ASTM tubing and pipe specifications do not include any recommended service or collapsing pressure for tubes subject to external pressure. The formulas below were developed to predict lap welded tubing failure due to external pressure by R. T. Stewart (Dean of the Mechanical Engineering department of the University of Pittsburgh)1. The predictions are reported to be accurate if the tube length between supports is greater than 6 times the diameter.
Lap welded tubing would normally be expected to fail before butt welding tubing due to the inherent bending moment created by the lap joint. This moment will not exist for butt welded tubing. Working pressures or allowable pressures are arrived at using a safety factor (SF) to reduce the pressure from a level where collapsing failure is very likely to where an acceptable level of risk is achieved. This is a complex matter where many issues must be considered such as: personnel hazard assessment, and general safety; corrosion; fatigue; fabrication changes (bends, flares); codes and insurance; seismic stability; and temperatures, to name a few. A number of these issues are addressed in a free publication (N° 9024) from the Nickel Development Institute (NiDI). These formulas are merely a tool for estimation or prediction under ideal conditions near room temperature. Note that actual values may vary from these theoretical calculations for a number reasons. Tubing which is out of round for any reason would be expected to fail prematurely relative to ideal circumstances or estimates.
A significant source of real-world collapsing type failures is thermal distortion caused by partial filling (or ID boiling) in horizontal tubulars in a heat exchanger environment. This type of situation can cause failures even with positive internal pressures, making these calculations mute.
It is interesting to note that material strength is not a factor in these formulas; this is common for buckling type calculations or predictions.
Actual wall thickness or conservative wall estimates should be used. For example 16 gauge or 0.065" average wall tubing is likely to measure 0.060" thick, or near the low end of the allowable range. Actual pipe ODs should be used rather than Nominal Pipe Sizes (3 nps is nominally 3.5" OD). Consult the RathGibson's pipe page for help regarding pipe dimensions.
1 – Machinery's Handbook 25, page 271.
P = 86,670*T/OD - 1386 for P > 580 psi
P = 50,210,000*(T/OD)3 for P < 580 psi
Where:
