Pressurising the fluid in a pipe (of diameter, D, and wall thickness, t) will create stresses in the pipe wall. The internal pressure, p, will cause two large stresses (biaxial (two) stress state) in the wall of the pipe (Figure 1):
- a ‘hoop (circumferential) stress’, h; and,
- an ‘axial (longitudinal) stress’, a.
The hoop stress is higher than the axial stress:
- h = pD/2t;
- a = pD/4t = 0.5h (if the pipe is free to expand; e.g., when the pipe is above ground with no restriction to its movement).
- a = 0.3h (if expansion of the pipe is restricted; e.g., it is buried and restrained by the surrounding soil).
where D = pipe diameter, and t = wall thickness.
There are other loads on the pipelines that can create stresses:
- temperature effects, caused by the substance temperature, pressure reduction, etc.;
- external loads, caused by ground subsidence in mining areas, vibration, etc.;
- water movement over submerged pipelines (tidal movement, river current);
- weight effects, caused by backfill over the pipeline, weight of substance carried, etc.
|Figure 1. Main Stresses caused by Internal Pressure.|