January 2019 - Chemical Boiler

Common Boiler Deposits

Description

Deposition in power plants is most likely the least understood aspect of the operational cycle chemistry applicable to these units during commissioning. Having a complete understanding of the subject is a key factor for the performance of an effective chemical cleaning and the chemical treatment program to be applied. This will represent the fundamental building blocks for the success of the steam conditioning process and further reliability of the chemistry cycle during operation.

For this matter, knowing the nature of the deposit is of significant importance aspect to take into consideration. Most prevalent deposits compounds found in power plants are corrosion products of iron oxides, cooper oxides and derivatives formed as a result of lack of conservation during construction, improper preservation procedures, and deficient water treatment programs.
The following table lists crystalline compounds and other materials that have been observed in water formed deposits of steam generation equipment

Commissioning

Ineffective Cleaning?

Ineffective Chemical Cleaning?

Introduction

Ineffective chemical cleaning? Not really… we have encountered situations where engineers and operators complaint about the cleanliness of the boiler as they see a black liquid coming off the blowdown system during first fire.

It´s not difficult to get carried away by a first impression and put into question the effectiveness of the chemical cleaning.

Explanations

This dark watery constituent is in reality a very welcome natural corrosion-protection product called Magnetite and it is formed as the steel surface is exposed to water under certain conditions.

The formation of Magnetite is an electrochemical process controlled by a continuous diffusion of iron ions from the steel surface across the water boundary layer and its stability, morphology and porosity depends on the oxidation-reduction potential, pH, oxygen concentration, pressure, temperature and the influence of the make-up water chemistry (CO2, Cl, Si, Na, Ca, Mg…)

Solubility rises with temperature up to 150ºC, then decreases with a steep drop to 300°C. Pressure promotes the growth of magnetite and thickness Having said that, the reason of the dark appearance of the blowdown during a boiler ramp-up is mainly due to the deposition of low quality magnetite since steady conditions has not been reached yet.