The mechanical stresses (internal pressure, external loading) to which pipelines are subjected in service can be accurately evaluated. It is much more difficult however to predict with certainty the stresses the pipes will be subjected to, over the course of time. For this reason, PAM has chosen high safety factors to ensure the maximum possible life for its ductile iron pipes.
Minimum specified safety factors
The pipes are designed to National Standard GB/T 13295 and International Standard ISO 2531:
- internal pressure: the working stress in the pipe wall must not exceed one-third of the tensile strength (420 MPa, which also corresponds approximately to one-half the yield strength). The minimum safety factor, for the calculation of internal pressure, is 3.
- external loading: the deformation must not result in either a stress greater than half the yield bending strength (500 MPa), or maximum vertical ovality of 4 %. The maximum deformation of 4 % is recommended by National Standard GB/T 13295 to guarantee the resistance of the cement mortar (mainly for DN > 800).
Experimental safety coefficients
The actual safety of PAM pipes is greater in practice than the nominal service levels (allowable operating pressure, depth of cover). Indeed:
- the material’s ductility gives ductile iron pipes a high capacity to absorb work or energy beyond its actual elastic limits;
- the methods used to calculate parts are conservative and include high safety coefficients.
This is clearly illustrated by the following two graphs on which it can be seen that the bursting pressures observed in experiments are more than three times the allowable operating pressure.