2.2.3 Consideration of Nozzle Loads
The previous discussion has primarily concerned the effect of supports on system stresses. The engineer is also interested in determining loads on supports and nozzles, in order to select the appropriate support hardware, to check the overloading of equipment, or to calculate vessel stresses.
A review of the restraint loads shows that the hanger loads are on the order of 2000 to 3000 pounds. These loads would be used as an upper limit for the selection of the support hardware for example, the rods, clamps, brackets, supporting steel, etc. must be capable of resisting these loads at a minimum.
A review of the weight load (Y-force) on the nozzle at node point 10 (see Figure 2-14) shows a relatively small load, of only 237 pounds, which should be acceptable for most types of equipment. However, closer inspection shows that the sign is positive, indicating that the piping system is pushing up on the support, rather than down. This seems unnatural for a gravity load, and in fact is due to the unbalanced elbow at node point 30 pivoting about the hanger at node point 22. Therefore, even though the nozzle load is low, this is not an optimally supported system.
The system support can probably be improved by moving the hanger a bit closer to the elbow to reduce pivoting — but how close is enough? Figure 2-15 shows the restraint loads for a configuration with the restraint at node point 22 moved 2'-0 closer to the elbow (i.e., 3'-0 from the end of the valve). The sign is now correct (indicating a reasonably balanced system), but the load on the nozzle is now 495 pounds, larger than before. It is not certain that this is an improvement.
However, this exercise demonstrates that support and nozzle loads may be tailored by adjusting the locations of the supports. The best location for the hanger may be estimated by interpolating between the two results, in order to minimize the load acting on the nozzle. This shows that the best (where "best" is defined as minimizing Y-direction weight force on the nozzle at node point 10) location for the hanger is:
Tuning nozzle loads may also be done by varying the support loads, rather than the support locations. This is done by refusing to allow the system weight to settle on its own, but rather by forcing weight unbalance at certain support locations. In this way, if the support at node point 22 is under loaded, the system is less likely to push up on the support. For example, if the support at node point 22 only takes -1725 pounds, the short fall will be split up between the nozzle at node point 10 and the support at node point 32, with the bulk of the shortfall going to the nozzle, which is closer. This shortfall, of approximately -300 pounds, will reduce the upward load at node point 10 by approximately 225 pounds (with the support at node point 34 being reduced by the remaining 75 pounds), down to approximately zero pounds. (Proof of this is left to the reader.)
The load at selected supports can be forced to be unbalanced through the use of pre-loaded springs (i.e., the loads are set to something other than the naturally distributed weight load), thus influencing the resulting loads on the nozzles. This is most easily done by releasing degrees-of-freedom at anchor points during the restrained weight phase of hanger design, as discussed in Section 2.4 of these pipe stress analysis notes notes.
Piping Design for Loading Types
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