The maximum allowable working pressure for vessels or vessel parts for which the strength cannot be computed can be established by burst testing in accordance with ASME VIII-1 UG-101(a)(1), or through Finite Element Analysis (FEA) in accordance with ASME VIII-2.
Burst testing and FEA may only be used for the purpose of establishing a maximum allowable working pressure of those elements or components for which code calculations cannot be applied. Code calculations must be completed for all elements or components which they may be applied.
Determining the MAWP (maximum allowable working pressure)
Each code has a basic pressure rating calculation, for example VIII-1 uses UG-101(m):
(1) P = (B / 4) X ((Su * E) / (Suavg)) - OR - (2) P = (B / 4) X ((Su * E) / (Sur)) Where B = Burst test pressure, or pressure at which test was stopped E = Efficiency of welded joint as per ASME VIII-1 UW-12 (if applicable) Su - Specified minimum tensile strength at test temperature Suavg = Average tensile strength at test temperature Sur = Maximum tensile strength at test temperature
Burst testing must be completed for each material of construction unless they can be grouped by the same p-numbers. The burst test starts with a 4x factor on the burst pressure, but once all of the factors listed above and below are included, the factor is often 6 to 8x. Where applicable, regular code calculation methods have a lower factor of safety.
The calculated MAWP must be reduced by the elevated temperature reduction, corrosion reduction and casting efficiency if applicable:
Elevated temperature reduction as per ASME UG-101(k)
(3) MAWP = P * (S / S2) Where S = Maximum allowable material stress at design temperature S2 = Maximum allowable material stress at test temperature (Values S & S2 are listed at ASME IID Table 1A & 1B)
Where information is not available, proof testing may also be completed at the elevated temperature.
Corrosion reduction as per ASME VIII-1 UG-101(i)
(4) MAWP = P * ((t - c)n / t n) Where c = corrosion allowance n = 1 for curved surfaces, 2 for flat surfaces t = nominal thickness of material at the weakest point
Casting efficiency as per applicable code, i.e. ASME VIII-1 UG-24, ASME B31.3 Table A-1A
Brittle fracture failure must also be addressed if the component is subject to operation temperature lower than proof test temperatures. Minimum temperature values are provided for ASME / ANSI listed materials in some ASME codes i.e. ASME IID, ASME B31.3. If materials are unlisted documentation must be provided to identify the brittle fracture transition temperature. If documentation is not available impact testing or a low temperature proof test may be required.
As mentioned, once all of the factors listed above and below are included, the test pressure is often 6 to 8x the design pressure.