Thermalmechanical
numerical
simulation
Effects of differential
thermal expansions of
bolts on flanged joints
tightness
sabato 4 marzo 2023
Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
The Problem
▪ Bolts material of many welding neck flanged couplings, originally selected as A193
Gr.B7 (1Cr 1/5Mo), is replaced with material Inconel 625 (N06625), having different
mechanical and thermal characteristics: to ensure continuity of the seal, some
thermo-mechanical numerical simulations are conducted.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
3D Symmetric Model
▪ The flanged coupling is modeled by exploiting the axial symmetry of the assembly
and involves the presence of flanges, resisting section of bolts, and gasket.
Flange Code
Rating
Flange
size
NPS
ASME B16.5
150
18
Bolt Size
1 1/8
Root Area Root Area
(in2)
(mm2)
0,7276
469,4
No. of
Bolt
16
Bolt
Pressure
Tensile Internal
Pressure
Thrust
Gasket ID Gasket OD Gasket Area
Stress Pressure
Thrust
Area
(mm)
(mm)
(mm2)
[N]
Diameter [Mpa]
[mm2]
[mm]
474,7
527,1
41.229
24,45
0,65
150846 98049,9
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Materials Data
▪ An equivalent homogeneous material is defined based on the spiral wound
steel/graphite package
Young Modulus
[Mpa]
Yield Strength [Mpa] @
Temperature
20 °C 100 °C 200 °C 300 °C 400 °C
315 SS (16Cr 12Ni) homogeneous material
200000
172
145
121
109
101
316 SS (16Cr 12Ni) + Graph 98% pack equivalent
98900
85
72
60
54
50
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Materials Data – A105
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Materials Data - 316 SS (16Cr 12Ni) + Graph 98% pack
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Materials Data - A193 Gr.B7 (1Cr 1/5Mo)
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Materials Data - Inconel 625 (N06625)
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Materials Data - Inconel 718 (N07718)
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Simulation Setup - 3 steps
▪ STEP 1: BOLTS TIGHTENING FOR GASKET SEATING
▪ STEP 2: COLD PRESSURISATION WITH BOLTS LOCKED
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Simulation Setup - 3 steps
▪ STEP 3: INTRODUCTION OF 400°C TEMPERATURE FIELD
+
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Results – stress fields for A193GrB7 Vs Inconel 625 bolts
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Results – stress fields for A193GrB7 Vs Inconel 718 bolts
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Results with A193 Gr.B7 bolts – contact pressure
Gasket contact pressure is well above the
seating value during cold bolts tightening,
while it decreases during cold pressurization
and decreases further with the introduction of
the thermal field, due to the differential
thermal expansion of the various materials.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Results with Inconel 625 bolts – contact pressure
Qualitatively, the same considerations as for
the A193 Gr.B7 bolts case apply, with
negligible numerical differences.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Results with Inconel 718 bolts – contact pressure
Qualitatively, the same considerations as for
the A193 Gr.B7 and Inconel 625 bolts cases
apply, still with negligible numerical
differences.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Evaluations
1.
Stress fields resulting from using bolts with three different materials, all other
conditions being equal, are globally and locally very similar.
2.
Numerical simulations confirm the well-known phenomenon of contact pressure
relaxation on the gasket between the tightening phase and the phase of subsequent
pressurization of the flanged coupling.
3.
The tightening force value, while meeting good engineering criteria (70% of the yield
stress of the threaded rod), is slightly insufficient to ensure a good tightening of the
gasket, whose surface is not uniformly flattened: this since the empirical formulas do
not consider the rotation of the flanges around the circumferential axis.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Evaluations
4.
The different thermal expansion of materials, resulting from the introduction of the
thermal field, changes the distribution of internal forces and consequently the distribution
of stresses on the assembly; note that the difference between the thermal expansion
coefficients of A193 Gr.B7 and Inconel 625/718 is about only 7.4%/3.9% at 400°C
Thermal Expansion Coefficients
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
12.0
11.0
10.0
0
50
A105 (carbon)
5.
100
150
316 SS (16Cr 12Ni)
200
250
A193 Gr.B7 (1Cr 1/5Mo)
300
350
Inconel 625
400
450
Inconel 718
In any case, the difference between the contact pressure between the cases (A193 Gr.B7
Vs Alloy 625/718 bolts) is negligible.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
Conclusions
▪ The impact due to differences of thermal expansion coefficients between A193 Gr.B7 and
Alloy 625/718 is expected to be negligible on the flanged couplings under investigation.
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Saipem Classification - General Use
Effects of differential thermal expansions of bolts on flanged joints tightness
References
▪ ASME Boilers & Pressure Vessels Code Section II Materials
▪ ASME B31.3 Process Piping
▪ ASME B16.5 Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24
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Saipem Classification - General Use