GB2295882A - Steam valve - Google Patents

Abstract

A steam valve for use in the control of steam turbines. The flow of steam from an annular chamber (1) to a pipe (5) through a valve seating provided by the mouth of the pipe, is controlled by a valve head which has a fixed central spigot (21) profiled (23) to provide a smooth passage for steam in the fully open condition. A sleeve (25) slides on the spigot (21) between an open position in which the sleeve surface (27) is continuous with the spigot surface (23) and a closed position in abutment with the valve seating. At intermediate positions the sleeve provides a discontinuity in the steam passage which suppresses unstable forces on the valve head otherwise arising. <IMAGE>

Description

STEAM VALVE This invention relates to steam valves for use, for example, in the control of steam turbines. In such applications there is a common requirement for a steam valve to control a steam supply through a range of values from 'off to full 'on'. The valve commonly has the form of a head (or plug) which pulls away from a seat to provide a controllable area for the passage of steam. In a typical arrangement steam passes from a chamber into a pipe when the pipe is not sealed by the plug.

A problem arises in that high pressure loss may be incurred by the use of a simple, axially movable plug in a sleeve because of the poor aerodynamic share of the resulting passage when the valve is fully open. An attempt has been made to overcome this problem by the use of a valve head design as shown in the accompanying Figure 1. This is a cross-section of such a valve in which the left hand side shows the valve in a closed condition and the right hand side in an open condition.

In this design the valve is required to control the flow of steam from a chamber region 1 surrounding the valve, into an exit chamber 3 provided by a pipe 5. The mouth of the pipe is provided with a seating insert 7, the mouth and seating being shaped to provide a smooth entry for steam into the pipe.

The valve head comprises a plug 9 which is mounted on a driving shaft 11 axially movable in a bearing sleeve 13 in the valve body 15. The plug 9 slides in a sleeve 17 which extends down to support the plug as far as possible and thus approaches the mouth of the pipe 5.

The leading end 19 of the plug 9 is profiled to provide a smooth and continuous wall confronting the seating and thus the problem mentioned above of excessive pressure loss is to a large extent overcome. Unfortunately however, when the valve is required to throttle for control purposes the separation of flow from the profiled head 19 can become unstable due to this very smoothness of the head. This separation of the flow is the boundary layer leaving the surface and occurs when there is too great a diffusion in the flow (ie too great a pressure rise).

The point of separation determines the pressure distribution on the valve head and thus the forces on it. The point is affected by the shape of the head and whether the head is moving (from side to side). Any movement or imperfection in shape can therefore cause a disparity in the separation points on opposite sides of the plug, which causes unequal force distribution and possible reinforcement of the movement. It is possible for energy to be fed into the side-to-side motion by the change in pressure forces occurring as a result of that motion.

At high steam pressures this instability can produce unsteady forces on the valve head and consequent mechanical damage. This could result in considerable expense and inconvenience in shutting down the system for repairs.

It is an object of the present invention to provide a steam valve which combines the features of low pressure loss and substantial immunity to unstable forces when operating partially open.

According to the present invention, a steam valve comprises a valve seating providing a passage to an exit chamber, and a valve head directed axially at the seating, the seating being shaped to permit smooth flow of steam from a region around the valve head to the exit chamber, and the valve head comprising a fixed spigot shaped to provide, with the seating, a smooth walled annular passage from the region to the exit chamber, the valve head including a sleeve movable axially relative to the spigot from a position in which it provides substantially no obstruction to steam flow through said passage to a position in which it engages the sealing and substantially prevents the passage of steam to the exit chamber.

The sleeve is preferably shaped so that when it is fully withdrawn from the seating the spigot and the sleeve provide a substantially continuous surface defining one wall of said passage.

The sleeve may be mounted on an axial drive shaft and be slidable on a cylindrical outer surface of the spigot. The spigot may be mounted on leg members extending axially from a coaxial cylindrical portion fixed to the valve body, the sleeve having a radial web fixed to the drive shaft and apertures in the web to accommodate the leg members.

One embodiment of the steam valve in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 (already described) is a cross-sectional elevation of a previously proposed steam valve designed to give low pressure-loss; and Figure 2 is a similar view of the inventive embodiment.

As explained above, the plug 9 of the valve in Figure 1 is profiled (19) to provide a smooth passage for the flow of steam from the region 1 around the valve head to the exit chamber 3 of the pipe 5. In the embodiment of Figure 2 the valve head incorporates a fixed spigot 21 also with a smooth profile 23 to provide a low loss passage from the region 1 to the exit chamber 3.

Closure of the valve is achieved not by movement of the spigot 21 but by axial movement of a cylindrical sleeve 25 which slides on the spigot 21. As shown in the drawing, this sleeve 25 has a profiled lower end 27 which, in the fully open position (shown to the left of the centre line) provides a continuous smooth surface with the profiled end 23 of the fixed spigot and consequent low pressure-loss.

In the closed condition of the valve, the profiled end 27 of the sleeve 25 engages the valve seating to close off the steam passage. In the partially open valve condition, with the sleeve 25 positioned as shown in broken line (27'), separation of the flow is forced to occur at the end of the sleeve, so producing 'clean' separation and a stable separation point. This discontinuity introduced into the passage wall does, it has been found, suppress to a large extent the instability in the forces to which the valve head is subjected, with a consequent reduction in vulnerability to mechanical damage.

The construction of the valve head is as follows. The spigot 21 is mounted on a member 29 which has a cylindrical end 31 (at the top end) from which project, axially, a number of legs 33. The cylindrical end 31 is fixed into the valve body and the lower ends of the legs 33 engage in a circumferential groove in the 'rear' of the spigot 21. This groove has a return flange 35 which traps a complementary shoulder 37 on the leg 33. The legs are assembled to the circumferential groove by way of slots or keyways (not shown) cut in the flange 35. A key 39 is inserted circumferentially between leg and spigot to lock the spigot in place on the legs.

Before assembling the spigot 21 to the supporting legs 33 the legs are inserted through apertures 41 in a radial web 43 integral with the sleeve 25. It is this web which supports the sleeve, the web providing a collar which is mounted on the driving shaft 11 by means of a shoulder 45 and fixing nut 47.

Axial movement of the driving shaft 11 drives the sleeve 25 between the positions shown to the left and right of the centre line, the legs 33 not obstructing such movement by virtue of the web apertures 41.

In the closed position it is important that steam does not bypass the sealing sleeve 25.

Seals 49 are therefore provided between the spigot 21 and the sleeve 25. The sleeve is also a close sliding fit in an outer sleeve 51 which is an extension of the vdlve body.

Claims (5)

1. A steam valve comprising a valve seating and a valve head directed axially at the seating, the seating providing a passage to an exit chamber, and being shaped to permit smooth flo .v of steam from a region around the valve head to said exit chamber, and the valve head comprising a fixed spigot shaped to provide, with the seating, a smooth walled annular passage from said region to said exit chamber, wherein said valve head includes a sleeve movable axially relative to said spigot from a position in which it provides substantially no obstruction to steam flow through said passage to a position in which it engages the seating and substantially prevents the passage of steam to said exit chamber.

2. A steam valve according to Claim 1, wherein the sleeve is shaped so that when it is fully withdrawn from the seating the spigot and the sleeve provide a substantially continuous surface defining one wall of said passage.

3. A steam valve according to Claim 1 or Claim 2, wherein said sleeve is mounted on an axial drive shaft and is slidable on a cylindrical outer surface of said spigot.

4. A steam valve according to Claim 3, wherein said spigot is mounted on leg members extending axially from a coaxial cylindrical portion fixed to the valve body, said sleeve having a radial web fixed to the drive shaft and apertures in the web to accommodate said leg members.

5. A steam valve substantially as hereinbefore described with reference to the accompanying drawings.