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)
where:
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).
Tip for the Month (Nov 2000)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 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.
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