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Learn MoreCollapsing Pressure for Tubing Subject to External Pressures
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. 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-welded tubing due to the inherent bending moment created by the lap joint — a moment that doesn't exist for butt-welded tubing. Working 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 an acceptable level of risk. This is a complex matter involving personnel hazard assessment and general safety, corrosion, fatigue, fabrication changes (bends, flares), codes and insurance, seismic stability, and temperature, among other issues. Several of these are addressed in a free publication (N° 9024) from the Nickel Development Institute (NiDI). These formulas are a tool for estimation under ideal conditions near room temperature — actual values may vary, and tubing that is out of round for any reason would be expected to fail prematurely relative to ideal estimates.
It's worth noting that material strength is not a factor in these formulas — this is common for buckling-type calculations.
A significant source of real-world collapsing failures is thermal distortion caused by partial filling (or ID boiling) in horizontal tubulars in a heat exchanger environment. This can cause failures even with positive internal pressure, making these calculations moot in that scenario.
Actual wall thickness, or a conservative estimate, should be used rather than nominal gauge. For example, 16 gauge (0.065" average wall) tubing is likely to measure closer to 0.060" — near the low end of the allowable range. Similarly, use actual pipe OD rather than Nominal Pipe Size (e.g. 3 NPS is nominally 3.5" OD).
Source: Machinery's Handbook 25, p. 271.
Open Collapsing Pressure CalculatorBursting or Working Tubing or Pipe Pressures
ASTM tubing and pipe specifications do not include any recommended service or burst pressure requirements. Barlow's formula, however, is commonly used in the industry to approximate the bursting pressure of ductile, thin-wall tubular (Wall/ID) < 0.1 or cylindrical materials due to ID pressurization, and generally yields conservative results. Other calculations are appropriate to heavy-wall and brittle materials.
Working or allowable pressures are arrived at using a safety factor (SF) to reduce the pressure from a level where bursting failure is very likely to an acceptable level of risk — the same complex set of considerations (safety, corrosion, fatigue, fabrication, codes, seismic stability, temperature) discussed above applies here as well, and several are addressed in the same NiDI publication (N° 9024). Barlow's formula is a tool for estimation under ideal conditions at room temperature; actual values may vary.
Many engineers use ultimate tensile strength (UTS) to estimate bursting pressure, and yield strength (YS) with a safety factor when estimating working pressure. Using yield strength with no safety factor (SF = 1) calculates the approximate theoretical pressure at which the tubing will begin to plastically deform.
As with collapsing pressure, use actual wall thickness (or a conservative estimate) rather than nominal gauge, and actual pipe OD rather than Nominal Pipe Size.