Abstract
Large diameter non-metallic pipes, designed and engineered as conventional aboveground piping to various international standards, have extensive applications. The support points for such piping are designed with cautions such as smaller support spans and avoiding point loads as compared to metal piping. Avoiding point loads is accomplished by providing a cradle, or a fully encirclement clamp shoe support. These support designs such as one diameter length of the cradle support and encirclement clamp shoe are based on arbitrary practice rather than a quantifiable limit Non-metallic pipes are not isotropic in behavior. Non-metallic pipes have different behavior for hoop strength and longitudinal strength. Non-metal pipes have a modulus of elasticity one hundredth to one thousandth as large as steel pipes. The longitudinal strength of these non-metal pipes will be used to determine the pipe support spans, but this assumes that pipe retains its circular cross-section at the support point. Non-metallic pipes will ovalize at the support points. Ovalized pipe has a reduced section modulus compared to circular pipes and hence higher stresses. Beyond a certain increase in pipe load, either due to increased span, heavy intermediate components or high-density fluid carried, the pipe support location will ovalize or/and buckle. This ovalizing and shell buckling is prevented by providing a cradle support or holding the non-metallic pipe in a rigid/metal full encirclement saddle or clamped shoe. Such a support design reduces ovalizing of the non-metal pipe and maintains its load-carrying ability by ensuring that entire non-metallic pipe cross-section is taking up pipe load. Stresses can therefore be assessed according to the basic rules of beam design from strength of materials concepts. Induced bending stresses can then be calculated for comparison against longitudinal strength limits. But the means to determine the need for cradle or a clamped shoe is complicated. ASME design codes NM-1 for plastics and NM-2 do not provide such guidance and this is left to the designer. ASME Code case N-886 discusses PE pipe design and cautions against axial buckling from compressive stresses but does not provide guidance on the support design.
This paper is an analytical study on pipe support aspects of non-metallic pipes as compared to metal pipes. The guidance and set of formulae generated can be applied to the range of non-metallic pipes available and in industry use today. This paper provides a basis that can be adopted by engineers to decide a threshold point below which bending moments should be kept on a piping system. If the piping system at the supports is going to a see a bending stress higher than the buckling stress limit, then strengthen the support by providing a full encirclement clamp type of support.
