2.2.2 Use of Standard Weight Spans
Implementation of the preceding analysis provides a simple way to design for weight loading. The engineer may first support all concentrated loads in the system as closely as possible, reducing the stresses due to those loads to near zero. Next, converting the formula
If the piping system is then supported, such that no straight span exceeds Lall, the engineer can be sure that allowable weight stresses are not exceeded in the system, and no analysis per se need be done.
In order to save even the brief time required to calculate Lall, the Manufacturer Standardization Society of the Valve and Fitting Industry has calculated allowable piping spans for various piping configurations, and published them in their standard MSS SP-69 (Figure 2-10). They have calculated the maximum allowable piping weight spans based upon the following criteria:
1 - the pipe is assumed to have standard wall, with insulation, 2 - the maximum moment is calculated as Mmax = wL^2/10, 3 - no concentrated loads are present, 4 - there are no changes of direction in the spans, which are assumed to run in the horizontal plane, 5 - the maximum allowable stress is assumed to be 1500 psi, combined bending and shear, 6 - maximum deflection of the span under load is limited to 0.1", and 7 - stress intensification factors of components are not considered.
Due to the low allowable stress value used, there is sufficient factor of safety that this standard span may be applied to a wide range of piping configurations.
If the engineer supports a piping system such that no span in the system exceeds the standard spans listed in the table, it is virtually certain that the system is adequately supported for weight loading. However, it is rare that a piping system has no concentrated
loads, consists of only horizontal runs with minimal changes in direction, etc. Therefore, standard practice dictates that standard spans be applied subject to the following four caveats:
Supports should be located as close as possible to concentrated weights. The theoretically best location for a support is directly on the concentrated load; however, this is usually impractical.
A developed length of 3/4 of the standard span or less should be used when the piping run changes direction in the horizontal direction, in order to minimize eccentric moments. The theoretically best location for a support is on an elbow; however, this is not recommended due to the bend stiffening and increased local stresses associated with attachments on a bend.
The standard span doesn't apply on risers, since no moment (and thus no stress) develops regardless of the riser length. The number and location of supports should be determined by the location and strength of building steel. However, it is preferable to locate supports above the center of gravity of long risers in order to prevent toppling.
Support locations should be selected as close to building steel as possible in order to simplify support configuration.
The steps involved in supporting a piping system for sustained loads can be illustrated with an example. In Figure 2-11, the system consists of a 12" diameter, standard schedule steel pipe filled with water, with a design pressure of 150 psi, and a design temperature of 350°F, which runs between two equipment nozzles.
The engineer first must determine the standard span for the system. For 12" diameter, water filled pipe, the standard span is shown in MSS SP-69 to be 23 feet. For changes of direction, 3/4 of this span is 17 feet-4 inches.
Next, the engineer locates supports. The first concern is to locate them near concentrated loads — supports should be located as close as possible to the two valves (for example, near node points 20 and 70). The first of these is optional, depending on whether the nozzle at node point 10 is assumed to act as an anchor, and whether it is desirable to minimize the nozzle loads on the equipment.
The next concern is the placement of supports on the riser. Assume that the capacity of the building steel dictates that the weight of the riser be split between two supports. It is recommended that one of these be placed above the center of gravity of the riser (for example, 15 feet below the top of the riser).
Now supports can be located elsewhere in the system, starting at the nozzle at node point 10. A support was located near node point 20 earlier; we now want to locate the next one downstream within the standard span. It is evident that pipe changes direction within 23 feet, so the developed length to the next support should be maintained as less than 17 feet-4inches. The next run of pipe accommodates a full 23 foot run, so two supports can be located between node points 30 and 40. The line of action of the supports on the riser provide support to the end of the horizontal 30-40 run, so no additional support is required at node point 40. Support locations can be continued to be selected in this manner until all locations meet the selection criteria; one solution is shown in the Figure 2-12.
Once completed, what does this accomplish? By using the standard span criteria, the engineer can assume that the maximum stress in the piping system due to weight loading does not exceed 1500 psi. Therefore, substituting this value for the weight component of the stress equation:
Ssus = PAi/Am + 1500 = 150(113.1)/14.58 + 1500 = 2664 psi < 20,000 psi
Piping sag is not a problem, since displacement is limited to 0.1 inches.
Therefore the engineer has demonstrated that this piping system meets the sustained stress criteria, without having to do any actual "work".
This can be confirmed by actually doing an analysis of the supported system. The results in Figure 2-13 show that the maximum sustained stress actually calculated for the configuration shown in Figure 2-12 by CAESAR II is 2418 psi, showing that the shortcut analysis is reasonably accurate, yet conservative. The CAESAR II analysis also shows a maximum vertical displacement under weight of 0.0046", which is also conservative.
A further implication of this approach is that in order to eliminate a stress or deflection problem due to weight loadings, the best solution is usually to reduce the unsupported span of the piping — i.e., add more supports.
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