McIlvaine Hot Topic Hour
Power Plant Cooling Towers
& Cooling Water Issues Matthew L. Haikalis
February 16, 2012
Agenda
Impact Considerations for Cooling Tower Systems • Corrosion, Scale, Microbiological
• Holding Time Index
• Induction Time
Best Available Technologies – Focus on Microbiological Control • Chlorine or Sodium Hypochlorite
• Chlorine Dioxide
• Mixed Oxidant
• Bio-Dispersants
• Stabilized Bromine
2
Impact Considerations for Cooling Tower Systems
Cooling Towers
Due to the evaporation and cycling factor of cooling towers, the water balance shifts when compared to once-through cooling
Makeup Water
Evaporation
Blowdown
Recirculating Water
4
Water Balance
Recirculation Rate 100,000 gpm; T = 15
Cycles Evaporation (gpm) Blowdown (gpm) Makeup (gpm)
1.5 1,500 3,000 4,500
2 1,500 1,500 3,000
3 1,500 750 2,250
4 1,500 500 2,000
5 1,500 375 1,875
6 1,500 300 1,800
7 1,500 250 1,750
8 1,500 214 1,714
0
50
100
150
200
250
300
350
400
450
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8
Makeup
Cycles
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Challenges
Cooling towers present challenges not experienced with once-through systems due to the “cycling” factor as a result of evaporation
Microbio
Corrosion
Scale Fouling
6
Corrosion
Fe(OH)3
Fe(OH)2
Anode
O2Fe++
e - Electron Flow
e -
e -
Cathode
Water - H20 (Electrolyte)
OH - O2
Corrosion of Carbon Steel
7
Scale & Corrosion Rates
Tem
pe
ratu
re
In general, for every 10°C in water temperature, chemical reaction rates double
Scale & Corrosion
CT
Makeup
Cooling
Tower
Total Hardness CaCO3 168 672
Calcium Hardness CaCO3 120 480
Magnesium Hardness CaCO3 58 232
Phenolphthalein Alkalinity CaCO3
Total Alkalinity CaCO3 104 416
Sulfate SO4 50 200
Chloride Cl 75 300
Silica SiO2 8 32
Total Phosphate PO4 0.2 0.8
Iron Fe 0.25 1.00
pH S.U. 7.8 8.8
Conductivity S/cm 440 1,760
As cycles rise, the contaminants in the makeup water increase proportional, not considering effects of scaling tendencies on bulk water concentrations, affecting corrosion control
8
Scale
Deposition and fouling involves both: • Inorganic contaminants
• Organic contaminants
9
Microbiology in Cooling Water Systems
MB organisms are introduced to cooling towers in many ways:
• Airborne from soil, particulates, dust
• Makeup water
• Piping and existing biofilm
Types of MB organisms are varied; slime formers lead to biofilm, and some such as sulfate reducing bacteria create corrosive environments, both leading to microbiologically influenced corrosion (MIC)
Exist in ALL areas of a cooling system
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Microbiology in Cooling Water Systems
Biofilm, in general, is comprised of mixed populations of bacterial cells within a fibrous matrix of ionic polymer (slime)
Bulk water
Aerobic biofilm
Anaerobic biofilm
Section of pipe
Time
Bulk water
Aerobic biofilm
Anaerobic biofilm
Section of pipe
Time
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Holding Time Index or Half-life
Important considerations for scale control:
• Holding Time Index (HTI) is the turnover rate of water and constituents in a cooling tower circuit. It can be understood as the amount of time required to dilute added chemical to 50% of its original concentration:
• HTI = 0.7 X Volume of System (gallons)
Blowdown Rate (GPM)
• Organic chemicals such as phosphonates and polymers lose their effectiveness over time, so HTI needs to be considered when determining dosages and how effective they will be
• In once through waters, the antiscalants and dispersants only have to be effective for a matter of seconds. In cooling towers, holding time can be hours to days or even longer so higher dosages are needed
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Holding Time & Effects on Water Treatment Chemistry
As you can see, the cycling effect of a cooling tower system has a direct impact on water treatment considerations and must be balanced with economics to develop the best strategy for control
As a rule of thumb, most cooling water treatment programs can be managed effectively when HTI is < 3 days
0
50
100
150
200
250
300
350
400
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Ho
ldin
g Ti
me
(h
ou
rs)
Cycles
Holding Time
0
1000
2000
3000
4000
5000
6000
7000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Blo
wd
ow
n (
gpd
)
Cycles
Blowdown Blowdown (gpd) Makeup (gpd) Evaporation (gpd)
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Induction Time – How Long Before Scale Occurs
The demand for phosphonate and polymers increases with an increase in HTI. They can delay scale, but not indefinitely
Phosphonate concentrations in a cooling system are limited by saturation, degradation due to oxidant levels, hydrolysis, biological activity, and demand
Polymers can be used to assist phosphonates in scale control and must be dosed appropriately
14 Ferguson, Robert J. “The Kinetics of Cooling Water Scale Formation and Control.” AWT Annual Meeting Atlanta, GA Sept. 14-17, 2011
Calcium Carbonate Threshold Inhibition - pH 8.8
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
110.00
120.00
1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50
Percent
Cycle of Concentration
10ppm New Polymer 10ppm AA/AMSA 10ppm PMA 3ppm PBTC
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Power GEN & Heavy Industry Requirements
High water temperatures and high heat exchanger skin
temperatures require stronger scale and corrosion
inhibition.
Biofilm and bacteria control extremely important
Greater stress requires high performance polymers
Key Performance Indicators (KPIs) used to evaluate
Good monitoring and control equipment
Proper service and maintenance program
16 16
Chemical Treatment Strategy
Anti-scalants for scale control
Dispersants for scale, sludge, suspended solids fouling
Corrosion inhibitors for mild steel, copper most generally, then others such as galvanizing or aluminum
Organic dispersants and surfactants for system cleanliness and to assist in biocide treatment
Biocides for MB control
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Questions?
THANK YOU!