Pipe anchor

An anchor holds the pipe completely at one point: it restrains all three translations and all three rotations. It thereby divides the line into sections whose thermal expansion is absorbed separately and defines the direction in which each section moves.

Anchors protect sensitive connections from the movements of long lines and carry the pressure thrust of unrestrained expansion joints. In practice there are also supports that restrain movement along the pipe axis only; these are called line stops or axial stops and are not full anchors.

An anchor collects what the adjoining sections deliver: the spring forces of expansion loops or joints, the friction of all sliding supports between it and the next point of expansion compensation, the pressure thrust of unrestrained expansion joints, and wind, earthquake and pressure surges.

For lines with axial expansion joints a distinction is made between main and intermediate anchors. The main anchor – at the end of a line, at a change of direction or at a reducer – carries the full pressure thrust. At an intermediate anchor between two identical expansion joints the pressure forces cancel; it carries only the difference in spring and friction forces.

The classic mistake is a straight pipe between two anchors. It cannot expand and builds up a stress of modulus of elasticity times coefficient of thermal expansion times temperature difference. For steel that is about 250 N/mm² for a temperature rise of 100 kelvin – regardless of the length of the run.

The associated force is stress times pipe cross-section and, even at medium sizes, reaches magnitudes no ordinary steel structure will carry. Between two anchors there must therefore always be some means of compensation: a change of direction, an expansion loop or an expansion joint.

An anchor is only as fixed as what it is attached to. The forces have to be given to the structural engineer with magnitude, direction and load case; a beam that deflects under the anchor force changes the behaviour of the whole line.

At the pipe itself the anchor introduces high loads over a small area. Welded lugs, trunnions or clamps create local stresses in the pipe wall, which are verified separately for thin-walled pipes and high loads.

Anchors are more often overused than underused.