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Tips October - December 2000
Stress Intensification Factors (SIF) – A Few Remarks
If you have not read Ron Haupt's commentary on SIFs, you may want to do so now.
A few remarks pertaining to SIFs would be appropriate at this point:
1. A.R.C. Markl's fatigue tests were done on full-scale 4 in. pipe assemblies involving piping components of various shapes and proportions conducted over a five year period.
2. Markl found that the fatigue behavior of piping components tested could be expressed mathematically as:
iS = 245,000 / N^(0.2)
i := SIF,
S := nominal Failure strength (cyclic moment divided by the section modulus of the matching pipe),
N := Number of cycles to failure.
3. SIFs given in various piping codes are based on Markl's tests. These values are, at most, approximate. Results of 4 in. pipe tests have been extrapolated to cover all pipe sizes ranging from 0.5 in. to 72 in.
4. For components other than those covered by the code, suitable SIFs may be assumed by comparison of their significant geometry with that of the components shown in the code.
5. New equations based on current research for SIFs will be introduced in the code in the future.
Paraphrased from SST's Piping Design and Analysis seminar notes (SST 101).
B31.3 Interpretation Re. SIF Use in Sustained Stress Calculation
Should one use SIFs for SL calculation since the code does not address the issue directly? Someone had asked this question a long time ago and the reply was published as a code interpretation (see below). However, there are engineers who have interpreted the interpretation to mean that SIFs should not be used at all in the SL equation. Ron Haupt, our NQA Manager (code committee member), clarifies:
B31.3 Interpretation 6-03R, Question (2)
Question (2) of B31.3 Interpretation 6-03R, issued May 24, 1988 states:
Question: In accordance with ANSI/ASE B31.3, para. 302.3.5(c) when calculating the longitudinal bending stresses due to sustained loads, what stress intensification factors should be applied?
Reply: ANSI/ASME B31.3 does not address the application of stress intensification factors for longitudinal stress due to sustained loads; but see ANSI/ASME B31.3, paras. 300(c)(3) and (5).
The reply should not be construed to mean that B31.3 does not require stress multipliers of some sort in calculating sustained load stresses, only that B31.3 does not currently say anything about them. A careful reading of the B31.3 paras. referred to provides the only guidance in the matter that can be provided because an interpretation should only reflect what is currently in the Code, not what the B31.3 committee wishes was in the Code. Para. 300(c)(3) essentially states that the Code generally provides a simplified design approach. This is why finite element methods, inelastic analysis, or fracture mechanics approaches are not explicitly incorporated in the Code. Para. 300(c)(5) further states:
The engineering design shall specify any unusual requirements for a particular service. Where the service requirements necessitate measures beyond those required by this Code, such measures shall be specified by the engineering design. Where so specified, the Code requires that they be accomplished.
In the case of longitudinal stresses due to pressure, weight, and other sustained loadings (para. 302.3.5(c)), the Code uses a simplified approach to assure that a piping system will not collapse. Using competent engineering judgement (see the B31.3 Introduction) it is obvious that a bend or elbow will collapse more readily than a straight pipe and, thus, some sort of nominal stress multiplier should be used to penalize the bend or elbow relative to straight pipe. What that stress multiplier should be, the B31.3 committee has not yet agreed upon. In other words, at the present time it would be considered an unusual requirement, in B31.3 terms. (It is not unusual in B31.1 terms, which uses a stress multiplier of 0.75i in B31.1 sustained stress calculations to penalize components relative to straight pipe). Thick-wall piping systems, well supported, and containing few components susceptible to collapse may, by inspection, not require nominal stress multipliers to help in identifying locations where a collapse concern could exist. In general, however, the designer may just wish to automatically use some nominal stress multiplier to remove the need to apply judgement, which at times may be difficult to defend.
It should be seen then, in an indirect manner and the only way available to the B31.3 committee, that the reply to Interpretation 6-03R, Question (2), by referencing paras. 300(c)(3) and (5), requires more than just a nominal stress check. That is, it infers that a stress multiplier, if using a consistent simplified approach, or some other method to evaluate the collapse potential of the piping system should be used. Para. 302.3.5(c) is a required, but simplified guard against collapse; alternatively more rigorous methods (see para. 300(c)(3)) or competent engineering judgement (see the B31.3 Introduction) must be used to provide an adequate margin of safety against collapse. For example, if pressure plus weight (being the only sustained loading) stress calculations for a thick-walled, well supported piping system disclosed very low nominal stresses, further collapse considerations may not be necessary. On the other hand, if high nominal stresses were calculated, some further effort would be necessary to evaluate components that are susceptible to collapse to meet B31.3 Code requirements. This further effort could be anything from component testing, to the use of a nominal stress multiplier for components that relates to the collapse potential of such components, to the use of elasto-plastic finite element analysis methods.
Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.
Harmonic Analysis – Notes
Harmonic loads can be loads from any sinusoidal loading, from rotating equipment, or reciprocating pumps on a pipeline.
The magnitude of the loading needs to be determined before analysis. If only one compressor is on a line, then only one harmonic load is input. If two loads act on the same line at different locations, then the phase (angle) or the separation in timing of application of each harmonic load is important (for e.g., the two loads may be equal and opposite to each other thus canceling out any dynamic imbalance, or the two loads can be in the same direction, say +X and separated by 30 deg.-phase angle).
So, the situation must be carefully analyzed before imposing the loads. In CAEPIPE, the load can be imposed as a Force (FX/FY/FZ) at a phase angle. User has to contact the manufacturer to get more information on the harmonic loading (mass, rpm, eccentricity etc.).
While viewing results, you need to bear in mind that the harmonic response is an unsigned case, and you will not see a sign (+/–) for this case.
Presently (as of v5.02G), Time history and Harmonic analyses do not include the missing mass correction. It is included in the Response spectrum analysis, however.