Since hoop pressure stresses are approximately twice as large as longitudinal pressure stresses, pipe wall thicknesses are initially sized for hoop stresses. Because of this, pressure design of components is usually done far before, and therefore in isolation, from the pipe stress analysis phase of piping design. Because of this, pipe stress software such as CAESAR II does not normally handle this part of the design effort. A discussion of pressure design of components is included here for the sake of completeness, and is based upon an amalgam of the requirements of various codes. Note that pressure design of piping components must be done according to the requirements of the user's specific code, not to the rules described here!
Because the pipe wall is sized for the large hoop stress, this usually provides sufficient margin between the allowable stress and the longitudinal pressure stress to accommodate the weight stresses. The requirement for the minimum pipe component wall thickness is: tm = t + c Where:
tm = minimum wall thickness, in t = minimum wall thickness required for pressure, in c = sum of allowances for thread or groove depth, corrosion, erosion, and manufacturer's tolerance, in
For thin wall (t < D/6), straight pipe under internal pressure, t may normally be calculated, through various approximations of Lame's equation, as:
t = PD / 2(SE + PY), or: t = PD / 2SE, or: t = (D/2) x (1 - [(SE - P) / (SE + P)]^1/2), or: t = P (Dj + 2c) / [2(SE - P(l-Y)]
P = design pressure, psig D = outside diameter, in Di = inside diameter, in S = basic allowable stress at design temperature, psi E = casting or longitudinal weld quality factor (typically ranges from 0.8 to 1.0) Y = material coefficient, with a value (depending upon the specific code requirements) to be interpolated from:
Requirements for pressure design of other piping components are described in the following sections. (For B31.3 y = 0.0, for B31.1 y = .4. The CAESAR II check uses 0.4 for all codes except B31.3, where y = 0.0.)