Page 131 - Pressure Vessel Design Manual

Design of Vessel Supports 111

of 120". The tnaxinium efficient saddle angle is NO", since lugs are attached can be made thicker to reduce the local
the weiqht and saddle splitting force go to zero above the stress.
belt lin;. In effect, taking into account the 6" allowed for There are two solutions presented here for analyzing the
reduction of stresses at the horn for wear plates, the Max- shell stresses caused by the eccentric lug action. Method 1
imum angle becomes 168". was developed by Wolosewick in the 1930s a.. part of the
Saddles may be steel or concrete. They may be bolted, penstock analysis for the Hoover Dam Project. This method
welded, or loose. For the loose type, some form of liner utilizes "strain-energy'' concepts to analyze the shell as a thin
should be used between the vessel and the saddle,.The typi- ring. Thus, this method is frequently called "ring analysis."
cal loose saddle is the concrete type. Usually one end of the Ring analysis looks at all the loadings imposed on the artifi-
vessel is anchored and the other end sliding. The sliding end cial ring section and the influence that each load exerts on
may have bronze, oiled, or Teflon slide plates to reduce the the other.
friction caused by the thermal expansion or contraction of Method 2 utilizes the local load analysis developed by
the vessel. Bijlaard in the 1950s, which was further refined and
Longitudinal location of the saddles also has a large effect described in the WRC Bulletin 107. This procedure uses
on the Lagnitude of the stresses in the vessel shell aS well as the principles of flexible load surfaces. This procedure is
;I bearing on the design of the: saddle parts themselves. For more accurate, but more mathematically rigorous as well.
large diameter, thin-walled vessels, the saddles are best When making decisions regarding the de5ign of lugs, a
placed within 0.5R of the tangent line to take advantage of certain sequence of options should be followed. The follow-
the stiffening effect of the heads. Othrr vessels are best ing represents a ranking of these options based on the cost to
sripported where the longitudinal bending at the midspan fabricate the equipment:
is approximately equal to the longitudinal bending at the
saddles. Hoc\7ever, the niaximum distance is 0.2 L. 1. 2 lugs, single gusset
2. 2 lugs, double gussets
3. 2 lugs with compression plate
Lugs and Ring Supports 4. Add-reinforcing pads under (2) lugs
5. Increase size of (2) lugs
6. 4 lugs, single gusset
Lugs 7. 4 lugs, double gussets
8. 4 lugs with compression plates
Lugs offer one of the least expensive and most direct ways 9. Add reinforcing pads under (4) lugs
of supporting pressure vessels. They can readily absorb &a- 10. Increase size of (4) lugs
metra1 expansion by sliding over greased or bronzed plates, 11. Add ring supports
are easily attached to the vessel by minimum amounts of
welding, and are easily leveled in the field.
Since lugs are eccentric supports they induce compressive, Ring Supports
tensile, and shear forces in the shell wall. The forces from
thc. eccentric moments may cause high localized stresses that In reality, ring supports are used when the local stresses at
are combined with stresses from internal or external pres- the lugs become excessively high. As can be seen from the
sure. In thin-walled vessels, these high local loads have been previous list, the option to go to complete, 360-degree stif-
known to physically deform the vessel wall considerably. fening rings would, in most cases, be the most expensive
Such deformations can cause angular rotation of the lugs, option. Typically, vessels supported by rings or lugs are con-
which in turn can cause angular rotations of the supporting tained within a structure rather than supported at grade and
steel. as such would be subject to the seismic movement of which
Two or four lug systems are normally used; however, more they are a part.
may be used if the situation warrants it. There is a wide Vessels supported on rings should only be considered for
variety of types of lugs, and each one will cause different lower or intermediate temperatures, say below 400 or 500
stress distributions in the shell. Either one or two gussets degrees. Using ring supports at higher temperatures could
can be used, with or without a compression plate. If a com- cause extremely large discontinuity stresses in the shell
pression plate is used, it should be designed to be stiff immediately adjacent to the ring due to the differences in
enough to transmit the load uniformly along the shell. The expansion between the ring and the shell. For elevated tem-
base plate of the lug can be attached to the shell wall or perature design, rings may still be used, but should not be
unattached. Reinforcing pads can be used to reduce the directly attached to the shell wall. A totally loose ring system
shell stresses. In some cases, the shell course to which the can be fabricated and held in place with shear bars. With this