Page 130 - Pressure Vessel Design Manual

110 Pressure Vessel Design Manual

Almost any number of legs can be used, but the most Saddle Supports
common variations are 3, 4, 6, 8, 12, 16, or 20. Legs
should be equally spaced around the circumference. Usually, horizontal pressure vessels and tanks are sup-
Leg supports may be braced or unbraced. Braced legs are ported on two vertical cradles called saddles. The use of
those which are reinforced with either cross-bracing or sway- more than two saddles is unnecessary and should be avoided.
bracing. Sway braces are the diagonal members which trans- Using more than two saddles is normally a stress-related
fer the horizontal loads, but unlike cross braces, they operate issue, which can be solved in a more conventional manner.
in tension only. The diagonal members in a sway-braced The reason for not using more than two saddles is that it
system are called tie rods, which transfer the load to each creates an indeterminate structure, both theoretically and
adjacent panel. Turnbuckles may be used for adjustments of practically. With two saddles, there is a high tolerance for
the tie rods. soil settlement with no change in shell stresses or loading.
Cross braces, on the other hand, are tension and compres- Even where soil settlement is not an issue, it is difficult to
sion members. Cross braces can be pinned at the center or ensure that the load is uniformly distributed. Obviously
unpinned, and transfer their loads to the legs via wing plates there are ways to accomplish this, but the additional expense
or can be welded directly to the legs. is often unwarranted. Vessels 40-SO ft in diameter and 150 ft
Bracing is used to reduce the number or size of legs long have been supported on two saddles.
required by eliminating bending in the legs. The bracing As with all other types of supports, the ASME Code does
will take the horizontal loads, thus reducing the size of the not have specific design procedures for the design of saddles
legs to those determined by compression or buckling. or the induced stresses in the vessel. While the ASME Code
The additional fabrication costs of bracing may not warrant does have allowable maximum stresses for the stresses in the
the savings in the size of the legs, however. Bracing may vessel shell, the code does not specifically address the
also cause some additional difficulties with the routing support components themselves. Typically, the allowable
of any piping connected to nozzles on the bottom of the stresses utilized are those as outlined in the AZSC Steel
vessel. Construction Manual.
Legs may be made out of pipe, channels, angles, rectan- A methodology for the determination of the stresses in the
gular tubing, or structural sections such as beams or col- shell and heads of a horizontal vessel supported on saddles
was first published in 1951 by L. P. Zick. This effort was a
umns. Legs may be welded directly to the vessel shell or continuation of others' work, started as early as the 1930s.
head or may be bolted or welded to clips which are directly This procedure has been used, with certain refinements
attached to the shell. It is preferable if the centroid of the leg since that time, and is often called Zicks analysis, or the
coincides with the center line of the vessel shell to minimize stresses are referred to as Zicks stresses.
the eccentric action. However, this may be more expensive Zicks analysis is based on the assumption that the sup-
from a welding and fit up viewpoint due to the coping and
contouring necessary to accomplish this. ports are rigid and are not connected to the vessel shell. In
reality, most vessels have flexible supports which are
Very large vessels and tanks may require a circumferential attached to the vessel, usually by welding. Whatever the
box girder, compression ring, or ring girder at or near the reason, and there are a myriad of them, Zicks assumptions
attachment point of the legs to distribute the large localized may yield an analysis that is not 100% accurate. These results
loads induced by the columns and bracing. These localized should, however, be viewed more in terms of the perfor-
stresses at the attachment point should be analyzed for the mance they have demonstrated in the past 45 years, than
eccentric action of the legs, overturning moments, torsion of in the exact analytical numbers they produce. As a strategy,
the ring, as well as the loads from any bracing. the procedure is successful when utilized properly. There
Whereas skirt-supported vessels are more common in are other issues that also would have an effect on the out-
refinery service, leg-supported vessels are more common in come of the numerical answers such as the relative rigidity of
the chemical industry. This may be due in part to the venti- the saddle-from infinitely rigid to flexible. The answers
lation benefits and the toxicity of the stored or processed should be viewed in light of the assumptions as well as the
chemicals. Legs should not be used to support vessels in necessity for 5-digit accuracy.
high-vibration, shock, or cyclic service due to the high loca- The saddle itself has various parts: the web, base plate,
lized stresses at the attachments. ribs, and wear plate. The web can be on the center line of the
Legs are anchored to the foundations by base plates, saddle or offset. The design may have outer ribs only or
which are held in place by anchor bolts embedded in the inner ribs only, but usually it has both. For designs in seismic
concrete. For large vessels in high seismic areas, a shear areas, the ribs perform the function of absorbing the lon-
bar may be welded to the underside of the base plate gitudinal, horizontal loads. The saddle itself is normally
which, in turn, fits into a corresponding recessed groove in bolted to a foundation via anchor bolts. The ASME Code
the concrete. does specify the minimum included arc angle (contact angle)