Patent Number: 052805069
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, general aspect and conventional art of the present invention will be first described with reference to FIGS. 18 to 20. First, referring to FIG. 18, generally, in a boiling water reactor (BWR) plant, a reactor pressure vessel 1 is directly connected to a steam turbine 3 through a plurality, four for example in the illustration, of main steam lines 2, and first and second main steam isolation valves 5 and 6 are incorporated to each of the main steam lines 2 inside and outside of the reactor container 4. The reactor pressure vessel 1 is isolated as occasion demands by closing the main steam isolation valves 5 and 6. The steam used in the steam turbine 3 is condensed into condensate in a condenser 7 and the condensate is then returned to the reactor pressure vessel 1 through a condensate return line 8. In FIG. 18, a third main steam valve 9 is also incorporated on the way of the steam line 2, and reference numeral 10 denotes a header. Referring to FIG. 19, in a conventional main steam isolation valve 5 or 6 provided with a valve body 11, inlet and outlet end portions 11a and 11b of the valve body 11 are connected to each of the main steam lines 2 and a cylindrical valve disk 12 is accommodated in a valve disk accommodation portion 11c of the valve body 11 in an axially reciprocal manner. The valve disk 12 is provided with a valve shaft 13 which extends and is inclined inwardly, in an illustrated and installed state, by about 45.degree. with respect to the flow direction, arrowed in FIG. 19, of the main steam thereby to reduce flow resistance. The valve shaft is connected at one end to a driving means 14 to move reciprocatingly the valve disk 12 thereby to open or close the fluid passage. As shown in FIG. 20, the reciprocating motion of the valve disk 12 is guided by means of a central guide rib 15 and a bilaterally pair of side guide ribs 16a and 16b inwardly projecting at rear side portions of the valve body 11 circumferentially apart from the central guide rib 15 with a separation angle of about 120.degree. with each other. In such a structure, when the valve disk 12 is rested on a valve seat 17, the valve disk 12 is fully closed and the valve disk 12 is upwardly lifted thereby to fully open a valve port. The conventional structure of the main steam isolation valve, however, involves problems or defects described hereinbefore. Accordingly, the present invention conceived for substantially eliminating the described problems or defects encountered in the prior art will be described hereunder by way of preferred embodiments in conjunction with FIGS. 1 to 17, in which like reference numerals are added to those elements or members corresponding to those of the respective embodiments. FIG. 1 shows a longitudinal sectional view of a first embodiment of the main steam isolation valve according to the present invention. Referring to FIG. 1, a main steam isolation valve 21 includes a valve body 22 having a valve disk accommodation portion 22a in which a valve disk 23 is accommodated. The valve body 22 is provided with inlet and outlet end portions 22b and 22c connected to the main steam line 2, FIG. 18, and the main steam flows as shown by arrows from the inlet end portion 22b towards the outlet end portion 22c in the valve body 22. The valve disk 23 is composed of a bottomed cylindrical member and accommodated in the portion 22a so that a valve shaft 24 connected to the central portion of the bottom of the valve disk 23 inwardly, as viewed, i.e. in a steam flow direction, inclines by about 45.degree. with respect to the axis of the inlet end portion 22b. A flange portion 25 is formed to the upper, as viewed, end of the valve disk accommodation portion 22a to air-tightly close the opening thereof. A valve disk driving mechanism 26 is mounted to the flange portion 25 and comprises an output shaft 26a connected to the valve shaft 24 thereby to reciprocate the valve disk 23 in the axial direction thereof along the inner peripheral surface of the valve body 22 to open or close a valve port, i.e. fluid passage. Namely, as shown in FIG. 1, when the driving mechanism 26 is actuated to obliquely lift the valve disk 23, the valve port is fully opened, whereas when the valve disk 23 is seated on a valve seat 27 disposed around the valve port, the valve port is fully closed. Such reciprocating motion of the valve disk 23 is guided by a central guide rib 28 and a pair of side guide ribs disposed apart, in a circumferential direction of the valve disk 23, from the central guide rib 28 with a separation angle of about 120.degree. with each other. A coupling 29 is secured to the valve body 22 and detachably fitted to the opening of the valve disk 23 so as to abut against the flange portion 25. Referring to FIGS. 2A and 2B, the valve disk 23 has a bottomed cylindrical shape and has an upper open end having an inner tapered surface 23a. As shown in FIG. 1, the coupling 29 has a cylindrical body 29a having a lower open end portion having an outer tapered surface having a shape in conformity with the shape of the tapered surface 23a of the valve disk 23. Accordingly, when the coupling 29 is fitted into the valve plug 23, both the tapered surfaces 23a and 26a are engaged with each other. According to such a structure, when the valve disk 23 is fully open, the upper open end thereof is fixedly engaged with the coupling 26 and the valve disk 23 is secured to the valve body 22 through the coupling 29 and the flange portion 25, so that, when the valve disk 23 is fully opened, the oscillation thereof is significantly reduced even if the lower portion of the valve disk 23 projects towards the main steam passage. Accordingly, the friction of the valve disk 23, the central guide rib 28 and the side guide ribs caused by the oscillation of the valve disk 23 are reduced, resulting in the reduction of the wearing thereof, thus preventing the fluid from leaking during the fully closing operation of the main steam isolation valve 21. The coupling 29 may be formed as shown in FIGS. 3, 5 and 6 by reference numeral 30. Namely, referring to FIGS. 3 and 4, a tapered surface 30b is formed to the inner peripheral portion of a cylindrical body 30a of the coupling 30 and an outer tapered surface 23b is formed to the outer peripheral portion of the upper open end portion of the valve disk 23 so as to be firmly engaged with the inner tapered surface 30b of the coupling body 30a. In this modified embodiment, substantially the same effects as those described hereinbefore can be attained. As shown in FIGS. 6A and 6B, the coupling 30 may be composed of a plurality of arcuate coupling compartments each having a tapered surface as described above. FIGS. 7 and 8 represent a second embodiment of the main steam isolation valve 31, the driving means of the structure shown in FIG. 1 being eliminated, in which FIG. 7 is a longitudinal section thereof and FIG. 8 is a sectional view taken along the line VIII--VIII of FIG. 7. In the second embodiment, as shown in FIGS. 7 and 8, the main steam isolation valve 31 has such main feature as that a tubular wall 32 is provided integrally with the valve body 22 so as to surround the outer periphery of the valve disk with a slight gap therebetween. Namely, as shown in FIG. 8, the tubular wall 32 extends rearwardly, leftwardly as viewed, along the peripheral surface of the valve disk 23 and is integrated with the valve body 22 at the rear end portion of the tubular wall 32. According to this structure, since substantially the entire structure of the valve disk 23 is accommodated in the tubular wall 32 in the fully opened state of the valve disk 23, the direct striking of the steam flow on the valve disk 23 can be prevented, thus reducing the oscillation of the valve disk 23. This results in the reduction of the friction between the valve disk 23 and the central and side guide ribs, thus also reducing the wearing. Accordingly, the fluid can be prevented from leaking during the fully closing operation of the valve disk 23, thus improving the reliance of the valve disk 23 and, hence, the main steam isolation valve 31. In a modification shown in FIG. 9, the valve disk 23 may be formed so as to have outwardly arcuate bottom 33 thereby to effectively reduce resistance against the steam flow striking the outwardly protruded bottom 33 of the valve disk 23, thus stabilizing the steam flow and hence further improving the oscillation suppressing performance. FIG. 10 is a view similar to that of FIG. 8 and represents a third embodiment of a main steam isolation valve 41 according to the present invention. Referring to FIG. 10, reference numerals 42a and 42b denote fluid passages in the valve body 22 with the guide rib 28 being disposed at the central position of the passages. In this embodiment, one side peripheral wall of the valve body 22, on the side of the fluid passage 42a in the illustration, is formed to be thick. Namely, an inwardly protruded portion 43 is integrally formed to the valve body 22 and the cross sectional flow area of the passage 42a is hence made smaller than that of the passage 42a, thus providing bilaterally asymmetric cross sectional area. In FIG. 10, reference numerals 44a and 44b denote a pair of side guide ribs for guiding the reciprocating motion of the valve disk 23 at the rear portion thereof. The side guide ribs 44a and 44b are formed integrally with the inner surface of the valve body 22 and disposed circumferentially apart from each other with a separation angle of about 120.degree. with the center being the central guide rib 28. According to this embodiment, when the main steam isolation valve 41 is fully opened, the flow velocity of the main steam passing the fluid passage 42a is made always lower than that flowing in the fluid passage 42a and, accordingly, the static pressure in the fluid passage 42b is always high. For this reason, there always causes one-sidedly unidirectional force for forcing the valve disk 23 from the passage 42b of high static pressure side towards the passage 42a of low static pressure side, so that the oscillation of the valve disk 23 reciprocating between these passages 42a and 42b can be then suppressed. In a further modification, it may be possible to form a protruded portion to the central guide rib 28a as shown in FIG. 11 instead of the location of the protruded portion 43 of FIG. 10. Referring to FIG. 11, the central guide rib 28a has a bilaterally asymmetric shape of substantially triangle so as to have a portion on the side of the fluid passage 42a having a sectional area smaller than that of a portion on the side of the other fluid passage 42b. Accordingly, this embodiment can attain substantially the same effect as that attained by the main steam isolation valve 41. The shape of the central guide rib 28a is not limited to such triangular shape and other modifications may be made as far as substantially the same effect is attained. FIGS. 12 and 13 represents further modifications of the central guide ribs of the present invention in which like reference numerals are added to elements or members corresponding to those used for the disclosure of the aforementioned embodiments or modifications. Referring to FIG. 12, the central guide rib 28b is disposed on the side of the steam flow passage 42a from the central portion of the flow passages 42a and 42b. According to this modified embodiment of the central guide rib 28b, substantially the same effect as that attained by the embodiment represented by FIG. 10 or 11 can be attained. Referring to FIG. 13, two central guide ribs 28c and 28d are arranged to the front, rightward as viewed, side of the valve disk 23 in symmetrical arrangement with respect to the valve shaft 24. According to this structure, the fluid pressure of the main steam is made stable between the two central guide ribs 28c and 28d, whereby the oscillation of the valve disk 23 can be significantly suppressed. FIG. 14 represents a fourth embodiment of the main steam isolation valve 51 according to the present invention, in which an inlet end portion 52a of a valve body 52 of the main steam isolation valve 51 stands vertically, as viewed, so as to be substantially parallel to side walls of the reactor pressure vessel 1 and the reactor container 4 (FIG. 18) and an outlet end portion 52b is mounted to a horizontally near portion with a predetermined angle. In this embodiment, the inlet side duct 53 connected to the inlet end portion 52a can be constructed to have a long length and it is also possible to make large a bent angle of an outlet side duct 54 connected to the outlet end portion 52b. According to these structures, the variation of pressure of the main steam flowing in the valve body 52 can be effectively made small, thus suppressing the oscillation of the valve disk 23. It is to be noted that the embodiment of FIG. 14 may be applied to the afore- mentioned embodiments. FIG. 15 shows an arrangement of the main steam isolation valve 51 shown in FIG. 14, in which the isolation valve 51 stands vertically so as to be substantially parallel to the side walls of the reactor pressure vessel 1 and the reactor container 4 (FIG. 18), and the valve shaft 24 is directed to the circumferential direction of the reactor pressure vessel 1. According to such arrangement, the length of the inlet side duct 53 is made sufficiently long and the bent angle of the outlet side duct 54 can be made large, whereby the variation of pressure of the main steam flowing in the valve box 54 can be significantly reduced and the oscillation of the valve disk 23 can be hence suppressed. FIG. 16 shows a portion of the valve body 22 in which the paired side guide ribs 44a and 44b are provided with cutout slits 60 for escaping pressure along the circumferential direction of the valve disk accommodation portion 22a. These slits 60 are formed to the side guide ribs 44a and 44b in their longitudinal directions with predetermined pitches. According to this structure, the main steam flows between the inner peripheral surface of the valve body 22 and the rear side of the valve disk 23 through the slits 60, thus the pressure of the main steam being made substantially even. Accordingly, the force for forcing the valve disk 23 against the side guide ribs 44a and 44b can be reduced, resulting in the reduction of the friction and wear between the side guide ribs 44a and 44b and the outer rear peripheral surface of the valve disk 23 and preventing the main steam from leaking at the time of fully closing the valve, and the reliance of the main steam isolation valve can be hence improved. As shown in FIG. 17, the valve disk 23 may be formed so that the valve disk body 23a thereof has a drum shape having a reduced diameter from the end portions towards the axially central portion thereof. According to this structure, the main steam is flows, as indicated by arrows, towards the rear side of the valve plug disk body 23a, i.e. the paired side guide rib sides, through the reduced diameter portion of the valve plug disk body 23a, whereby the pressure of the main steam can thus be made even and substantially the same effects as those described with reference to FIG. 16 can be achieved. It is to be understood that the present invention is not limited to the described embodiments and modifications and many other changes and modifications may be made without departing from the scopes of the appended claims. For example, embodiments or modifications constituted by the combinations of the described embodiments and modifications may be within the scope of the present inventions.