2.4.7 CAESAR II Hanger Design Control and Options
CAESAR II provides a number of user specified options for controlling its automatic hanger design. The control options may, for the most part, be applied to the system globally, or at specific locations. These options are fully described in the CAESAR II User's Manual, but are discussed to some extent here:
Actual cold load calculation — This is described in more detail above. The user should specify Yes, if:
The spring installation load is to be adjusted with the pipe resting on the spring and free to move vertically otherwise (i.e. there isn't a steel strap around the spring base and the load flange, preventing movement of the load flange when the spring is adjusted in the cold position).
The piping adjacent to the spring is very flexible and/or the stiffness of the spring is very high.
Fluid filled systems are installed and set empty, and the user wishes to know the empty installation load.
Use short range springs — CAESAR II's hanger design algorithm first tries to select for an application a short range spring, followed by a mid-range, and then a long range, spring. On some construction sites short range springs are considered specialty items, and are only used where available spring installation clearance is small and where travel from cold to hot is small. In these cases, the user may instruct the design algorithm to bypass consideration of short range springs (and start with mid-range springs first) unless space limitations require it.
Allowable Load Variation — As noted above, this is computed as:
Var = I CL-HL I /HL = I k Δ th I /HL
The maximum possible load variation inherent in recommended ranges of the spring tables approaches 100% when the Hot Load is less than the Cold Load, and is approximately 50% when the Hot Load is greater than the Cold Load. Typical values for the permissible load variation range from 10% to 25%. A constant support may be forced at a location by specifying a minuscule load variation requirement at that location.
Rigid Support Displacement Criteria —Where feasible, rigid supports are considered preferable to springs supports, for reasons of economy (purchase, installation, and maintenance) and vibration prevention. Therefore, if a rigid support can be chosen instead of a spring at a location, the engineer will usually want this to occur.
One definition of a spring support is: "a restraint that supports a given load through some thermal travel". If the thermal travel is zero, or very small, then it is hypothesized that a rigid support can be used in place of the spring. This is indeed true providing that the surrounding pipe is relatively flexible as compared to the rigid rod.
The extent to which rigid supports are chosen can be controlled by this criteria. At any support location where the vertical displacement calculated during the operating load case for hanger travel is less than the specified Rigid Support Displacement Criteria, a rigid rod will be selected and used in subsequent load cases.
Note that this may not be desired at spring locations in the vicinity of pumps or other rotating equipment or on risers, since this may result in high nozzle loads or thermal lockup/liftoff of the support. It is best if this criteria is used in conjunction with some pre-design of support locations, such as that discussed in Section 2.4.5 of these seminar notes.
Free Anchors/Restraints — Often a major objective of hanger design is the minimization of equipment nozzle loads due to weight. This is done by forcing an unbalanced hot load (usually an overload) at the hanger location nearest to the equipment nozzle. This unbalanced force pulls on the nozzle, thus relieving it of some of the weight that would normally fall on it under a natural distribution — ideally, the hanger would be sufficiently unbalanced to make the load on the equipment nozzle as close to zero as possible. In an attempt to force this unbalance, anchors at equipment nozzles are often "freed" during the restrained weight case, forcing all of its weight to the hot load of the nearest support.
This technique should be used sparingly in those configurations where no hangers are located within three pipe diameters or so in a horizontal direction from the nozzle being released. It is also recommended that care be taken when releasing more than just the Y-direction force at a anchor/restraint, as release of additional degrees-of-freedom may cause gross angular and vertical displacements, resulting in unrealistic hanger design loads.
Manufacturer's Tables — This entry is used to designate the manufacturer of the springs (and thus the hanger table) to be used, as well as certain design criteria relating to selection of the hangers within this table. The selection criteria include:
use of maximum (vs. recommended) load range,
centering of the spring in the table, and
cold load (vs. hot load) design.
Most hanger vendors provide hanger tables with two ranges defined: 1) a restricted, or recommended load range, and 2) a maximum allowed load range. In order to provide margin against analytical uncertainties, it is best to use the recommended range. The maximum allowed load range may be used in certain situations, such as to permit the use of variable support hangers instead of the more expensive constant effort support, or when an already-owned spring is to be used over a new one.
In cases where the expected analytical uncertainty is especially high, maximum margin may be provided by selecting the spring which most closely centers the loads in the hanger table.
Cold load design balances the weight loads in the cold, rather than the hot, condition. This may be desired in those systems where installation is difficult due to flange fit-up problems caused by unbalanced cold loads, and where nozzle operating loads are not critical.
Available Space — In certain cases, the distance between the top of the pipe and the steel overhead; or between the bottom of the pipe and the foundation or platform below, govern the type (and number) of springs which may be used at a specific location. This value may be specified at individual hanger locations for use in spring selection. Figure 2-34 defines the available space as used in the CAESAR II spring design.
The available space option together with the "number of springs allowed" option lets the user design multiple spring support systems.
Number of Allowed Springs — If there is physically more than one spring can at a given hanger location, that number may be specified here. Likewise, the maximum number of springs that the user will permit may be specified (in the event that CAESAR II has to split the load in order to meet space criteria). In the case of multiple springs, CAESAR II will split the load evenly among all springs.
User Defined Operating Load — In some piping configurations the program selected operating (or hot) load on the spring doesn't unload the equipment nozzle sufficiently to satisfy manufacturers allowables. In these situations the user can force a hot load (higher or lower), overriding the program calculated value in an attempt to tune weight distribution and bring the equipment loads within the allowables. The user's entry in this case should normally be a variation of the value initially proposed by the program spring selection algorithm. Before adjusting the operating load the user should determine if a preferable course of action is freeing the problem nozzle during the restrained weight case (as discussed above).
Old Hanger Redesign — In cases where part of a piping system is redesigned, it is preferable that the hanger design algorithm re-select the existing springs in the system wherever possible. Where they can be re-used, new load ranges may be identified for them, and only a readjustment of the load flange in the field may be required. Where the existing springs can't be used, new ones will be recommended. The Old Hanger Redesign capability allows the user to do this.
Multiple Load Case Spring Hanger Design—This option is useful whenever the piping system has multiple thermal states that are sufficiently different such that the results from each thermal state should be considered when doing the spring hanger design. Figure 2-35 illustrates this idea:
The hanger at "A" should be designed with the main pump running, and the hanger at "B" should be designed with the backup pump running. Once the springs are designed for their respective thermal cases they are inserted into the piping system and the various operating conditions run to check for stress or equipment overloads.
The options available in CAESAR II for combining data from the various design load cases are shown below:
Design per thermal load case 1
Design per thermal load case 2
Design per thermal load case 3
Design for maximum operating load
Design for maximum travel
Design for average load and average travel
Design for maximum load and maximum travel
Piping Design for loading Types