Piping max deflection
Piping max deflection
(OP)
Dear all
I got 2 questions about max piping deflection:
1) The famous Kellogg Design of Piping Systems states:
deflection = 17.1 * (w * (L^4) / (E*I) (inches)
w = weight per foot of pipe+liquid+insulation (lbs/ft)
L = pipe span (ft)
E = modulus of elasticty (psi)
I = moment of inertia (in^4)
Somebody in other post wrote that this is formula is very easy to find, for example in "Roark's Formulas For Stress And Strain". I searched, but I didn't find it. Could somebody please explain me where this formula comes from?
2) Why usually the max deflection is 1/2"?
Thank you all in advance :)
I got 2 questions about max piping deflection:
1) The famous Kellogg Design of Piping Systems states:
deflection = 17.1 * (w * (L^4) / (E*I) (inches)
w = weight per foot of pipe+liquid+insulation (lbs/ft)
L = pipe span (ft)
E = modulus of elasticty (psi)
I = moment of inertia (in^4)
Somebody in other post wrote that this is formula is very easy to find, for example in "Roark's Formulas For Stress And Strain". I searched, but I didn't find it. Could somebody please explain me where this formula comes from?
2) Why usually the max deflection is 1/2"?
Thank you all in advance :)





RE: Piping max deflection
1. ?
2. Good common sense
RE: Piping max deflection
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.
**********************
"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) http://virtualpipeline.spaces.live.com/
RE: Piping max deflection
RE: Piping max deflection
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.
RE: Piping max deflection
RE: Piping max deflection
**********************
"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) http://virtualpipeline.spaces.live.com/
RE: Piping max deflection
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.