Pipe Under External Pressure

The strength of the pipe under external pressure is a function of the physical properties of the pipe material at the operating temperature and its geometrical parameter, such as the unsupported length, L, pipe thickness t, the outside diameter, D, and the pipe out-of-roundness.

The behaviour of thin-wall cylindrical shell under uniform external pressure varies in accordance to the cylinder length as follows:

Very Long Cylinder.

The critical collapse pressure is given by:

The Critical Length, L_c, which is the minimum unsupported length beyond which P_cis independent of L, is given by:

Intermediate Cylinders with L < L_c

The critical Pressure, Pc is a function of the collapsed contour and the two characteristic ratios (t/d) and (L/D). For practical design, the empirical equation below is given:

Short Cylinders:

The cylinder will fail in this case by plastic yielding, where the critical pressure, Pc will be given from the equation below:

ASME Charts: Geometric Chart

For L > L_c, the Tangential Stress Sc at collapse pressure Pc (given by equation for long cylinder above), can be written as below:

For L ≤ L_c, the Tangential Stress Sc at collapse pressure Pc:

Both above equations were plotted to develop the geometric chart as shown on Fig. UGO-28.0 of the ASME Boiler and Pressure Vessel Code, Section VIII Division 1.