Basic Cooling Water Treatment principles

John Cowpar Area Manager GE Water and Process Technologies

USING WATER

POTENTIAL PROBLEMS



CORROSION



DEPOSITION

-

Fouling Biofouling Scaling

Scale Formation

 Results in loss of heat transfer efficiency  Increased running costs  Danger of under deposit corrosion  Increased maintenance costs  Danger of bacteria  Health implications

Corrosion

 Destruction of plant  increased maintenance costs  Fouling  loss of efficiency due to increased pumping costs  loss of heat transfer efficiency  Increased Biological Nutrients  fouling and health implications

Fouling

 Loss of heat transfer efficiency  increase in running costs  Under deposit corrosion  increase in maintenance requirements  Increased biological nutrients  health implications  Blockages in system  increased operating costs and downtime

Objectives of Water Treatment

 MINIMISE SCALE  MINIMISE CORROSION  MINIMISE FOULING  MINIMISE BIOFOULING  MAXIMUM SAFETY  MAXIMUM EFFICIENCY  NON-POLLUTING

WHAT CAUSES OUR PROBLEMS?

DISSOLVED SOLIDS

 e.g. CALCIUM  MAGNESIUM  SODIUM  CHLORIDE  BICARBONATE  SULPHATE  SILICA  IRON

DISSOLVED GASES

 e.g. OXYGEN  CARBON DIOXIDE  NITROGEN  SULPHUR DIOXIDE

SUSPENDED MATTER



DUST/DIRT



CONTAMINANTS e.g. OIL



BIOLOGICAL e.g. ALGAE, FUNGI, BACTERIA

TYPICAL WATER ANALYSIS CHART

Water Analysis Result pH 7.7

Colour Turbidity Solids - Suspended Chloride as Cl Alkalinity as CaC03 Ammoniacal Nitrogen as N Iron (Total) as Fe Manganese (Total) as Mn Nitrate as N Total Hardness as CaC03 3.00 HAZEN 9.00 F.T.U.

5 mg/l 44 mg/l 144 mg/l 0.140 ug/l 311 ug/l 65 ug/l 4.0 mg/l 207 mg/l Sulphate as S04 62.3 mg/l Silica - Reactive as Si02 6.9 mg/l Sulphide as S 0.015 mg.l

Carbon Dioxide - Free Solids - Total Diss. at 180C 2.50 mg.l

347 mg/l D.O. Concentration (Field Det.) Coliforms E. Coli Faecal Streptococci Sulphite Red. Clostridia 10.7 mg/l <10 /100ml <10 /100ml <1 /100ml 300 /20ml

Hardness

 Hardness is due to calcium and magnesium salts dissolved in water  All hardness salts are less soluble in hot water than in cold water (they show inverse solubility)  Different hardness salts have different levels of solubility  Hardness is normally reported as calcium carbonate

MAKE UP M = E + W + B EVAPORATION WINDAGE BLEED

Useful Equations

E=R/100 x Temp Drop(degF)/10 W=R x 0.2/100 ( Forced Draught) W=R x 0.6/100 (Natural Draught) B=E/(C-1) -W M=E + B + W

SCALE FORMATION

SCALE CAN BE CONTROLLED BY:  PRE-TREATMENT  CHEMICALS  CONCENTRATION FACTOR

CORROSION

 Iron ore is found in nature and requires a large input of energy to convert it into steel.  Steel corrodes in order to get back to its natural (lower energy) state  Corrosion is an electrochemical process

CORROSION CAN BE CONTROLLED BY:

 REMOVAL OF OXYGEN ?

 ADDITION OF CHEMICALS  CONTROL OF pH

Biofouling

What is Biofouling caused by?

•

FUNGI

•

ALGAE

•

BACTERIA

FOULING/BIOFOULING

 Can be controlled by  Filtration  Control of Concentration Factor (bleed)  Dispersants  Biocides

Open Cooling

 When evaporation occurs, the heat of evaporation is used to drive off the vapour  The loss of this energy results in a cooling effect in the water  Pure water is evaporated (gases may also be lost)  Dissolved solids remain in the water Customer Training WT200C Page 23

Cooling Water

WATER DROPLET COOLS BY: EVAPORATION RADIATION CONVECTION Customer Training WT200C Page 24

Control of Concentration

 The number of times the solids build in the system water is termed the concentration factor (CF).

 CF is controlled by bleed  to increase CF - decrease bleed  to decrease CF - increase bleed Customer Training WT200C Page 25

Bleed Control

 Effect of too much or too little bleed:  Too much bleed :  low concentration factor  waste of water  waste of treatment  Too little bleed:  high concentration factor  danger of scale and fouling  increased nutrient in system  danger of biofouling Customer Training WT200C Page 26

Water Use x

x

While increasing concentration factor reduces water use, it also increases nutrients in the system water, encouraging growth of bacteria and slimes. Therefore, we normally run most cooling systems between 2 and 5

x x x x 1 2 3 4 5 6

Concentration Factor

Customer Training WT200C Page 27

Non-biological Fouling

 Treated by addition of dispersants  dispersants (antifoulants) coat the particles and so keep them apart  The dispersed particles are then removed from the system water  either with the bleed or via a side stream filter Customer Training WT200C Page 28

Non-biological Foulants

 Silt  Rust  Process contamination  all removed by dispersant/bleed  Oil  Grease  a different chemical is required but the principle is the same Customer Training WT200C Page 29

MICROBIOLOGY

Customer Training WT200C Page 30

Microbiology in Industrial Cooling Systems

• • • • •

Problematic Microorganisms The Biofouling Process Water Treatment Biocides Biocide Programming Monitoring and Control

Customer Training WT200C Page 31

FUNGI

• • • • •

Although yeast and some aquatic fungi are normally unicellular, most fungi are filamentous organisms Fungi form solid structures which can reach a considerable size Some wood destroying fungi exist, associated with deterioration of tower timber Fungi require presence of organic energy source Exist at between 5 to 38 C and pH 2 to 9 with an optimum of 5 to 6

Customer Training WT200C Page 32

ALGAE

• •

Classified as plants as they grow by photosynthesis Range in size from unicellular microscopic organisms to plants that can be up tp 50m in length Single cells

Customer Training WT200C

Multi cellular

Page 33

ALGAE

•

Algae cannot survive in the absence of air, water or sunlight

• •

Basic difference is that algae utilise CO2 and water using sunlight as the energy source to assimilate food Large quantities of polysaccharides (slime) can be produced during algal metabolism

•

Plug screens, restrict flow and accelerate corrosion

•

Provide excellent food source

•

Exist between 5 to 65 C and pH 4 to 9

Customer Training WT200C Page 34

BACTERIA

• • • • • • •

Universally distributed in nature Great variety of micro organisms Multiply by cell division Slime formation Pseudomonas (utilise hydrocarbon contaminants) Sulphur bacteria - anaerobic sulphate reducing bacteria Nitrogen cycle bacteria

Customer Training WT200C Page 35

FACTORS CONTRIBUTING TO MICROBIAL GROWTH

• • • • • • •

Rate of incoming contamination Amount of nutrient present pH Temperature Sunlight Availability of oxygen/carbon dioxide Water velocities

Customer Training WT200C Page 36

THE BIOFOULING PROCESS

• • •

Bacteria prefer to colonise surfaces

–

enables production of biofilm which acts to protect and entrap food sources Planktonic bacteria

–

free swimming in bulk water Sessile bacteria

–

attached to surfaces

Customer Training WT200C Page 37

EFFECTS OF BIOFOULING

• • • • •

Fouling of: tower, distribution pipework, heat exchangers Reduction in heat transfer efficiency Lost production Under deposit corrosion Inactivation/interference with inhibitors

Customer Training WT200C Page 38

WATER TREATMENT BIOCIDES

•

Oxidising Biocides

–

Have the ability to oxidise organic matter eg. protein groups

•

Non-Oxidising Biocides Prevent normal cell metabolism in any of the following ways :

–

Alter permeability of cell wall

– – –

Destroy protein groups Precipitate protein Block metabolic enzyme reactions

Customer Training WT200C Page 39

OXIDISING BIOCIDES

• • • • •

Sodium Hypochlorite Hypobromous Acid Chlorine dioxide Ozone Hydrogen Peroxide

Customer Training WT200C Page 40

Oxidising Biocides

• • •

Rapid kill Cost effective Tolerant of contamination e.g. Bromine, Chlorine Dioxide

•

Minimal environmental impact

• • •

e.g. Bromine, Ozone, Peroxide, Chlorine Dioxide Ineffective against SRB’s Low residual toxicity Counts approaching potable water standards possible

Customer Training WT200C Page 41

Non Oxidising Biocides

• • • • • • •

Screen water Select alternating biocide to prevent resistant strains from developing Effective against SRB’s Can protect system long after dosing.

Contain biodispersant Higher dosage for kill possible Environmentally some have rapid breakdown e.g. DBNPA

Customer Training WT200C Page 42

BIODISPERSANTS

• • • •

Improves penetration of biocide within bacterial slime Disperse released bacteria and biofilm into bulk water for removal by blowdown Reduces ability for bacteria to attach to system surface Improves performance of both non oxidising and particularly oxidising biocides

Customer Training WT200C Page 43

Physical Methods

Ultra Violet and Ultra Filtration

•

Only Effective At Point Of Use

•

Cannot Kill Sessile Organisms

•

Offer No Protection To Isolated Parts Of System (Static Areas)

•

Environmentally Acceptable.

Customer Training WT200C Page 44

Control of Concentration

 The number of times the solids build in the system water is termed the concentration factor (CF).

 CF is controlled by bleed  to increase CF - decrease bleed  to decrease CF - increase bleed Customer Training WT200C Page 45

Customer Training WT200C Page 46

Customer Training WT200C Page 47