Chapter IX of ASME B31.3 only applies when the owner specifies its use. It applies to piping in High-Pressure Fluid Service. Note that the definition of High-Pressure Fluid Service simply requires that the owner specify use of Chapter IX. Some guidance is provided in K300 (a), which states that “High pressure is considered to be pressure in excess of that allowed by the ASME B16.5 PN 420 (Class 2500) rating for the specified design temperature and material group.” This is not a requirement, and the base Code may be satisfactorily used at pressures higher than ASME B16.5, PN 420 (Class 2500). However, the base Code rules become increasingly conservative and, in fact, impossible to use as the pressure approaches the allowable stress (including quality factors). See Section 17.3 for a discussion of pressure design of straight pipe. By the same token, Chapter IX may be used at lower pressures; it has been used with high-strength steels with pressures as low as 28,000 kPa (5,000 psi).
The rules provide a combination of considerations. Although reduced wall thicknesses and provisions that are specific to the needs of high pressure (e.g., not including thread depth as an allowance under specific conditions) are provided, additional material toughness, analysis during design, inspection, and testing are required. For background on these rules see Sims (1986).