U.S. patent number 3,851,906 [Application Number 05/179,703] was granted by the patent office on 1974-12-03 for head closure system.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Harry N. Andrews, Erling Frisch.
United States Patent |
3,851,906 |
Frisch , et al. |
December 3, 1974 |
HEAD CLOSURE SYSTEM
Abstract
In order to reduce the time required for refueling a nuclear
reactor, the studs utilized for securing the head of the reactor
pressure vessel are provided with modified breechblock threads at
their lower ends, and also at their upper ends. After being
detensioned, the studs are rotated 60.degree. for disengagement
from corresponding threads in a flange at the top of the pressure
vessel and are lifted out of the stud holes when the head is
removed. The studs are tensioned and detensioned by a plurality of
portable hydraulic stud tensioning devices or tensioners which are
attached to the upper ends of the studs by the modified breechblock
threads at the upper ends of the studs. The tensioners are carried
by a separate hoist system mounted on the closure head for the
vessel.
Inventors: |
Frisch; Erling (Pittsburgh,
PA), Andrews; Harry N. (Export, PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
22657622 |
Appl.
No.: |
05/179,703 |
Filed: |
September 13, 1971 |
Current U.S.
Class: |
292/256.73;
220/327; 376/263; 376/463; 976/DIG.167; 292/251; 376/305;
976/DIG.175 |
Current CPC
Class: |
G21C
13/02 (20130101); B25B 29/02 (20130101); G21C
13/06 (20130101); B23P 19/067 (20130101); Y02E
30/30 (20130101); Y02E 30/40 (20130101); Y10T
292/1099 (20150401); Y10T 292/221 (20150401) |
Current International
Class: |
B23P
19/06 (20060101); G21C 13/00 (20060101); G21C
13/06 (20060101); G21C 13/02 (20060101); B25B
29/02 (20060101); B25B 29/00 (20060101); E05c
005/04 () |
Field of
Search: |
;52/224 ;85/1L,1T,42
;176/87 ;220/3,44,55D,46R,46MS ;292/251,256,256.67,256.73 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Moore; Richard E.
Attorney, Agent or Firm: Campbell; J. R.
Claims
We claim:
1. A head closure system for a generally cylindrical pressure
vessel and a closure head having outwardly extending mating flanges
with a plurality of vertically aligned holes therein, comprising
studs rotatably disposed in said holes to retain the head on the
vessel, each stud having a plurality of separate parallel
horizontally extending interrupted threads at its lower portion and
also at its upper portion, each hole in the vessel flange having
threads therein corresponding to the threads at the lower portion
of each stud, said threads being aligned in vertical sections with
gaps between sections having a greater angular extent than the
angular extent of the thread sections, said stud being rotatable to
align the thread sections on the studs with the thread sections in
the vessel flange, portable tensioning means adapted for mounting
on said closure head for applying tension on each stud to retain
the head on the vessel, and said tensioning means having threads
therein corresponding to the threads at the upper portion of the
stud for attaching the tensioning means to the stud.
2. The head closure system defined in claim 1, including a
rotatable member mounted in the tensioning means and containing the
threads corresponding to the threads at the upper portion of the
stud; and a device on said tensioning means for turning said
rotatable member to selectively align the threads thereon with the
stud upper portion threads to thereby permit said tensioning means
to selectively elongate said stud.
3. The head closure system defined in claim 2, wherein the
rotatable member comprises a vertically disposed shaft, and
including thrust bearing means at the lower end of the shaft
supporting the shaft on the upper end of the stud.
4. The head closure system defined in claim 3, including an
extension on the shaft extending below said bearing means and
containing the threads corresponding to the threads at the upper
portion of the stud.
5. The head closure system defined in claim 2, including additional
means for rotating said stud through a predetermined number of
degrees separately from the rotatable member in the tensioning
means.
6. The head closure system defined in claim 5, including locking
means for preventing rotation of the stud during rotation of the
rotatable member in the tensioning means.
7. The head closure system defined in claim 4, wherein the
tensioning means includes at least one hydraulically actuated
piston surrounding the shaft.
8. The head closure system defined in claim 7, including a member
fixed on the shaft for transmitting thrust from the piston to the
shaft, and an adjustable member on the stud below the extension on
the shaft to which the stud loading is transferred when the
hydraulic pressure is removed.
9. The head closure system defined in claim 8, including mechanical
means for actuating the adjustable member.
10. In a head closure system for a generally cylindrical pressure
vessel and a closure head having outwardly extending mating flanges
with a plurality of vertically aligned holes therein, in
combination, holding studs disposed in said holes to retain the
head on the vessel, each stud having a plurality of separate
parallel horizontally extending interrupted threads at its upper
portion, portable tensioning means for applying tension on each
stud to retain the head on the vessel, said tensioning means having
threads therein corresponding to the threads at the upper portion
of the stud for attaching the tensioning means to the stud, said
threads being aligned in vertical sections with gaps between
sections having a greater angular extent than the angular extent of
the thread sections, a rotatable member mounted in the tensioning
means and containing the threads corresponding to the threads at
the upper portion of the stud, means for rotating said member to
align the thread sections in the member with the thread sections on
the stud, a generally cylindrical shroud structure mounted on the
head, a circular track supported by the shroud structure, and at
least one hoist mechanism movably mounted on the track for handling
the portable tensioning means.
11. The combination defined in claim 10, including alignment guides
attached to the shroud structure for aligning the tensioning means
with reference to the studs.
12. The combination defined in claim 10, wherein the circular track
includes an I-beam having a rail secured to its top flange, the
hoist mechanism comprises a trolley carriage having a flanged
roller riding on the rail, and safety latch means disposed in close
proximity to the bottom flange of the I-beam to prevent derailment
of the trolley carriage.
13. The combination defined in claim 12, wherein the latch means
includes a shaft rotatably mounted on the trolley carriage and
means operable by the shaft into close proximity to the bottom
flange of the I-beam.
14. The combination defined in claim 13, including a handle for
rotating the latch shaft, and a spring-biased latch pin for
retaining the handle in preselected positions.
15. A tension load distribution system, comprising a holding member
having a hole therein for receiving a tension stud member, said
stud member having a plurality of separate parallel horizontally
extending interrupted threads thereon, said holding member having
threads in the hole corresponding to the threads on the stud
member, said threads being aligned in vertical sections with gaps
between sections having a greater angular extent than the angular
extent of the thread sections, one of said members being rotatable
to align the thread sections on the stud member with the thread
sections in the holding member, stud tensioning means mounted on
said holding member for elongating the stud member when the stud
member and holding member threads are in alignment, and said stud
member having an axially extending hole therein with varying
diameters which respectively correspond with said threads mounted
on the stud member, thereby to permit the application of equal
loading on all stud members threads when the stud member is
elongated by said stud elongation means.
16. The tension load distribution system defined in claim 15,
wherein the hole is substantially coextensive with the threads on
the stud.
17. The tension load distribution system defined in claim 15,
wherein the diameters become smaller in steps extending from the
entrance to the hole.
18. The tension load distribution system defined in claim 17,
wherein the decreasing hole diameter steps provide corresponding
stud sections having increasing cross-sectional areas.
19. The tension load distribution system defined in claim 18,
wherein the number of stud sections corresponds to the number of
threads on the stud.
20. The tension load distribution system defined in claim 15,
wherein the holding member thread pitch is the quantity (P + Y)
where P is the stud member thread pitch and Y is the elongation per
pitch under maximum stud load.
21. The tension load distribution system defined in claim 20,
wherein the loading on all threads is substantially equal when
maximum load is applied on the stud.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application relates to an application, Ser. No. 179,645
entitled "Head Closure Mechanism" filed Sept. 31, 1971 by Erling
Frisch, Harry N. Andrews and P. B. Haga and assigned to the
Westinghouse Electric Corporation.
BACKGROUND OF THE INVENTION
This invention relates, generally, to pressure vessels and, more
particularly, to systems for securing and releasing the closure
heads of nuclear reactor vessels
Heretofore, the closure heads of large nuclear power reactors have
been secured to the pressure vessel by a large number of studs
which are threaded into the vessel flange and extend through
corresponding holes in the head flange. The necessary loading on
the studs has been achieved by hand tightening of the upper nuts
while the studs are being preloaded by a small number, usually
three, of portable hydraulic tensioning devices which are applied
to the studs in a prearranged sequence until all studs are equally
loaded.
In order to remove the closure head for refueling, the studs must
be tensioned until the nuts become unloaded and can be backed off
by hand. Following this, all studs are completely removed and
stored before the closure head is lifted off and refueling
operations can commence. On a typical reactor, the studs are seven
inches in diameter and there are eight threads per inch covering
nine inches of stud length. Therefore, approximately 140 complete
revolutions of each stud are required for removal and subsequent
replacement.
Also, each of the portable tensioners is connected to a stud by
means of a six-inch long nut, forming part of the tensioner
actuator shaft, to an upper threaded extension of the stud.
Accordingly, approximately 100 revolutions of the shaft are
required for each application of the tensioner and two applications
per stud are normally required to obtain the proper stud
loading.
Considering that a typical four-loop reactor has 56 studs, each
weighing 600 pounds, and that great care must be taken during the
turning to prevent galling of the threads, it is realized that
considerable time is required to remove the closure head after
shutdown and to replace it after refueling. Actually, the time
required for each of these operations averages three days if there
are no complications. In order to make possible more rapid
refueling of a nuclear reactor, it is necessary to reduce this
time.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention, the necessary
reduction in the time required to remove and to replace the closure
head by an operating crew is achieved by replacing the prior studs
with a smaller number, for example 36, of larger diameter studs and
by utilizing a modified breechblock system for attaching the studs
to the pressure vessel flange as well as for connecting them to
portable hydraulic tensioners. The tensioners are carried by a
separate hoist system mounted on a shroud structure encircling the
reactor control rod drive mechanisms. The shroud structure is
mounted on top of the closure head. The engagement or disengagement
of the studs to either the pressure vessel or to the tensioners is
accomplished by a 60.degree. rotation of the studs or the tensioner
shafts, requiring only a relatively short time on the part of the
operating crew. After all studs have been detensioned or unloaded
and rotated to the "clear" position, they are simultaneously lifted
out of the stud holes when the closure head is removed. The studs
remain in place on the closure head during the refueling operation,
thereby saving the time previously required for separate removal
and storage. Because of the relatively large clearances provided
between adjoining modified breechblock threads when the studs are
detensioned, there is a minimum possibility of galling of the
threads.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the nature of the invention,
reference may be had to the following detailed description, taken
in conjunction with the accompanying drawings, in which:
FIG. 1 is a view, in elevation, of a head closure system embodying
principal features of the invention;
FIGS. 2a and 2b, when joined end-to-end, constitute a sectional
view through a portion of a reactor vessel and closure head and one
of the holding studs and a portable tensioning device;
FIG. 3 is a view, in plan, of the structure shown in FIG. 2a.,
FIG. 4 is a view, in section, taken along the line IV--IV of FIG.
2b.,
FIG. 5 is an enlarged view, in section, taken along the line V--V
in FIG. 2b.,
FIG. 6 is an enlarged detail view taken along the line VI--VI in
FIG. 1, partly in section and partly in side elevation, of a hoist
for one of the portable tensioning devices;
FIG. 7 is a view, in front elevation, of the hoist shown in FIG.
6;
FIG.8 is a view, in section, of a portion of a modified holding
stud; and
FIG. 9 is a view, in section, taken along the line IX--IX in FIG.
8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, particularly to FIGS. 2A and 2B, the
structure shown therein comprises a portion of a generally
cylindrical reactor vessel 10 having an integral outwardly
extending flange 11, and a closure head 12 having an integral
outwardly extending flange 13 which mates with the flange 11 on the
vessel 10. The vessel 10 may be of a type, well known in the art,
suitable for use in a nuclear power system.
As shown more clearly in FIG. 1, the head 12 is retained on the
vessel 10 by means of a plurality, for example 36, of holding studs
14. As shown in FIG. 2B the studs 14 are disposed in vertically
aligned holes 16a and 16b in the head flange 13 and the vessel
flange 11, respectively. The hole 16a extends through the head
flange 13 and the hole 16b terminates in the vessel flange 11. As
shown in FIG. 1, the reactor is in operating condition with all
studs 14 in position on the flange 13 of the head 12 with tension
applied. The necessary tensioning of the studs is produced by a
small number, for example three, of portable hydraulic tensioning
devices 22 which are applied to the studs in a prearranged
sequence.
As explained hereinbefore, it is desirable to reduce the operating
time required to remove and to replace the closure head during a
refueling operation. In accordance with the present invention, the
necessary reduction in operating time is achieved by utilizing a
modified breechblock system for attaching the holding studs to the
pressure vessel flange as well as for connecting them to the
hydraulic tensioning devices or tensioners. Engagement or
disengagement of the studs to either the pressure vessel or to a
tensioner is accomplished by a 60.degree. rotation of the studs or
the tensioner shaft.
As shown in FIGS. 2A and 2B, each stud 14 has a plurality of
separate parallel horizontally extending interrupted threads 17a at
its lower portion and similar threads 17b at its upper portion.
Each hole in the vessel flange has threads 18a therein
corresponding to the threads at the lower portion of the stud.
Likewise, threads 18b, corresponding to the threads 17b in the
upper portion of the stud, are provided in a generally cylindrical
extension 19 on an actuator shaft 21 of the hydraulic tensioner 22
which will be described more fully hereinafter.
It should be noted that the interrupted threads 17a and 17b at the
lower end and the upper end, respectively, of the stud actually are
generated by cutting separate parallel grooves in the stud rather
than a continuous helical groove, which is the case with standard
breechblock threads. This modification is utilized to prevent any
contact with the corresponding threads when rotating the stud or
the tensioner actuator shaft to the "lock" position.
Because of the extreme difficulty in machining threads of this
type, to the required tolerances, directly in the vessel flange,
the threads 18a are cut in a separate bushing 23 which, in turn, is
attached to the flange 11 by standard or Acme threads 24 which may
be machined in the flange without difficulty. The completely
finished bushing 23 is threaded into the flange to the proper depth
and is locked in place by a pin 25 so that the interrupted portions
of the threads assume the same position on all bushings relative to
the vessel centerline. The threads 18b may be machined directly in
the extension 19 which is threaded onto the lower end of the
actuator shaft 21 and secured thereto by a pin 26.
The use of the bushings 23 has the additional advantage of
permitting selection of a material better suited for the
application than the vessel material. The bushings may also, if
desired, be plated with a corrosion resistant material, such as
nickel or zinc, to reduce the corrosive effect of water when the
vessel is submerged during the refueling period. It also makes it
possible to replace a bushing in case of accidental damage to the
main threads.
As shown more clearly in FIG. 5, in which the stud is shown in the
"clear" position, the threads on the stud and in the vessel flange
or bushing are aligned in vertical sections with gaps between
sections having a greater angular extent than the angular extent of
the thread sections. In the present structure the threads are
arranged in three vertical sections. Thus, after the studs have
been inserted in the bushings, they must be rotated 60.degree.
until thread sections 17a and 18a overlap as indicated in FIG.
5.
In order to avoid any change of interference between the threads
during the rotation, relatively large clearances are provided
between the horizontal surfaces of adjoining threads. As shown in
FIG. 4, each stud 14 may be rotated by means of a straight
cylindrical handle 27 projecting radially from the stud and
attached thereto by threading into the stud. Two stop pins 28,
attached by threading into the underside of the head flange 13, as
shown in FIG. 2B, limit the rotation in either direction. A hinged
locking bar 29 prevents accidental movement of the stud from either
position. A support 31 for the locking bar 29 is attached to the
flange 13 by screws 32. In order to avoid interference with normal
rotation of the stud, the locking bar 29 may be swung out of the
way to a position indicated by dot-dash lines in FIG. 2B.
As explained in the aforesaid copending application, stud rotation
is facilitated by reducing the frictional forces to a minimum. For
this purpose, a thrust ball bearing 33 is attached to the lower end
of each stud. The bearing 33, shown more clearly in FIG. 8, is
attached to the stud by means of a clamp plate 34 and a bolt 35. A
circular plate 36 having a large central hole serves as a seat for
the thrust bearing when the stud is in the inserted position. The
plate 36 is attached to the bushing 23 by means of several bolts
37. The vertical distance from the seat 38 of the support plate to
the bushing threads is so selected as to obtain maximum axial
clearance between stud and bushing threads in this position. The
stud is centered in the bushing by a taper 39 in the support plate
hole.
As explained hereinbefore, the necessary tensioning of the studs is
achieved by utilizing a small number, for example three, of the
portable hydraulic tensioning devices 22 which are applied to the
studs in a prearranged sequence. When in position for service, as
illustrated in FIG. 2A, the tensioner 22 rests on the flat upper
surface of the head flange 13 and is rotationally aligned with the
stud by an alignment pin 41. In general, the tensioner 22 comprises
two pistons 42 disposed in two cylinders 43 supported between a
base plate 44 and a top plate 45 which are held together by four
tie rods 46 as shown in FIG. 3. The pistons are mounted in tandem
to obtain the required stud loading without exceeding a hydraulic
pressure of 10,000 psi. A heavy cylindrical bushing 47 is attached
to the base plate 44, as by welding, and serves as the support for
the tensioner when it rests on the head flange 13. The bushing 47
encloses the upper portion of the stud 14.
The tensioner actuator shaft 21 is surrounded by the pistons 42. As
explained hereinbefore, the cylindrical extension 19 is fixed on
the lower end of the shaft 21. The extension 19 contains the
threads 18b corresponding to the threads 17b on the upper portion
of the stud 14. Sufficient clearances are provided between the
horizontal surfaces of the threads 17b and 18b to prevent
interference during subsequent rotation of the extension 19 to the
engaged or "lock" position. A thrust ball bearing 48, similar to
the bearing 33, is attached to the lower end of the actuator shaft
21. When the tensioner is positioned, the outer bearing race 49
rests on the upper flat surface of the stud 14 and the entire
weight of the actuator shaft assembly is carried by the bearing.
The bearing location is selected to obtain maximum axial clearance
between threads 17b and 18b. The bearing also reduces the effort
required to rotate the actuator shaft to the engaged or "lock"
position by means of a hand wheel 51. The correct amount of
rotation in either direction is insured by a pointer 52, fixed on
the actuator shaft, and two stop pins 53 located on the top plate
45.
After the actuator shaft assembly is in the "lock" position, the
tensioner is gradually pressurized by supplying hydraulic fluid
underneath the pistons 42 through openings 54 in the cylinders 43.
The developed upward thrust is transmitted through a spherical
washer 55 and a fixed member or nut 56 on the actuator shaft
assembly which moves freely upwardly until contact is established
between threads 17b and 18b. As shown, the nut 56 is threaded on
the upper portion of the shaft 21 and is fixed by a set screw 57.
Continued movement causes lifting of the main stud 14 until stud
threads 17a contact bushing threads 18a. Further application of
pressure results in tensioning of the stud until the desired
loading is obtained. At this point, the pressure is maintained at a
constant value while an adjustable member or main nut 58, which is
threaded on the stud 14, is tightened by hand until firm contact is
established with a spherical washer 59 and the head flange 13. When
the hydraulic pressure is reduced, the stud load is transferred to
the adjustable member 58 and the tensioner pistons are returned to
the lower position by application of hydraulic pressure above the
upper piston through an opening 61 in the upper cylinder 43.
The rotation of the adjustable member or main nut 58 is
accomplished in this example by a hand wheel 62 which drives a gear
63. The gear 63 causes rotation of a thin-walled tube 64 which is
constructed to permit a small vertical movement. Teeth 65, provided
in a thickened portion at the lower end of the tube 64, are
slidably disposed in slots 66 provided in the upper outer surface
of the main nut 58 to rotate the nut.
The actual value of the stud loading may be estimated by a
micrometer 67 on top of the actuator shaft which indirectly
measures the elongation or strain of the stressed part of the stud
by comparing its length with that of an unstressed rod 68 of the
same material located in a central hole in the stud. Since this
method is well known in the art, no further explanation of its
operation will be furnished herein.
After a stud has been properly tensioned and secured by the main
nut 58, the hand wheel 51 is rotated to the "clear" position to
disengage the threads 17b and 18b. The tensioner 22 may now be
lifted straight up by means of lifting lug 69 and hoist hook 70 and
transferred to another stud.
In preparation for removing the closure head 12 for refueling, the
tensioners 22 are again positioned on all studs in a prearranged
sequence and the following procedures executed to unload the studs.
The hand wheel 51 is moved to the "lock" position and hydraulic
pressure is applied under the pistons thereby stretching the stud
14 until the main nut 58 is completely unloaded. Next, the nut is
backed off a short distance, the piston pressure is removed, and
the pistons are returned to the lower position. This causes return
of the main stud 14 and the actuator shaft assembly to the lowest
position, as illustrated on the drawing. Following this, the
actuator shaft may be rotated to the "clear" position and the
tensioner removed. After all studs have been unloaded in this
manner, they are rotated to the "clear" position by means of the
handles 27 to disengage the stud threads, and the head may be
lifted with all studs in place in the head.
The upper, external part of a reactor with the herein described
Head Closure System is shown in FIG. 1. As illustrated, the reactor
is in operating condition with all studs 14 in position on the
flange 13 of the head 12 with tension applied. Control rod drive
mechanisms 71, extending from the top of the head to a missile
shield 72 are encircled by a generally circular shroud structure 73
which may be made of angle irons 74 and steel plates 75. For
convenience, the shroud structure may be made in three sections and
it is secured to the head 12. The missile shield 72 is attached
directly to the head 12 by several heavy rods 76. Ventilating holes
77 are provided near the bottom of the shroud for a natural
circulation cooling of the mechanism coils. The wiring for the
control rod mechanisms and also for nuclear instrumentation is
brought through multi-conductor cables 78 to a cable tray 79 which
is attached by hinge joints to the shroud structure 73.
The cable tray, which is at least 7 feet wide in a typical
structure, makes it impossible to service six or more of the head
closure studs, directly below the tray, by any overhead crane. For
this reason, and for convenience, a separate hoist system is
provided below the cable tray 79. A hoist mechanism 81 is provided
for each of the portable stud tensioners 22. The hoist mechanisms
travel on a circular track 82 which is supported on brackets 83
attached to the angle irons 74 of the shroud structure. The track
diameter is such that the hoist hooks 70 are directly on the stud
circle. Thus, the stud tensioners may be easily and rapidly lowered
into position on any of the closure studs by means of the hoist
mechanisms 81.
In order to avoid interference between the interrupted threads at
the upper part of the studs and on the tensioner actuator shaft
extension, the tensioner must be accurately aligned rotationally
during the lowering operation. This is achieved by providing
alignment guides 84 which are attached to the shroud structure 73.
An alignment guide 84 is provided for each stud 14. The guides 84
enter a slot 85, cut in the tensioner base plate 44, as indicated
in FIG. 3. If desired, the tensioners may be removed from the
reactor during normal operation by moving the hoists with the
withdrawn tensioners to a position on the track 82 accessible to an
overhead crane sling 86. This permits removal of hoist and
tensioner as a unit without requiring any transfer of load. The
removal of a stud may be accomplished in the same manner.
The details of a suitable hoist mechanism 81 are shown in FIGS. 6
and 7. Each hoist mechanism comprises a trolley carriage 87 having
two heavily flanged trolley wheels 88 which ride on top of the
circular track 82. The track 82 includes an I-beam 89 having a rail
91 secured to the top flange of the beam. The I-beam 89 is attached
to the brackets 83 by gusset plates 93. Motive power is provided by
an electric gear motor 92 connected to one of the trolley wheels
88.
In order to prevent accidental derailing of the trolley carriage
87, a safety latch 94 is provided to limit any upward movement to a
safe value. The safety latch 94 includes a shaft 95 rotatably
mounted on the trolley carriage 87 and a roller 96 rotatably
mounted on a pin attached to the end of a lever arm 97, secured to
the shaft 95. In the latched position, the roller is brought in
close proximity to the bottom flange of the I-beam 89. When it is
desired to remove the hoist from the track, the latch is made
inoperative by a 180.degree. movement by means of an operating
handle 98 which rotates the shaft 95 and increases the distance
between the roller 96 and the I-beam 89. A spring-biased latch pin
99 in the handle 98 enters holes 101 spaced 180.degree. apart in
the trolley carriage 87 to retain the handle in a preselected
position.
A lifting unit 102, which may be of the chain type, is supported by
the trolley carriage 87. The lifting unit is powered by an electric
gear motor 103 and is provided with a magnetic brake. Electric
power to the motors and brake is delivered through a cable (not
shown) from a floor level near the top of the pressure vessel.
In order to reduce the threaded length of each stud to a minimum,
it is important that the total load on a stud be distributed
equally on all threads. The modified stud structure shown in FIGS.
8 and 9 meets this requirement. The stud member 14A is shown lifted
from its bottom position until the lower thread 17a of the stud is
in contact with the lower thread 18a of the bushing of holding
member 23. However, no actual load is applied on the stud. It will
be noted that a gradually increasing clearance of Y to 5Y is
provided between successive mating threads, where Y represents the
elongation per pitch under maximum stud load. In effect, this means
that bushing thread pitch is (P + Y) as compared to P for the stud
thread pitch. In order to make this possible, while assuming
equally distributed load on all threads, it is necessary that the
stresses (psi) resulting from the maximum stud loading be the same
over the whole length of the threaded portion of the stud.
As illustrated, this is achieved by machining a centrally located
axially extending hole 106 with varying diameters in steps in the
lower portion of the stud 14a so that the remaining stud area is
proportional to the load carried by any particular section of the
stud. As shown, the diameters become smaller in steps until the
upper stud thread is reached. For a six-thread stud the lower
section, which carries only one-sixth of the total load, will have
a cross-sectional area of one-sixth of the maximum stud area. The
next section will have two-sixths of the maximum area and so on
until maximum area is reached at the top of the hole which is
substantially coextensive with the threaded portion of the stud.
Thus, the number of stud sections corresponds to the number of
threads on the stud.
If load is gradually applied to the stud, the lower stud section
will lengthen until the gap 1Y is closed. At this time, the load on
the lower thread is one-sixth of the total load and will not
increase beyond this value. Further increase in the stud load will
cause closing of gap 2Y and so forth until all gaps are closed when
full stud loading is reached with all thread loadings being
equal.
Because the corresponding sections of the bushing 23 have a much
larger area than the lower stud sections, it will have a negligible
effect on the value of Y. Also, the same holds true for the upper
section where the load is transferred to the pressure vessel flange
11 through threads 24. The stud hole 106 is covered at the lower
end by a cap 107 attached to the stud by bolts 108. The thrust ball
bearing 33 is centered on a hub of the cap and is secured by the
clamp 34 and bolt 35. The central rod 68, which is used in the
measurement of stud loading, now continues through the threaded
section and rests on the upper surface of the cap. This is
permissible since the elongation per inch of this section now is
the same as that of the stud itself. The plate 36, which serves as
a seat for the thrust bearing when the stud is in the bottom
position, is attached to the bushing 23 by bolts 37 as previously
described. The modified stud structure may also be used at the
upper stud end for connection to the hydraulic tensioner.
From the foregoing description, it is apparent that the invention
provides a head closure system which makes it possible to remove
and to replace the closure head of a nuclear reactor pressure
vessel in relatively short periods of time with a relatively small
amount of equipment. Therefore, the reactor refueling time and the
cost of the equipment are materially reduced by utilizing the
present system in which provision is made for quickly attaching
portable tensioners to holding studs and detaching the tensioners
from the studs.
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