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S. Thanheiser:
"Modeling, Simulation and Failure Analysis of the Natural Circulation System at Ridham Docks";
Supervisor: M. Haider, A. Werner, H. Walter; Institut für Energietechnik und Thermodynamik, 2017.

This master thesis investigates the circulation issues that were encountered at a natural circulation
steam generator at the Ridham Docks site (England), causing heat damage. The evaporator of the
steam generator was modeled in detail and numerically simulated using state of the art calculation
software. While no conclusive evidence was found that could explain the damages unambiguously,
other instabilities were discovered that may have contributed to the phenomenon. General design
guidelines were deducted as possible solutions to the problem.
The damages occurred at a convective evaporator of the steam generator. Investigations identified
the cause of the damages to be overheating. Measurements showed increased tube wall
temperatures at several locations of the convective evaporator. They only occurred after system
startups and also disappeared after another shutdown and restart.
The convective evaporator was modeled with the highest attention to detail; the rest of the
evaporator (residual heat exchangers) was modeled with less precision to investigate its influence
on the convective part. Model parameters were introduced to investigate different configurations of
the model, like different designs of the convective evaporator.
The calculation software Apros was used to simulate the model. It uses a state of the art six
equation model, capable of simulating liquid and gas flows separately in an axially discretized
network of pipes.
Simulation of the stationary status already revealed flow oscillations in one of the residual heat
exchangers. A sudden increase in heat flux can stabilize these oscillations. Integrating that heat
exchanger into other parts of the evaporator is likely to suppress the instabilities.
Numerous experiments were conducted with the model. The investigations included uneven
heating conditions, pressure variations, heat flux variations in one of the residual heat exchangers,
hot startups with different configurations, the influence of foreign objects and the possibility of local
flow circulations within the convective evaporator. Except for hot startups, all experiments led to
stable systems.

Natural Circulation System, Steam Generator, Heat Damage