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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)?
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.
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.
