Control of Boiler Water Chemistry

Earlier chapters discuss many of these standard internal treatments and provide waterside chemistry and program guidelines in the form of charts, tables, and accompanying notes. 

NOTE The principal guides to internal treatment chemistry found in earlier chapters are  

Table 9.3 Specification for Water for Higher Pressure Boilers p341 

Table 10.2 Carbonate Cycle BW Carbonate Reserves p4I5 

Table 10.3 Carbonate Cycle BW Control Limits p4I7 

---

The water chemistry of CANDU reactors embraces control of corrosion and corrosion-product transport in the coolant system, control of radiolytic decomposition of the moderator (51) and control of the concentration of soluble neutron absorbers used to adjust reactivity and control of boiler-water chemistry to minimize tube corrosion (52). The major chemical engineering effort has dealt with coolant technology and I will confine this review to that aspect of water chemistry. 

Consensus on Operating Practicesfor The Control of Feedwater and Boiler Water Chemistry in Modem Industrial BoilerSs The American Society of Mechanical Engineers, New York, NY, 1994. 

If the rate of blowdown is too high, heat (fuel) and treated water are wasted and the boiler water chemistry may be less than desirable for optimum waterside control. 

In addition to external conditioning processes and the need to provide internal chemical treatments to some or all steam-water circuits within the steam cycle, the scope of boiler water treatment includes, as mentioned earlier, the provision of suitable technical resources sufficient to control the steam-water chemistry within defined limits appropriate for the boiler plant under consideration. Because these steam-water control limits tend to narrow considerably with increase in boiler pressures (and heat-flux densities), suitable monitoring and control procedures may require implementing actions with knife-edge precision. 

The opposite scenario also may occur, whereby a localized pre-boiler water chemistry problem can affect the downstream, mechanical operation of a boiler. As an example, difficulties in the control of pretreatment equipment may lead to treated MU water instability, causing downstream after-precipitation, leading to FW line blockages, and finally resulting in boiler cutout as a result of low water. 

It is generally agreed that the causes and effects of poor water chemistry, mechanical problems, boiler section corrosion, metal failure, and poor boiler plant operation are all closely interrelated. Thus, effective control over the various corrosion processes that may occur in a boiler and its auxiliary equipment is fundamental to the realization of the full life expectancy and safe operation of the plant. Corroded and wasted metal cannot be replaced easily, and the failure of a boiler in service is both potentially dangerous and expensive. 

At later dates and as a result of specific water chemistry or new boiler design problems, various derivations of coordinated phosphate programs appeared. The basic philosophy relating to waterside control through the use of coordinated treatment programs still holds true today (albeit with modifications), and controlled-pH phosphate programs remain the principal choice for internal treatment of higher pressure boilers. 

Tvedt Jr., T. J. Holloway, Robert T. Control of Industrial Boiler Water Chemistry A New ASME Consensus. Industrial Water Treatment, Tall Oaks Publishing, Inc., USA, May/June 1996. 

The function of boiler water treatment, therefore, is to control the waterside chemistry of boiler plant systems within certain agreed and relevant parameters and specifications. As these adverse processes are by no means limited to the boiler itself, in practice, boiler water treatment also includes pre-boiler and post-boiler functions and further requires that all the various types of water utilized are controlled through a comprehensive treatment and proactive management program. 

Other notaries are the Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modem Industrial Boilers (1994 edition), published by the American Society of Mechanical Engineers, and BS 2486 1997 Recommendations for Treatment of Water for Steam Boilers and Water Heaters from the British Standards Institution. The 1994 Consensus (with its engineering background) and the 1997 version of BS 2486 (with its strength in operational chemistry) complement each other well. I consider that the tables and propositions contained in these two booklets jointly represent a true standard for boiler water treatment operational control. Consequently, I am pleased to be able to reproduce in this book all the tables from both publications, having received permission from the respective organizations to do so. 

Removing a portion of boiling water to control boiler water chemistry. 

American Society of Mechanical Engineers (ASME). Research and Technology Committee on Water and Steam in Thermal Power Systems (1994). Consensus on operating practices for the control of feedwater and boiler water chemistry in modem industrial boilers. ASME I (X)367. ASME, Washington, DC. 

Concensus Operating Practices for Control of Foedwater/Boiler Water Chemistry in Modem INDL Boilers (CRTD34)... 

They can only carry out these functions consistently and efficiently if the feedwater (FW) and boiler water (BW) chemistry and the overall boiler operation are properly controlled. Thus, over the years, in addition to improvements in engineering design and practice, many different forms of external water treatment processes and internal water treatment programs have been developed. 

Apart from bottom blowdown valves (main BD valve), other types of valves are often employed. Blowdown valves also can be used to control high water levels, drain the boiler for cleaning or inspection purposes, and maintain chemical concentrations and water chemistry below maximum permitted levels. 

Utility boilers generally require waterside chemical cleaning of all boiler surfaces every 300 to 500 days of operation, and this work may be carried out by specialist contractors. It is regarded as a routine function, irrespective of water chemistry, laboratory involvement, or the quality of FW treatment and water management provided. Chemical cleaning of utility boilers is designed to permit the boilers to operate at peak performance and within knife-edge control parameters. 

Corrosion anywhere within the steam-water circuits results in metal wastage and possible equipment failure. And if the boiler plant is not operated correctly or if the water chemistry is not maintained within certain control parameters, the generated steam may contain contaminants in a cause-and-effect process that ultimately affects the utilization of the steam, reducing quality in a number of areas, and increasing fuel, manufacturing, or maintenance costs. 

---