Boiler Feed Water and Cooling Tower Chemicals Dosing

How are the chemicals dosing calculated and decided by the chemicals manufacturer for boiler water and cooling towers?

What are the parameters for the dosing calculations and what is the formula which gives the best approximation? 

Taxonomy

• Treatment
• Chemical Treatment
• Water Treatment & Control
• Cooling Systems
• Chemical Materials
• Utility Chemicals
• Treatment Chemicals

8 Answers

1. With proprietary chemicals one would be best to follow the manufacturers guidelines.
   If chemical compounds and concentrations are known, I would suggest to run PHREEQC models (available free from USGS website) to model your water and focus on the solubility indexes (to identify scaling issues) and the reduction potential (Eh, to identify corrosion issues).
   
   Good thing about PHREEQC is that you can batch run mixtures of your source water and your additives to generate graphs you can use to identify optimal mixtures for your conditions.

   Please note that -to the best of my knowledge- chloride and it's compounds are not included in their main libraries, so that may cause issues depending on your additive's chemical composition.

2. We have Software program for these calculations, when we enter the system values, the program calculates the suitable dosage rate and suggests the suitable products. 

3. Why to use chemicals for Boiler/Cooling water? Pl. install Hydropath ( a device that keeps water under suspension) hence no scale is formed in Boiler tubes/Heat-exchangers in cooling towers. I shall send U ref. if U require.

4. For Boiler, its depend on the manufacturer and phase of the product liquid or solid also the mixtured purity components. Usually for checking for routine test PO4, N2H4,SO3, Polymer or MoO4 etc, and for steam line depend on total alkalinity in the make up water which is defined from calculation of COF its similar for Open Cooling Water for non oxidizing biocide should be dosed by slug

5. There are some interesting manuals containing around 1000 pages each one of those, explaining in detail all parameters.

   There is not enough spare here.... neither for indexing. 

6. For Boilers and Cooling Towers, one of the primary parameters is Cycles of Concentration.  This is represents the increase in conductivity from the feed to the blow-down.  For example, if your feed water is 200 micro-mho and your blow-down is 400 micro-mho, that wold be two COC.  This is important when controlling scale in the boiler/tower.  Then there are also the concentration of scale forming constituents like Ca, Mg etc and their individual COC.  That will determine the chemical feed rate, but it will be unique to each manufacturers product.  So, there is no set equation.

   Additionally for Boilers, you have to be concerned with dissolved oxygen and corrosion in the condensate lines.  To that end, bisulfite is injected and used as an oxygen scavenger.  An amine is injected to neutralize carbonic acid in the condensate and is dependent on things like feed alkalinity and COC.  Don't know off hand any equations you can use.  The actual dosage is going to vary with each manufacturer if they use different strengths of the chemicals involved.

   Additionally for Cooling Towers, you will be concerned with biological growth.  This is a totally separate animal and is completely dependent on each individual system.

7. To promote optimum performance downstream, filtration of boiler feed water streams allows an industrial boiler or power plant to meet the stringent requirements for more efficient boiler performance. Treatment of boiler feed water is required for preventing excessive heat transfer equipment fouling and erosion of turbine blades.

   The most common form of corrosion is due to attack of steel by oxygen.   Oxygen attack is accelerated by high temperature and by low pH.  Another form of corrosion is alkali attack which can occur where caustic is concentrated.  Direct attack by steam on the boiler metal can occur at elevated temperatures according to the following reaction:

                                                   4H2O + 3 Fe  ->  Fe3O4 + 4 H2

    This type of corrosion can occur in areas of restricted water flow where the entire metal surface is blanketed with steam.  Monitoring the levels of hydrogen in the steam is a simple method of detecting this form of corrosion.

   In our experience, some of the operational issues that can severely impact performance include:

   • Operation outside rated capacity
   • Improper or damaged pressure-reducing valves
   • Failure to maintain continuous positive steam pressure
   • Infrequent or improper venting
   • Leaks in the external vent condenser
   • Contaminating deaerated water with oxygenated water

   Dosing

   For Oxygen removing

   Hydrazine  is dosed

   N2H4 O2 = N2 + H2O

   For High PH

   Ammonia dosed or as shown in attached figure

   For Cooling water Treatments 

   1. To protect the system against damaging corrosion.
   2. To control scale formation and fouling which will impair cooling efficiency.
   3. To control the growth of harmful microbes like Legionella bacteria.

   1. Corrosion inhibitor like zn based

   2. Dispersent dosing

   3. Chlorine dosing

    

   1 Comment

   1. Hydrazine is forbitten for use since some years. 

8. Chemical treatment for boilers and cooling towers is based upon makeup water quality as well as system operating conditions and goals of said treatment.  Once preferred treatment chemistry is identified, dosing is based upon system losses (intentional and unintentional blowdown) and target concentration of treatment chemistry (as prescribed by a water treatment professional), taking into account the active ingredient concentration of the chemical being applied.