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3.6.2 Fiberglass Reinforced Plastic Pipe

The characteristic of wrapped fiberglass reinforced plastic (FRP) pipe which distinguishes it from other piping materials is that it is orthotropic—the material has different properties in the longitudinal and hoop directions — and can therefore not be modeled by the standard CAESAR II pipe element. This is because of the method by which this type of pipe is made.

For wrapped FRP pipe, the wall thickness is built up by wrapping layers of glass and matrix at several pre-specified angles about the pipe axis pipe. Twenty to thirty layers, wrapped at different angles, are usually used to obtain the desired pressure carrying capacity and bending strength. Because the layers are at different angles, and because the glass/matrix sheets are only capable of axial load carrying capacity, the resulting pipe has different strength characteristics in the hoop and longitudinal directions. For example, if the pipe was only wrapped at 90 degrees to the pipe axis, the resulting pipe could only contain hoop pressure stresses, with essentially no resistance against longitudinal pressure or bending loads. Because there is no standard angle of wrap, the pipe properties must be gotten from the manufacturer. Obtaining these properties is fairly straight forward, as there are typical relationships that are used to calculate global properties based upon the local characteristics of the glass and matrix, given the angle and the degree of wrap.

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Note that today a non-wrapped form of FRP pipe is also available. In this type of pipe, the glass, in very small pieces, is enclosed in the matrix, and then sprayed into a piping mold. This method of construction provides for essentially isotropic properties, as the glass fibers are oriented at random in the matrix. In this instance, the standard CAESAR II pipe element can be used. Other plastics, such as PVC, also exhibit isotropic properties, and can therefore be modeled by the standard element as well.

The CAESAR II plastic pipe element is based upon a model of wrapped fiberglass reinforced plastic pipe. When this element is requested, the following additional material properties are required:

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When analyzing plastic pipe, one primarily looks for points where the pipe is under-supported both horizontally and vertically. Piping designers used to working with steel pipe tend to under-support plastic pipe because they often use rules of thumb and a "design eye" for the much stronger steel pipe. As a result supports near vertical risers may be placed too far from the vertical leg, causing excessive bending, and leakage at weak joints. Horizontal supports should be provided liberally because they are inexpensive and lightly loaded, and because they prevent the pipe from buckling or moving into a position that is potentially dangerous. In fact, any horizontally unsupported line can "walk" its way off of supports, into neighboring lines, etc., if the designer is not careful. These types of problems seem to be exacerbated when working with plastic pipe.

Practically, the pressure stresses in plastic pipe should be considered before any flexibility analysis is done (this is consistent with the way any other pipe stress analysis is done). These pressures determine the required thickness of the pipe, and the degree of wrap. Next the pipe stress analysis should be done. There are few explicit piping codes or allowable stresses available for plastic pipe; it is up to the user to determine the appropriate flexibility and stress intensification factors, load combinations, and allowable stresses. CAESAR II models flexibility factors for plastic pipe elbows as 1.0, since the hoop modulus is generally considerably higher than the axial modulus, thus resisting cross sectional ovalization. Intersections and curved fittings are generally assumed to be approximately three times as thick as the matching pipe. When this is done an SIF of 2.3, a value recommended by Ciba-Geigy for plastic pipe systems, is typically used. If the user has better stress intensification factor data, those values may be specified at individual fittings.

It is conservative, and a practical approach, to combine all simultaneous loadings together to determine the maximum stress in the pipe. This includes the effects of weight, pressure, and thermal effects. (When pressure is specified in plastic pipe, CAESAR II always activates the Bourdon pressure effect. This accounts for the displacements due to pressure elongation of the pipe, which can be significant in plastic pipe.) Preferably, the analysis results should be reviewed with the plastic pipe manufacturer to verify that the model is accurate and that the pipe supplied is capable of withstanding the stresses, pipe forces and moments, and restraint reaction loads. Otherwise, the resulting operating stresses (both code and bending stresses) can then be compared to the maximum allowable stresses as specified by the pipe manufacturer.

In some cases, the manufacturer does not provide allowable stresses, but rather, maximum allowable pressures and maximum recommended weight spans. In this case, the user can convert this information to allowable stresses by proceeding through the steps outlined below:

  1. The user should look up in the manufacturer's specifications the maximum pressure and allowable weight span for plastic pipe (of the appropriate diameter), filled with water.

  2. Next a plastic pipe model comprised of straight pipe elements resting on vertical supports should be built. The model should include at least six equally spaced supports, with a node point placed at the midpoint of the middle span. The distance between supports should be equal to the maximum allowed pipe span obtained from the manufacturer's data.

  3. Making sure that the modeled pipe is filled with water and pressurized at the maximum allowable pressure (obtained from the manufacturer's data), a weight plus pressure analysis on the pipe should be run.

  4. The largest code stress and the largest bending stress found on the three node points of the middle span should then be used as the limits to the operating bending and code stresses found in the analysis of the actual system.

The real benefit of analysis of plastic pipe is that it helps to eliminate poorly supported systems that will eventually leak, or will cause distortion problems with the line. Whereas hot steel pipe can be easily over-supported, plastic pipe typically cannot. The tendency is to under-support it, or to support it incorrectly, producing large thermal moments at intermediate elbows. Both of these design flaws should be discovered easily with a stress analysis of the system.

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