Pipe Stress Analysis
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The reasons one does a pipe stress analysis on a piping system are as follows
- to comply with legislation
- to ensure the piping is well supported and does not sag or deflect in an unsightly way under its own weight
- to ensure that the deflections are well controlled when thermal and other loads are applied
- to ensure that the loads and moments imposed on machinery and vessels by the thermal growth of the attached piping are not excessive
- to ensure that the stresses in the pipework in both the cold and hot conditions are below the allowables
The piping system is modelled using analysis software such as CAESAR II, available from Chempute Software. The model is constructed from piping general arrangement drawings, piping isometric drawings and piping and valve specifications. Once the system is accurately modelled, taking care to set the boundry conditions, comprehensive stress analysis calculations are done, modifications to the model are made to ensure compliance with the above requirements.
The modifications may include one or more of the following tools
A device which prevents, resists or limits the free thermal movement of the pipe. Restraints can be either directional, rotational or a combination of both.
A rigid restraint which provides substantially full fixety, ie encastre or built-in, ideally allowing neither movements nor bending moments to pass through them.
True anchors are usually difficult to achieve. A seemingly solid gussetted bracket welded to a house column does not qualify as an anchor if the column does not have the strength to resist the loads applied to it.
A purpose designed device which absorbs thermal growth; usually used in combination with restraints and cold pulls.
Neutral Planes of Movement
This refers to the planes on the 3 axes of a turbo machine or pump from where expansion of the machine starts eg the fixed end of a turbine casing. This information is normally provided by the equipment manufacturer. If not available from this source, the fixed points of the machine must be determined by inspection and an estimation of the turbine growths calculated.
A pipe restraint positioned in line with a neutral plane prevents differential expansion forces between the pipe and the machine.
Cold Pull or Cold Spring
This is used to pre-load the piping system in the cold condition in the opposite direction to the expansion, so that the effects of expansion are reduced. Cold pull is usually 50% of the expansion of the pipe run under consideration. Cold pull has no effect on the code stress, but can be used to reduce the nozzle loads on machinery or vessels.
Used to support a piping system that is subjected to vertical thermal movements. Commercially available single coil spring units are suitable for most applications. Supplier's catalogues adequately cover the selection of these springs. According to Hooke's law, the spring's supporting capacity will vary in direct proportion to the amount of displacement the spring undergoes due to thermal movement. This variation between cold and hot should be between 25 and 50% of the hot loaded condition.
Solid Vertical Support
In places where vertical thermal movement does not create undesirable effects, or where vertical movement is intentionally prevented or directed, solid supports in the form of rollers, rods or slippers are used.
It is important that free horizontal movement of the pipe is not impeded unless horizontal restraint is desired. Slipppers and rollers must be well designed and lubricated.