February 2019 - Inerchem.com

Erosion-Corrosion

Introduction

Erosion degradation on the outside of cold reheat tubes are due to mechanical action, frequently by impingement of wet steam and/or high-velocity flow. Impingement can be controlled by placing baffle plate tray protections upstream the heat exchanger module; high velocity flow can be handled by oversizing inlet nozzle diameter. However, the effects depicted in the photo represent a more complex failure mechanism commonly encounter during the commissioning of a power plant.

Conclusions

A comprehensive investigation conducted revealed the presence of high amount of iron citrate deposits in most of the impingement cavities. Also it became apparent the existence of strong and general abrasion in the interface tube-baffle. The reason of this failure was due to a combination of an unsatisfactory chemical cleaning operation and an incorrect steam blowing design.

Commissioning

Steam Blowing Insights

Some Insights About Steam Blowing

Introduction

The science behind steam blowing involves the optimization of the drag force of the steam and the chemical conditioning for corrosion control.

Fludization Aspects
The fluidization potential (drag force) can be enhanced by minimizing the boundary layer thickness of the fluid, all along the surface of the pipe, as follow:

1. Running at high Reynolds will increase turbulences, adjust the velocity profile and boost terminal velocity near the surface.
2. Rising temperature will reduce the viscous layer, and so, the shear wall stress.
3. Increasing density will maximize the momentum (kg/m·s2) of the steam. This can be done by injecting water in the superheater outlet to blow with saturated steam.

Chemical Aspects
Chemistry also plays an essential role in the performance of steam blowing. The dynamic nature of this operation makes impossible to control carryovers, deposition of salts, oxides formation and diffusion of impurities. Shutdown periods for thermal cycles can also increase the concentration of O2 in the boundary layer. As a result, corrosion can be quite active during these stage of commissioning.

Suggestions

As suggestion, it is recommended to first carry out a chemical cleaning in the steam system. Then, start-up the steam generator for a few days to improve chemistry conditioning. Finally, proceed with the execution of steam blowing.