High-pressure structure made of rings with peripheral weldments of reduced thickness

A high-pressure structure having a circular cylindrical metal shell made of metal rings joined together by weldments and which have peripheral areas of reduced shell thickness at the weldments which permit a reduction in the amount of weld metal deposited while still maintaining sufficient circumferential or hoop stress strength.

This invention relates to circular cylindrical metal shells, especially for 
high-pressure uses such as for high-pressure vessels and flow conductors, 
such as penstocks and blast tubes. More particularly, this invention is 
concerned with an improved high-pressure structure having a circular 
cylindrical metal shell made of metal rings joined together by weldments 
and which have peripheral areas of reduced shell thickness at the 
weldments which permit a reduction in the amount of weld metal deposited 
while still maintaining sufficient circumferential or hoop stress 
strength. 
BACKGROUND OF THE INVENTION 
Circular cylindrical metal shells are widely used in large civil 
engineering projects as, for example, penstocks in dam projects, blast 
tubes and in the fabrication of high-pressure structures, including 
vessels. 
One type of high-pressure vessel which is suitably employed in many 
industrial processes has a circular cylindrical shell body, which is 
generally positioned horizontally or vertically, and end closures which 
can be hemispherical, elliptical or conical shells or even be flat ends. 
Many pressure vessels of the described shape are shop fabricated and then 
transported to the site for erection. However, the large size and weight 
of some such high-pressure vessels prohibits shop fabrication so field 
fabrication at the site is necessary. 
Whether the high-pressure structure, such as a vessel, is shop or field 
fabricated, the cylindrical shell is generally made from metal rings which 
are joined together in consecutive order by weldments which connect 
abutting edges of adjacent rings. Because the metal rings often are made 
of metal plates 3 to 12 inches or more thick, and have a diameter of 3 to 
30 feet or more, the weldments used to join the rings are time consuming 
and costly to make. Furthermore, 8 inch thick rings are generally the 
maximum joined together in field or site fabrication. While vessels 
greater than 8" thick can be site erected, vessel construction gets more 
complicated and therefore more costly for vessels greater than 8" thick. 
Some of the complications are: 
A. Eight inches (8") is a practical limit for isotope radiation sources for 
radiographic examination of weldments. Therefore, when the welds are 
thicker than 8", a linear accelerator must be used for radiography which 
will require extensive shielding of the area during radiography. This will 
complicate the completion of these welds making the costs higher and 
requiring longer schedules. 
B. SA-387 Grade 22 Class 2 steel is one of the commonly referenced vessel 
materials used in the petroleum refining industry. This material requires 
post weld heat treatment (PWHT) hold times which are a function of weld 
thickness (Table AF-402.1 of ASME Section VIII, Division 2). As the 
material gets thicker more PWHT time is required and it is more difficult 
to provide material that can withstand these long PWHT times and still 
maintain the specified strength properties after PWHT. Therefore, for very 
thick walled vessels, the materials are more expensive per unit weight and 
are also available from fewer sources. 
C. For large site-erected heavy-wall vessels, individual cylindrical 
sections of the shell are assembled on the ground or at a shop 
manufacturing facility and then shipped to the site. These rings are then 
lifted into place and the girth seams welded together. The time required 
to complete these girth seam weldments has a very direct relationship to 
the construction schedule for heavy-wall vessels. The time and labor to 
complete the girth seam weldment increases as a quadratic function of the 
weld thickness. The overall cost and schedule for constructing very thick 
walled vessels has a negative impact on overall plant costs. 
Much of the above discussion also applies to the fabrication of metal 
penstocks, blast tubes and similar large size structures having metal 
shells which are open-ended. 
From the above it is clear that a need exists for improved circular 
cylindrical metal shells which can be used in high-pressure structures, 
including penstocks, blast tubes and similar open-ended shells, and also 
as part of thick-walled high-pressure vessels, which can be shop and field 
fabricated with lower costs and in a shorter time than has been previously 
possible. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention there is provided a circular 
cylindrical solid walled metal shell with inner and outer surfaces and 
being capable of withstanding an internal pressure for which the shell is 
designed, the cylindrical shell comprising a series of consecutive metal 
rings of essentially equal maximum thickness positioned in axial 
arrangement with abutting ends of adjacent rings being joined together by 
a weldment; and the axial length of the cylindrical shell portion 
comprising the abutting end portions of adjacent rings, and the weldment 
joining the abutting ends together, having a reduced thickness which is 
not more than about 0.90 of the maximum thickness of the rings. 
The described circular cylindrical shell can be open or closed at one or 
both ends. It can be used as a flow conductor, such as a penstock, blast 
tube, or for similar uses. 
According to a second aspect of the invention a solid walled high-pressure 
structure having a circular cylindrical shell with inner and outer 
surfaces and end closures and being capable of withstanding an internal 
pressure for which the structure is designed is provided in which the 
cylindrical shell comprises a series of consecutive metal rings of 
essentially equal maximum thickness positioned in axial arrangement with 
abutting ends of adjacent rings being joined together by a weldment; and 
the axial length of the cylindrical shell portion comprising the abutting 
end portions of adjacent rings, and the weldment joining the abutting ends 
together, has a reduced thickness which is not more than about 0.90 of the 
maximum thickness of the rings. 
Whether the shell is used as part of a high-pressure vessel, or a flow 
conductor, such as a penstock, blast tube or for some other use, the 
reduced thickness at the joints, in general, should not be less than 0.50, 
and desirably 0.67, of the maximum thickness of the rings. 
An important feature of the invention is that the axial length of each ring 
be 2.5 times the square root of the cylindrical shell internal radius 
times the maximum thickness of the cylindrical shell. This sizing of the 
rings assures that the grooves or areas of reduced thickness are not 
located too close together to provide the desired shell design strength. 
Only the internal surface, or only the external surface, of the cylindrical 
shell can be a substantially smooth cylindrical surface at the weldment 
joining abutting ends of adjacent rings. However, the reduced thickness of 
the axial length of the cylindrical shell can be located radially inward 
from the outer surface of the shell and radially outward from the inner 
surface of the shell. Thus, the reduced thickness area at the weldment can 
be located inwardly from both the outer and inner surfaces of the rings 
and shell. Furthermore, a shell can be fabricated having some of the 
joints smooth on the inside surface and some joints smooth on the outside 
surface. Thus, some of the areas of reduced thickness at the weldments can 
be only inside, and some only outside, the shell. 
The area of reduced thickness can define a peripheral groove extending 
around the outside of the structure with the groove having a maximum width 
axial of the structure adequate to deposit at least a portion of the 
weldment from outside the shell. However, the area of reduced thickness 
can also define a peripheral groove extending around the inside of the 
structure, with the groove having a maximum width axial of the structure 
adequate to deposit at least a portion of the weldment from inside the 
shell. Furthermore, the area of reduced thickness can be defined by 
peripheral grooves on both sides thereof so that one groove is on the 
outside, and another groove is on the inside, of the shell. 
The shape of the peripheral groove can vary and it can be symmetrical or 
asymmetrical. It can have a flat or curved bottom when viewed in section 
parallel to the vessel longitudinal axis and, in addition, the sides of 
the groove when similarly viewed can be sloped in a curved manner or have 
tapered surfaces. 
Regardless of the particular shape of the groove, the end portion of each 
ring wall from its thickest part to its thinnest part at the weldment, 
must be able to withstand the design maximum circumferential or hoop force 
for which the vessel is to be used. 
By producing a cylindrical shell from metal rings joined together by 
weldments as described very significant savings in fabrication costs are 
achieved without sacrifice in structure strength. Since the radial 
thickness of each weldment is reduced about 0.1 to 0.50 of the maximum 
shell thickness, less welding is needed and this reduces the cost and 
fabrication time substantially. Additionally, examination of weldment 
quality is more readily achieved, especially in the field, because of the 
reduced radial thickness of the weldment. 
The thickness of the rings generally will be in the range of 1 to 20 
inches, and usually will be at least 3 inches, and desirably 5 inches, 
thick for best utilization of the advantages of the invention. 
By use of the invention, it is possible to more readily fabricate shells 
for pressure vessels, penstocks, blast tubes and the like both in the shop 
and in the field. As an example, shell rings 12 inches thick can be joined 
together conveniently because the weldments need only be about 6 to 10.8 
inches thick at the peripheral groove at each of the weldments. 
The use of reduced thickness girth seam weldments can be most easily 
understood by a simplified review of the pressure load on a cylindrical 
shell. The required thickness for the cylinder in the circumferential 
direction can be approximated as t.sub.c =PR/S where; 
P=design pressure 
R=vessel radius 
S=allowable design stress 
The required thickness for the longitudinal direction is approximated by 
t.sub.1 =PR/2S. Therefore the nominal thickness for a cylindrical shell is 
controlled by the stress in the circumferential direction and is twice the 
required thickness for the longitudinal direction. Providing a local area 
of reduced thickness to 0.67, and even to just above 0.50, of the maximum 
shell thickness will still provide excess thickness for longitudinal 
stress. 
The reduced thickness area at the groove will create higher stress locally 
in the circumferential direction. However, since the total amount of 
material removed is small when compared to the remaining material, the 
ultimate capacity will not be significantly reduced. As the thinned area 
tries to stretch due to the higher stress, it will be restrained by the 
thicker material on either side of it. Therefore, some of the pressure 
loading in the thin area will be shifted to the thicker material. Bursting 
for ductile materials will not occur until all the material has fully 
yielded and is at the ultimate capacity of the material.

DETAILED DESCRIPTION OF THE DRAWINGS 
To the extent it is reasonable and practical, the same numbers will be used 
to identify the same are similar elements. 
With reference to FIG. 1, the vertical solid walled high-pressure vessel 20 
has a vertical shell or body 22, a hemispherical bottom closure 24 and a 
hemispherical top closure 26. The vessel is supported by legs 28. 
The vessel shell or body 22 is fabricated from a series of ten consecutive 
metal rings A to J of essentially equal maximum thickness positioned in 
axial arrangement with abutting ends of adjacent rings being joined 
together by a weldment 30 (FIG. 3). 
The adjacent abutting edges of rings E and F before weldment 30 is 
deposited is illustrated by FIG. 2. It should be understood that all the 
abutting adjacent edges of all the rings A to J are similarly shaped 
before welding. The maximum width of the area of fully reduced thickness 
100 at the joint is shown in FIG. 2 as W. The maximum depth of the area of 
reduced thickness 100 is shown in FIG. 2 as X. The weldment joining 
together the abutting edges of the rings E and F will have the thickness 
Y. X plus Y equals the radial thickness Z of rings E and F. The thickness 
Y, which equals the thickness of the weldment (FIG. 3) has a reduced 
thickness which is not more than about 0.90 of the ring thickness Z. 
However, Y is not less than 0.50 of Z. It is desirable for the edges of 
the rings E, F to be tapered or curved outwardly from the thinnest portion 
Y of each ring to the thickest part of each ring Z. Thus, as shown in 
FIGS. 2 and 3, each ring E, F is tapered outwardly 50, 52. As shown in 
FIG. 2, the adjacent abutting edges of rings E and F are beveled 36, 38, 
40, 42 to provide space for deposit of weld metal through the full 
thickness of the joint. Such beveled surfaces can have any suitable shape. 
Returning now to FIG. 2, the thickness Z of the rings used to fabricate 
shell 22 may be increased in thickness above the thickness V used for 
rings where the joints are made the full thickness of the rings to 
reinforce the joint area surrounding the area of reduced thickness. Thus, 
Z can be about 1 to 1.2 times the thickness V. 
The second embodiment of the invention illustrated by FIG. 4 is similar to 
that shown in FIGS. 1 to 3. Rings E1 and F1 in FIG. 4 are comparable to 
rings E and F in FIGS. 1 to 3. The embodiment of FIG. 4, however, will be 
seen to have the area of reduced thickness 100 on the inside of the shell 
rather than on the outside as shown in FIGS. 1 to 3. 
The third embodiment of the invention is illustrated by FIG. 5. In this 
embodiment, an area of reduced thickness 102 is located radially inward 
from the outer surface, and an area of reduced thickness 104 is located 
radially outward from the inner surface of the rings E2, F2 of shell 22. 
It is to be understood that rings E2, F2 are like rings E, F and E1, F1 
except for the adjacent edge portions of reduced thickness. 
FIG. 6 illustrates the shell 22 fabricated as described in connection with 
FIGS. 1 to 3 but with each end open so that it can be used as a flow 
conductor, such as a penstock, blast tube or for a similar purpose. 
EXAMPLE 
A pressure vessel is fabricated as illustrated by FIGS. 1 to 3 for a design 
pressure of 1800 psig and design temperature of 850.degree. F. using 
SA-387, Grade 22, Class 2 steel. The vessel is designed to meet the code 
requirements of ASME Section VIII, Division 2, para. AD-201. For an 
internal radius of 8 feet 8.5 inches, the thickness V of the rings A-J for 
full thickness weldments is 9 inches. However, by utilization of the 
invention, the joint thickness Y can be reduced to 6 inches or less, with 
W equal to 4.5 inches. To provide some reinforcement about the joints, 
rings can be used with Z equal to more than 9 inches. For Z equal to 
9.61", the extra thickness adjacent to the joint provides for full area 
replacement within the square root of the product of the radius and the 
thickness. 
The final dimensions of Z and Y as well as the slope and location of the 
reduction levels 36, 38, 40, 42 are to be determined by detailed stress 
analysis per the Appendix 4 criteria of ASME Section VIII, Division 2 or 
other similar design procedures. A variety of acceptable configurations 
can be provided. The actual configuration utilized will be determined by 
an overall cost analysis of the material costs, shop labor and field labor 
for a particular structure. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood 
therefrom, as modifications will be obvious to those skilled in the art.