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© CADFEM 2016
3D solid bolt analysis for pressure vesselsAndy Howson (Pacson Valves)
1st CADFEM ANSYS Simulation Conference, United Kingdom2nd & 3rd November 2016 – Oxford Spires Hotel, Oxford
Basic manual bolt calculations
Finite Element modelling of example joint
Example applications from project work
Contents
𝜎𝑛𝑜𝑚 =𝐴𝑝𝑝𝑙𝑖𝑒𝑑 𝑡𝑒𝑛𝑠𝑖𝑙𝑒 𝑙𝑜𝑎𝑑 (𝑒𝑥𝑐𝑙𝑢𝑑𝑖𝑛𝑔 𝑝𝑟𝑒𝑙𝑜𝑎𝑑)
𝐵𝑜𝑙𝑡 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎
Applied loads
Gasket seating force for non-self-energising gaskets
Pressure acting across seal diameter
External tensile and bending loads
Pros/cons of method
+ quick and easy (ie, cheap!)
- provides little understanding of joint behaviour
- can produce overly conservative designs
Bolt sizing - nominal bolt force and bolt stress𝑭𝒆𝒙𝒕
𝑭𝒆𝒙𝒕
In a preloaded joint, the bolt sees only a portion of the applied external load:
Preload & external concentric load
Forc
e
Bolt stretch Clamped member compression
𝐹𝑝𝑟𝑒𝑙𝑜𝑎𝑑
Deformation
∆𝐹_𝑏𝑜𝑙𝑡= 𝐹𝑒𝑥𝑡𝑘𝑏𝑜𝑙𝑡
𝑘𝑏𝑜𝑙𝑡 + 𝑘𝑗𝑜𝑖𝑛𝑡
𝐹𝑒𝑥𝑡
Model valid only for concentric load applied at load
plane directly under nut
Applies only prior to a critical load (joint
separation) after which behaviour is as-per non-
preloaded bolt
∆𝐹_𝑏𝑜𝑙𝑡
A real joint does not typically apply a force directly under the nut and in line with the bolt axis
Load offset causes the flange to act as a lever
Bending induced in the clamped components and bolt
Bolt force can be increased due to prying effect
Bolt force response becomes nonlinear prior to flange separation
Effect of eccentric load
FE geometry prepared from CAD geometry in SpaceClaim
Local details removed from clamped components – stiffness important, local stress raisers not important
Shank represented as plain cylinder, using nominal minor diameter of external thread (for fully-threaded stud)
Bolt volumes sectioned to suit hex meshing
Method - pre-processing
Material models
Linear elastic bolt components
Elastic-plastic clamped components (ASME BPVC
VIII-2 Annex 3-D)
Method - pre-processing
Contact
Frictional between clamped components
Bonded between nuts and clamped components
Mesh in clamped components relatively coarse
Element size in free shank based on known-good shank diameter to max element size ratio
Valid for shank lengths > 1*shank diameter
Method - pre-processing
Loading
Pretension force applied at load step 1
Pretension displacement locked at load step 2
Thermal load applied at load step 3 (if applicable)
Ramping pressure/structural loads from load step 4
External load increased over at least 20 load steps
Method - pre-processing
3 simulations (min, nominal, max) to account for preload scatter
Average bolt stress
Maximum bolt stress (excluding concentrations)
Alternating stress (excluding concentrations)
Bolt load
Flange-flange contact gap/contact pressure
Method – post processing
Static stress criteria
Fatigue criterion
Leakage/service criterion
Thread shear/bearing stress criteria
Extracted with APDL macro
Extracted in Workbench
APDL Macro extracts bolt results
Element results used to generate average and maximum stress for each element ‘layer’ at each load step
Results close to singularities removed using an average stress convergence
Nodal results used to calculate alternating stress between 2 user-defined load steps
Bolt force extracted from pretension element
Maximum flange-flange contact gap/contact pressure extracted with contact tool in Workbench
Method – post processing
Method – post processing
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Max
imu
m c
on
tact
gap
(in
)
Internal pressure (psi)
Flange-flange contact gap
Min preload Nominal preload Max preload
30000
35000
40000
45000
50000
55000
60000
65000
70000
75000
80000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Equ
ival
ent
(vo
n-M
ises
) b
olt
str
ess
(psi
)
Internal pressure (psi)
Bolt stress
Average stress, min preload Maximum stress, min preload
Average stress, nominal preload Maximum stress, nominal preload
Average stress, max preload Maximum stress, max preload
Extracted results allow the nonlinear response of the system to be examined
Note the effect of preload scatter on results
Example application - bolted bonnet on swing check valve
Bolt stress exceeded allowable values at 10000 psi design pressure by manual calculation
Design verified in accordance with API Specification 6A bolt stress limit (𝑆𝐴 = 0.83 × 𝑆𝑌) using FE method
Example application – tie rods on hydraulic actuator
Tie rods added to existing hydraulic actuator design to increase load capacity using existing actuator body
Manual method not suitable to determine proportion of load transferred via each load path
FE method used to verify existing bolting and additional tie rods
Covered:
Basic manual bolt calculations
Finite Element modelling of example joint
Example applications from project work
Any questions?