Dual-shank attachment design for omega seals

An improved apparatus and process for attaching welded omega seal segments to reactor heads, standpipes, mechanisms, and plugs comprises a first shank in combination with a second shank to attach an omega seal at a metal-to-metal interface.

FIELD OF THE INVENTION 
This invention relates to omega seals. More particularly, this invention 
relates to toroidal omega seals which form flexible fluid pressure 
boundaries at metal-to-metal interfaces and the process of attachment 
welding such seals. The seals of this invention are particularly well 
adapted to form fluid pressure boundaries at reactor head penetrations or 
standpipes. 
DESCRIPTION OF THE PRIOR ART 
Current practice is to attach omega seal halves to their respective 
foundations or support structures by welding, which enables the seals to 
be premachined into toroidal configuration before attachment and to be 
replaced if necessary in the event of damage. For example, as depicted in 
FIGS. 1a and 1b a mechanism omega seal comprises an outboard seal half 10 
and inboard seal half 12. The outer half is attachment welded to the head 
or standpipe 14, and the inner half 12 is attachment welded to the 
mechanism motor tube 16. 
Prior art attachment designs currently in use are the long-shank 18 and 
short-shank 20 designs as respectively shown in FIGS. 1a and 1b . Each 
design 18 and 20 utilizes a weld attachment of only a first shank 21. 
After attachment welds 22 of the seal halves are complete, a seal weld 
joint preparation groove 24 is machined into the crown 26 of each half as 
shown in FIG. 1. Upon final assembly, this joint is seal welded, forming a 
complete pressure boundary. To allow subsequent temporary removal of the 
mechanism or other component, this weld joint must be cut, reprepared and 
rewelded. 
Incumbent in this prior art method are distortions in both the radial and 
vertical directions which occur during the attachment welding of the seal 
halves to their foundations. Circumferential variations result in 
"wobble", which, in turn, causes irregularities in the final-machined seal 
weld preparation as shown in FIGS. 2a and 2b . As illustrated in FIGS. 2a 
and 2b , variations 28 in the amount of vertical shrinkage and variations 
30 in the amount of radial shrinkage and rotation around the circumference 
(from location 12 to location 12') result in irregularities in preparation 
cross section when the uniform preparation machining is performed. These 
irregularities lead to problems in producing seal welds of the required 
degree of integrity owing to the large accompanying variations in heat 
absorption patterns. The "wobble" producing these irregularities is 
especially pronounced in the case of short-shank attachment. 
An additional problem is the large inward radial shrinkages of the outer 
seal half due to seal welding, notably in the long-shank omega seal 10 
(FIG. 1a). These large radial shrinkages limit the ability of a seal weld 
to be cut and rewelded. For example, referring to FIGS. 3a, 3b and 3c, the 
variation in radial positions of the outer seal 10 through several rewelds 
is illustrated in relation to the constant-diameter seal weld preparation 
24. With each successive reweld, the radial shrinkage results in added 
detrimental imbalance of the preparation cross-section areas. The original 
lip diameters of the seal weld preparation 24 must be maintained for each 
successive cut. It has been shown that the flexibility of the long 
attachment shank contributes markedly to this shrinkage. A long-shank 
attachment weld is illustrated in U.S. Pat. No. 2,792,241 issued to R. J. 
Bondley et al. on May 14, 1957. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of this invention to provide an 
improved design for omega seal segments. 
It is a further object of this invention to provide a design for attaching 
omega seals which reduces the distortions in both the radial and vertical 
directions in response to attachment welding. 
A still further object of this invention is to control inward radial 
shrinkage of the outer half of the omega seal in response to seal welding. 
Yet, another object of this invention is to provide an improved method for 
attaching omega seal segments in a metal-to-metal interface. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be learned by practice of the invention. The 
objects and advantages of the invention may be realized and attained by 
means of the instrumentalities and combinations particularly pointed out 
in the appended claims. 
To achieve the foregoing objects and in accordance with the purpose of the 
invention, as embodied and broadly described herein, the omega seal 
structure of this invention comprises an omega seal structure comprising a 
pair of arc shaped spaced-apart members, each of the members oriented to 
each other to form approximately one-half of an omega seal and having one 
end disposed for welding at the apex thereof, each of the members 
including a first extended shank attachment at the second end respectively 
disposed for welding to respective first and second support structures and 
at least one of the members having a second shank attachment disposed from 
45.degree. to 135.degree. to the first shank suitable for welding at the 
second end to one of said support structures. 
Preferably the second shank is disposed at 90.degree. to the first shank. 
It is also preferred that an omega seal structure segment comprises an arc 
shaped member, the member defining a first apex end and a second end, a 
first extending shank welding attachment to the second end, and a second 
shank welding attachment attached to the second member end, the second 
shank disposed from 45.degree. to 135.degree. with respect to the first 
shank. 
The preferred method of attaching omega seal segments to spaced-apart first 
and second structures to form an omega seal, the omega segments having a 
pair of arc shaped spaced-apart members, each of the members oriented to 
each other to form approximately one-half of an omega seal and having one 
end disposed for welding at the apex thereof, each of the members 
including a first extended shank welding attachment, and a second shank 
welding attachment disposed from 45.degree. to 135.degree. to the first 
shank, consists of (1) welding each of said first shank segment 
attachments to the respective first and second support structures; (2) 
welding the second shank segment attachment to the respective first and 
second support structures; (3) machining a seal weld joint preparation 
groove at the apex of the segments; and (4) welding each segment together 
at the apex groove to form a closed omega seal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. 
Referring now to the drawings, FIG. 4 and FIG. 5 show the arc shaped omega 
seal segment of the present invention which uses a second shank 32 in 
combination with the first shank 34 and features an attachment weld 22 at 
each end thereof. 
In accordance with the invention, the first and second shanks 32 and 34 act 
in combination to limit attachment welding distortions. The second shank 
is disposed from 45.degree. to 135.degree. to the first shank. In this 
manner, the tendency of each of the attachment welds 22 to shrink in the 
width direction pulls each attachment shank in contact to a first support 
structure surface for support. For instance, as the first attachment weld 
22 cools, it tends to draw the seal downward, pulling the second shank 32 
against the foundation surface. This contact, in turn, restrains the 
completed seal from displacing vertically. The horizontal shank has the 
additional function of restraining the radial shrinkage of the seal due to 
the subsequent seal welding operations. 
Preferably shanks 32 and 34 are oriented in respective horizontal and 
vertical directions, or alternatively at right angles to one another. It 
is also apparent that shanks may be spaced at angles ranging from 
45.degree. to 135.degree. to each other with one in a horizontal or 
vertical plane and this spacing of the shanks is equally effective in 
reducing welding distortions. Preferably, each of the horizontal shanks 
may be of equal length to the length of the vertical shanks. In addition, 
the horizontal shank thickness can be less than the vertical shank 
thickness, but greater than the wall thickness of the omega seal portion. 
In this manner, the radial distortions and shrinkages are minimized. 
As here embodied, the seal is machined integrally with its attachment 
shanks from a forged ring, using vacuum-melted material for high seal 
integrity. Although the present invention is illustrated only in an outer 
seal application, it is equally applicable to inner seal halves as well. 
The concept need not be restricted to seals which bridge a vertical 
interface as in the case of mechanism seals, but could also be applied in 
instances where the sealed interface is horizontal as in the case of 
position indicator coil housing-to-motor tube seals in nuclear reactors. 
In accordance with the invention, the pair of omega seal segments having 
the apex of each point towards one another form an omega seal structure. 
In FIGS. 5a and b welds 22h and 22v at each shank and apex weld 36 form 
the completed omega seal between first and second spaced-apart support 
structures 14 and 16. In accordance with the invention, the method of 
attaching the omega seal to the spaced-apart structures consists of 
welding each of the first shank segments to each respective first and 
second support structures, welding each of the second shank segment 
attachments to the respective first and second support structure, 
machining a seal weld joint preparation groove 36 at the apex of the 
segments, and welding each segment together at the apex to form a closed 
omega seal. One of the dual shank segments may also be used in combination 
with a single shank segment where space limitations preclude the disposal 
of the second shank. 
According to the present invention, an attachment weld distortion is 
maintained of only 0.008 .+-. 0.002 inch vertical shrinkages as measured 
around the circumference of dual shank mechanism outer seals. This 
tolerance is in marked contrast with shrinkages as large as 0.052 .+-. 
0.007 inch obtained with a short-shank outer seal and 0.083 .+-. 0.016 
inch obtained with long-shank outer seals. The average radial shrinkage 
due to seal welding was 0.031 inch for the dual-shank outer seal as 
compared to 0.038 inch for a long-shank seal of the same geometry. 
As here embodied, the dual-shank seals with integral shanks and attachment 
weld preparations are machined using standard practices. There is adequate 
space between adjacent mechanism seals in most reactor designs to 
accommodate attachment welding and weld repair operations on adjacent 
horizontal shanks. 
The subsequent servicing operations of seal welding, seal weld cutting, and 
seal weld repreparation will be facilitated by the reduced seal welding 
distortions. The outer attachment weld can be interrupted as necessary to 
accommodate holddown studs adjacent to the seal. 
From a stress viewpoint, the dual-shank seal is nearly the same as an 
integrally attached seal in response to foundation displacements and 
pressurization loading. The dual-shank design is superior both to the 
short-shank and the long-shank attachment seal designs in that the latter 
designs sustain parasitic bending stresses due to pressurization. These 
stresses occur owing to the radial flexibility of the stilted shanks, 
which displace under pressurization and induce bending in the seal 
toroids. Further, the dual-shank seal attachment design exhibits a width 
shrinkage tendency pursuant to the attachment welds such that the shanks 
are pulled into supporting contact with their mating foundation surfaces. 
The shrinkage obtained in welding is thus used to advantage in this 
instance rather than endured as a usual welding hindrance. 
The invention provides a design for omega seal segments, an omega seal 
structure that reduces the welding distortion in radial and vertical 
directions, controls inward radial shrinkage, and provides an improved 
method for attaching omega seal segments in a metal-to-metal interface. 
Thus, it is apparent that there has been provided in accordance with the 
invention a dual-shank attachment omega seal that fully satisfies the 
objects, aims, and advantages set forth above. While the invention has 
been described in conjunction with specific embodiments thereof, it is 
evident that many alternatives, modifications, and variations will be 
apparent to those skilled in the art in light of the foregoing 
description. Accordingly, it is intended to embrace all such alternatives, 
modifications, and variations as fall within the spirit and scope of the 
appended claims.