Impact of degradation of the moisture separators on the overall performance of the moisture separator reheater in a nuclear power plant

• Main Author: Saaymans, Natalie
• Other Authors: Fuls, Wim
• Format: Dissertation
• Language: English
• Published: University of Cape Town 2021
• Subjects: Nuclear Power
• Online Access: http://hdl.handle.net/11427/32511

The moisture separator forms part of the moisture separator reheater (MSR) component used in a steam cycle in a nuclear power plant, to reduce the risk of erosion of the low-pressure (LP) turbine and to improve cycle efficiency. The performance and optimisation of moisture separators is well studied in literature; however, there have been few investigations on the impact of moisture separator degradation on MSR performance. To investigate this impact a mathematical model, representing the steam flow through the MSR, is developed and used to simulate and analyse the impact of degradation conditions. The mathematical model was developed for design conditions, calibrated and validated against manufacturer specifications. The model was then augmented to include two moisture separator degradation conditions. The first degradation condition is the partial blockage of separator vane channels due to fouling, and the second is separator material deterioration resulting in steam bypass of the moisture separator. The model uses known properties of the MSR inlet steam and predicts the properties of steam exiting the MSR, given the simulated degradation of the moisture separator. The outcomes of the model simulations demonstrated that partial blockage of moisture separator vane channels increases steam velocity though the separator and consequently improves MSR performance, but with a noted pressure drop. The velocity increased until a theoretical upper limit, above which re-entrainment of droplets back into the steam flow reduces MSR performance. It was concluded that there is margin in the separator surface area design, where a minimal reduction in separator surface area (represented in the model as blockage of the vane channels) would improve the performance of the MSR, while still allowing for a buffer against the re-entrainment velocity upper limit. Equally, an unexpected improvement in MSR performance may be an indication of blockage of separator vane channels that, if not monitored and managed, could surpass the critical velocity limit where re-entrainment adversely affects the MSR performance. The simulation results demonstrated that steam bypass of the moisture separator is a credible degradation condition which affects MSR performance. It was found that steam bypass of the moisture separator leads to a decline in the quality of steam exiting the separator and a decline in MSR performance. The simulation of a fully bypassed moisture separator showed that the MSR performance declines by more than three times the design value when compared to the scenario where there is no bypass of the moisture separator.