Steam control valve with improved muffler portion

A steam control value for a steam chest such as used in a nuclear plant with an improved muffler portion. The muffler portion includes window openings of predetermined dimensions. A ring portion is below the window opening having a plurality of pockets. Each pocket is aligned with one of the window openings. This design greatly reduces vibrations which may damage the valve.

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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 there is shown a steam control valve 10 for a power 
plant steam chest 12. The steam chest 12 includes a hollow elongated 
central member 14. Central member 14 has a steam inlet means 16 passing 
therethrough in predetermined position as shown in FIG. 1. The central 
member 14 also has a steam outlet means 18 passing therethrough in 
predetermined position. Steam inlet means is at a relatively higher 
pressure than the steam outlet means. For example the pressure at the 
steam inlet 16 may be 1000 psia and the pressure at the steam outlet 18 
200 psia, for example. In a nuclear power plant the throttle valve 
controls the flow of steam from the steam generator to the steam chest as 
is known in the art. The central member has control valve aperture means 
22 passing therethrough in predetermined position. The control valve 10 
includes a cylindrical outer housing means 24 having a lip member 26 
affixed to the upper end 28 thereof. 
The lip member 26 has an outside diameter such as 40 inches which is 
greater than the control valve aperture 22 which typically has a diameter 
of 25 inches. The outer housing 24 passes through the control valve 
aperture and is maintained in predetermined position by the lip member 26. 
The outer housing 24 includes a bottom muffler portion 30. The bottom 
muffler portion 30 has eight window opening means therethrough 32 of 
predetermined dimensions such as 9.times.6.5 and spaced about the 
circumference of the bottom muffler portion. The bottoms 34 of the window 
openings 32 are a predetermined distance such as 4.0 inches above the 
bottom 36 of the bottom muffler portion 30. The bottom muffler portion 30 
of the outer housing 24 has an initial inside diameter such as 20 inches 
which is greater than the final inside diameter which is 19.4 inches of 
the remaining portion of the outer housing 24 above the muffler portion. 
A movable plug 38 is provided for controllably sealing the high pressure 
inlet means 16 from the relatively low pressure outlet means 18 as is 
known in the art. The movable plug 38 is coaxially aligned with the outer 
housing 24 and is slidable within the interior 40 of the outer housing. 
Shaft means 42 for moving the plug is provided as is known in the art. The 
shaft means 42 for moving the plug is affixed at one end 41a to the plug 
in predetermined position as shown in FIGS. 1 and 2. The shaft means 42 is 
affixed at the other end 41b to lifting means 43 which is conventional. 
The plug 38 preferably has groove means 44 therein about the circumference 
thereof in predetermined position as shown in FIG. 2. Circular seal ring 
means 46 is provided preferably split pressure rings. The circular seal 
ring means 46 is sized to be insertable into the groove means. The steam 
control valve 10 is described thus far as conventional. 
The improvement comprises a bottom ring portion 48. The prior art bottom 
ring portion is shown in FIGS. 2 and 3. As discussed previously the bottom 
ring portion of the prior art comprised a plurality of flowholes 49 
disposed therethrough and upper band 51 without flowholes adjacent the 
window openings. The bottom ring portion of the present invention is shown 
in FIGS. 4-8. The bottom ring portion 48 of the present invention has a 
plurality of pockets 50 therein about the inner circumference 52 thereof. 
Each of the tapered pockets 50 is of predetermined dimensions such as 
about 5 inches wide, 3.8 inches high and 1.8 inches deep at the top of the 
ring. Each of the pockets is aligned with one of the window openings 32. 
As shown in FIG. 6 each of the pockets 50 has a steam exit portion 52 
having an initial central diameter 54 substantially equal to the inside 
diameter of the bottom muffler portion 30 of the outer housing 24. Each of 
the pockets 50 has a final central diameter 56 at the steam entrance 
portion 58 of the pocket 50 a predetermined amount larger than the initial 
central diameter of the steam exit portion 30 of the outer housing 24 with 
the steam entrance portion 58 of each of the pockets 50 being disposed 
with in bottom 34 of the aligned window openings 32. The exit portion 52 
of the pocket 50 being a predetermined distance such as 0.20 inches above 
the bottom of the outer housing 24. A channel 60 is provided at the bottom 
of the muffler portion for engaging said central member 14 as shown in 
FIGS. 5 and 6 in the same manner as in the prior art. 
With reference to FIGS. 4 and 5 utilizing the present invention when the 
plug 38 is initially lifted by the lifting means 43, steam at the high 
pressure inlet 16 flows through the window openings 32 and through the 
pockets 50. As the plug is raised the steam flow through the pockets is 
increased without damaging vibration of the outer housing means and plug. 
Preferably the wall portion 62 of the ring portion 48 from the entrance 
portion 58 to the exit portion 52 of each of the pockets is offset from 
the vertical. The entrance portion 58 to the exit portion 52 is typically 
at an angle offset 26.degree. from the vertical as shown in FIG. 6. 
Preferably the side portions 64 of the ring portion 48 proximate said 
pockets have a predetermined curvature such as a 1.25 inches radius.