Minimum radius of sag bends in a pipeline

Question

Minimum radius of sag bends in a pipeline

cpwrocky (Petroleum)

(OP)

22 Jun 01 14:34

I am trying to determine the amount of deflection allowable in a buried pipeline, using the trench to flex the pipe, without exceeding the maximum allowable stress on the pipe. I am taking into account internal pressure. The temperature is not a factor, as the fluid is at ambient ground temperature. I am using 12.75" o.d., API 5L-X42, 0.25 w.t. pipe at 150 psig internal pressure, and have done a hand calculation based on maximum bending moment. I came up with 41" deflection per 100 ft of pipe. I just don't have the references to be confident that what I have done is correct. Any suggestions?

mikolay (Civil/Environmental) · Answer 1 · 2001-06-26 09:22:34

Have you consulted the pipe supplier or manufacturer? Usually they have guidelines.

JeffH (Mechanical) · Answer 2 · 2001-06-26 12:17:31

Chris,

The Pipeline Rules of Thumb Handbook, Gulf Publishing Company, Houston, TX has a section that addresses stress calculations for cold bends and would be a valuable addition to your reference library.
It also contains a section on strain calculations for laying pipelines in a curved trench.
Basically figure the amount that the pipe will be stretched on the outside of the arc as it is bent, compute the srain from the change in length (deltaL/L), and then the stress can be found from E=stress/strain
Jeff

cpwrocky (Petroleum) · Answer 3 · 2001-06-26 14:02:34

Thanks, JeffH - I found the Rules of Thumb Handbook, and sure enough there was exactly what I was looking for! I feel good that my method was basically on target - I was within 2' of their answer. Nothing like reinventing the wheel! By the way, the ROT Handbook doesn't take into account the stress imparted by the pressure of the gas in the pipeline.

Thanks again - what a geat resource!

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Minimum radius of sag bends in a pipeline