Evaluation of glassfibre reinforced plastic pipe couplings for high pressure applications

Gutmayer, Johannes
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This research was conducted in order to establish the feasibility of manufacturing GRP pipe couplings for high pressure GRP piping systems.The use of GRP pipes in high pressure piping systems is desirable due to their light weight, high corrosion resistance and their extremely low friction factors. To date no official standard exists that details the design of GRP pipes or GRP couplings for high pressure applications in excess of 7 MPa. In this report various existing steel and GRP couplings are discussed. From this the GRP coupling tested during the present investigation was developed. Tensile testing of couplings was carried out at each stage of the development phase in order to assess design modifications and to obtain an overview of the behaviour of GRP couplings. The test specimens were made from 400 rom long E-glass/epoxy pipe sections which an internal diameter of 50 rom, a wall thickness of 8 rom and a fibre orientation of ±55°. The most suitable design suggested in this report consists of a pipe with ? machined step and a flange laid up directly onto the pipe. The load is transferred from the pipe to the flange via adhesive shear stresses and through a mechanical interaction between the pipe and the flange. The seal is made with a hydrostatic "U"-type rubber seal, which is located on the outside of the pipe. An outer split clamp clamps over the flanges on adjacent pipes, thus holding these together. A thin sleeve over the split clamp secures the clamp. The maximum load bearing capacity for this coupling was 75 kN, which is equivalent to an internal pressure of 22 MPa. Three different failure modes occurred, viz. interlarninar shear of the pipe, buckling of the fibres on the inside of the pipe and compression of the fibres on the face of the step. The type of failure depended on the location and depth of the step on the pipe. Since failure occurred only in the pipe, it was concluded that the pipe needs to be tailored at the ends in order to achieve higher pressure capabilities. A finite element. model was used to simulate one of the experimental pipe/coupling configurations. This allowed experimental and ·theoretical results to be correlated, and a prediction of the coupling performance to be made.
A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Master of Science in Engineering.