Piping imposes loads on equipment nozzles. These loads
may exceed the allowables given by the manufacturer or codes such as API
610.
The following guidelines may be helpful in reducing these piping loads
on nozzles connected to equipment.
1. If the dead loads exceed the allowable,
• Ensure the piping system is adequately supported,
• Remove unneeded supports; they may be the cause of the problem.
2. If the thermal loads exceed the allowable,
• Check the design and operating temperatures. Consult the process
engineer to obtain correct or reasonable values for different operating
conditions.
3. Try modifying the piping support system and layout
• Add expansion loops if apt,
• Use expansion joints or other flexible joints,
• Consider spring mounted pumps,
• Modify the layout of piping by rerouting,
• Use guides or anchors at strategic locations,
• Use reinforcing pads on vessel nozzles.
A rod hanger in CAEPIPE functions as a limit stop,
that is, it functions as a non-linear one-way restraint. It is rigid in
-Y direction and fully flexible in +Y direction (in a Y-vertical system).
The rod hanger offers no resistance in +Y direction.
Rod hanger results are included in the hanger report which reports results
for the first operating case (W+P1+T1). In the hanger report, a rod hanger's
spring rate may be shown either as Rigid or zero. The zero spring rate
often confuses users. It simply means that there is liftoff at the hanger
location for the first operating case. One can confirm this by studying
the output Y displacement at the support in the first operating case (which
will be 0 or positive).
Liftoff (i.e., zero spring rate and a zero or positive operating condition
displacement) indicates the support may not be needed and could be removed.
Contemporary commercial piping analysis programs deal differently
with the problem of apparent lift-off of an operating pipe at a rod hanger
or a one-way vertical support, such as a pipe on a support rack. A few
programs provide error messages; others show a vertical movement with
a possible increase in sustained (weight) stress (see NOTE below for CAEPIPE).
A proper understanding of the standard piping design practice is the key
to correct interpretation of these results from different programs. Such
standard piping design practice was generally understood when the sustained
and flexibility analysis rules were introduced in the 1955 Edition of
the ASME B31 Code for Pressure Piping.
The problem with lift-off is compounded by the intention of the piping
analysis being performed - whether the intent is to design new or revamp
existing piping or the intent is to analyze as-built. The intention of
the various sections of ASME B31 Code (B31.1, B31.3, etc.) is to provide
guidance for new construction. Note, since the publication of the 1935
Edition of ASME B31.1 (which included the predecessor of B31.3 as a chapter,
Paras. 101.6 and 121.4 and their predecessor paras.) state:
Piping shall be carried on adjustable hangers
or properly leveled rigid hangers or supports, and suitable springs...
Hangers used for the support of piping, NPS 2½ (NPS 2 in 1935 edn.)
and larger, shall be designed to permit adjustment after erection while
supporting the load.
While not quite as explicit, the current ASME B31.3
Para. 321.1.1 states:
The layout and design of piping and its
supporting elements shall be directed toward preventing... piping stresses
in excess of those permitted by in this Code;... unintentional disengagement
of piping from its supports;... excessive piping sag in piping requiring
drainage slope;...
These paragraph excerpts define standard practice
in piping design. That is, during operation, it is neither the intention
of the code nor standard practice to allow piping to lift-off. Piping
is normally designed to be supported in the operating condition. The means
to achieve this is through proper adjustment of the supports during operation.
This is important in piping because unadjusted supports will permit the
pipe to sag and create locations in steam or condensable gas piping where
condensates can collect or concentrate. And it is especially important
for piping operating above 800 degF, where unadjusted supports will allow
the pipe to permanently deform (creep) over time.
Small gaps are inevitable in actual construction because of fabrication
and installation tolerances and would normally be closed by support adjustments.
But, so long as the pipe is prevented from significant lateral movement,
small gaps below pipe during operation (¼ inch and less in moderate
size piping) may be tolerable because the weight analysis is a very simplified
and conservative method that the ASME B31 codes use to guard against collapse.
Stresses caused by takeup of a small gap below the pipe could even be
considered expansion or building settlement type stresses and thus would
not need to be considered in the weight analysis. Weight analysis with
the intent of designing pipe normally considers all the weight supports
perform their intended function. Any significant gaps determined by analysis
could either indicate that a support is not required, or that adjacent
supports need to be modified, or that an alternate means of support is
needed, e.g., a variable or constant spring should be used.
However, if the purpose of an analysis is not to design a new or revamp
an old piping system, but to evaluate an as-built and maintained piping
system, small gaps may have more significance in as much as they would
indicate that the pipe support system may not be acting as designed and
maintained. A lack of or improper adjustment of the supports in the operating
condition may cause lift-off at rigid supports. Improperly designed or
adjusted or maintained or degraded variable or constant spring supports
may cause lift-off, too.
The interpretation of the results of the analysis of as-built piping systems
need not necessarily conform to the rules of the ASME B31 codes. Remember,
the rules in the B31 codes are required for new construction, not the
evaluation of existing piping. It is understood that a greater factor
of safety is required for the design process because the pipe and its
components are not yet available to be measured and materials confirmed,
as well as the knowledge of how the piping is to be actually used. The
interpretation of the analysis results of as-built piping may be able
to take advantage of what the actual piping dimensions and materials are
and how the piping has been operated. Competent engineering judgement
based on knowledge of the intent of the respective ASME B31 codes must
then become part of the evaluation process.
For the reasons noted, it is important to distinguish between the design
and analysis of piping. If designing, certain assumptions are normally
made with regard to whether the piping is supported in the operating condition.
Such assumptions might include tolerating a small gap at a given support
but realizing that the installation of the given support will require
adjustment. Alternately, a larger gap at the given support may require
support relocation to be effective or the selection of a different type
of support, most typically a constant or variable spring. If merely analyzing
existing piping, no assumptions need be made regarding supports acting
and analysis gaps may become important considerations. That said, however,
the analyst must realize that the piping analysis model is a very idealized
estimation of the as-built piping and for the analysis results to be meaningful,
the analyst needs to consider how well the results correlate with the
actual performance of the in-situ piping.
NOTE: In case of lift-off, CAEPIPE will show
a gap and possibly increased sustained stresses. The user must interpret
the gaps according to whether the user is designing new or revamping existing
piping or is analyzing an existing condition.
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.
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