JPS6042321B2 - Main steam stop valve with bypass valve - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a main steam stop valve with a bypass valve, in which a bypass valve body for controlling steam flow rate is provided within the main valve body.

Steam turbines perform all-round injection operation to reduce thermal stress during startup.

For this reason, a bypass valve with a small opening ratio is provided in the main valve body of the main steam stop valve, and at startup, all downstream steam control valves are left fully open, and the steam flow rate is controlled by the bypass valve in the main valve body. Control. Since this bypass valve is built into the main valve body of the main steam valve, its valve stem is provided on the downstream side, and the valve body is opened by pushing up with the valve stem.

In addition, by controlling a relatively small flow rate range, the valve lift is also small.
The pressure difference before and after the valve body is also very large. Therefore, the flow rate of steam passing through the valve is extremely high, and as a result, the valve is often subject to erosion due to heavy objects such as minute amounts of condensate and scale contained in the steam flow, resulting in part of the valve being damaged. However, the fragments may cause damage to downstream parts, such as the turbine nozzles and blades. The damage caused by this erosion increases as the pressure of the main steam increases, and when the pressure of the main steam exceeds 169 kglc#ig, the bypass valve is often replaced every year. Next, a conventional main steam stop valve with a bypass valve will be explained with reference to FIGS. 1 and 2. A main valve chamber 2 is formed inside a valve box 8 having an inlet pipe section 1 and an outlet pipe section 9, and the main valve chamber 2 is used to prevent foreign matter such as metal rust contained in the main steam from entering the turbine. A cylindrical strainer 3 is attached to prevent water from entering the tank.

A main valve seat 5 is attached to the wall of the valve box inside the strainer 3.
A main valve body 4 is mounted to the main valve seat 5. A bypass valve body 10 screwed onto a valve stem 11 is built into the center of the upper part of the main valve body 4, and a steam chamber 22 is provided therein.

A bypass valve seat 21 is formed in a portion of the upper wall of the steam chamber 2 . The valve stem 11 is a guide bush 1 attached to the main valve body 4
2 so as to be slidable in the axial direction, and the guide bush 12 is held down by a keeper ring 13. The main valve body 4, keeper ring 13, and valve stem ill are prevented from rotating by a key 14 and a pin 15, respectively. A valve cap 16 having a stopper portion 18 for receiving the shoulder portion 17 of the bypass valve body 10 is fixed to the upper part of the bypass valve body 10 by bolts 32 . On the outer periphery of the valve cap 16, there is an annular steam chamber 20 formed by the valve cap 16, the bypass valve body 10, and the main valve body 4.
The main valve body 4 is provided with a passage 23 that connects the steam chamber 22 and the inside of the expansion tube 7. When the drive device is actuated by a signal from a bypass valve control device (not shown) and the valve stem 11 is pushed up, the bypass valve body 10 rises together with the valve seat 21.
move away from

That is, the bypass valve opens. At this time, the main valve body 4 remains fully closed. Further, the valve stem 11 rises, and the shoulder portion 17 of the bypass valve body 10 moves to the stopper portion 18 of the valve cap 16.
By pushing it up, the main valve body 4 separates from the main valve seat 5 and opens. On the other hand, when the bypass valve first opens, steam enters the annular steam chamber 20 through the passage section 19 of the valve cap 16.

Therefore, the pressure in the circumferential direction is equalized, and the bypass valve body 1
0 and the bypass valve seat 21, and enters the annular steam chamber 22 in the main valve body 4 centered on the valve stem 11, where the pressure in the circumferential direction is equalized, and then the passage It flows into the expansion tube 7 through the section 23, and further flows out to the turbine through a downstream control valve. By changing the lift of the bypass valve mentioned above, the area of the valve throttle part is changed to control the steam flow rate. However, because the flow rate is in a small flow range, the pressure difference before and after the valve is large, and the flow passing through the valve is Become supersonic.

Therefore, in this valve structure in which the flow downstream of the valve largely changes the flow direction near the surface of the valve stem 11, the valve stem 11
is subject to large shocks due to changes in the flow vector. Therefore, if heavy foreign matter such as drain or metal oxide is included in the steam, the impact will cause erosion as described above, and in extreme cases, the valve stem 11 will break.
An object of the present invention is to provide a main steam stop valve with a bypass valve that can reduce erosion of the valve stem.

Hereinafter, one embodiment of the present invention will be explained with reference to FIG. In this figure, the same reference numerals as in FIGS. 1 and 2 represent the same or equivalent elements. A point A, a closing position between the bypass valve body 10 and the bypass valve seat 21;
The minimum cross-sectional area point 24 of the flow path is shifted in the axial direction, and both points A. ．． In the middle between point B, or in other words, on the downstream side of point A, there is an annular steam chamber 27 formed between the main valve body 4 and the bypass valve body 10.

In other words, a cut-off point A with a large inclination angle of at least 60 degrees with respect to the vertical valve shaft is provided at a position with a relatively large radius, and the jet velocity is softened downstream of the cut-off point A, thereby making the pressure uniform in the circumferential direction. (Furthermore, the flow path wall is tilted so that the angle 1 with respect to the valve shaft becomes gradually smaller, and at the point where the radius becomes small, the steam chamber 27 is narrowed with an angle of inclination of 30 degrees or less relative to the valve shaft.) At point B, there is a gap δ of 0.5 to 1.h in the axial direction when the bypass valve body 10 is fully closed.In this case, the radius of point A is R1
, the inclination angle is α1, the radius at point B is R2, the inclination angle is α2, and the lift height of the valve is h. If the following conditions are satisfied, the cross-sectional area of the flow path at point B is greater than the cross-sectional area of the flow path at point A. will also become smaller.

In equation (1), even if h is small, it is structurally not difficult to make the value on the left side larger than the value on the right side, so when the valve is closed, B
If the value δ of the axial gap allowed at the point is set to a certain small value, the above conditions can be satisfied with a considerably small value of the valve lift.
The upper part of the bypass valve body 10 is hollow, and a steam chamber 25 is formed by attaching a bypass valve cap 31 to the head thereof.

This steam chamber 25 is connected to an annular steam chamber 27 via several passages 29 that are machined in the bypass valve body 10 at right angles to the axis. Further, a small passage hole 26 in the axial direction, a steam chamber 30 connected to the passage hole 26, and a passage groove 29 connected to the steam chamber 30 are provided on the cylindrical surface portion of the bypass valve body 10 at the lower side of the steam chamber 25. The steam chamber 22 communicates with the inside of the expansion tube 7 via a passage section 23. A lower cylindrical portion of the bypass valve body 10 projects into the steam chamber 22, and a threaded portion of the valve rod 11 is fitted into a threaded hole provided therein.

That is, the screw fastening portion C between the bypass valve body 10 and the valve stem 11
is located far below, so that the valve stem is not affected by the jet flow from the bypass valve. As a result, even if erosion occurs due to the jet flow, only the bypass valve body 10 needs to be replaced. As the valve stem 11 rises, the bypass valve body 10, which had been closed at point A, begins to open.

At this time, point B has a certain gap. When the gap at point A becomes slightly larger, the minimum cross-sectional area of the flow path shifts to point B. Therefore, up to point B, the steam flow is below the speed of sound and has relatively little turbulence. Further, the flow is partially separated in the steam chamber 27 and enters the steam chamber 25 via the passage section 18 . Here the flow stagnates, the pressure is equalized, and the axial passage hole 26,
The pressure in the circumferential direction is made uniform through the steam chamber 30, and then flows out from the passage groove 29 as a cylindrical axial jet along the surface of the valve barrel. This jet stream prevents the supersonic flow from point B from colliding with the valve stem 11 and the surface of the lower part of the bypass valve connected thereto, thereby suppressing the occurrence of erosion. After that, the steam is pressure-equalized in the steam chamber 22,
It flows out downstream of the main valve body 4 through the passage portion 23 . In the valve of this invention, since there are two throttle parts in the steam flow path of the bypass valve, if the pressure ratio before and after the valve is equal, the velocity of the flow jetting out from point B will be considerably smaller than that of the conventional valve.

This is because the steam that has passed through the throttle at point A loses most of its kinetic energy in the steam chamber 27, and this reduction in flow velocity further reduces the occurrence of erosion. Furthermore, the angle of the jet flow from point B to the valve shaft is considerably smaller than that of conventional valves. It is known that when this angle is 30 degrees or less, erosion progresses very slowly. According to the invention described above, erosion of the valve stem can be suppressed and the gap for maintenance can be increased.

[Brief explanation of the drawing]

FIG. 1 is a vertical sectional view showing a conventional main steam stop valve with a bypass valve, FIG. 2 is an enlarged sectional view showing the main valve body and bypass valve body portions in FIG. 1, and FIG. 3 is an embodiment of the present invention. It is a longitudinal cross-sectional view showing an aspect. 7... Enlarged pipe, 10... Bypass valve body, 11... Valve stem, 12... Guide bush, 13... Keeper ring, 14... Key, 15... Pin, 17... Bypass valve body shoulder, 18... Stopper part, 19... Passage part, 20... ... Steam chamber, 21 ... Bypass valve seat, 22 ... Steam room, 23 ... Passage section,
24... Minimum flow path cross-sectional area point, 25... Steam chamber, 26... Passage hole, 27... Steam chamber, 2
8... Passage portion, 29... Passage groove, 30...
...Steam chamber, 31...Bypass valve cap.

Claims (1)

[Claims] 1. In a main steam stop valve with a bypass valve having a bypass valve body within the main valve body, the radial position between the closing position of the bypass valve body and the minimum cross-sectional position of the flow path is spaced apart in the axial direction,
With a bypass valve that forms a steam chamber that equalizes the steam pressure between both positions, and that the connection part of the bypass valve body to the valve stem is housed in the steam chamber that is formed below the flow path at the minimum cross-sectional position of the flow path. Main steam stop valve.