Thermoplastic Pipe Expansion

Swimming pool piping is design is notorious for including long straight runs of pipe.  For pools that do include long runs of straight pipe sections, consideration may be given to the use of expansion loops or mechanical expansion joints to accommodate thermal expansion and contraction.  The ASTM Standard 2774 references “Underground Installation of Thermoplastic Pressure Piping”. This ASTM Standard 2774 contains specific information on the topic of expansion in pool piping.  Expansion and contraction of thermoplastic pipe is typically due to temperature changes to either the surrounding pipe embedment materials or changes in the pool water traveling in the pipe system. When temperature changes of the thermoplastic pipe does occur, changes to the piping network in the form of elongation or contraction will occur.  Therefore, in piping networks where there may exist sections of the pipe that are straight and without bends, expansion and contraction of the pipe can cause failures in the system.  Swimming pool piping networks that include PVC pipe used in straight runs nearing 100 LF should take special consideration to the potential expansion and contraction of the pool pipe.  In these instances the design of the pool piping system should have the coefficient of expansion calculated for determination if design accommodations for the expansion and contraction should be included.

In cases where expansion and contraction will be significant, inclusion of an expansion loop or even a mechanical expansion joint can be installed.  A pipe loop is a relatively easy item to include within the piping network.   This pipe loop, or offset, can alleviate the bending stresses that can occur when thermoplastic piping systems experience expansion and contraction.  If expansion joints are required, a detail for the expansion loop may include a specific dimension for the offset lengths.  Mechanical expansion joints work like a piston allowing the pipe to elongate or contract within the pipe run.  The installation of mechanical expansion and contraction joints is critical, particularly cleaning debris (sand, rock, dirt) out of the moving parts.  If small particles come in contact with the mechanical joint mechanism the joints seal can become compromised.

In most instances a linear pipe runs without interruption of bends, joints or pipe size changes is not common.  However, in situations where conditions do allow long pipe runs, your design should pay special attention for need of expansion and contraction loops and mechanical joints.

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