Glossary

Pipe support spacing

Support spacing is the distance between two pipe supports. It influences deflection, pipe stresses and support loads.

Also: Support span · Pipe span · Span length

Also known as

Support span
Pipe span
Span length

Four criteria limit the span

How far apart two supports may be is not decided by a single limit. Four criteria act together, and the strictest one governs the spacing.

Bending stress: weight bends the pipe between the supports; the stress has to stay below the allowable value.
Deflection: sag must not cancel slope and drainage – contents remain standing in a low point.
Natural frequency: the greater the spacing, the lower the natural frequency and the more susceptible the line to vibration.
Support load: each support and the steelwork below carry the weight of half the spans on either side.

The calculation behind it

The pipe between two supports is a beam under a uniformly distributed load w – the weight of pipe, contents and insulation per metre. Depending on the end restraint, the maximum bending moment lies between w · L² / 8 for a simply supported single span and w · L² / 12 for a span fixed at both ends. For continuous pipe the intermediate value w · L² / 10 is often used.

The deflection correspondingly lies between 5 · w · L⁴ / (384 · E · I) and w · L⁴ / (384 · E · I). More important than the coefficients are the exponents: the moment grows with the square of the span, the deflection with its fourth power. A span 20 per cent longer doubles the sag.

Span tables and their assumptions

In practice the span is rarely calculated; it is taken from a table. ASME B31.1 contains a table of suggested support spacing; operators have their own standards, and so do the manufacturers of pipe supports.

Each of these tables rests on assumptions: straight horizontal pipe, a certain wall thickness, a certain fill, a permitted sag, moderate temperature, no concentrated loads. This is why different tables give different spans for the same pipe size. Anyone using a table adopts its assumptions – and has to check whether they apply to the line in question.

Where the table does not apply

Six cases occur in almost every plant and are not contained in any standard table.

Concentrated loads: valves, flange pairs and actuators need a support close by.
Changes of direction: the spacing is reduced before and after bends.
Gas lines: for the hydrostatic test, the weight of the water fill counts.
High temperatures: modulus of elasticity and allowable stress of the material decrease.
Plastic pipe: much smaller, temperature-dependent spans; small sizes are supported continuously.
Vertical lines: the issue there is not sag but guiding against wind and vibration.

Span and steelwork

Actual support spacing rarely follows the table; it follows the grid of the steelwork. On a pipe rack the distance between the cross beams sets the span. Large pipe sizes bridge it with ease, small ones do not – they need intermediate supports or are run in bundles. Support spacing is therefore not fixed for each line separately but together with the structure.

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Overview

Influences on the permissible span

Influence	Effect	Consequence for the span
Larger pipe size	Higher bending stiffness	Longer
Heavier contents, thick insulation	Higher distributed load	Shorter
Higher temperature	Lower modulus of elasticity, lower allowable stress	Shorter
Sloped line that has to drain	Sag must remain smaller than the slope	Shorter
Vibration excitation	Natural frequency has to be high enough	Shorter

Frequently asked questions

Frequently asked questions about Pipe support spacing

Is there a formula for the maximum pipe span?

There are the beam equations for bending moment and deflection. The span follows from them once allowable stress and permitted sag have been defined. These two limits are set by the code or the project – which is why there is no single correct span.

Why does a line sag although the span has been observed?

Because the table permits a sag that may be visible, or because the assumptions have changed: insulation added later, an additional valve, or a neighbouring support that lifts off in operation and no longer carries load.

Does every additional support improve the line?

For weight, yes; for thermal expansion, not necessarily. A support that blocks a movement increases the restraint forces. Additional supports therefore have to permit the movement the line performs in operation.

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