Patent Number: 052805069
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a main steam isolation valve particularly of a boiling water reactor (BWR) plant for suppressing oscillation of a valve disk of the steam isolation valve. Generally, in a BWR plant, a reactor pressure vessel is directly connected to a steam turbine through a plurality of main steam pipes, and first and second main steam isolation valves are incorporated to each of the main steam pipes inside and outside of the reactor container. The reactor pressure vessel is isolated as occasion demands by closing these main steam isolation valves. The steam used in the steam turbine is condensed into condensate in a condenser and the condensate is then returned to the reactor pressure vessel. In a conventional main steam isolation valve, inlet and outlet end portions of a valve body are connected to each of the main steam pipes and a valve disk is accommodated in the valve body in an axially reciprocal manner. The valve disk is provided with a valve shaft which is inclined inwardly, in an installed state, by about 45.degree. with respect to the flow direction of the mainsteam thereby to reduce flow resistance. The valve shaft is connected at one end to a driving means to reciprocatingly move the valve disk thereby to open or close the fluid, i.e. steam, passage. The reciprocating motion of the valve disk is guided with a central guide rib and a bilateral pair of side guide ribs inwardly projecting at rear side portions of the valve body circumferentially apart from the central guide rib with a separation angle of about 120.degree. with each other. In such a structure, when the valve disk is rested on a valve seat, the valve disk is fully closed and the valve disk is upwardly lifted thereby to fully open a valve port. In the conventional structure of the main steam isolation valve, since the bottom portion of the valve disk is held, at its fully opened state, with substantially the half portion thereof being projected into the fluid passage, the projected bottom portion is exposed to the steam flow, thus being subjected to fluid pressure in the main steam flowing direction and a direction normal thereto. Because the fluid pressure is in proportion to two squares of the fluid velocity, the valve disk may be oscillated in case of high fluid velocity. When the valve disk is oscillated, the central guide rib is rubbed with the valve disk with each other, resulting in the wear thereof and hence causing the leakage of the steam even in the case of the fully closed state of the valve. Furthermore, since the steam flow is prevented by the paired side guide ribs and the valve disk, the pressure is locally increased at the upstream side of the steam flow, and since a force for pressing the valve disk against the side guide rib sides, friction force between the guide ribs and the valve disk is increased, thus the degree of wear therebetween being also increased, resulting in the increased possibility of causing the leakage of the steam. SUMMARY OF THE INVENTION An object of the present invention is to substantially eliminate the defects and drawbacks encountered in the prior art described above and to provide a steam isolation valve, particularly of a main steam isolation valve for a reactor power plant, capable of effectively suppressing oscillation of a valve disk of the isolation valve thereby to improve a reliance thereof during the valve disk opening and closing operations. This and other objects can be achieved according to the present invention by providing, in one aspect, a steam isolation valve comprising a valve box provided with inlet and outlet portions through which a steam flows and an end opening, a valve disk accommodated in the valve body to be reciprocatingly movable therein along an inner peripheral surface of the valve body for opening and closing the steam flow inlet portion, a driving mechanism secured to the valve body and operatively connected to the valve disk for reciprocatingly moving the valve disk body in the valve body, and a coupling member applied to the end opening of the valve body for holding the valve disk when the valve disk is shifted to a position fully opening the inlet portion. The open end of the valve disk has a tapered surface which is firmly engaged with a tapered surface of the coupling member. In another aspect of the present invention, there is provided a steam isolation valve comprising a valve body provided with inlet and outlet portions through which a steam flows and an end opening, a valve disk accommodated in the valve disk body to be reciprocatingly movable therein along an inner peripheral surface of the valve body for opening and closing the steam flow inlet portion, a driving mechanism secured to the valve body and operatively connected to the valve disk for reciprocatingly moving the valve disk body in the valve body, and a tubular wall member integrally formed with the valve body, the valve disk being accommodated in an inner hollow portion of the tubular wall member with a gap therebetween when the valve disk is shifted to a position fully opening the inlet portion. In a further aspect of the present invention, there is provide a steam isolation valve comprising a valve body provided with inlet and outlet portions through which a steam flows and an end opening, a valve disk accommodated in the valve body to be reciprocatingly movable therein along an inner peripheral surface of the valve body for opening and closing the steam flow inlet portion, a driving mechanism secured to the valve body and operatively connected to the valve disk for reciprocatingly moving the valve disk body in the valve box, and a tubular wall member integrally formed with the valve body, and a steam flow guide means disposed for the valve body for guiding the steam flow from the inlet portion of the valve body. The guide means comprises a central guide rib disposed at the inlet portion of the valve body and side guide ribs formed integrally with an inner peripheral surface of the valve body at portions circumferentially apart from the central guide rib. The central guide rib may be formed so as to have a various arrangement for changing the steam flow velocity on both the side of the central guide rib at the inlet portion of the valve body. According to the characters of the present invention described above, in one aspect, when the valve disk is fully opened, the coupling member is detachably engaged with the valve disk. Since the coupling member is secured to the valve body, the valve disk is firmly secured to the valve body through the coupling member when the valve disk fully closes the inlet portion of the valve body, thus the oscillation of the valve disk due to the steam flow being suppressed. Accordingly, the friction and, hence, the wearing between the valve disk and the associated members slidably engaged with the valve disk can be significantly reduced, resulting in the prevention of the steam from leaking and in the improved reliance of the steam isolation valve itself. In another aspect, when the valve disk is fully opened, the valve disk is accommodated in the tubular wall member formed integrally with the valve body, so that the direct striking of the steam flow against the valve disk can be prevented. Accordingly, the oscillation of the valve disk due to the steam flow can be suppressed. The friction and, hence, the wearing between the valve disk and the associated members slidably engaged with the valve disk can be significantly reduced, resulting in the prevention of the steam from leaking and in the improved reliance of the steam isolation valve itself. In a further aspect, since the steam flow sectional areas are made asymmetric around the valve disk by the specific location or arrangement of the guide rib, the flow velocities of the steams on both the sides of the guide rib differ from each other and, hence, there causes a difference between the static pressures of the steam flowing on both the sides of the guide rib. Accordingly, unidirectional force is always caused from the high static pressure side steam from to the low static pressure side steam flow, thus effectively suppressing the oscillation of the valve disk. Furthermore, substantially the same effects as described with respect to the above aspects can be also attained.