A Primer on Pipe supports

Expansion joints absorb thermal expansion or contraction to lessen loads on anchors or restraints. But restraints on both sides of an expansion joint must bear the fluid force from the pipe; and friction forces in slip type or deflection forces in corrugated expansion joints.

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Fluid force consists of pressure and momentum components. Pressure is greatest during hydraulic testing, typically 1.3 times the design pressure. And momentum forces cannot be ignored in high-flow-velocity lines. Most support failures near expansion joints are due to neglecting the fluid force, which tends to separate an expansion joint much like a hydraulic jack.

For large pipes like a penstock, offsetting or loops are impractical. Expansion joints must be used even for environmental temperature variations and short pipes between anchors when expansion is restricted. More information on expansion joints is available from manufacturers like U.S. Bellows Inc., Houston.

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Differential anchor movement arises because anchors on different foundations move at different amplitudes (and they never move in synch during earthquakes). Differential thermal movements of vessels or main pipes are similar, although they occur more slowly. Supporting pipes near connections from a vessel, or keeping the first support of a branch pipe a good distance from the run pipe, reduces loads on the connection.

Snubbers

Snubbers resist fast-acting loads much like rigid struts but allow slow thermal movement with little resistance. The devices effectively resist shock, water hammer, and earthquake loads. Snubbers are sized based on load capacity and maximum travel.

A hydraulic snubber consists of a main cylinder with a piston, and a compensation cylinder or reservoir. The main cylinder and piston rod, respectively, are connected to opposite ends of the snubber unit. A port connects valves at both ends of the cylinder. When the pipe moves slowly, the piston pushes fluid through the valves to the back side with little resistance. When the piston reaches a threshold velocity, the outlet valve closes to stop the flow and the unit behaves like a rigid strut. Reverse motion works in a similar fashion.

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