Wednesday, May 6, 2015


Boiler Chemistry control and treatment of feed water

I am delighted response from a few guys. out of many requests I find that majority wanted short 
discussions on role of Instrumentation on boiler chemistry. Keeping the view in mind I started the 
discussions on the same. In the first instalment, I discuss basic requirements. In chapter V (http://www.sciencedirect.com/science/book/9780128009406of our book "Power plant instrumentation and control handbook" (http://store.elsevier.com/Power-Plant-Instrumentation-and-Control-Handbook/Swapan-Basu/isbn-9780128011737/I tried to cover the issues in details, so interested person may go to the desired link to download the book as a whole or required chapter. I plant to cover the discussions in 4-5  regular instalments. So in next few instalments lets concentrate on boiler chemistry.

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 fig 1 (Courtesy: http://store.elsevier.com/Power-Plant-Instrumentation-and-Control-Handbook/Swapan-Basu/isbn-9780128011737/). 

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 http://store.elsevier.com/Power-Plant-Instrumentation-and-Control-Handbook/Swapan-Basu/isbn-9780128011737/.
Here only treatment part is discussed, with special reference to Supercritical boilers.
1.      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.
Diethylhyrdoxylamine (DHA) has unique properties such as volatility, passivating steel surface, coupled with low toxicity makes DHA a good Oxygen Scavenger in modern days power plants. Some of the features of DHA shall include:
Rapid scavenging of O2 at FW temperature and pH
Passivate internal surfaces of boiler
Volatility with steam similar to neutralizing amine with ability for distilled of boiler and protects steam and condensate equipment
Under heat it yields two neutralizing amines to raise pH value.
Low toxicity hence less health hazard.
Reaction with Oxygen: 4(C2H5)2NOH+9O2     =     CH3COOH+2N2+6H2O. In hydroxide alkalinity of boiler acetic acid does not pose any problem and acetates are formed. Only shortcoming is its sluggishness in reaction kinetics of oxygen capture at lower temperature.


Some of EPRI*  Recommended values related to various parameters are as follows:

Parameter at  Main steam / Reheat steam
Normal
Cat Cond. µS/cm
<0.15
Silica ppb
<10
Sodium ppb
<2
Sulfate  ppb
<2
Chloride ppb
<2
TOC ppb
<100
*EPRI: Electric power research institute



For this, control limit at Feed water will be around
Parameter at  Feed water
Normal
Cat Cond. µS/cm
<0.15
pH
~ 9.0
Do at Economizer inlet ppb
30-150
Fe  ppb
<2

Typical sampling and dosing points in power cycle has been shown in fig 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. Expected parameter with their ranges have been discussed in “Power plant Instrumentation and control handbook”: Elsevier.  Interested persons may follow either of the following links to get the required details viz. whole book or required chapter.

With this basics I close the discussions for the day! 

Watch for the next instalment on the same topic shortly ! 
Please feel free to communicate . 
I am available at linkedin Facebook and tweeter also . In Linkedin power plant profession group also.
have a nice time ahead
Swapan 




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