Pup joints are different sections of pipe with grooved ends and usually a constant wall thickness of 0.4 inch, but can be made to any custom thickness to suit. Pup joints are used to make short pieces of pipe, or nipples, and can be cut and re-grooved to close gaps less than 19 feet in newly constructed, or broken pipelines and in pumping-station manifolds. Repair crews sometimes carry pup joints of assorted lengths. When a damaged section is removed (especially a surface-laid, grooved-coupled-type pipeline), the two ends are likely to spring apart. This occurs most often on long downhill slopes during very cold weather because of pipe contraction. The gap is likely to be 1 or 2 inches longer than the standard 19-foot length of grooved pipe section.
Please refer to the pup joint pdf for further details.