Restrained versus Unrestrained Pipe (Difference in Gas vs. Liquid)
- Drew Lafleur (Deactivated)
ASME B31.4 liquid and B31.8 gas codes include calculations for the net longitudinal compressive stress that must be applied only for a restrained line that equates to a low (less than 2%) longitudinal strain. This stress status is characteristic to underground pipelines located some distance away from above ground piping facilities.
Unrestrained lines means those above ground sections of piping without axial restraint as with buried pipe with soil. In others words the soil exerts substantial axial restraint, but not fully restrained. See pictorials below:
Restrained Unrestrained
Therefore, what is the difference between liquid and gas pipelines when running the calculations in the Pipeline Toolbox?
The differences in ASME B31.4 and ASME B31.8 in these calculations:
- Bending moments considered in B31.8 Gas
- Net Longitudinal Stress use different calculations in Gas & Liquid codes
- Internal loads including axial, shear forces and bending moments create stresses which are known as thermal expansion or contraction stress. It is calculated in such a way all temperature range of the system, from minimum to maximum system temperature is covered in B31.4.
- Combined Biaxial Stress in Gas B31.8.
To assure the integrity of a piping system, knowledge of stress and deflection calculations as well as their criteria in the codes must be understood to run these calculations.