Satish Lele

Cold Pull / Self Springing
Cold Pull is another term for the more common term, Cold Spring. Cold Spring (and Cold Pull) is a technique used to compensate for the effect of thermal expansion in long pipe runs. Cold Pull tends to reduce stress on the elbows at the change of direction (corner) or at expansion loops in piping systems and lowers the forces at the anchor points. To do this, First step: Calculate the amount of expansion caused by thermal growth during full operation. Second step; Remove 1/2 the calculated growth from the actual dimension of the pipe run from the anchor to the corner or loop at the ambient temperature. Third step: Spring (or pull) the pipe back at the corner or loop and make the weld.

The amount of movement to be taken up by the piping and any device incorporated in piping can be reduced by 'cold pull'. The total amount of expansion is first calculated for each section between fixed anchor points. The pipes are left short by half of this amount, and stretched cold by pulling up bolts at a flanged joint, so that at ambient temperature, the system is stressed in one direction. When warmed through half of the total temperature rise, the piping is unstressed. At working temperature and having fully expanded, the piping is stressed in the opposite direction. The effect is that instead of being stressed from 0 to +1 units of force, the piping is stressed from -1/2 to + 1/2 units of force.
In practical terms, the pipe work is assembled cold with a spacer piece, of length equal to half the expansion, between two flanges. When the pipe work is fully installed and anchored at both ends, the spacer is removed and the joint pulled up tight.

The remaining part of the expansion, if not accepted by the natural flexibility of the pipe work will call for the use of an expansion fitting.

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