Continued from part 1….Click here to read part 1:
EXPANSION LOOPS: “U” expansion loops are provided for long straight pipeline runs, as shown in the figure (Fig. 7) below:
The recommended spacing between axial guide supports close to the expansion loop are also shown in the drawing. Other supports shall be spaced following other calculations (beam load).
Various kinds of standard expansion joints can be used. Low stiffness expansion joints are preferable since they develop a low reaction in correspondence with relatively big displacements. GRP pipes expand more than steel pipes but have much lower thrusts.
Using stiff expansion joints would reduce the stresses in the pipe only by a little
We suggest rubber joints with one or more waves, possibly with limiting travel devices, with an activation load lower than the Pcr load calculated in part 1 of this article, and with a working travel equal to the total expansion.
Horizontal pipes should be supported according to the spacing suggested by the support spacing data or according to a specific project.
Pipe span is defined as the distance between two consecutive pipe supports or anchoring devices.
The maximum span length for every pipe size and class is suggested by the Technical Department of GRP Vendor for standard pipes or according to a specific project.
The span length is limited by the following considerations:
- the maximum axial strain must not exceed the allowable value;
- the mid span deflection has to be smaller than 1/300th of the span length and anyway not exceed 15 mm which is the minimum value.
If factor (b) is the determinant factor, then the distance between supports must not be changed by reducing the working pressure.
Often the spacing between the supports is set by other reasons, as for instance joint spacing or existing bearing structures. Normally the 6 meters half-length span is the maximum that is used, even for large diameter pipe, for which a theoretical longer span could be used. The maximum support span in meters is shown in the following table (Fig. 8), for different pipe sizes and pressure classes:
The maximum span has to be evaluated for a continuous span length when the joint can transmit axial loads.
In this case the span is the distance LC between two supports of a pipeline, placed at a distance from the joint that in general shall not exceed the value of 1.2 m for pipe size up to DN 200 or 2 m for pipe size above DN 200.
Following are suggested the basic rules to design and for the positioning of supports, anchors and guides.
- Loads with linear and punctiform contacts have to be avoided, therefore curved supports that bear at least 120 degrees of the bottom part of the pipe and that have a maximum bearing stress of 600 kPa have to be used. Unprotected pipes are not allowed to press against roller supports or flat supports. Do not bear any pipe directly against ridges or other points of the support’s surface. Protective sleeves have to be used in these cases.
- To protect pipes against external abrasion between the pipe and the steel collar, a PVC saddle (Fig. 9) or a protective rubber layer has to be positioned in-between. The PVC saddle is necessary when free axial sliding of the pipe must be permitted (axial guides).
- Valves and other heavy equipment must be supported independently in both horizontal and vertical directions.
- The clamps must fit firmly but must not transfer excessive force to the pipe wall. This could result in deformations and excessive wall stresses
5. Vertical runs have to be supported as shown in Fig. 9. Excessive loading in vertical runs have to be avoided. It is preferable to design a “pipe in compression” than a“ pipe in tension”. If the “pipe in tension” method cannot be avoided, take care to limit the tensile loading below the maximum tensile rate recommended for the pipe. The guiding collars will have to be installed by using the same space intervals used for horizontal supports.
An anchoring point must efficiently restrain the movement of the pipe against all of the applied forces. Anchors can be installed in both horizontal and vertical directions. Pipe anchors divide a pipe system into two sections and must be attached to some structure that is capable of withstanding the applied forces. In some cases pumps, tanks and other similar equipment function as anchors.
However, most installations require additional anchors where pipe sizes change or where fiberglass pipes join another material or a product from another manufacturer. Additional anchors are usually located on valves, pipeline changes of direction and major branch connections.
It is a good practice to anchor long, straight runs of aboveground piping at intervals of approximately 90 m.
In any case the correct positioning of anchor points has to be decided only after a detailed stress analysis.
The pipe must be able to expand radially within the pipe clamps.
To secure the pipe to the clamp it is suggested to apply a GRP lamination (as shown in Fig. 10 below) on each side of the clamp. If the movement of the pipe has to be restrained only in one direction, it is sufficient to apply only one overlay ring of GRP in the opposite position.