Tall pipe risers are used in skyscrapers, mines, etc. There are two points that the piping stress engineer should consider:
- Fluid weight load distribution
- Pipe longitudinal stability
Fluid weight load distribution
The most of widely used piping stress analysis software, including START-PROF, apply fluid weight as uniform load along the pipe. This approach is correct for horizontal piping and short vertical pipes. But it is incorrect for tall risers and may lead to incorrect support loads and incorrect stress distribution along the vertical pipe length.
The picture below (a) shows axial force F diagram caused by uniform load q ath the vertical pipe that is supported at the bottom. If fluid weight is applied as uniform load q, the axial force diagram is incorrect. Software will show great axial stresses from sustained loads q*L/A, A – pipe crossection area. But in real world fluid pressure will act only on the bottom of the pipe and axial stresses along the pipe should be zero. The correct results can be obtained by applying of concentrated force at the bottom of the pipe instead of uniform load.
In case of pipe supporting at the top, uniform load gives incorrect results too. Application of fluid weight as uniform load leads to underestimation of axial stresses at the lower part of the pipe.
In case of supporting riser by several springs, the application of fluid weight as uniform load will lead to uniform load distribution between supports. But in real world fluid weight will act in the bottom bend and lowest support will hold greatest weight load.
Now let’s see how to model vertical riser correctly in PASS/START-PROF piping stress analysis software:
- Fluid density should be zero
- Add concentrated force q*L in the lower bend
Pipe Longitudinal Stability
The second problem concerns vertical risers that is supported at the bottom. Pipe is compressed by big pipe and insulation weight. Compression force may lead to pipe buckling as shown on the drawing (a) below.
To avoid buckling we should add additional V-stop supports (d) to decrease the compressive force or add guides (c) to prevent buckling.
The START-Elements has a special procedure allows to check buckling of vertical or horizontal pipe. For vertical pipe we should set sliding support friction factor to zero and enter the compressive load N value. Software will calculate the span between guides Lcr needed to prevent buckling.