Thermal-Stress Analysis of Reactors, Turbine Generators, Composite Mandrels, Pressure Vessels and CFRP Structures

Figure 1: Thermal-Stress Analysis of 500kw Turbine Generator per ASME Section VIII, Division 2 Classification

This high-power, compact generator spins at speeds up to 50,000 RPM thanks to friction-less magnetic bearings. The turbine side is pressurized and is considered an ASME-type pressure vessel and was classed under the ASME Section VIII, Division 2 rules. The generator side was water cooled and unpressurized. The thermal-stress challenge was managing the thermal expansion on the turbine’s hot side against the water-cooled generator side. Besides the thermal loads, the analysis included bolt loads, turbine pressure loads and a 10g shock load. Our FEA consultants collaborated with the client to determine realistic thermal loads and assisted in the redesign of the water cooling channels. Upon re-analysis, stress margins were within ASME code specifications and allowed our client to certify the generator for general use in standard industrial environments with no special safety precautions.  The generator has gone into service and is operating successfully.

 

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Figure 3: Thermal-Stress Analysis of Hydrogen Reactor

The biggest challenge on this project was the mapping of the client’s experimental thermal loads onto a logical set of FEA boundary conditions. The reactor is used to convert natural gas into high-purity hydrogen gas to power a fuel cell. The client’s equipment is typically located in remote sites and with a nominal supply of natural gas, can provide backup power for weeks. The reactor develops a high thermal gradient where the natural gas fuel source is fed into the reactor. The thermal-stress results were interrogated to determine weld joint stresses. A fatigue analysis was performed based on the varying operational loads of the reactor. Some analysis judgement was required to differentiate between real and artifact stresses along the weld boundaries. The design was modified through several iterations via collaborative face-to-face meetings with the client’s design team. The final design is in service today.

 

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Figure 5: ASME Thermal-Stress Analysis of Thick-Walled LOX and RP1 Propellant Tanks

Thick-wall, high-pressure liquid oxygen (LOX) tanks experience thermal fatigue conditions rarely seen in normal pressure vessels. Our FEA consultants teamed with our client’s design team at Taylor-Forge and SpaceX to develop realistic cryogenic transient-thermal profiles for the liquid oxygen filling and nitrogen back-filling of the tanks. These loads were then used to drive the stress model. All results were classed under ASME BPVC Section VIII to certify the tanks as fit-for-service under operating, upset and fatigue conditions. 

 

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Figure 7: Thermal-Stress Analysis of Water-Cooled Stoker Grate

We would be remiss not to include a classic example of thermal-stress analysis with this water-cooled stoker grate. The grate uses cast iron castings facing the burning material with their undersides cooled with the array of piping shown above. The complete system is vibrated to move the ash load off of the grate and into a conveyor system. Our FEA consulting group has done the vibratory analysis work along with this thermal-stress work. The objective of the thermal work was to reduce weld breakage due to cyclic thermal loading. The thermal differential between the cold pipes and the hot grate tends to tear apart the grate unless carefully managed. FEA results were then used to minimize the stresses along critical weld lines and have significantly improved fatigue life for the stoker grate. The reduction in warranty costs directly paid for our FEA service work.  

 

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