2. PH & Alkalinity
• pH adjustment (IS 8188 : 1999)
• The make up water ph is around 7.8. In Cooling
tower as water evaporates alkalinity increases
and ph goes upto 8.5 to 9.0
• pH is so adjusted by dosing H2SO4 that the
Langelier Saturation index of the cooling water is
always slightly on positive side either + 0.2 or
maximum + 0.6 Slight deposit or CaCO3 formed in
this way on the metal surface will act as
protective layer to minimize further corrosion.
3. Effects of high ph
• In case the pH goes above 8.5, the scaling
tendency will substantially increase and hence
it is necessary to increase the concentration of
dispersants/anti-scalents. If high pH persists
for a longer time, microbial growth will
increase, as high pH is favorable for their
growth and chlorine is less effective at high pH
In such cases, it is advisable to give shock dose
of non oxidizing biocide.
4. How much acid is Reqired
• Use of Sulphuric acid is also limited to a
maximum dose of 600 ppm, otherwise
chances of sulphate attack to the concrete
5. Chlorine Dosing
• In chemically pure water, molecular chlorine
reacts with water and rapidly hydrolyzes to
hypochlorous acid (HOCl) and hydrochloric acid
• Cl2+H2O- HOCL+HCL
• Both of the acids formed by hydrolysis react with
alkalinity to reduce buffering capacity of water
and lower pH. Every pound of chlorine gas added
to water removes about 1.4 lb of alkalinity.
• HOCL is mainly responsible for killing of bacteria
6. Chlorine Dosing
• For all practical purposes the reaction is
irreversible. Hypochlorous acid is a weak acid and
dissociates to form a hydrogen ion and a
• HOClH+ + Ocl
• Between pH 6.5 and 8.5, the dissociation reaction
is incomplete, and both hypochlorous acid and
hypochlorite ions are present. The equilibrium
ratio at any given pH remains constant even if the
hypochlorous acid concentration is decreasing. At
constant pH and increasing temperature,
chemical equilibrium favors the OCl - ion over
7. • The primary oxidizing agents in water are hypochlorous
acid and the hypochlorite ion, although hypochlorite
has a lower oxidizing potential. Oxidizing potential is a
measure of the tendency of chlorine to react with
other materials. The speed at which these reactions
occur is determined by pH, temperature, and
oxidation/reduction potential. As pH increases, the
chemical reactivity of chlorine decreases; as
temperature increases, reactions proceed more rapidly.
The oxidation reactions of chlorine with such inorganic
reducing agents as sulfides, sulfites, and nitrites are
generally very rapid. Some dissolved organic materials
also react rapidly with chlorine, but the completion of
many organic-chlorine reactions can take hours.
8. Chlorine Demand
• Chlorine demand is defined as the difference
between the amount of chlorine added to a
water system and the amount of free available
chlorine or combined available chlorine
remaining at the end of a specified time
• Free available residual chlorine is the amount of
chlorine which exists in the treated water system
as hypochlorous acid and hypochlorite ions after
the chlorine demand has been satisfied
• NOTE- Under some conditions, hypochlorous acid
is 80 times more effective in controlling bacteria
than the hypochlorite ion. Hypochlorous acid
predominates below a pH of 7.
10. Chlorine Demand
• Amount of chlorine consumed by following impurities before free chlorine
appears in Cooling Water.
• Organic Matters
• Ammonia - forms Chloroamines
• Dead Algae, Slime
• Other Oxidizable Substances
• Care should be taken not to feed excessive amounts of
halogen that will adversely affect corrosion rates and
delignification of wood. Chlorine feed rates should not
exceed 0.6 ppm based on recirculation rate.
11. Few points of Chlorine
• a part of it reacts with organic matters and iron in water and get
consumed. This is chlorine demand
* a part of the added chlorine reacts with ammonia or organic
nitrogen [urea] if present and form chloramines known as
monochloramine, dichloramine, and trichloramine. Chloramines are
known as combined chlorine
* after meeting chlorine demand and combined chlorine
requirement, there is a free residual available chlorine as HOCl and
OCl- ions in water for dis infection – this is called free available
chlorine. Free chlorine is measured to find out if there is enough
disinfectant in water.
* Total chlorine = Free chlorine + Combined chlorine
12. How Chlorine works
• Bacteria are protected from its environment
by a membrane, the integrity of the
membrane is essential for their survival.
Oxidizing biocides destroy cell walls. They also
oxidize protein groups within the cell,
resulting in loss of normal enzyme activity
necessary for respiration and cell metabolism.
• The superiority of HOCl over Ocl is due to
smaller molecular size and electrical neutrality
which allow it to pass through cell membrane.
13. Stress Corrosion Cracking
• Chloride is the main contributor to SCC of
stainless steels. High chloride concentrations,
resulting from high chloride levels in the
makeup water and/or high cycles of
concentration, will increase susceptibility.
Although low water temperatures generally
preclude cracking, SCC of stainless steels can
occur in cooling systems
14. Selection for SS
• Level of Chlorides (Cl-) and temperatures.
This is your first step : depending on the levels
of both factors (Cl- and Temp), you are more
or less agressive to the wide vairety of S-Steel.
A rule applied to most common stainless
steels says as Cl- are going up and Temp is
growing, in a project go from basic 304, to
304L then 316, next 316L. At very high levels
of Cl- and high temperatures, it may be worth
the price to pay using Titanium
15. Something more on SSC
• Stress Corrosion Cracking (SCC) – chloride in water with high
Presence of high chloride in water with high temperatures leads to
stress cracking . The stresses can be tensile, resulting from the
application of loads, or residual stresses from the method of
fabrication. Austenitic group of steels , 300 series, is more commonly
affected by stress cracking. The duplex stainless steels, alloy 2205, are
resistant to stress corrosion cracking (SCC) than austenitic grades. Alloy
2205 is approximately 50% harder than SS316. Duplex 2205 is a
nitrogen enhanced duplex stainless steel that was developed to
combat common corrosion problems encountered with the 300 series
stainless steels. The ferritic grades, 400 series, are more resistant to
temperature and free of SCC. This group of SS in the 400 series
contains 10.5% to 20% chromium for corrosion resistance and
resistance to scaling at elevated temperatures
16. WHY Stainless Steel CORRODES
• All 300 series SS [austenitic stainless steels]
contain a small amount of carbon in solution in
the austenite. Carbon is precipitated out of the
steel, in the temperature range of 565° C to 870°
C. This is a typical temperature range during the
welding of stainless steel. The precipitated carbon
combines with the chromium in the stainless
steel to form chromium carbide, starving the
adjacent areas of the chromium they need for
17. Definition of Corrosion
• Corrosion is an electrochemical process where
metals are converted to their more stable
• The process requires an anode, a cathode, and
an ionic conduction path through an
electrolyte such as water
19. The Corrosion Process
Occurs due to the presence of local cells with anodic and cathodic sites on the metal
mediated by electron transfer through an electrolyte.
In an aerated, neutral solution, the overall reactions are :
Fe Fe++ + 2e- (1)
O2 + 2H2O + 4e- 4OH- (2)
Fe++ + 2OH- Fe(OH)2
ELECTRON FLOW CATHOD
Water / Electrolyte
20. Corrosion Inhibitors
– zinc, poly/Meta/Pyro phosphate, organic
phosphorus compounds providing controlled
• Anodic inhibitors
– Molybdate, ortho phosphate, Nitrite, Silicates and
Chromate in the past
21. Phosphate Technology Corrosion
• Orthophosphate - anodic inhibition via the
promotion of a gamma iron oxide film due to
formation of a calcium phosphate protective film
• This protective layer is haemetite based.
• The simple basis is: controlling calcium phosphate
precipitation to allow an inhibitor film to be
formed without forming calcium phosphate
sludge The program depends on using the proper
phosphate stabilizing polymer. Without calcium
at levels > 100 ppm (as CaCO3), the film
formation is weak.
• These programs also provided corrosion
protection because phosphate will react with
ferrous ions (Fe+2) produced at anodic sites to
form a protective barrier, while [Ca3(PO4)2]
precipitates in the local alkaline environment
at cathodic sites. Zinc was a common
corrosion protection supplement, as zinc ions
will also precipitate (as zinc hydroxide
[Zn(OH)2] at cathodic sites) thus enhancing
the barrier film.
• Phosphates exist in water as orthophosphates,
polyphosphates [ condensed phosphate ] and
organic phosphates. Filtered phosphate [ 0.45
µm filter ] is the true “soluble”
orthophosphate. Unfiltered phosphate is a
measure of total phosphate in the water
24. Δ PHOSPHATE
• Comparing the value of soluble phosphate with
total phosphate is the basis of control of ‘ delta
orthophosphate’ or ‘ delta phosphate ‘
Delta Orthophosphate = Total Orthophosphate –
High delta orthophosphate may be the result of
high pH excursions, low polymer dosages,
excessive orthophosphate and over cycling.
• Maintain below 1.2 ppm
25. Need to check Δ PHOSPHATE
• The delta orthophosphate is a good safeguard against
feeding too much phosphate to achieve a filtered
target. If the delta orthophosphate is at or above a
threshold value, corrective action is generally
necessary to avoid calcium phosphate deposition. The
acceptable delta orthophosphate threshold
concentration typically ranges max. 1.5 mg/L PO4,
depending upon whether the cooling water chemistry
is managed in an alkaline or neutral pH mode.
• A lower value indicates higher dispersing action, higher
polymeric dispersant and higher corrosive tendency. A
higher value is an indication of higher deposition
tendency, particularly in low velocity areas.
26. Other Importance
• one problem that high delta phosphate can cause is ,
lowering of conductivity of cooling water by precipitation of
calcium phosphate. The conductivity blowdown controller
sees this as a lowered cycles value and thus does not
blowdown to maintain a “true” cycles value, resulting in
over cycling and generation of additional scale, which then
further lowers the conductivity, increasing true cycles even
more. This results in a cycle effect leading to more and
more scale formation in the cooling system.
This clearly explains why it is important to test phosphate
both in filtered and unfiltered water
• zinc is strong corrosion inhibitor by making a
film protection over metal surfaces.
• Zinc is amphoteric in nature.
• Zn Inhibitor is ph sensitive.
• De-zincification has been found to occur with
waters having a pH of over 8.2.
• Zn is heavy metal and discharge is often
• Zn precipitates at the zone of high heat flux.
• Scaling is the precipitation of hard and
adherent salts of water soluble constituents,
like calcium and magnesium, on the metal
surface. These salts have very poor thermal
conductivity and their control is therefore
absolutely essential for proper heat transfer
• The most commonly encountered scale is
calcium carbonate and it forms an extremely
hard and adherent deposit.
• Scale deposits are formed by precipitation and
crystal growth at a surface in contact with water.
Precipitation occurs when solubilities are
exceeded either in the bulk water or at the
• Although they may be completely soluble in the
lower-temperature bulk water, these compounds
(e.g., calcium carbonate, calcium phosphate, and
magnesium silicate) supersaturate in the higher-
temperature water adjacent to the heat transfer
surface and precipitate on the surface.
30. • Scaling is not always related to temperature.
Calcium carbonate and calcium sulfate scaling
occur on unheated surfaces when their
solubilities are exceeded in the bulk water .
Metallic surfaces are ideal sites for crystal
nucleation because of their rough surfaces
and the low velocities adjacent to the surface.
• Scale control can be achieved through
operation of the cooling system at
subsaturated conditions or through the use of
31. • Calcium bicarbonate is present in all cooling
waters. At higher temperatures and pH the
bicarbonate decompose to calcium carbonate
and carbon dioxide. Calcium carbonate is highly
insoluble in water and precipitates at the hot
spots of the heat exchanger forming a dense
• Calcium sulphate does not pose much of a
problem because of its higher solubility. This
solubility is the basis of scale control by acid feed.
Adding sulfuric acid, replaces the alkalinity with
sulfate ions enabling operation at higher cycles of
concentration without exceeding the carbonate
• Fouling occurs when insoluble particulates
suspended in recirculating water form
deposits on a surface. Fouling mechanisms are
dominated by particle-particle interactions
that lead to the formation of agglomerates.
• Foulants enter a cooling system with makeup
water, airborne contamination, process leaks,
and corrosion. Most potential foulants enter
with makeup water as particulate matter, such
as clay, silt, and iron oxides.
• Deposit accumulations in cooling water systems
reduce the efficiency of heat transfer and the
carrying capacity of the water distribution
system. In addition, the deposits cause oxygen
differential cells to form. These cells accelerate
corrosion and lead to process equipment failure.
Deposits range from thin, tightly adherent films
to thick, gelatinous masses, depending on the
depositing species and the mechanism
responsible for deposition.
• Deposits are broadly categorized as scale or
• Dispersants are materials that suspend particulate
matter by adsorbing onto the surface of particles and
imparting a high charge. Electrostatic repulsion
between like-charged particles prevents
agglomeration, which reduces particle growth. The
presence of a dispersant at the surface of a particle
also inhibits the bridging of particles by precipitates
that form in the bulk water. The adsorption of the
dispersant makes particles more hydrophilic and less
likely to adhere to surfaces. Thus, dispersants affect
both particle-to-particle and particle-to-surface
• The most effective and widely used dispersants are low
molecular weight anionic polymers.
36. Cooling Tower Silica Levels
• The safe levels of silica in cooling tower would
depend on following factors
* chemistry presence of magnesium silicate,
magnesium , calcium, iron and aluminum
scaling ions in the water
37. Complexities of pH and temperature balancing - limitations
• Temperature factor
Silica shows normal solubility characteristics. Its
solubility increases proportionally to
temperature, in almost a straight line relation, in
contrast, magnesium silicate exhibits inverse
solubility. While silica solubility rises with
temperature, at say a typical cooling tower water
temperature of 30 degc, the solubility of silica is
about 120-130 ppm. This fixes the silica level as
far as the temperature is concerned.
38. pH factor
• pH factor
Silica scale formation is usually favored at a pH level of less than 8.5. Whereas
magnesium silicate scale forms at pH greater than 8.5. Silica solubility is mostly
independent of pH between 6-8 range and then it increases. This pH region has
minimum solubility of silica and maximum rate of silicic acid polymerization.
Between 6-8 pH the solubility of silica is nearly 130-140 ppm. Operation at a high-
pH does not help in presence of magnesium , calcium and other scaling ions. A pH
more than 8.5 results into very high precipitation magnesium silicate if high levels
of magnesium ions are present or in calcium carbonate calcium phosphate
precipitation which are very common. Silica precipitation can also be intensified by
the presence of metal ions such as iron and aluminum. These limitations restrict
water pH at 8 and the silica level at 130-140 ppm.
Generally, in a cooling tower operating at a pH level of less than 7.5, soluble silica
should be maintained below 150 ppm (as SiO2). For a pH level higher than 7.5,
soluble silica should be maintained below 100 ppm (as SiO2). One should take Mg
ions levels into account at a pH level greater than 7.5.