FIELD OF INVENTION
The present invention discloses a pressure balanced steam control valve for steam turbines. Steam control valves generally act as flow regulating devices in steam turbines. The invention, specifically deals with the development of a modified control valve with zero leakage and a considerable pressure balance across it. Zero leakage is required for controlling the over-speeding issue during transients. Larger pressure balance across the control valve also reduces the net actuation force requirement and fail-safe closure even in case of spring failure.
BACKGROUND OF THE INVENTION
The present invention relates to the Steam turbine control valves in particular to a control valve where the vibrations of the control valve body are suppressed in the entire operating region of the control valve. The present invention also allows higher pressure balance across the control valve which in turn reduces the net valve actuation force requirement. The purpose of this article is to develop a modified control valve with a larger pressure balance across it. Larger pressure balance across the control valve to reduces the net actuation force requirement.

As indicated in the fig.1, a conventional steam control valve consists of a valve casing (1), into which the valve seat diffuser (8) is secured. The valve cone assembly (A) is seated on the valve seat diffuser (8). The valve cone assembly (A) consists of a main control valve cone (10) and valve spindle (6) secured to it permanently by welding or made integral to the control valve cone (10). The valve cover (7) is secured on the control valve casing (1) with the help of threaded studs (15) which also ensure that there is zero steam leakage from the valve body (8). The valve cover (7) houses the spindle packing assembly (9) which is provided to reduce the steam leakage through the spindle seal clearances and allowing the free movement of the spindle in the axial direction of the valve cone (10).
The valve spindle (6) is provided with external threads at its other end. The valve spindle is locked after insertion into the internal threads provided in the fork and secured with the help of anti-rotation pin. The fork (13) is coupled to the servomotor (3&4) through a lever (3) pivoted at the control valve bracket (14). The servomotor consists of a servo valve and a hydraulic cylinder (3) which can also be pneumatic cylinder and electrical actuators. The pressurized hydraulic oil provides the required effort to lift the valve cone assembly against the valve forces due to steam pressure forces and spring (12) compression. The pilot valve (4) positions the valve to meet the steam flow demand. The net force required for control valve actuation is directly dependent on the steam inlet pressure. As the steam pressure

at the valve inlet (1a) is increased, the net force required for control valve actuation also increases. The steam leaks through the guide bush valve cone clearance space (11a) between the valve cone (10) and guide bush (11) into the valve cone space (7a) formed by the valve cover (7) and valve cone (10). The leakage of steam is eliminated when the pressure in the region (7a) formed by the valve cover (7) and valve cone (10) becomes equal to the inlet steam pressure. Initially the steam forces experienced by the control valve is very large in magnitude and act in the closing direction of the control valve.
In order to reduce net actuation force required to lift the valve from fully closed position, the valve cone (10) is provided with plurality of small pressure balancing holes (10a) on its top surface. This allows for a small quantity of continuous leakage flow to be established through clearance (11a) between the guide bush (11) and valve cone (10). This continuous leakage also reduces the pressure in the space (7a) formed by the valve cover (7) and valve cone (10). This reduces the net actuation force requirement but ensures a continuous leakage through the control valve even at fully closed conditions. The steam leakage quantity between the clearances (11a) of the guide bush (11) and valve cone (10) increases with the increase in the inlet steam pressure. The reduction in the actuation force requirement due to the plurality of through holes (10a) on the cone top surface is offset by the problem of

continuous leakage through the control valve which increases with the increase in the inlet steam pressure. In order to eliminate this problem of continuous valve leakage and along with the control valve vibrations and reduce the net actuating forces requirement, a new control valve with an inbuilt pilot valve was developed.
US patent 5119859 has attempted to eliminate the problem by providing a pilot valve in the control valve assembly. The main valve cone (15) could only open when the landing (5) on the valve spindle (14) comes in direct contact with bottom face 6a. The steam forces acting in (valve closing direction) on the valve spindle at the point of pilot valve lift is small in magnitude. The steam pressure forces rise sharply as the main valve cone (15) starts to open. The steam pressure forces drastically reduce when the control valve starts lifting and are become very low when the control valve is at its maximum opening. This would create a possibility of separation of the valve spindle lands (5) with the main cone surface (6a) can arise. To avoid the separation of the valve spindle (14) with main control valve cone (15) on account of reduced engaging force at nearly full open condition, James M Sudolnik et al. provided the control valve cone (15) is provided with extending axial grooves (23) which communicating with the upper pressure chamber formed by the valve rod and the control valve cone after a fixed lift (LR) for elimination of valve

vibration problem. For making the motion of the valve body more stable, in a steam governing valve, a plurality of compression springs is arranged above the valve body in the pressure chamber at equal circumferential intervals, and adapted to press the valve body downwardly when the valve body moves beyond a predetermined valve opening degree.
US patent 4928733 attempted to reduce the control valve actuation forces by providing a pilot valve and a pressure balancing chamber to reduce the control valve actuation forces by providing a sleeve (66), integrally formed between balance piston (58) and main valve (12) which permits a small leakage into the balance chamber till the main valve (12) is lifted. The control valve is provided with a pilot valve formed by valve stem (20), pilot valve seat (18). On opening of the pilot valve by upward movement of the valve stem (20), causes the steam to flow through the plurality of through holes (49) provided on the main valve cone (38). As the pilot valve (16) is unseated from the pilot valve seat (18) steam pressure in valve chamber (40) also begins to fall due to steam flow through the pilot valve (16). When the pilot valve land (46) forms abutting contact with the main cone surface (44), the main control valve (38) starts to lift. As the main valve (12) lifts, the flange (60) is unseated from the cylindrical wall (64) of the pressure chamber (37) causing live steam to flow into the balance chamber (37) through the grooves (74) without any restriction. The net downward force on the main control valve is reduced.

This reduces the actuation force requirement. In this situation, the differential pressure aiding upward balance acts across the relatively larger area defined by sealing flange (60). Thus, beyond the crack-open position, a greater balancing force is generated by steam valve (56), as is desired. This ensures that at nearly full lift there exists a considerable steam force acting in the downward direction to ensure the control valve stability.
OBJECT OF THE INVENTION
The objective of the present invention was to design a highly pressure balanced steam turbine inlet control valve to eliminate any flow induced vibrations during the valve operation and also reduce the actuation force requirements.
Another object of the invention is to develop a pressure balanced steam control valve that ensures higher pressure balancing across it.
A further object of the invention is to ensure easy replacement of valve components.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Conventional throttle control valve for steam turbine.
Figure 2 Control valve assembly in an US prior art (US 5119859A).
Figure 3 Control valve assembly in an US prior art (US 4928733).
Figure 4 Control valve assembly of the present invention.

Figure 5 Valve cone assembly of the present invention.
Figure 6 Valve cone assembly in fully lifted condition.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 5 shows a cross section of newly developed steam turbine throttle valve. Referring to FIG. 5, control valve assembly consists of valve casing (8) into which other valve internals are assembled. The valve cone assembly (A) consists of a modified valve cone (1), valve cone bush (3), valve spindle (4), flanged bush (6) and threaded ring (2). Valve cone (1) sits on the valve seat which in turn is secured into landing provided in the valve casing permanently by welding or with the help of a locking pin. The valve cone (1) houses a valve cone bush (3) into which the seating surface for the valve spindle (4) is machined. The assembly of the valve cone bush (3) with the valve cone (1) forms a damping chamber (1a). The seating surface of the bush (3) with the valve cone (1) is made steam tight to ensure there is no leakage into the damping chamber (1a). The damping chamber (1a) provided with a plurality of peripheral holes (1b) which establishes a continuous steam supply into the control valve outlet (8b) on opening of the pilot valve (15) evenly providing an even distribution of pressure around the valve cone surface (1c). The valve cone bush (3) itself consists of two rows of equally distributed peripheral holes (1a) to ensure the supply of live pressure steam to above and below the valve spindle (4) collar as shown in the figure 4. The spindle (4) is assembled into the valve cone

bush (3) and locked with the help of a flanged bush (6). This arrangement fixes the pre-lift of the valve spindle. The valve cone bush (3) is provided with a slot (3c) to ensure uniform distribution of live steam to the holes of the valve bush even if they are not aligned with the peripheral holes of the valve cone (1d). The valve cone bush (3) after assembly is secured in its position by installing of threaded ring (2) into internal threads machined on the valve cone (1).
The proposed design always ensures that the control valve cone assembly always experiences a force in the closing direction throughout its stroke. The valve cone (1) sits on valve seat (7) which is in turn secured into the landing machined in the valve casing (8). The control valve assembly is mounted as shown in the Fig. (5).
The valve cover bore is machined for mounting of the spindle packing assembly to providing the spindle sealing. The spindle packing assembly consists of a bottom bush (10), a serrated spacer bush (11), set of graphite sealing rings (12) and a locking bush (13) for providing a minimum pre-compression on the sealing rings (12). The locking bush (13) is externally threaded and installed the threads machined in the valve cover (9). The minimum pre-compression is dictated by the designed live steam pressure. The main valve opening can start only after the pilot valve (15) (formed by seating of valve spindle (4) on bush (3) seating surface (3d)) is fully opened. This can only happen when the valve spindle (4) profiled face back butts against the flanged bush (6).

The spindle (4) experiences a steam pressure force acting in the opening direction due to the steam pressure acting on the valve spindle seating surface. The valve cone (1) and hence the valve cone bush (3) experience a steam pressure force in the closing direction on account of live steam pressure (Pfd) in region (9a). The valve spindle always remains butted with the flanged bush bottom surface (6a) throughout the valve travel.
The net actuating force required for pilot valve (15) opening is quite small as compared to the valve opening forces for conventional control valves. Once the pilot valve (15) has opened, the steam accumulates in the damping chamber (1a). Continuous steam leakage from the damping chamber (1a) occurs through a plurality of equally spaced holes (1b) provided in the valve cone. This small amount of steam leakages reduces the pressure in the damping chamber (1a) but induces a back pressure on the cone surface (1c). The valve cone assembly experiences a downward force due to live steam pressure as seen in enclosed space (9a). The steam force generated by back pressure experienced by the valve cone bottom surface (1c) acts in the upward direction in opposition to the steam force acting on the valve cone enclosed space (9a). As the valve cone (1) starts opening, it experiences a reduced steam force in the downward direction on account of the back pressure generated by the leakage steam. This reduces the net actuating force requirement to be provided by the control valve actuator which in-turn reduces

actuator size. The valve cone assembly incorporating this design experiences a net downward steam force throughout its working stroke. Any malfunction at the actuator end or breakage of coupling will lead to the closing of the valve assembly due to the steam forces acting on the valve.

WE CLAIM
1. A pressure balanced steam control valve assembly for steam turbine comprising; - a valve cone assembly (A) housed in a valve casing (8); the said valve cone assembly (A) comprising: a modified valve cone (1); a valve cone bush (3); a valve spindle (4);
- the valve cone bush (3) configured inside the valve cone (1); the said valve cone (1) disposed on valve seat (7) which in turn secured into landing provided in the valve casing (8) permanently by welding;
- the valve cone bush (3) provided with a surface (3d) which is machined for the seating of valve spindle (4);
- the upper side of the spindle (4) guided by a flange bush (6) fitted inside valve cone bush (3) and then through a flange bush (2) having external thread fitted in the inside thread of the upper portion of valve cone bush (3);
- a valve cover (9) placed on top of valve cone assembly which is secured on the valve casing (8) with the help of locking bolts;
- the valve cone assembly placed between inlet passage (8a) and outlet passage (8b);

- the assembly of the valve cone bush (3) with the valve cone (1) forms a damping chamber (1a) in a manner such that the seating surface of the valve cone bush (3) with the valve cone (1) is made steam tight to ensure that there is no leakage into the damping chamber (1a);
- the assembly being provided with plurality of equi-spaced holes (1b) in the valve cone (1) for passage of steam to outlet area (8b) that induces back pressure on surface (1c) of valve cone (1) in upward direction in opposition to the steam force acting on the valve cone assembly due to steam occupied into the space (9a) which is filled initially by the steam passing through the holes (3a, 3b) provided in the valve cone bush (3);
- a pilot valve (15) disposed at the bottom of valve spindle, characterized in that the pilot valve (15) opens when the valve spindle (4) is lifted from its seating surface (3d) allowing the inlet steam from region (8a) to enter the damping chamber (1a) through plurality of holes (3a) provided in the valve cone bush (3) followed by lifting of valve cone (1), when valve spindle (4) collar is butted to the bottom surface (6a) of the flanged bush (6) allowing the flow of steam from inlet region (8a) to outlet region (8b) through the annulus formed between the valve seat (7) and the valve cone (1) ensuring the valve assembly to experience a reduced downward force at fully opened condition of pilot valve (15) wherein the steam

pressure forces acting on the control valve cone surface (1c) in the upward direction due to back pressure (Pb) in direct opposition of the downward force exerted by the pressure (Pfd) in the region (9a) and inlet region (8a) on the valve cone assembly (A) resulting the valve assembly to experience a reduced downward force at fully opened condition of the pilot valve (15).
2. The valve assembly as claimed in claim 1, further comprising spindle packing assembly for providing sealing conditions of bottom bush (10), a serrated spacer bush (11), a set of graphite sealing ring (12), a locking bush (13) for providing a minimum pre-compression on the sealing rings (12).
3. The valve assembly as claimed in claim 1, wherein the damping chamber is provided with a plurality of holes (1b) which ensure the leakage of steam from damping chamber (1a) into the outlet region (8b) leading to the stabilization of the flow under the valve cone (1) and ensuring the symmetric distribution of steam flow in the outlet region (8b) leading to the damping of the pressure pulsations being generated in the valve downstream.

4. The valve assembly as claimed in claim 1, wherein the valve cone bush (3) is provided with horizontal hole (3a) for allowing the steam to pass into the damping chamber (1a) for exerting an upward force on the valve spindle collar surface causing it to butt against the bottom surface (6a) of the flanged bush (6).
5. The valve assembly as claimed in claim 1, wherein the valve cone surface (1c) provided with a plurality of holes (1b) for reduction in pressure pulsations in outlet region (8b) that experiences a back pressure (Pb) which being always lesser than the inlet steam pressure (Pfd) induces a net steam pressure force in closing direction of the valve thereby ensuring the butting of the valve spindle (4) with bush bottom surface (6a) for stable valve operation throughout the valve’s designed lift.