ASME B31.1-2024 Occasional Loads — FAQ (¶104.8.2, ¶102.3.3, ¶101.5)

Question 1

What counts as an occasional load under B31.1?

Accepted Answer

A load that is not always there, evaluated together with the sustained loads that are. ¶104.8.2 says the dynamic effects of ¶101.5 may be treated as occasional loads if the k factor's time limits are met, and ¶101.5 lists them: impact including water and steam hammer from a rapid valve operation (¶101.5.1); wind on exposed piping (¶101.5.2); earthquake on the piping, supports and restraints (¶101.5.3); vibration, handled by arrangement and support rather than a stress allowance (¶101.5.4); and discharge reactions from pressure and momentum during normal operation and anticipated transients — the safety-valve case (¶101.5.5). ¶102.3.3(a) adds temporary supporting of extra weight as another example. Two things people wrongly file as occasional: snow and ice, which are live load under ¶101.6.1, and a pressure or temperature excursion above design, governed by the separate ¶102.2.4 hoop-stress allowance, not eq. (16). The duration test is the gate — an event recurring more than eight hours at a time or 800 hours a year is no longer occasional, and its stress belongs in the sustained check against Sh without any k.

Question 2

What is the allowable stress for the occasional case — where do 1.15 and 1.2 come from?

Accepted Answer

Figure 104.8-1 eq. (16) limits the stress due to pressure, weight, other sustained loads and the occasional load to k·Sh, and ¶104.8.2 sets k by duration: 1.15 for no more than 8 hours at a time and 800 hours a year, 1.2 for no more than 1 hour and 80 hours a year. Both trace to ¶102.3.3(a), which lets occasional loads exceed the Allowable Stress Table values by those amounts and durations. These are the same percentages ¶102.2.4 uses for pressure and temperature excursions — one philosophy of short-term overstress applied to two different stresses, and an event can invoke both at once. The contrast with B31.3 ¶302.3.6 is the number most engineers carry: process piping allows a flat 1.33 × Sh (with an elevated-temperature alternative), not graded by hours, so a line passing at 1.33 has not been shown to pass at 1.15. Wind, at a few hundred hours a year, generally lands in the 1.15 tier; a relief lift or design earthquake in the 1.2 tier — but that classification is the designer's, and it should be written down.

Question 3

How is the occasional stress S_o actually assembled?

Accepted Answer

Equation (16) has the same shape as the sustained eq. (15), with the occasional loads added into the force and moment terms. The first term is the axial index Ia times the magnitude of the longitudinal pressure stress Slp plus Fb/Ap, where Fb is the longitudinal force from weight, other sustained loads and the occasional load, compression negative; to that is added the SRSS of Ii·MiB and Io·MoB over Z, the moments being sustained plus occasional; that bracket is squared and added to the square of It·MtB/Z, and the root of the sum is So. The indices are the ¶104.8.1 sustained indices — the greater of 0.75 × the B31J i and 1.00 — not the full expansion SIFs. ¶102.3.2(a)(3) adds a detail easy to miss: the pressure P in Slp for eq. (16) is the pressure coincident with the event evaluated — relieving pressure for a relief case, surge pressure for water hammer. ¶104.8.4(b) settles the sign question for dynamic results: where the method returns only magnitudes, the most conservative combination of signed sustained and unsigned occasional forces and moments is used.

Question 4

Do wind and earthquake have to be applied together, and do I apply the ASCE 7 ASD reduction?

Accepted Answer

Not together, and — under B31.1-2024 — not reduced. ¶101.5.2 and ¶101.5.3, both revised in 2024, now say the same three things for wind and earthquake: the loads may be as described in ASCE/SEI 7, but no Allowable Stress Design reduction factor applies; local jurisdictional values are the minimum design values; and wind need not be considered concurrent with earthquake, nor earthquake with wind. Local meteorological or seismological data may refine the loads, and ¶101.5.3 names ASME B31E as an alternate method of seismic qualification, or as guidance. The ASD point changes numbers: ASCE 7 Chapter 13 component forces and Chapter 26–29 wind pressures are strength-level loads, and the ASCE 7 allowable-stress combinations apply 0.7 to earthquake and 0.6 to wind before comparing with an allowable — B31.1 says skip that reduction and check the full load against k·Sh. On this site's ASCE occasional-load card the ASD factors default to B31.3-practice 0.6/0.7; for a B31.1-2024 run enter 1.0 for both. B31.3 ¶301.5.2 and ¶301.5.3 are silent on the reduction, leaving it to the owner's specification there.

Question 5

How are water hammer and steam hammer treated?

Accepted Answer

As impact loads under ¶101.5.1, which requires impact forces from all external and internal conditions to be considered and names the pressure wave from a sudden change in fluid momentum — water or steam hammer from a rapidly opened or closed valve — as one example, warning other causes exist. B31.1 gives no method for computing the load and no separate allowable; the resulting stress enters eq. (16) as an occasional load if its duration fits the k limits, and a design-basis valve closure or turbine trip fits the 1.2 tier comfortably. The method is the designer's: the honest hierarchy runs from a Joukowsky screening estimate of surge pressure, through an unbalanced-force calculation on each straight run with a dynamic load factor, to a time-history hydraulic analysis feeding a dynamic stress run. Two power-piping cautions: steam hammer from condensate slugs in warm-up and from rapid stop-valve closure on reheat and extraction lines has broken supports never designed for an axial load, and ¶101.5.4 with ¶120.1(c) require vibration and resonance to be addressed by arrangement, dampers and restraints — a support-design problem, not a stress-allowance one.

Question 6

Is the safety valve reaction an occasional load?

Accepted Answer

Yes — it is the ¶101.5.5 discharge reaction, and on steam headers it usually governs the occasional case. The force comes from the Nonmandatory Appendix II method for an open discharge — momentum plus pressure at the elbow exit, stamped capacity increased by 1.11, dynamic load factor never less than 1.1 — and the moment at the branch connection under the valve may not be taken less than DLF · F1 · Lo. That moment enters eq. (16) with the sustained-plus-occasional indices, checked against k·Sh, and Appendix II-4.1 restates the same three checks for every critical point. The duration tier is the designer's call: a valve that lifts rarely sits in the 1.2 tier, one that cycles through every transient may not. Multiple-valve headers must be checked for each valve and the combinations that can occur; closed-discharge systems are a different problem, with pressure waves the simplified method does not capture. The separate B31.1 PSV reaction force FAQ on this site walks the appendix — the point here is that the reaction is an eq. (16) load, and the header and its supports, not just the branch, must carry it.

Question 7

How does the 15 % / 20 % pressure–temperature allowance differ from the k factor?

Accepted Answer

They share the hours but govern different stresses. ¶102.2.4 says a system may be considered safe for occasional short periods above design pressure or temperature when the circumferential pressure stress stays within 15 % of the maximum allowable stress at the coincident temperature, for events of no more than 8 hours at a time and 800 hours a year, or within 20 % for no more than 1 hour and 80 hours a year — subject to any tighter component or manufacturer rating. That is a hoop-stress allowance for a pressure excursion, letting a relieving pressure above design pass the wall-thickness check. The k factor of ¶104.8.2 is a longitudinal-stress allowance for an added load, applied to all of eq. (16). A relief event can invoke both — relieving pressure through ¶102.2.4, reaction force through k — each checked against its own limit. Neither extends a flange, valve or other component rating; ¶102.2.4 says so up front, and the component rating usually binds first. The B31.3 counterparts, ¶302.2.4 and ¶302.3.6, carry different numbers — don't cross the code boundary with either.

Question 8

What about the hydrostatic test — is that an occasional case?

Accepted Answer

No, it has its own limit, and occasional loads are excluded from it. ¶102.3.3(b) says that during a ¶137 pressure test the hoop stress shall not exceed 90 % of yield strength at test temperature, and the sum of longitudinal stresses from test pressure plus the live and dead loads present at test — excluding occasional loads — shall not exceed that same 90 % of yield. ¶137.1.4 repeats that no part of the system may exceed the limit at any time during the test. So the test case is checked against yield, not k·Sh, and wind or earthquake are not combined with it; B31.3 ¶302.3.6(b) says the same in its own words. The test case still matters: on a vapor line filled with water it is the heaviest weight case the pipe ever sees. ¶137.2.2 requires temporary supports where the test liquid weight demands them, ¶120.2.2 requires springs capable of the test load or supplemented, and ¶121.2(j)(2) lets support material reach 80 % of room-temperature yield during the test — a different allowable, a different set of supports active.

Paragraph	What it sets
¶104.8.2	Sustained plus occasional stress per eq. (16) not to exceed k·S_h; k = 1.15 for ≤ 8 hr at a time and ≤ 800 hr/yr, 1.2 for ≤ 1 hr and ≤ 80 hr/yr; ¶101.5 loads may be treated as occasional when those limits are met.
¶102.3.3(a)	The occasional-load allowance during operation, by the amounts and durations of ¶104.8.2.
¶101.5.1 – ¶101.5.5	Dynamic effects: impact including water and steam hammer; wind and earthquake per ASCE/SEI 7 without the ASD reduction factor, jurisdictional minimums, B31E as alternate; vibration; discharge reactions.
¶104.8.4(b)	Where a dynamic analysis returns magnitudes without sign, use the most conservative combination of signed and unsigned forces and moments.
¶102.2.4	Pressure–temperature variation allowance: hoop stress may exceed the allowable by 15 % (≤ 8 hr, ≤ 800 hr/yr) or 20 % (≤ 1 hr, ≤ 80 hr/yr) — a different rule from k.
¶102.3.3(b)	Test condition: hoop and longitudinal stresses limited to 90 % of yield at test temperature, occasional loads excluded.
¶120.1(b), ¶121.2(j)	Supports designed for the concurrently acting loads including wind and earthquake; 20 % allowable increase for short-time overloading.