Patent Number: 047050713
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to power plant steam chests and in particular, to a steam control valve for a power plant steam chest with an improved muffler portion. In power plants such as nuclear plants, steam control valves are utilized in the steam chest which is in fluid communication with the saturated steam coming from the steam generator. The control valve includes a plug which controls the flow of steam to the outlets of the steam chest. The plug is raised to permit flow of steam to the outlets of the steam chest. The typical steam control valve has an outer housing including a bottom muffler portion having window openings therethrough. The bottom muffler portion both guides the valve plug and attentuates valve flow noise. The bottom muffler portion of the outer housing frequently experiences mechanical damage during operation. The types of damage encountered include cracking of the muffler strut portion and wear and cracking in the the bottom ring area. It has been thought that flow-induced vibration of the valve plug and/or muffler portions is the cause of the damage. It has been found in model tests that large-amplitude plug vibrations initiate above a threshold pressure ratio (or flow rate) for a given plug lift (valve opening). Below the threshold pressure ratio the vibrations appear to have a random character such as would be expected to be caused by turbulence; while above the threshold pressure ratio they have essentially a sinusoidal character, such as would be expected from a self-excited, flow-induced vibration mechanism. Above the threshold pressure ratio, the vibration of the plug increases rapidly and heavy impacting occurs between the plug and the muffler portion. One present muffler portion bottom ring design includes a series of flow holes with an upper band without flow holes adjacent the window openings. It has been found in model tests that this design has an inherent instability threshold pressure ratio (or flow rate) for a given valve lift, above which self-excited vibration initiates. The vibration is due to modulation of the flow through the flow holes and through the annular gap between the plug and the muffler portion. The modulation is caused by the relative vibratory motions of the plug and muffler portion. These vibrations may lead to valve damage and the field damage can be reasonably explained by the self-excited vibration mechanisms identified. It has been found that the self-excited vibration mechanism acting at low valve lifts is related to pneumatic hammer, a known cuase of severe vibration of shafts supported by hydrostatic bearings that use a gaseous fluid or steam. The bottom ring portion of the prior art muffler in the area of the flow holes is similar to a hydrostatic bearing while the plug is similar to a shaft. The self-excited vibration mechanism acting at higher lifts when the plug is in the upper band of the bottom ring is due to gap modulations. SUMMARY OF THE INVENTION The present invention eliminates the possibility of pneumatic hammer by removing the hydrostatic bearing features of the plug guide, i.e. flow holes, while still providing mechanical guidance for the valve plug in a flow opening that varies with plug lift. It also eliminates the possibility for gap modulation by removing the upper band in the bottom ring. The present invention is in combination with a steam control valve for a power plant steam chest. The steam chest typically includes a hollow elongated central member. The central member has a steam inlet means passing therethrough in predetermined position and a steam outlet means passing therethrough in predetermined position. The steam inlet means is at a relatively higher pressure than the steam outlet means. Throttle valve means are positioned proximate the inlet means. The central member has control valve aperture means passing therethrough in predetermined position. The steam control valve typically includes a cylindrical outer housing having a lip member affixed to the upper end thereof. The lip member has an outside diameter greater than the control valve aperture. The outer housing passes through the control valve aperture and is maintained in predetermined position by the lip member. The outer housing includes a bottom muffler portion. The bottom muffler portion has window openings of predetermined dimensions therethrough spaced about the circumference thereof. The bottoms of the window openings are a predetermined distance above the bottom of the bottom muffler portion. The bottom muffler portion of the outer housing has an initial inside diameter greater than the final inside diameter of the remaining portion of the outer housing. A movable plug for controllably sealing the high pressure inlet means from the low pressure outlet means is provided. The movable plug is coaxially aligned with the outer housing means and slidable within the interior of the housing. Shaft means are provided for moving the plug and affixed at one end thereof to the plug. Lifting means are provided affixed to the other end of the shaft means for lifting the plug. The plug has grooves therein about the circumference thereof in predetermined position. Circular seal rings of predetermined circumference are provided. The seal rings are sized to be insertable into the grooves. The steam control valve described thus far is conventional. The improvement comprises a bottom ring portion disposed between the bottom of the window openings and the bottom of the bottom muffler portion. The bottom ring portion has a plurality of pockets therein about the inner circumference thereof. Each of the pockets has a steam exit portion having an initial central diameter equal to the inside diameter of the bottom portion of the outer housing. Each of the pockets have a final central diameter at the steam entrance portion of the pocket a predetermined amount larger than the initial center diameter of the outer housing. The steam entrance portion of each of the pockets are disposed within the bottom of the aligned window openings, the exit portion being a predetermined distance above the bottom of the outer housing, whereby when the plug is initially lifted by the lifting means, steam at the high pressure inlet means flows through the window openings and through the pockets. As the plug is raised the steam flow through the pockets is increased without damaging vibration of the outer housing and plug. Preferably, the side of the pocket between the entrance portion of the pocket and the exit portion of the pocket is offset at an angle, typically 26.degree. from the vertical.