Page 129 - Pressure Vessel Design Manual

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Design of Vessel Supports

SUPPORT STRUCTURES

There are various methods that are used in the support used for elevated temperatures to minimize discontinuity
structures of pressure vessels, as outlined below. stresses at the juncture by maintaining a uniform tempera-
ture in the region. In addition, skirts for elevated tempera-
0 Skirt Supports ture design will normally be insulated inside and outside for
1. Cylindrical several feet below the point of attachment.
2. Conical There are various methods of making the attachment weld
3. Pedestal of the skirt to the shell. The preferred method is the one in
4. Shear ring which the center line of the shell and skirt coincide. This
0 Leg supports method will minimize stresses at the juncture. Probably the
1. Braced most common method, however, is to make the OD of the
a. Cross braced (pinned and unpinned) skirt match the OD of the shell. Other methods of attach-
b. Sway braced ment include lap-welding, pedestal type, or a shear ring
2. Unbraced arrangement. The joint efficiency of the attachment weld
3. Stub columns also varies by the method of attachment and is usually the
0 Saddle Supports governing factor in determining the skirt thickness. This
0 Lug supports weld may be subject to cracking in severe cyclic service.
Ring Supports Because the skirt is an attachment to the pressure vessel,
0 Conbination Supports the selection of material is not governed by the ASME Code.
1. Lugs and legs Any material selected, however, should be compatible with
2. Rings and legs the vessel material in terms of weldability. Strength for
3. Skirt and legs design is also not specified for support material by the
4. Skirt and ring girder ASME Code. Usually, in the absence of any other standard,
the rules of the AISC Steel Construction Manual will be
Skirt Supports utilized. For elevated temperature design, the top three
feet of skirt at the attachment point should be of the same
One of the most common methods of supporting vertical material as the shell.
pressure vessels is by means of a rolled cylindrical or conical The governing conditions for determining the thickness of
shell called a skirt. The skirt can be either lap-, fillet-, or the skirt are as follows:
butt-welded directly to the vessel. This method of support 1. Vessel erection
is attractive from the designer’s standpoint because it mini- 2. Imposed loads from anchor chairs
mizes the local stresses at the point of attachment, and the 3. Sktrt openings
direct load is uniformly distributed over the entire circum- 4. Weight + overturning moment
ference. The use of conical skirts is more expensive from a
fabrication standpoint, and unnecessary for most design
situations. Leg Supports
The critical line in the skirt support is the weld attaching
the vessel to the skirt. This weld, in addition to transmitting A wide variety of vessels, bins, tanks, and hoppers may be
the overall weight and overturning moments, must also resist supported on legs. The designs can vary from small vessels
the thermal and bending stresses due to the temperature supported on 3 or 4 legs, to very large vessels and spheres
drop in the skirt. The thinner the skirt, the better it is able up to 80 feet in diameter, supported on 16 or 20 legs.
to adjust to temperature variations. A “hot box” design is Sometimes the legs are also called columns or posts.

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