Coupled thermomechanical analisys of electrofusion fittings and electric sealings for plastic piping. G. Bizzarrini, M.Domaneschi, A.

Transcription

1 Coupled thermomechanical analisys of electrofusion fittings and electric sealings for plastic piping G. Bizzarrini, M.Domaneschi, A. Marzi

2 ELECTROFUSION FITTING OPTIMIZATION: PRODUCT DESCRIPTION 1. Electrofusion fitting: common device for a plumber to join PE and PP plastic pipes 2. Fitting: basically is a plastic ring with a metallic 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 source metallic coil

3 ELECTROFUSION FITTING OPTIMIZATION: PRODUCT DESCRIPTION 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

4 ELECTROFUSION FITTING OPTIMIZATION: 3D CAD MODELLING 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.

5 ELECTROFUSION FITTING OPTIMIZATION: ANALYSIS CONDITIONS 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 fitting due an electric current in the steel coils of the electric coupling Structural analysis : radial expansion of the pipe due to the temperature to calculate the time when air gap closes Step 2: coupled analysis Thermo-mechanical analysis : heat propagation in the thickness of the pipe and of the fitting due to an electric current in the steel coils even across the air gap between the pipe and the fittings; radial expansion of the pipe due to the temperature to calculate the time when air gap closes

6 ELECTROFUSION FITTING OPTIMIZATION: STEP 1 THERMAL ANALYSIS MESH GENERATION 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.

7 ELECTROFUSION FITTING OPTIMIZATION: STEP 1 THERMAL ANALYSIS INITIAL CONDITIONS: the whole body at the temperature of 20 C BOUNDARY CONDITIONS: 1. EDGE FILM 2. EDGE FLUX 3. EDGE RADIATION 4. CAVITY RADIATION TYPE B ELEMENT CLASS/TYPE: quad 4, n. 40, full integration LOAD CASES: - transient analysis - total loadcase time - fixed steps JOBS: axisimmetric analysis

8 ELECTROFUSION FITTING OPTIMIZATION: STEP 1 STRUCTURAL ANALYSIS GEOMETRIC MODEL A simple axialsimmetric model of a portion of the pipe thickness of the pipe divided in 6 elements EXTERNAL DIAMETER MECHANICAL PROPERTIES OF PE Young modulus dependent on temperature: Table E = f(t) INTERNAL DIAMETER

9 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.

10 ELECTROFUSION FITTING OPTIMIZATION: STEP 1 RESULTS Manicotto Akatherm Diametro [mm] INTERVALLO DI SALDATURA EFFICACE Tempo [s] Temperatura [ C] Spira Tubo Temperatura int. tubo Temperatura ext. tubo Temp. spira Temp. aria

11 ELECTROFUSION FITTING OPTIMIZATION: STEP 1 RESULTS 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.

12 ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO-MECHANICAL ANALYSIS MODEL GENERATION 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. 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. 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.

13 ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO-MECHANICAL ANALYSIS RESULTS

14 ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO- MECHANICAL ANALYSIS RESULTS These curves provide all the necessary information about the fitting you are designing:

15 ELECTROFUSION FITTING OPTIMIZATION: STEP 2 THERMO-MECHANICAL ANALYSIS Comparison of calculated curve with the thermal analisys 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

16 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 (*) PATENT PENDING 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.

17 Mesh and materials definition ELECTRIC SEALING (*) DESIGN Boundary conditions, Heat flux applied to coils (*) PATENT PENDING

18 ELECTRIC SEALING (*)DESIGN Temperature distribution at the end of the heating phase (*) PATENT PENDING

19 ELECTRIC SEALING (*) DESIGN Radial stress Axial stress (*) PATENT PENDING

20 (*) PATENT PENDING ELECTRIC SEALING (*) DESIGN

21 ELECTRIC SEALING (*) DESIGN 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(T2 4 -T1 4 ) + (Hct(1-dq)+HblDq)(T2-T1) (*) PATENT PENDING