance space is provided between the downstream flange of the bobbin-shaped valve head extension and the valve housing into which steam passes from the axial bobbin hole via ports at its downstream end when the balance valve is opened.

12. A steam turbine control valve substantially as described herein and as shown in the accompanying drawings.