The Logic Behind Our Internal Yield (Burst) Pressure Calculator

If you’ve used our Internal Yield (Burst) Pressure calculator, you’ve seen it spit out a number in about a second. This post walks through what’s actually happening behind that number — the formula, where each term comes from, and why we built in a wall factor you can adjust.

The starting point: Barlow’s equation

Pipe body burst pressure comes from a thin-wall pressure-vessel formula usually credited to Barlow. In its plain form it says the hoop stress in a pipe wall under internal pressure is proportional to pressure times diameter, and inversely proportional to wall thickness. Rearranged to solve for pressure, and written the way API TR 5C3 presents it:

P = factor × 2 × Yp × t ÷ D

Where P is internal yield pressure, Yp is the pipe’s minimum yield strength, t is nominal wall thickness, D is nominal outside diameter, and factor is a wall-thickness adjustment we’ll get to below. Get any one of those wrong and the answer is meaningless, so it’s worth knowing exactly what each one means before you type a number in.

Why there’s a wall factor at all

API 5CT allows pipe manufacturers a wall-thickness manufacturing tolerance of minus 12.5%. That means a pipe stamped as 0.361 in. wall could legitimately leave the mill at 0.316 in. and still be in spec. If you calculated burst pressure using the nominal 0.361 in. figure, you’d be overstating the pressure rating of the thinnest pipe that’s still API-compliant.

API TR 5C3 handles this by multiplying nominal wall thickness by 0.875 (that is, 1 minus 0.125) before it goes into the formula. That’s the industry-standard “minimum wall” design case, and it’s what you want for a genuine pressure rating you’re going to rely on.

Our calculator defaults the factor to 1.0 instead, on purpose. A 1.0 factor gives you the nominal-wall figure — useful for a quick screening estimate or for comparing pipe on a like-for-like nominal basis — but it is not the conservative design value. Switch the factor to 0.875 any time you need the number that reflects API’s minimum-wall tolerance.

A worked example

Take 5-1/2 in. J-55 casing with a 0.361 in. nominal wall — the same defaults our calculator opens with. J-55’s minimum yield strength is 55,000 psi.

That roughly 900 psi gap between the two numbers is the entire point of the wall factor — it’s the margin you’d otherwise be quietly assuming away.

What this number doesn’t tell you

This is the pipe body internal yield pressure at zero axial load — nothing else. It doesn’t account for combined loading (axial tension or compression stacked with internal pressure, or bending), it doesn’t apply any temperature derating, and it says nothing about the connection. On a made-up string, the connection is very often the weaker link, not the pipe body. Treat this calculator as a fast first check, not a final design number.

If you’re past the screening stage and need a full triaxial (von Mises) load-case analysis or a connection re-rating, that’s a conversation for one of our engineers rather than a calculator.

Try the Internal Yield (Burst) Pressure calculator yourself, or get in touch if you’d rather talk it through with an engineer.