External Pressure Collapse Calculator
Critical collapse pressure and allowable external pressure for a pipe or cylinder under vacuum or jacket pressure, per ASME BPVC VIII-1 thinking.
When to use this calculator
Use whenever a line or vessel can see pressure from the outside — a vacuum service, a steam line that draws vacuum on cooldown, a jacketed pipe, or a subsea or buried line with external head. Internal-pressure rules do not apply: failure is elastic buckling rather than yielding, and it depends on the length between stiffeners as much as the wall thickness. The tool evaluates both the intermediate and long-tube regimes and takes the lower critical pressure.
Required inputs
- Outside diameter Do and wall thickness t
- Unsupported length L between stiffeners or ends
- Modulus E and Poisson's ratio ν at temperature
- Design factor of safety
Expected outputs
- Critical collapse pressure in the intermediate regime
- Critical collapse pressure in the long-tube regime
- Governing critical pressure and the allowable external pressure
- Hoop stress at collapse
Formula overview
SI: pressures in MPa, Do, t, and L in mm, E in MPa. ν is dimensionless, typically 0.3 for steel.
Intermediate length — collapse depends on L:
P_cr = 2.6·E·(t/Do)^2.5 / [ L/Do − 0.45·√(t/Do) ]
Long tube — collapse independent of L:
P_cr = 2·E·(t/Do)³ / (1 − ν²)
P_allow = min(P_cr) / FS
σ = P_cr · Do / (2t)Worked example
20" line under full vacuum, Do = 508 mm, t = 6.35 mm,
L = 6 000 mm between stiffeners, E = 200 000 MPa, ν = 0.3, FS = 3
t/Do = 0.0125, L/Do = 11.81
P_cr(intermediate) = 2.6 × 200 000 × 0.0125^2.5 / (11.81 − 0.0503)
= 9.095 / 11.76 = 0.773 MPa ← governs
P_cr(long tube) = 2 × 200 000 × 0.0125³ / 0.91 = 0.859 MPa
P_allow = 0.773 / 3 = 0.258 MPa (2.58 bar)
Full vacuum is 0.101 MPa → acceptable with margin.Common mistakes
- Applying internal-pressure thickness rules to an external-pressure case. Collapse is a stability problem, not a strength one — a wall that is comfortable at 20 bar internal can buckle under one atmosphere of vacuum if the unsupported length is long enough.
- Overlooking how a line gets into vacuum. Steam and vapour lines draw vacuum when they condense on cooldown, and a line can be pulled down by draining or by a closed outlet on a falling-temperature service — none of which appear on the design pressure line of a datasheet.
- Forgetting that stiffener spacing is the strongest lever. Collapse pressure in the intermediate regime is roughly inversely proportional to L, so adding a stiffening ring is usually far cheaper than going up a schedule.
FAQ
Learn
Related articles
Pipe Sizing Calculation คืออะไร
ทำความเข้าใจหลักการเลือกขนาดท่อจาก flow rate และ velocity พร้อมสมการพื้นฐานที่วิศวกรใช้จริง
Pressure Drop Calculation
คู่มือคำนวณการสูญเสียความดันในระบบท่อ (Pressure Drop) ด้วย Darcy–Weisbach Equation, Reynolds Number, Friction Factor และ Minor Losses พร้อมตัวอย่างคำนวณจริงสำหรับวิศวกร
วิธีคำนวณ Hydrotest Pressure
หลักการคำนวณความดันทดสอบ hydrostatic ตาม ASME B31.3 สำหรับงาน commissioning