FE calculations for the bolted helium vessel - CERN

FE calculations for the bolted helium vesselMay 6th 2015

F. Carra, L. Dassa, N. Kuder

FE calculations for the bolted helium vessel 206/05/15

Intro

Safety

Safety valve

● PS=1.8 bara (=0.8 barg)

● Vtot = 160 L

Intro

Load cases

Pre-tuning always present

06/05/15 FE calculations for the bolted helium vessel 3

Intro

Scheme1 - Main model

with bolts,

highest preload

2 - Cavity sub-model

(linear)

7 - analytic submodel for

welded bolt covers on

plates

6- analytic sub-model for

welded bolt covers on

flanges

4 - analytic sub-model

for bolts (according to

VDI 2230 Part 2

5 - Simplified FEM model

for sealing plate weld

8 - Main model with

bolts, smallest

preload

3 - Cavity model Elasto-

plastic

Strength

assessment

9 - Cavity model thermo-

mechanical

Not yet

performed!!!Not yet

performed!!!


Helium vessel

Baseline

Model simplified win order to facilitate the FE analyses

Thickness of the cavity reduced according to material removal during BCP


Combining effect of bolt pretension, pressurization and pretuningBolt pretension 3800 N

Gravity 9806.6 mm/s2

0.2 mm

Pressure 0.18 MPa

Thermal expansion 0.2 mm

Fixed support

Scale

x100

Cavity submodel

Helium vessel

Loads and boundary

conditions


Helium vessel

Stress

Maximum stress: 110 MPa

Max allowable stress: 187 MPa (Ti Grade 2)


Helium vessel

Deformation

Maximum deformation: 0.62 mm


Cavity

Stress (1)

Stress Intensity: “preload + pressure + pretuning”


Cavity

Stress from the submodel (1)

FE calculations for the bolted helium vessel 10

Stress Intensity: “preload + pressure + pretuning” -> sub-model

It is mandatory to split the

total stress in :

• primary stress

• secondary stress

06/05/15

Stress due to pressure (primary stress)

𝑃𝑚 < 𝑓𝑃𝑙 < 1.5𝑓 -> no local stress 𝑃𝑙 + 𝑃𝑏 < 1.5𝑓∆(𝑃 + 𝑄) < 3𝑓

LINEARIZATION Cavity

Stress (2)

Stress due to pre-tuning (secondary stress) not present


Cavity

Stress from the submodel (2)

Pressure ONLY

Stress due to pre-tuning (secondary stress) not present

ONLY primary stress


Cavity

Stress Linearization

LINEARIZATION on primary stress (only pressure)

𝑃𝑚 < 𝑓𝑃𝑙 < 1.5𝑓𝑃𝑙 + 𝑃𝑏 < 1.5𝑓∆ 𝑃 + 𝑄 < 3𝑓

FE calculations for the bolted helium vessel

• Red: membrane stress

• Green: bending stress

• Blue: membrane + bending stress VERIFIED

06/05/15

Cavity

Elastic-plastic analysis (1)

THIS IS NOT A STRENGHT ASSESSMENT.

Analysis performed only to have an idea about the plastic

behaviour

Equivalent stress


Cavity

Elastic-plastic analysis (2)

THIS IS NOT A STRENGHT ASSESSMENT.

Analysis performed only to have an idea about the plastic behaviour

Total principal structural strain 1: 0.00511 / -6.07e-06

Total principal structural strain 2: 0.000694 / -0.000378

Total principal structural strain 3: 3.6e-06 / -0.00519


Cavity

Buckling

Load multiplier = 48 wrt to p = 0.18 MPa


before

preload

direction of relative plate movement

Welds modelled as the edge-face contacts. Contacts established after the bolt preload to prevent the weld prestressing.

Weld seams for plates

Model


Weld seams for platesANSYS reaction moments and forces attached to the 3D solid weld seam.

Results from FEM


Fixedsurface

Surfacewhere theloads areapplied

Weld seams for plates

Strength assessment in Ansys

Maximum stress (linearized): 142 MPa (with raw and fine mesh)

Max allowable stress: 187 MPa (Ti grade 2)


Bolts

Model

δ

Mb

-Mb

Sliding

δ

Pressure 0.18 MPa

Bolt line

Threaded hole

---- Fixed jointsWasher imprinted face

Frictionless contact

Bolt modeling in ANSYS


Bolts

For the maximum values of the bending moment and shear force from ANSYS:

Results form Ansys

Max. axial force PA [N] 3955

Max. bending

moment

Mb [Nmm] 1648

Max. shear force T [N] 245

Applied preload: 3800 N

Warning: the shear load evaluated

with simplified model is > 362 N

A part of the pressure load is

carried out by the weld seams.

The geometry of the beam corresponding to the bolt has

been derived from the VDI 2230 Part 2


Bolts

VDI strength assessment

Axial stress σa [MPa] 276

Bending stress σb [MPa] 129

Shear stress τ [MPa] 107

Equivalent stress σeq [MPa] 445

Safety factor k - 1.9

For the operating conditions

For the assembly conditions

Minimum preload: 2280 N

Maximum preload: 3800 N

(60% of scattering allowed)

Utilization factor: 45% (usually around 80%)

Assembly Permitted Preload: 4490 N

Warning:

Procedure for preload definition could be modified since it seems that

the preload required is even lower.

Minimum length of engagement

• Actual length: 9 mm

• Minimum length: 6.9 mm

Surface pressure

• Evaluated 97 MPa

• Maximum : ? (1340 for Ti Grade 5)



Bolts

Open questions

Washers:

• Yes? Not?

• Which type?

• Locking effect?

Galling ->Which coating to prevent it?:

• Dioxide?

• Molykote?

06/05/15

Weld seams for

bolts cover


Bolts

Contact between plates

The joints don’t open.

Contact between plates to be

improved (in some cases)



Thermal effects

06/05/15

Optimization

The scope

Promising results: we think we are able to reduce the weight by 30 kg without loosing in stiffness


Future tests

Test are planned for:

• Friction between titanium threads and between bolt head and

plate

• Traction test on bolts at room T and at 2 K

• Shear behaviour of the bolts at room T and at 2 K

• Friction between Titanium plates at room T and at 2 K

• Test of a dummy vessel

• Coatings on bolts

Cavity calculation will be performed again in order to verify the not

clarified points:

• with higher preload on bolts

• with friction

Very conservative hypothesis:

• No friction between plates (-> all the shear load carry out by the

bolts)

• Pressure (1.8 bara) due to the vacuum failure considered as

permanent load on the cavity

• Very low preload on the bolts


Tank RFD

Similar strength assessment will be performed for the tank of the RFD cavity


Documents

FE calculations for the bolted helium vessel - CERN