Titelmasterformat durch Klicken bearbeiten

© 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?