2.4.6 Notes on Hanger Design
In the event that a system which carries a fluid with a specific gravity less than 1.0 is to be hydro tested, the springs will generally have to remain pinned during the hydro test. The hanger hardware (clamps, rods, etc.) and supporting structure will have to be selected and/or designed to withstand the hydro test loads, which will normally be the controlling design loads for these supports.
When specifying the spring hanger's Hot and Cold Loads, the anticipated weight of additional hardware should be added to the loads calculated by CAESAR II, especially if it is expected to be significant (such as in the case of large stock clamps or a trapeze assembly made of structural steel). The spring must also support the hardware, and if this is not considered when specifying the spring parameters, the piping weight loading will be unbalanced by the weight of the hardware.
Horizontal movement at hanger locations must be considered when designing a support in order to assure that the pipe does not move so far that it falls off of the support. Additionally, support manufacturers typically limit the range of a hanger rod's arc in to values such as 6°, where the arc can be calculated as: Arc = Tan^-1 (horizontal movement / rod length) In cases where the horizontal movement is especially large, it may be advisable to install the support in an offset position to minimize the deviation of the line of support action from vertical in both the cold and hot positions.
In systems where installation is difficult due to flange fit-up problems caused by unbalanced cold loads, it may be preferable to adjust the springs in the field to carry the hot load once the system has been started up. In cases where nozzle operating loads are not critical, and fit-up problems are more of a concern, CAESAR II can provide Cold Load Design, where the weight loads are balanced in the cold, rather than the hot, condition.
CAESAR II provides the option of calculating both the "theoretical" and the "actual" cold loads for springs. The theoretical cold load is the load to which the spring should be preset prior to installation (usually this is done at the factory, and the spring is pinned to keep it at this value). This is the load which the spring will exert on the piping system in the cold condition, as long as there is no vertical displacement of the system at this location. Since the cold load is almost always unbalanced vs. the piping weight load, there will be a net load on the system at this location in the cold condition. If this net load is large, or the piping system is very flexible, the system may displace under the load, leading to extension or compression of the spring, and a corresponding change in the load plate reading. The new reading of the spring load is what CAESAR II calculates as the "actual" cold load. Or more simply, the "theoretical" cold load is the cold load to be specified in the factory order of the spring, while the "actual" cold load is an approximation of the reading of the spring load after pulling the pins upon initial installation. The actual installed load case is important if the springs are to be adjusted or checked in the cold condition, or if the spring's cold load is being set in position, rather than at the factory.
Excessive use of spring hangers may create a dynamically unstable (low natural frequency) system due to lack of restraint stiffness. These systems have essentially no horizontal support, and typically small vertical stiffnesses resisting movement in the Y direction. Note that constant effort spring supports have no dynamic effect on a piping system.
Selected hanger locations may actually hold the pipe down during the restrained weight case due to unbalanced parts of the system pivoting about other supports. CAESAR II flags these with a warning during the analysis and reports them as zero load constant effort supports in the hanger table during output. When this occurs, the offending supports should be removed, or the support locations in the vicinity should be reconsidered.
There are special provisions to consider when cold spring and hanger design exist in the same job. Cold spring should be omitted from the restrained weight case, and included in the operating load case for hanger travel. The actual installed load case should be run with the cold spring in order to determine the installed hanger settings in the presence of cold spring. It is the user's responsibility to verify that the displacements during the actual installed case are still within the manufacturer's recommended load range. Problems usually only arise when there is significant cold spring in a vertical run of pipe in the vicinity of one or more spring hangers.
In a liquid filled line, the springs may be installed when the system is empty. In this case it is necessary to ignore the "actual" cold load, and in some cases it may be preferable to adjust the springs in the field to carry the cold load once the system has been filled.
Piping Design for Loading Types