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

5

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

10

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

11

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

12

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)

13

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

15

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

17

Questions?

THANK YOU!