Coupled thermomechanical

analisys of electrofusion fittings

and electric sealings for plastic

piping

G. Bizzarrini, M.Domaneschi, A. Marzi

ELECTROFUSION FITTING OPTIMIZATION: PRODUCT DESCRIP TION

1. Electrofusion fitting: common device for a plumbe r to join PE and PP plastic pipes

2. Fitting: basically is a plastic ring with a metal lic coil close to the internal surface

3. Coil: a resistive metallic alloy which generates the heat energy necessary to melt the plastic when connected to an electric power sou rce

metallic coil

1) 3 basic coil layouts named TYPE A, TYPE B and TYPE C must be compared to get the best solution.

2) Each layout is strictly related to the manufacturing technology

3) Existing welding machine gives 2 constraints : - current intensity- welding time

4) It is impossible to make prototypes with no influence on thermomechanical properties

� only a Finite element modelling of the electric coupling is feasible to check compatibility with costraints and to optimize fitting properties

� MSC.MARC Mentat code has been used to perform the calculation

ELECTROFUSION FITTING OPTIMIZATION: PRODUCT DESCRIP TION

TYPE A

(COES current production)

The resistive coil is fully embedded in plastic but located externally to the welding surface.

TYPE B

The resistive coil is partially embedded in plastic and is located exactly on the welding surface.

TYPE C

The resistive coil is fully embedded in plastic and is located inside the welding surface.

ELECTROFUSION FITTING OPTIMIZATION: 3D CAD MODELLIN G

Comparison of 3 different layouts has been performed in two steps:

Step 1 : decoupled analysis

• Thermal analysis : heat propagation in the thickness of the pipe and of the fittingdue an electric current in the steel coils of the electric coupling

• Structural analysis : radial expansion of the pipe due to the temperature to calculatethe time when air gap closes

Step 2 : coupled analysis

• Thermo-mechanical analysis : heat propagation in the thickness of the pipe and ofthe fitting due to an electric current in the steel coilseven across the air gap between the pipe and thefittings; radial expansion of the pipe due to thetemperature to calculate the time when air gapcloses

ELECTROFUSION FITTING OPTIMIZATION: ANALYSIS CONDIT IONS

CAD model was converted in to a mesh of an axisimmetric model Symmetry with the middle perpendicular plane was also used

The air layer has been included in the mesh, a cavity around the air mesh defined.

ELECTROFUSION FITTING OPTIMIZATION: STEP 1 THERMAL ANALYSIS

MESH GENERATION

• INITIAL CONDITIONS: the whole body at the temperature of 20°C

• BOUNDARY CONDITIONS:1. EDGE FILM2. EDGE FLUX3. EDGE RADIATION4. CAVITY RADIATION

• ELEMENT CLASS/TYPE: quad 4, n. 40, full integration

• LOAD CASES: - transient analysis

- total loadcase time

- fixed steps• JOBS:

axisimmetric analysis

TYPE B

ELECTROFUSION FITTING OPTIMIZATION: STEP 1 THERMAL ANALYSIS

GEOMETRIC MODEL

thickness of the pipe divided in 6 elements

EXTERNAL DIAMETER

INTERNAL DIAMETER

A simple axialsimmetric model of a portion of the pipe

MECHANICAL PROPERTIES OF PE

Young modulus dependent on temperature:

Table E = f(T)

ELECTROFUSION FITTING OPTIMIZATION: STEP 1 STRUCTURAL ANALYSIS

• INITIAL CONDITIONS: 6 temperature time histories obtained by the thermal analysis in the corresponding nodes,

• BOUNDARY CONDITIONS: heat flux = 0 in X direction,

• LOAD CASES: - linear-static mechanical analysis

- total load case time• JOBS: axisimmetric analysis

• MODEL VALIDATION: 3 samples of fittings for each type have been welded in the laboratory recording the internal temperature of the pipe under the fitting versus the time.

An average value of the internal temperature of the pipe is represented in the next diagrams (dash curves) and compared with the same temperature calculated by MARC.

Calculation precision looks to be very good.

ELECTROFUSION FITTING OPTIMIZATION: STEP 1 STRUCTURAL ANALYSIS

ELECTROFUSION FITTING OPTIMIZATION: STEP 1 RESULTS

Manicotto Akatherm

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0 20 40 60 80 100 120Tempo [s]

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Tem

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tura

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Spira Tubo Temperatura int. tubo Temperatura ext. tubo Temp. spira Temp. aria

INTERVALLODI SALDATURA EFFICACE

Type C is uncompatible with constraint of welding time = 80s

Type B proves to be even more efficient than type A for temperature distribution.

Type B coil layout has been adopted.

ELECTROFUSION FITTING OPTIMIZATION: STEP 1 RESULTS

ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO-MECHANICAL ANALYSIS

Mesh representing air has been removed from basic model. Cavity also removed.

Mechanical properties of Pe and coil steel added to the thermal model.

Contact bodies definition and contact heat transmission coefficents input completed the model.

MODEL GENERATION

This semplification is possible because heat flux in X direction is negligeble around the middle coil

A comparison between the computed temperature curves of the full and the reduced model has confirmed the assumption: curves are identical.

To reduce computing time to match available hardware capability we decided to model only the central coil with a thickness equal to the step of the coils.

ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO-MECHANICAL ANALYSIS RESULTS

These curves provide all the necessary information about the

fitting you are designing:

ELECTROFUSION FITTING

OPTIMIZATION: STEP 2 THERMO-

MECHANICAL ANALYSIS RESULTS

Comparison of calculated curve with the thermal analisys

ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO-MECHANICAL ANALYSIS

•Pipe temperatures are the same

•Coil temperature is much lower at the end of the heating phase, as expected.

•Coil max temperature is now under laboratory investigation for additional confirmation of the model

ELECTRIC SEALING (*) DESIGN

Now we want to develop a new idea based on the concept of a metallic coil heated by an electric current to melt a plastic surface

to create an electric sealing that can replace a standard rubber seal for waste water piping. No reference for basic parameters are available for this item so we use the electrofusion fitting model to calculate the power necessary to work.(*) PATENT PENDING

Mesh and materials definition

ELECTRIC SEALING (*) DESIGN

Boundary conditions,Heat flux applied to coils

(*) PATENT PENDING

Temperature distribution at the end of the heating phase

ELECTRIC SEALING (*)DESIGN

(*)

PA

TE

NT

PE

ND

ING

Radial stress

ELECTRIC SEALING (*) DESIGN

Axial stress

(*) PATENT PENDING

ELECTRIC SEALING (*) DESIGN

(*) PATENT PENDING

Thermal contact in thermo-mechanical analyses

• Setting the near contact coefficients allows to define the near contact heat transfer between the contact

bodies.

• These coefficients to be defined are listed below:

• Hct = CONTACT HEAT TRANSFER COEFFICIENT

• Hcv = NEAR CONTACT HEAT TRANSFER COEF.

• Hbl = DIST. DEP. HEAT TRANSFER COEFFICIENT

• Hnc = NATURAL CONVECTION COEFFICIENT

• Bnc = NATURAL CONVECTION EXPONENT

• E = SURFACE EMISSIVITY

• S = Stefan-Boltzman coefficient

• T1 = temperature at touching body

• T2 = temperature at touched body

• D = distance between the bodies

• Dn = user defined near contact distance

• Dq = D/Dn

• q = flux

The heat flux is given by the following expression using the above variables :

q = Hcv(T2-T1) + Hnc(T2-T1)Bnc + SE(T24-T14) + (Hct(1-dq)+HblDq)(T2-T1)

ELECTRIC SEALING (*) DESIGN

(*) PATENT PENDING