GB2067722A

GB2067722A - Steam turbine governing valve

Info

Publication number: GB2067722A
Application number: GB8101213A
Authority: GB
Original assignee: Northern Engineering Industries PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 1980-01-23
Filing date: 1981-01-15
Publication date: 1981-07-30
Also published as: GB2067722B

Abstract

A steam turbine governing valve in which a valve member has a cavity and inlet 23 and outlet A ports leading to and from the cavity. The member also has a portion which is a close fit in an outlet formation 13 of the valve housing when the member is at or near its position in which the valve is closed. As the valve is opened flow can progressively increase through the inlet ports, the cavity and the outlet ports to increase the pressure at the valve outlet. The pressure in the outlet ensures that flow adheres to the wall of the valve seat and to the outlet formation as the valve opens. This reduces noise. Flow is possible only through the cavity until the portion clears the outlet formation, after which flow can occur past the outside of the valve member. <IMAGE>

Description

SPECIFICATION Steam turbine governing valve This invention relates to steam turbine governing valves.

According to the present invention a steam turbine governing valve comprises a valve housing and a valve member within the housing, the housing having a valve inlet, a seat and a valve outlet formation downstream of the seat, the valve member having an internal cavity and a portion which in the last stage of the movement of the valve member to its position in which the valve is closed enters and fits within the outlet formation, the valve member also having inlet ports leading to and outlet ports leading from said cavity, in which, in said position, communication between the valve inlet and the inlet ports is prevented by engagement of the valve member with said seat, said outlet ports communicating with the outlet formation in said position.

The invention will now be described, by way of example only, with reference to the accompanying drawing which is a longitudinal cross-section of steam turbine governing valve at an intermediate partialiy-open position.

The steam turbine governing valve has a hollow valve housing 10 having a steam inlet 11 and a steam outlet formation 1 2. The outlet formation 12 is downstream of an annular convex seat 13 and includes an outlet wall portion 13a. The valve also has a valve member 14 having a cylindrical stem 21 slidable within a guide 22.

The valve member 14 flares outwardly to a convex annular rib 1 4a and is shaped at 1 5 to conform to the shape of the seat 1 3 and a short portion 1 5a of the valve member 14 at its lower end is cylindrical.

The portion 15a is such that as the valve member 14 completes its approach to the fully closed position, the portion 15a, is a close running fit within the outlet port 12 at its minimum internal diameter.

Such a running fit requires a clearance of for example 0.80 mm (0.032 inch) between the parts for a 406 mm (16 inches) diameter valve. In general the clearance is 0.2% of valve diameter, for example.

The valve member 14 has screwed into it a flat base 16 having an annular array of equi-sized outlet ports 1 7 which lead from an annular cavity 1 8 surrounding a central hollow column 1 6a integral with the base 1 6 and defining a passage 1 9. The pattern, number and sizes of the outlet ports 1 7 is a matter of choice and may in modifications (not shown) be different from what has just been described. The column 1 6a has an outwardly flared upper end the outer edge of which sealingly engages an internal annular rib on the valve member 14 at 16c.The valve member 14 has through-ports 23 leading from the cavity 1 8 and located so that, when the valve member 14 is fully closed on the seat 13 the ports 23 are completely closed and no steam can pass through the ports 23.

The ports 23 are all of the same diameter, for example, and are distributed around the section 1 5. Three ports 23 are shown iying in a common radial plane. Similar groups of three ports 23 lie in other radial planes equidistant from the plane shown. For example, there may be fifteen such ports lying in groups of three in five respective radial planes. Other patterns, numbers, sizes and arrangements of ports may be provided in the section 1 5 in modifications (not shown).

The passage 19 leads downwardly from an upper cavity 24, and passages 25 through a wall 26 in the valve member 14 lead to the cavity 24.

The valve member 14 is connected to a valve rod 27.

A balancing pressure is provided owing to the passage 25 to reduce the forces required to open and close the valve.

The seating 13 and the valve member 14 include annular hardened inserts at 30 and 32, respectively, which are engageable when the valve member 14 is fully seated.

The valve member 14 is movable by a application of force by an actuator (not shown) at 50 to a lever 52 connected by a link 54 to the rod 27. The valve member 14 is biassed towards its closed position by a compression spring 56 acting on a plate 58 carried by the rod 27.

Operation In the closed condition of the valve the hardened insert 32 of the valve member 14 is seated against the insert 30 of the seat 13 so that there is no flow of steam through the ports 23 nor between the inlet 11 and the outlet 12. The drawing shows the valve member 14 in an intermediate, partially open position.

As the valve member 14 is lifted from the seat 13, as shown in the drawing, there exists a high steam pressure in the inlet 11 and the chamber 10, but until the cylindrical portion 15a clears the outlet wall portion 1 3a there is no steam flow past the outside of the valve member 14 except for an extremely slight leakage flow of negligible amount.

The ports 23 are uncovered, however, and steam flows through them, through the cavity 1 8 and through the ports 17 to the outlet 12. This flow raises the pressure in the outlet 1 2. However, as the valve member 14 is lifted further the portion 15a clears the portion 1 3a steam flows past the outside of the valve member 14 and the pressure in the outlet 1 2 ensures that the flow adheres to the wall of the seat 13, to the portion 1 3a and to the remainder of the wall of the outlet formation 1 2.

The flow at the wall of the seat 13 is at a relatively low velocity compared with the velocity which occurs in conventionai valves since the provision of the ports 23 and 17 linked to the cavity 1 8 absorbs much of the flow energy. This lowered velocity also reduces the tendency for the flow to detach from the walls.

As the valve is progressively lifted still further and the ratio of the inlet pressure to the outlet pressure falls, the flow remains attached to the seat and outlet wall. The short cylindrical portion 1 spa and the sharp edge at the lower end of the valve member 14 reduce the likelihood of any flow attachment to the member 14.

The valve member 14 described and shown is typically one which can be applied by way of "retro-fit" to existing valves as a straightforward replacement of the original valve member. The profile of the valve member may be chosen to suit most existing valve seats, so that the modifications to reduce valve noise and vibration simply entail the replacement of the valve member only without alteration of the remainder of the valve.

The invention therefore includes both new valves and valves modified as described above.

In other forms of the valve (not shown), the valve member 14 is different from that shown in the drawing.

For example, the stem of the valve member in one such form of the valve has a constant diameter from the part which slides within the guide 22 as far as the upper boundary of the shape at 15.

For example, in another form of the valve the valve member has a plain annular shoulder engageable with the planar part of the seat 13 radially outwardly of the radiussed part of the 13 to prevent flow past the valve member in the closed condition of the valve.

In another form of valve the wall portion 1 3a is cylindrical, the lower end of the valve member being a close fit within that cylindrical wall portion, in which case the cylindrical portion 15a may either be retained or may if preferred be dispensed with.

The lower end portion of the valve member may be other than cylindrical in further modifications.

In the valve as described with reference to the drawing the wall portion 1 3a may be cylindrical if desired, the seating above merging continuously with that cylindrical portion 13a, which then merges continuously with the outlet port 12.

Otherwise the wall portion 1 3a is the annular region where the seating 13 merges continuously with the slightly diverging wall of the outlet formation 12, such diverging wall being designed to function as a flow-diffuser.

The upward opening movement of the valve member 14, during which flow past the outside of the valve member is prevented by the close running fit of the valve member 14 within the wall portion 1 3a regardless of whether the valve member has a cylindrical portion 1 5a or the wall portion 1 3a is cylindrical), is a relatively small proportion of the total movement between the fully closed and fully open conditions. For example, flow past the outside of the valve member is typically prevented during the first half inch (12.7 mm) of a total travel of six inches (152.4 mm). The cylindrical portion 15a or 1 3a is typically 12.7 mm (0.5 inch) long in a 406 mm (16 inches) diameter valve. In general the ratio of the length to the valve diameter is typically 1:32.

Noise is reduced in valves made as described above because apparently the fluid flow adheres to the wall of the seating and the flow through the outlet is generally divergent in conformity with the outlet diffuser divergent wall and the flow through the outlet is largely uniformly symmetrically distributed about the central longitudinal axis of the outlet.

What turbulence arises immediately downstream of the exits of the outlets 1 7 is only small-scale since the jets passing through the outlets 1 7 are parallel and they generate no interaction noise.

This reduction in noise is in marked contrast to the very high levels of noised produced in known valves in which, during initial opening movement, very powerful convergent fluid flow occurs immediately below the valve member. The resultant interaction of the convergent fluid stream in such a flow mode involves quite large quantities of mechanical energy and relatively high levels of noise and vibration are therefore produced.

In such known valves, the ratio of the inlet pressure to the outlet pressure remains relatively high during the initial opening of the valve and the outlet pressure is not sufficient to maintain adherence of the fluid flow the outlet wall.

Although the details are not shown it will be understood that labyrinth seal arrangements are provided to prevent excessive loss of steam by flow between the stem 21 and the guide 22 or between the rod 27, through a very slight, negligible leak is likely to occur.

Claims

1. A steam turbine governing valve comprising a valve housing and a valve member within the housing, the housing having a valve inlet, a seat and a valve outlet formation downstream of the seat, the valve member having an internal cavity and a portion which in the last stage of movement of the valve member to its position in which the valve is closed enters and fits within the outlet formation, the valve member also having inlet ports leading to an outlet ports leading from said cavity, in which, in said position, communication between the valve inlet and the inlet ports is prevented by engagement of the valve member with said seat, said outlet ports communicating S with the outlet formation in said position.

2. A valve according to claim 1, in which said portion is cylindrical.

3. A valve according to claim 1, or claim 2, in which in the outlet formation the seating merges continuously with a divergent outlet port.

4. A valve according to claim 1 or claim 2, in which in the outlet formation the seating merges continuously with a cylindrical wall portion which merges continuously with a divergent outlet port.

5. A valve according to any preceding claim, in which in said position flow through the valve is prevented by engagement between the valve member and an annular convex surface of the seating.

6. A valve according to any claims of claims 1 to 4, in which in said position flow through the valve is prevented by engagement between the valve member and an annular plain surface of the seating.

7. A valve according to any preceding claim, in which the valve member and seating include respective hardened inserts engageable in said position.

8. A valve according to any preceding claim, in which inlet ports have entrances opening at an annular surface of the valve member which is shaped to conform to a convex surface of said seating.

9. A valve according to any preceding claim, in which said portion of the valve member is bounded by a plain end face of the valve member at which said outlet ports open.

10. A valve according to any preceding claim, in which the valve member has a stem of less diameter than the diameter at which the member engages the seating in said position.

11. A valve according to claim 10, in which the valve member flares outwardly to a convex annular rib between the stem and the diameter at which the member engages the seating, the maximum diameter of the rib being greater than said diameter and greater than the diameter of the stem.

12. A valve according to claim 1, substantially as described herein with reference to the accompanying drawing.