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Boiler Chemistry Control and Treatment of Feed Water
It is essential to make sure the efficient boiler start up and commissioning with the help developing suitable means and workable approach to meet the various chemistry limits set for boiler especially for once through supercritical boilers where boiler chemistry cannot be controlled by blow down controls. Therefore it is absolutely necessary to control water chemistry very rigorously. Corrosions and deposits are two phenomena which are related to each other. Corrosions & deposits in power cycle has been depicted in Figure 1:
Corrosion produces solid Metallic Oxides and Hydroxides which are carried away by the system flow to be deposited at another place. Also high concentrations of deposits cause corrosion. So some treatment of feed water is necessary. Details related to boiler water chemistry can be found in Chapter V of Power Plant Instrumentation and Control Handbook: A Guide to Thermal Power Plants (Here only treatment part is discussed, with special reference to Supercritical boilers.)
It is necessary to treat the boiler water to produce protective coat on the tubes required for good quality water & steam. For this it is necessary that the water shall be low controlled alkaline, very low level of dissolved Oxygen and very low presence of ions like Cl–. Silica from CW is another damaging factor as it deposits on turbine. Silica needs to be controlled at DM plant also. The percentage of impurity allowed in boiler water/steam is function of boiler operating pressure, so, more critical for high pressure boilers.
Two basic ways to regulate corrosion in water, are All Volatile Treatment (AVT) and Oxygenated Treatment (OT). Two types of AVT are; i) AVT (R) Using Ammonia (NH3) and as reducing agent like Hydrazine (N2H4), ii) AVT (O) same as AVT ® minus reducing agent i.e. N2H4. Ammonia is commonly used cheaper material to maintain pH in boiler water. N2H4 is used to scavenge Dissolved Oxygen and protect Copper alloys from Ammonia. It may be good for Cu alloy but form thick harder magnetite layer. In OT Ammonia is used to control pH and remove little Oxygen, but slightly oxidizing environment is maintained to promote formation of oxidizing layer on the metal surface. In supercritical plants as well as in subcritical plants with CPU, after start up Oxygen Treatment (OT) is done. This treatment feeds Oxygen to Condensate & FW to arrest corrosion as well as internal scale formation—hence less chemical cleaning and quicker start up. Normally after start up during operation such OT is done to reduce internal scale formation at operating temperature due to double oxide layer. This also controls pH, because high temperature & pressure NH3 breaks down & increase pH. There is direct relation between pH and conductivity.
Instead many use DEHA which has nearly 40 times less toxicity when tested in animals. Diethylhyrdoxylamine (DEHA) has unique properties such as volatility, passivating steel surface, coupled with low toxicity makes DEHA a good Oxygen Scavenger in modern days power plants.
Some of EPRI* Recommended values related to various parameters are as follows:
For this, control limit at feed water will be around:
Typical sampling and dosing points in power cycle has been shown in Figure 2:
Here all probable points have been shown, based on application necessary sampling points may be chosen for Steam and water analysis system (SWAS). Also blow down points have been shown. Interested persons may follow either of the following links to get the required details viz. whole book or required chapter below:
Power Plant Instrumentation and Control Handbook: A Guide to Thermal Power Plants is available for purchase on the Elsevier Store. Use discount code “STC215” at checkout and save up to 30% on y0ur very own copy!
About the Author
Swapan Basu is Chief Executive, Systems and Controls, Engineering and Consulting Kolkata, India. He has several decades of experience in practicing instrumentation & control systems for subcritical and supercritical thermal power plants, including combined cycle projects. Since 1979, he has led teams of engineers in India, Jordan, Singapore, South Korea, Syria, and the United States. He has a number of national and international technical papers to his credit.
You can follow Swapan’s thoughts, research and musings on his personal blog here.
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