Transactions, SMiRT-25
Charlotte, NC, USA, August 4-9, 2019
Division V
PRACTICAL ASPECTS OF DECOUPLING CRITERIA FOR SEISMIC
ANALYSIS OF SMALL BORE PIPING
Alexey Berkovsky1, Oleg Kireev2
1
2
Principal, CKTI-Vibroseism, Saint-Petersburg, Russia (aberkovsky@cvs.spb.su)
Principal, CKTI-Vibroseism, Saint-Petersburg, Russia (obkireev@gmail.com)
ABSTRACT
The paper deals with some aspects of the decoupling criteria applicable for the seismic analysis of the
small bore pipes (SBP). The reason for this research is a current situation in the industry: piping models
for the flexibility and stress analysis are transferred from 3D CAD Software to the piping stress analysis
program and contain pipes of different sizes. As result, the seismic loads calculated for the small bore
pipes include both: inertia part and loads due to seismic anchor motion (for the small bore pipe, it is a
movement of the big pipe it is attached). Separation of these combined loads on the primary and
secondary parts hardly can be done and consequently, results of the seismic analysis for the small bore
pipes become over-conservative.
The paper describes an effective numerical approach that may be realized in the frame of
conventional seismic calculation methods: Response Spectra Method or Time History analysis. The
numerical results gained from the nuclear piping design are illustrated and discussed.
INTRODUCTION
Different sources depending on the purpose of classification suggest the different definition of what could
be called as small bore pipes. Some of them just point on the Nominal Pipe Size (NPS) and pipe schedule,
others specify additional parameters such as temperature, material, etc. Table 1 summarizes different
definitions found in the literature.
WRC-300
Table 1: Definition of the small bore pipes
EPRI NP-6628
EPRI TR-101968S-V10
< 2''
< 2''
T, °C
Schedule
-
-
Material
-
-
Safety Class
-
NPS
ASME BPVC Class 2,3
(NC/ND), ASME B31.1
< 2''
(1)
(2)
T≤150°С
Sch 40,80,160
CS, AUS, [σ]≥100 МПа
ASME BPVC Classes 1,2,3
(NB/NC/ND), ASME B31.1
Notes:
(1) for ASME Section III Class 1 piping 1 inch and smaller
(2) for higher temperatures a more rigorous computer analysis should be considered
Decoupling criterion usually deals with the ratio of run to branch diameter or moment of inertia
or section modulus. The ratio for the moment of inertia typically ranged from 10:1 to 25:1, for diameter it
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
is 4:1. Being decoupled in such a way, it's assumed that SBP has a rigid anchor in the point of attachment
with a run pipe
Specific features of the SBP design and stress analysis.
The following features of the SBP affect their design, WRC-300 (1984):
1) The big amount of these pipes on the plant: the total length of SBP even for rather compact PWR
plant of the western design is more than 15 km. That leads to high expenses for the design,
manufacturing, installation and following maintenance of the lines and their supports.
2) Unavoidable intersections (collisions) of these lines with other equipment and distribution
systems (cable trays, ventilation ducts and pipes)
3) The discrepancy between design and installation: due to the tight arrangement of other
components SBP are often field routed, and their supports have to be relocated on the site
4) Restraining of the large valves with heavy operators needs special care from the designer: one
should provide the liberal conditions for the thermal expansions and at the same time put the
adequate seismic restraining
5) It’s important to define proper temperature used for the thermal flexibility analysis: using of the
operating temperature rather than design is preferable for establishing an arrangement of piping
supports: higher temperatures require more flexible piping layout that creates more severe
conditions for seismic design
6) Special consideration should be done for the accurate implementation of the SIFs in the branch
connections. Especially it is important when small bore piping is considered as decoupled from
the main line: in this case either SIF for the socket weld or SIF for branch connection should be
used. From the other hand, it’s required not to miss influence of the SBP on the main line: “the
branch pipe may cause significant local stresses in the run pipe, especially on larger diameter”
7) SBP are susceptible to high vibration. Socket welds to an attachment to a run pipe or a piece of
equipment are especially prone to vibration fatigue failures.
The current practice of the piping design is a routing in 3D CAD packages the large and small
bore pipes. Normally these models are transferred to the piping stress analysis software without
separation by their size. Having such a complex model it’s a question to analyst: use a coupled model as
is, or apply decoupling procedure. Each approach has own pros & cons. If the model remains coupled, no
special efforts are required for the establishing of the boundary conditions: run analysis and get results. It
works well enough for the static analysis under the weight and thermal expansions load. However, the
results of the seismic analysis may be over-conservative. From the other hand decoupling of SBP from
the large diameter pipes has benefits of more compact and understandable models and results, but
difficulties exist how to define seismic inertial input transferred from the run pipe.
DECOUPLING PROCEDURE
Dividing the seismic load into two parts is an acceptable approach for piping seismic analysis. These two
parts are:
−
−
primary loads due to inertia. For stress assessment inertia loads are combined with other sustained
loads, such as weight loads and internal pressure;
secondary loads due to seismic anchor movements (SAM). For the stress assessment, depending
on the Code, these loads may be used either together with inertia loads (conservative approach) or
being evaluated separately like other secondary loads, eg. loads form thermal expansions.
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
The seismic loads acting on the small bore pipes include both these parts: inertial loads
transferring via supports of SBP and SAM loads caused by the seismic motion of the main line and
transferred mostly through the attachment to the run pipe.
To reduce overall conservatism the following procedure is proposed and realized as a part of the
dPIPE calculations, dPIPE (2017):
1. Select piping segments that may be classified as “small bore pipes” with help of the decoupling
criteria. One of such criteria proposed in the literature is the ratio of run to branch pipe moment of
inertia, WRC-300 (1984):
𝐼
𝑅𝑆𝑆𝑆 = 𝑅𝑅𝑅 > 25
(1)
𝐼𝐵𝐵𝐵𝐵𝐵𝐵
2. Run conventional seismic analysis of the coupled system. Resultant loading on cross sections of
(1)
the small bore pipes 𝐹𝑆𝑆𝑆 would consist from the combination of the primary (inertial) seismic
𝑆𝑆𝑆
𝐼𝐼𝐼𝐼
loads, 𝐹𝑆𝑆𝑆
, and secondary (displacement based) components, 𝐹𝑆𝑆𝑆
:
(1)
𝑆𝑆𝑆
𝐼𝐼𝐼𝐼
(2)
𝐹𝑆𝑆𝑆 = 𝐹𝑆𝑆𝑆 + 𝐹𝑆𝑆𝑆
Under this analysis corresponding seismic equations are checked only for the large diameter pipes
(like EQ. 9 of ASME NC-3600).
3. Set to zero weight of the small bore pipes segments and run the seismic analysis again. Now
inertial seismic loads are excluded for small pipes and only secondary loads from the seismic
motion of the big pipes are applied:
(2)
𝑆𝑆𝑆
𝐹𝑆𝑆𝑆 = 𝐹𝑆𝑆𝑆
(3)
Results of this analysis are used for the check of stress equations that consider secondary loads for
(2)
small bore pipes. It should be noted that 𝐹𝑆𝑆𝑆 corresponds to the amplitude. For the stress
evaluation two times amplification is required to get the range.
4. Create load combination:
(3)
(1)
(2)
𝐼𝐼𝐼𝐼
𝐹𝑆𝑆𝑆 = 𝐹𝑆𝑆𝑆 − 𝐹𝑆𝑆𝑆 = 𝐹𝑆𝑆𝑆
(4)
These loads are used for the check of seismic stresses for the small bore pipes.
Resulting loads obtained from the above should be used for stress check according to the
applicable Code. Let’s consider the implementation of this procedure for the Class 2 pipe in accordance
with ASME BPVC Section III, Subsection NC, ASME BPVC 2017. Let’s seismic input corresponds to
Safe Shutdown Earthquake (SSE), and seismic stresses are verified according to NC-3655 “Consideration
of Level D Service Limits”.
Correspondingly, inertial loads from steps (2) and (4) are evaluated according to equation (9a),
NC-3653.1(a):
where:
𝑃𝐷
𝑀𝐴 +𝑀𝐵
�
𝑍
𝐵1 2𝑡 0 + 𝐵2 �
𝑛
≤ 𝑚𝑚𝑚�3𝑆ℎ ; 2𝑆𝑦 �
P – Service Level D coincident pressure;
DO – outside diameter of pipe;
tn – nominal wall thickness;
Z – section modulus of pipe;
MA - resultant moment loading on cross section due to weight and other sustained loads;
MB - resultant moment loading on cross section due to occasional loads;
(5)
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
Sh - material allowable stress at a temperature consistent with the loading under consideration;
Sy – material yield strength at a temperature consistent with the loading under consideration;
At the same time stresses resulting from the SAM (step 3 above) are evaluated according to NC3655 (b)(4). On this stage two equations are checked:
𝐶2
where:
𝑀𝐴𝐴 𝐷𝑂
2𝐼
< 6𝑆ℎ and
𝐹𝐴𝐴
𝐴𝑀
< 𝑆ℎ
(6)
MAM – the range of the resultant moment due to SAM, N*mm;
FAM – longitudinal force resulting from SAM, N;
Ам – cross-sectional area of metal in the piping component wall, mm2;
C2 – secondary stress index from Table NB-3681(a)-l; I – moment of inertia, mm4;
Sh – material allowable stress at a temperature consistent with the loading under consideration,
N/mm2
NUMERICAL SAMPLE
Described above procedure is considered on the sample of the nuclear safety related piping.
Evaluated system is shown in Figures 1 and 2. Small bore piping 18x2.5 mm is attached to the run pipe
159x9 mm. Input seismic excitation is defined in terms of the floor response spectra, Figure 3. Results of
the “convenient analysis” show overstressing of the SBP with factor ~ 1,3, Figure 4. After utilization of
the described here procedure all considered piping segments comply with Code requirements, Figure 5.
Figure 1. Sample pipe
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
Figure 2. Sample pipe, Stress Isometric.
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
Figure 3. Design Floor Response Spectra
Figure 4. Result of “conventional” analysis for the coupled system
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
Figure 5. Results of the decoupling procedure.
CONCLUSIONS
1) This paper presents procedure useful for automatization of the routine calculations of the complex
piping models consisting of the pipes of different sizes.
2) The procedure may be useful when the seismic capacity of the main line is already provided, but
the small bore pipes attached to the run pipe are still seismically overloaded. In this situation, the
procedure allows reducing conservatism of the calculations being at the same time in the line with
applicable Code.
3) The procedure described above has several obvious limitations:
a) Selection of the Code equations for inertial and SAM loads should correspond to the level of
the seismic input: it's not supposed to use Level D allowable for seismic input less than SSE
b) Use of 6Sh allowable stresses must be confirmed by checking the absence of elastic followup. Otherwise, 3Sh should be used as an allowable value.
c) Reference on the secondary stress index C2 for Class 2 piping has limitation for the fittings
not described for Class 1 piping (set of intersections of the different types that are not
addressed in NB-3600)
4) Even in the case if the system failed to be qualified, separate check of the equations for SAM and
for inertial loads allows getting insight on the preferable way of seismic upgrading. If equations
for SAM are not satisfied, then the main line should be seismically restrained for reducing of
seismic movements. Otherwise, restrains should be added for SBP.
REFERENCES
WRC Bulletin 300 (1984). Technical position n industry practice, The Design Process for Small Bore
Piping, December;
EPRI NP-6628 (1990), Projects Q101-16,-17, Procedure for Seismic Evaluation and Design of Small
Bore Piping (NCIG-14);
25th Conference on Structural Mechanics in Reactor Technology
Charlotte, NC, USA, August 4-9, 2019
Division V
EPRI TR-101968S-V10 (1996), Guidelines and Criteria for Nuclear Piping and Support Evaluation and
Design, Volume 10: Small Bore Piping Guidebook, May 1996
dPIPE, Flexibility and stress analysis of piping systems, Verification Report, VR01-07, CKTI-Vibroseism,
2017, www.dpipe.ru