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RECENT DEVELOPMENTS IN INTEGRAL JOINT RESTRAINTS

 

Guido Quesada

# 2021 Amsterdam

An important breakthrough for belled PVC pipe was made around 2006, when the first versions an integral joint restraint were released, after about two years in development. This type of restraint comprises a casing, usually made of metal such as ductile iron or stainless steel, a rubber seal, and a “C” grip ring, typically metal as well. The casing and the seal are placed together on a belling mandrel to make a PVC socket by heating it until soft and pushing it over the mandrel with the seal and the casing on it. When the bell is cooled and pulled out of the mandrel, the casing and the seal are integrated to it. Although it is feasible to conceal the grip ring inside the casing during belling, it is usually placed in the casing after forming the socket.

The seal will act between the socket and the spigot once inserted, while getting axial support from the casing. The casing has external ridges which prevent it from sliding along the socket. Inside the casing there is a raceway for the grip, with a mating conical interface which will cause the grip ring to wedge between the casing and the spigot, thereby preventing it from sliding out of the joint assembly by the action of external loads or internal pressure. The grip has internal ridges or teeth, designed to indent the spigot to maximize the gripping effect while avoiding excessive damage.

This paper reviews recent developments around the concept described above. These include grips with progressive tooth height and a backward-leaning profile, which improve engagement and optimize load distribution on the spigot. Also, a new casing profile was developed which can accept relatively slender lip seals intended for moderate pressures, in which relatively low assembly force is a priority. In connection with this there is a design variation adapted to PVC couplings, in which the socket is initially open to accept the restraining and sealing components, then closed into the final coupling geometry. Finally, the expected advantage of using a modified PVCformulation, such as PVC-A, was evaluated and validated for demanding applications such as Horizontal Directional Drilling (HDD) and cyclic loading.

Nonlinear Finite Element Analysis was used extensively during these developments. It is also used as a visual tool to explain how they led to the validation of new integral joint restraint products and success stories in the field.

Following up on these recent developments, the paper provides a preview of possible future enhancements to these integral joint restraints, aimed at improving their reliability, minimizing axial displacement between completed spigot insertion and full engagement, and maximizing socket and spigot life while subjected to a combination of pressure and axial traction loads.

Related keywords : PVC, PVC-A, Pipe Joint Restraint, Horizontal Directional Drilling, Analysis, Design.
Please note that the whole article content is available on PPCA website only

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