Design of exhaust channels. Seminar 7. February Kjell Olav Teien

Transcription

1 Design of exhaust channels Seminar 7. February 2007 Kjell Olav Teien

2 Content u Challenges failures u Failure mechanism u Possible analysis CFD FEM thermal FEM structural Vortex shedding u Criteria u Conclusion

3 Challenges - failures Cracks in plates adjacent to reinforcements/brackets

4 Challenges - failures Cracks in the connections between thin plate and heavy structure

5 Challenges - failures u Exceedance of the deflection capacity in expansion joints causing: Rupture of the sealing blanket, i.e. exhaust gas leakage Unintened forces into the exhaust channels, i.e. potential risk of cracks in the exhaust channels and the structural part of the expansion joint

6 Challenges - failures u Flange bolt failures Loose bolts, potential exhaust gas leakage Bolt rupture, potential exhaust gas leakage u Leakage in flanges Blow out of gasket Deformation (buckling) of flange

7 Challenges - failures u Failure / cracks in the supports of the exhaust channels Cracks/rupture in steel brackets Cracks/rupture in ceramic thermal isolators u Fibre escape In lined ducts In baffles in silencers u Abrasive wear In blankets for fibre packing Sliding supports of baffles

8 Failure mechanism u Cracks thin plate/heavy structure connections: During start up the thin plate get rapid temperature rise while the heavy structure get slow temperature rise Uneven temperature rise causes high temperature gradients, next causing high transient thermal stresses High thermal stresses can cause yielding, low cycle fatigue failure and rupture Flow induced pulsations can cause high fatigue failure

9 Failure mechanism u Failure in expansion joints: Thermal movement of one/both side of joint is larger than expected Internal movements of the exhaust system due to wind and motion accelerations is larger than expected External movement of the support system not taken into account Cold condition preset of joint not correct set during installation of the exhaust system The above mentioned movement not correctly communicated between the sub-suppliers of parts of a complete exhaust system

10 Failure mechanism u Flange bolt failures Uneven temperature rise in bolt and flange causes high transient thermal stresses High thermal stresses can cause yielding (loose bolts), low cycle fatigue failure and rupture Pulsations and vibrations can cause high cycle fatigue u Leakage in flanges Loss of preload due to loose bolts and deformation of flanges Loss of preload on the gasket causes blow out of gasket

11 Failure mechanism u Support failure Thermal movement of the complete exhaust system is larger than expected Internal movements of the exhaust system due to wind and motion accelerations is larger than expected External movement of the support system not taken into account Cold condition preset of supports not correct set during installation of the exhaust system The above mentioned movements/preset not correctly communicated between the sub-suppliers of parts of a complete exhaust system

12 Failure mechanism u Fibre escape High local flow velocity Inadequate packing of fibre Sharp edges on one side of the perforated plate in contact with the blanket can wear and tear the blanket u Abrasive wear Thermal movement Vibrations

13 Analysis u Computational Fluid Dynamics (CFD): u Input: Flow geometry Inlet flow conditions: - Mass flow v.s. time - Gas temperature v.s. time - Gas properties (density, viscosity, conductivity, heat capacity) - Velocity distribution and direction u Analysis cases: Start up / shut down transient Steady state u Output: Outlet velocity distribution and direction Pressure loss Pulsation pressure level Convection heat transfer coefficient

14 Analysis u Thermal analysis (FEA): u Input: Structure as FEM-model Material properties (conductivity and heat capacity) Start temperature of structure Bulk (gas) temperature v.s. time Convection heat transfer coefficient v.s. location v.s. time Radiation factors u Analysis case: Start up transient, steady state, shut down transient u Output: Temperature distribution v.s. time Time at maximum temperature gradient

15 Analysis u Structural analysis: u Input: Structure as FEM model (same as thermal analysis) Material properties (elastic modulus, yield properties, density, thermal expansion coefficient) Maximum temperature gradient during start up/shut down, and steady state External loads from gravity, motion accelerations and wind Support spring rate in 6 d.o.f. Deflection of supports in active d.o.f. Definition of how to combine thermal, gravity, motion acceleration and wind effects

16 Analysis u Structural analysis: u Analysis cases: Static analysis at maximum temperature gradient during start up / shut down down Static analysis at steady state Natural frequency analysis u Output: Stress v.s. location Support forces Deflections at supports and expansion joints Natural frequencies and mode shapes

17 Analysis u Vortex shedding Harmonic flow/wind pulsation u Input: Flow geometry Inlet flow conditions: - Maximum volume flow - Gas properties (density, viscosity) - Velocity distribution and direction Wind speed Structure as FEM model Material properties (elastic modulus,density) Support spring rate in 6 d.o.f. u Analysis case: - Critical wind/flow to give shedding frequency equal structural natural frequency u Output: - Margin between maximum flow/wind speed and critical flow/wind speed where flow/vibration interaction occur

18 Criteria u CFD: Outlet velocity distribution within predefined acceptance limits Pressure loss within predefined acceptance limits Plusation pressure level within predefined acceptance limits u Structural FEM: NORSOK standards N-001, N-003, N-004 Norsk standard NS3472 API616 Project requirement Maximum stress/strain below predefined acceptance limits Maxium cyclic stress/strain range within predefined acceptance limits (elastic limit or Low Cycle Fatigue-limit if material data is available) Support forces within the capacity limit of the supports Deflection in expansion joints and supports within the capacity limit Natural frequencies outside predefined ranges u Vortex shedding: Margin between maximum flow/wind speed and critical flow/wind speed where flow/vibration interactionoccur

19 Conclusion u Design of exhaust is more challenging than traditionally considered u The analysis tools are available today u Acceptance criteria not clearly defined