Piping max deflection 2

Answers"
1. ?

2. Good common sense

A simple supported beam's deflection at centerline with a uniform load is = 5 * w * L^3 / 384 /E /I

with L in feet, the above formula must include a conversion to inches 12, so that's (12*L)^3
Running out the constants yields,
So, 5 * 1728 / 384 = 22.5

They are using 17.5 (about 80% of 22.5), propably since a pipe is continuous across the supports, not simply supported, and that reduces the deflection.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

The practice is to use a case which is the average between a simple beam onto supports and a beam fully restrained at its ends.

The intention of the minimum deflection value is for drainage and process flow (empty the pipe when done) requirements between the most common support methods.

A "pipe" is fighting its own weight, its fluid's weight, its insulation weight, and the valves, fittings, and extra snow/ice/wind/etc loads that happen to fall on the thing up when it is vunerable up in the air. Plus impact load, thermal loads, expansion, contraction, vibration, ....

It also has to contain the internal fluid pressure, sometimes at high temperature (which reduce steel capacity) and sometimes at very low temperatures - which reduce reserve strength/toughness/brittle failure as well. Connections (flanges and welds usually) have to carry all this stress.

Restricting deflection by Code to a minimum value removes the "structural engineering" part of the problem from the piping engineer's "list of things that will break" the pipe. He (the pipist) then has a more manageable job of anticipating and fixing the rest of the problems that keep coming up.

Supporting pipes in a rack so there is only a little little defection in the most highly loaded pipe means the rest are also supported adequately. The most deflected pipe still drains when emptied - even if "flat" - and, if it has to drain by process definition, the slope between supports can be kept so the 1/2" max deflection doesn't interrupt flow.

The last "assumption" in the formula is pipe material and geometric strength: A round pipe of known material and known wall thickness with known material tolerances only has the material, insulation, and fitting weights as unknowns. Plus earthquakes, thermal expansion .....

When the pipe is draining there isn't much liquid load by the time it gets down to draining any deflected low spots, so that's not the reason, otherwise you would just use 1/2 in with pipe & insulation weight only.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

The 1/2" deflection comes from the desire to keep the natural frequency of the span above 4HZ to reduce the susceptibility that the span will resonate under a wind load or other minor vibration inducing source.

Carthago is also right about the formula being a compromise between the equations for a simply support beam and a fully restrained beam.

Both of these come from the old Kellogg Design of Piping Systems book. Since this question comes up from time to time, I've got a excerpt of the relevant section scanned and attached at the bottom.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.