Essential Cooling Tower Treatment

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Essential
Cooling Tower
Treatment
Why Treat The Water?
•To Control Corrosion
•To Prevent Scale
•To Control Algae and Bacterial Growth
•To extend equipment life and efficiency
What’s Wrong with Water?
•Water is a natural solvent, it dissolves thing.
•Natural waters contain significant amounts of
minerals like calcium and magnesium.
•Natural waters typically contain significant amounts
of oxygen.
•Water, particularly warm water, is a great
environment for living micro-organism
What are Solids?
•Any material that is or becomes dissolved in water
is referred to as a dissolved solid, or solids.
Common Dissolved
Solids
•Calcium and Magnesium
Salts, aka Hardness
•Iron
•Copper
•Sulfates
•Phosphates
•Silica and Silicates
More about Solids
The measure of all of the solids in a sample of
water is its Conductivity.
Typical Conductivity in parts of Missouri,
Arkansas and Kansas
St. Louis
430 micro Siemens
KCMO
480
Little Rock
440
St. Joe
700
Columbia
480
Lenexa
530
Cape Girardeau
500
Jonesboro
540
Another name for Conductivity
is Total Dissolved Solids. Both
are a measure of total mineral
content, but they differ in units.
TDS is expressed in parts per
million and Conductivity is
expressed in micro Siemens
Not All Solids Are Alike
Some solids are more problematic than others. Two of the most important
solids in water treatment are Calcium Carbonate and Magnesium
Carbonate, aka Hardness
Hardness
•Is naturally occurring in most all waters
•Account of nearly 40% of the Conductivity in natural
waters in the Midwest
•Is the primary component of white scale deposits
•Becomes less soluble as water gets hotter
Cycles of Concentration
When water evaporates is leaves it solids behind. If you completely
evaporate a sample of water and then refill the container with the same
water the mineral content doubles. This is called Cycling Up.
1 Cycle
More on Cycles
When water evaporates is leaves it solids behind. If you completely
evaporate a sample of water and then refill the container with the same
water the mineral content doubles. This is called Cycling Up.
As cycles increase the amount of solids
increase.
This means:
•The Conductivity increases.
•The total hardness increases
•The pH will go up as the alkalinity increases
•The risk of scale formation increases
Still More on Cycles
In a cooling tower system, evaporation is the main means to remove
heat. This evaporation causes the water to cycle up, or increase in
mineral content.
The cycles of concentration in a cooling tower system
are limited via bleed-off. The cycles limit is based on:
•The Conductivity of the makeup water
•The Total Hardness of the makeup water
•The Total Alkalinity (pH) of the makeup water
•The heat load or temperature differential of the system
•The type of water treatment chemistry used
Bleed Off
Bleed Off is the removal of high Conductivity water in favor of low
Conductivity makeup. Bleed off generally based on TDS or Conductivity.
This is because the maximum hardness or alkalinity of a given water is
relative to its Conductivity.
A Conductivity Controller and a Bleed Valve are generally
used to control cycles. It follows this process:
•Sensor continuously measures the Conductivity of the tower water.
•If the Conductivity is higher than the maximum, the bleed valve is opened.
•Bleed off removes high Conductivity water and forces the system to
makeup low Conductivity raw water.
•Bleed off continues as it dilutes the Conductivity is the system, until the set
point is satisfied.
Scale
Scale is the formation of mineral deposits. It can occur in any
area that experiences heat transfer or evaporation. Solids
precipitate when they reach the limit of their solubility; when
there is more of a solid than the water can keep dissolved
Common Scales
•Calcium / Magnesium Carbonate - Lime Scale
•Calcium / Magnesium Phosphate
•Calcium / Magnesium Sulfate
•Calcium / Magnesium Silicate
•Silica
Understanding
Scale Control
Scale Control works by changing the scaling solids to more
soluble solids, using water treatment chemistry
Assuming that the Conductivity is under control, scale
can be prevented by:
Converting calcium carbonate to more soluble calcium
phosphates and phosphonates
Keeping calcium phosphates part or “dispersed” through
the use of polymers
Keeping calcium carbonate dispersed
Keeping the pH and alkalinity in check
Key Points of
Scale Control
Scale Control Hinges on a Few Key Issues
Understanding the mineral content of the raw
water
Proper and complete Bleed Off
Maintaining system water alkalinity in “Scale
Safe” zone
Consistent addition of scale inhibitors
Consistent addition of scale dispersants
Corrosion Basics
Corrosion is the deterioration of a metal due to interaction
with its environment. Corrosion requires metal, oxygen
and water.
In cooling tower systems, the water contains significant amounts of
air. The oxygen carried by that air reacts with the metal at the
surface and starts a corrosion cell.
Water
oxygen
Iron
The key to
preventing this is
keeping the oxygen
from interacting with
with metal.
Corrosion Control
Since we can’t remove the oxygen from the tower water,
we must create a barrier between the metal and the water
and oxygen.
There are three basic kinds of barriers
•Passive
•Precipitated
•Film Forming
Water
oxygen
Iron
The protective layer
is thin and can be
very delicate.
More on Corrosion Control
The protective barrier prevents the oxygen from
contacting the metal. Thereby preventing corrosion.
Common Corrosion
Inhibitors
•Molybdate
•Phosphate
•Phosphonate
•Zinc
•Azoles
•Nitrite
A combination of
inhibitors is usually the
best approach
Crevice and Underdeposit
Corrosion
When the metal is trapped under dirt, debris or scale it does not have
normal access to corrosion inhibitors and the bulk water. This sets up a
specially recognized corrosion cell. Generally, chemical treatment does
not prevent or cure these corrosion issues.
Hot Spots for Crevice and Underdeposit Corrosion
Under dirt collected in the sump of the tower or on tube sheet
Leaking threads
Good and bad weld joints
Under epoxy coatings
Under seals and gaskets
Other Issues in Corrosion
Aside from general corrosion, as previously described,
there are other forms of corrosion that are equally
important.
Other Forms of Corrosion
Galvanic - Corrosion between
dissimilar or incompatible metals
Stress Crack - Corrosion caused
by mechanical stress such as
vibration
Flow Related - Cativation, and
erosion
None of these forms
of corrosion can be
controled chemically
Essential Cooling Tower Treatment
Good Tower Treatment
Starts with :
Good Tower Treatment
Ends with :
Good Conductivity Control
Clean condenser tubes
Proper selection of treatment
chemicals based on makeup
water and operating conditions
Good heat transfer
Consistent Chemical Levels
for Maximum Protection
Consistent and timely
monitoring of the water
chemistry
Minimized corrosion of
piping and surfaces
Extended equipment life
Annual PM, and Cleaning
Boiler Standby / Summer Storage
Proper Boiler Standby Storage Is
•Essential for the Longevity of the Boiler
•Prevention of Corrosion during offline periods
•Preventative Maintenance, but Low Maintenance
•On going during standby
Proper Boiler Standby Storage Is
NOT
•Shutting off the boiler at the end of winter
•Draining the boiler
•Something that is forgotten once started
•Costly or Time Consuming
Wet Storage
the Technique
•
Reduce treatment levels and blowdown heavily 2 days prior
•
Increase bottom blowdown to remove mud
•
Cool and drain, then wash down and refill (optional)
•
Fill boiler to header
•
Add oxygen scavenger, phosphate and alkalinity booster
•
Fire boiler at low fire until steam begins to form
•
Close header
•
Low fire boiler every 4 to 6 weeks to keep chemicals mixed and active
Dry Storage
the Technique
•
Reduce treatment levels and blowdown heavily 2 days prior
•
Increase bottom blowdown to remove mud
•
Cool and drain, then wash down
•
Open all drain plugs and inspection ports
•
Close heater and feedwater valves
•
Using a fan circulate air through boiler to completely dry
•
Place trays of desiccate at 10# per 1000 gallons of volume
•
Seal all drain plugs and inspection ports
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