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I was looking at high pressure pipes and their pressure ratings. I'd like to know how these pressure ratings are determined.

I assume that pipes are tested until failure, and the failure pressure is multiplied by some safety factor to determine its 'rated' pressure, but is there a formula to calculate what the failure pressure should be before testing it (based on the material, wall thickness, diameter, or other measurements of the pipe)?

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According to the ASME Process Piping Code (B31.3)

$$p = \frac{2 * t * S * E}{D - 2 * t * Y}$$

where

$p$ = internal pressure
$t$ = wall thickness
$S$ = material's tensile strength
$D$ = outer diameter
$Y$ = wall thickness coefficient (B31.3-1999, Table 304.1.1)
$E$ = material and pipe construction quality factor (B31.3-1999, Table A-1A)

Note that this equation does indeed have a safety factor included.

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  • $\begingroup$ Note that 2tS is the force per length holding two halves of the pipe together, and (D - 2t) p is the force per length trying to separate those two halves. Y and E are correction factors to account for safety. $\endgroup$ – Rick Apr 3 '15 at 19:11
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In the world of plastic piping, the formula is different, because the material doesn't yield. For isotropic plastics, B31.3 shows piping as:

$$ p = \frac{2St}{D-t} $$

Where D, t and S remain the same as above. However, the allowable strength (S) is given by an applicable ASTM specification, which functions the same as the yield stress - but is not always based on the materials ultimate strength.

Composite piping, being composed of Orthotropic laminates, doesn't have a well defined strength - the material is designed with the pipe. In these cases, the original assumption that pipes are tested until failure is absolutely correct. B31.3 states again:

$$ p = \frac{2SFt}{D-t} $$

Where a new factor, F is introduced. S is obtained from the Hydrostatic Design Basis - and it is essentially an S-N Curve for that particular sequence of lamination. F allows conversion between the two tests - 0.5 for the static test, 1.0 for the dynamic test.

ASME is currently reviewing this method - and this is an exciting new area of development for them as they are generating a new piping standard to relieve the industry of the expensive and extensive HDB testing requirement.

Quality testing mandated by ASTM D2996 / ASTM D2992 ensures the piping is made the same way - any change in the formula requires a re-test. Using this method, composite piping is typically designed for a 50 year life-cycle.

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