As noted earlier, piping systems must be designed to withstand primary and secondary loadings. Sustained loads were discussed as being the most common types of primary loads. There are additional requirements for the evaluation of occasional loads, or primary loads which are present for short time durations, typically 1% to 10% of the total operating time. Failure criteria are typically the same for occasional loads as for sustained loads, except that creep failure is not a concern for occasional loads. Because of this, the allowable levels for the absolute sum of sustained and occasional stresses are the same as those for sustained loads, but increased by a factor (typically 15% to 33%). For example, looking at the B31.1 equation for occasional stresses:
SLP + 0.75 i Ma/Z + 0.75 i Mb/Z < k Sh
SLP = longitudinal pressure stress, psi i = stress intensification factor Ma = resultant moment on cross-section due to sustained loads, in-lb Z = section modulus of pipe cross-section, in^3 Mb = resultant moment on cross-section due to occasional loads, in-lb k = occasional stress factor
= 1.2 for loads present less than 1% of time = 1.15 for loads present less than 10% of time
Sh = Basic allowable stress in hot condition
Typical of these types of loads are wind loads, earthquake loads, and quickly applied loads (relief valve, fluid hammer, etc.). These are dynamic (meaning that they change as a function of time) loads, and are therefore discussed in greater detail in Sections 4 and 5 of these Pipe Stress Analysis Notes. However, the easiest (but less accurate), and therefore most common means of analyzing dynamic loads is usually to model them as static (meaning that they are constant throughout time) loads, with the magnitude increased to reflect the dynamic load amplification.