New Proposal for Chemical Reaction System of Scale Components and Water Treatment Chemicals of Phosphate/Alkali Type in Marine Boilers
- For Automatic Control of Water Quality in Marine Auxiliary Boilers by Monitoring of pH and Electric Conductivity -
Yoshiharu Itami*・Eiichi Nishikawa**・Masayoshi Umeda***
We have carried out a series of experiments in order to investigate effects of seawater mixing on boiler water qualities, especially on pH, concentrations of PO43- and of Cl-, and Electric Conductivity, and also investigate the behavior of Mg hardness when OH- and/or PO43- coexists, and when silica was mixed. These experimental studies revealed that there is a limitation of the applicability of the conventional chemical reaction model developed so far. This paper considers, therefore, a new model regarding the chemical reaction model of scale components and water treatment chemicals of phosphate/alkali type. The validity of a new proposed model is confirmed by comparing with the experimental results in the case when seawater mixed into boiler water. Finally the paper gives some recommendation for the automatic control of water quality in marine auxiliary boilers.
Key words: Water treatment, Automatic control, Boiler water quality, Marine boilers, Monitoring
Water side problems of auxiliary boiler such as scaling, corrosion and carry-over could be prevented if the water quality components such as pH, PO43- concentration and Electric Conductivity EC are controlled properly. In our previous papers   , a series of experiments were carried out by using phosphate multipurpose chemicals with proper alkalinity in order to develop the automatic control of water quality in marine auxiliary boiler by monitoring of pH and EC.
The first and second papers  have investigated how the boiler water quality is affected by the mixing of seawater, especially on pH, concentrations of PO43- and of Cl-, and EC by the use of an actual boiler. Because, most of impurities of marine boiler are due to seawater contamination. The feed water of marine boiler is the distilled water made by the fresh water generator on board. The impurity components of distilled water are similar to that of seawater except SiO2. The generator is possibly, therefore, one of main causes of seawater contamination. The seawater contamination is also occurred due to seawater leakage from condenser, drain cooler, etc. Important features of seawater component are the higher content of Mg than Ca and very little content of SiO2. The experiment  have shown that there is a certain difference between theoretical prediction and experimental result regarding the change of PO43- concentrations.
In order to make clear the above discrepancies, therefore, in third paper , the behavior of Mg hardness in boiler water when OH- and/or PO43- coexisted, and when silica was mixed, was investigated experimentally by the use of an autoclav. The experimental results revealed that the present theory, that is, the conventional model of chemical reaction between scale components and water treatment chemicals of phosphate/alkali type could be applicable only in a certain limited conditions.
This paper considers, therefore, a new model regarding the chemical reaction system especially for the conditions of marine boiler, and the validity of a new model proposed here is estimated by the comparison with the experimental results  in the case when seawater mixed into boiler water. Finally the paper gives some suggestions in order to develop automatic techniques for controlling water quality of marine auxiliary boilers.
2. NEW PROPOSAL FOR CHEMICAL REACTION MODEL BETWEEN SCALE COMPONENTS AND WATER TREATMENT CHEMICALS OF PHOSPHATE/ALKALI TYPE
2.1 Conventional chemical reaction model developed so far
Eqs. (1) to (7) are the conventional system of chemical reaction between scale components and phosphate/alkali type treatment chemicals.
* Marine Technical College, Engine department
12-24 Nishikura-cho, Ashiya-city 659-0026 JAPAN
FAX: +81-797-32-5972, E-mail: firstname.lastname@example.org
** Kobe University of Mercantile Marine
*** Osaka Boiler Mfg. Co., Ltd.