Areas of Concern in Cooling Water Treatment

There are five primary areas of concern in recirculating cooling water treatment: Initial conditioning, Scale, Fouling, Corrosion, and Microbiological control.  A successful cooling water treatment program must be able to address all five areas.

1- Initial Conditioning

A Proper startup of a recirculating cooling system includes chemical conditioning that provides first, a thorough cleaning of all metal surfaces, second, the formation of a passive protective film on the metal surfaces, and third, maintaining the protection of these metal surfaces.

2-Scale

Scale is probably the most widely associated problem in cooling water systems.   This term has become so generic that it is often applied incorrectly to all solid accumulations in the system.  Scale, as originally defined, forms by crystallization of a dissolved salt when its concentration exceeds its solubility.  Most scale forming compounds have the unusual property of becoming less soluble as the water temperature increases.  Because of this, scale will generally deposit at the heat of the exchange surface and water interface-the very place which should be kept scale-free for efficient heat transfer to take place.

The most common scale will usually consist of calcium carbonate.  Its ingredients, calcium and alkalinity, are present in all natural water.  For any given water composition and temperature, the tendency to form calcium carbonate scale can be predicted from the temperature, pH, calcium hardness, total alkalinity, and the total dissolved solids (TDS).  This prediction calculates when a water of known composition would be in equilibrium with calcium carbonate at the indicated temperature.  The calculation is further correlated with field experience data to give a better picture of what might be expected of a given water under the specified conditions. The final answer is referred to as the Stability Index.  A number below 6.0 indicates scale will form, and become more pronounced the lower the index number, while as the number rises above 7.0, the water is said to be increasingly corrosive.

3- Fouling

Fouling is the accumulation of solid material other than scale in a way that hampers the operation of plant equipment or contributes to its deterioration.  Examples of fouling are:

• Dirt and silt
• Sand
• Corrosion Products
• Natural Organic
• Microbial Masses
• Aluminum Phosphates
• Iron Phosphates

The most important factors that have influence over fouling are water characteristics, temperature, flow, velocity, microbial growths, corrosion, and contamination.  Most waters contain the dissolved and suspended materials that can cause a significant fouling problem under certain conditions.

As the temperature increases, the tendency to foul increases.  Since heat transfer surfaces are hotter than the cooling water, they will accelerate the tendency to foul, and become the most likely area of fouling.

At low flow rates (1 foot per second or less), fouling occurs due to natural settling of suspended material.  At higher flow rates (3 feet per second), fouling can still occur, but usually is less severe.

Microorganisms can form deposits on any surface.  In addition, corrosive or iron-depositing bacteria will cause or utilize corrosion products, which subsequently deposit as voluminous foulants.  All microbial colonies act as a collection site for silt and dirt, causing a deposit of different foulants.

Corrosion can form insoluble corrosion products that migrate and mix with debris, process contamination, or microbial masses to aggravate fouling.

Material that leaks from the process side of heat exchange equipment can cause serious fouling problems in several ways:

• Depositing as insoluble products.
• Providing nutrients for microorganisms and causing severe microbial growths.
• Reacting with scale or corrosion inhibitors to form insoluble foulants

Fouling can be controlled mechanically or by the use of a chemical treatment program.   The best method depends on the type of fouling present.  Control of fouling in a cooling water system involves three steps:

1. Prevention - Whatever can be done to prevent foulants from entering the cooling system.
2. Reduction - Steps taken to remove or reduce the volume of foulants that unavoidably enter the system.  Examples would be side-stream filtering or periodic cleaning of the tower basin.
3. Problem Control - Taking regular action to minimize deposition of the foulants in the system.  This would include the use of specific treatments for control of fouling, air bumping or back-flushing of heat exchangers.

4-Corrosion

Corrosion is by far the greatest course of failure of cooling water equipment.  It takes place to some degree whenever water, metal and a depolarizing agent such as oxygen are brought together.  Also, water inherently contains aggressive species such as chloride and sulfate.  Corrosion is a chemical process, therefore, its rate of reaction will increase with temperature.  In general, a low pH will increase the rate of corrosion, while inhibiting scale.  A high pH  will decrease corrosion and promote scale formation.  Low pH can result from gases such as sulfur dioxide being scrubbed from the air by the cooling tower; acidic process fluids being leaked in at the heat exchangers; or an overfeed of acid, if being used to control the pH in the cooling tower.

Non-chemical factors such as water velocity, or deposit formation, also play a considerable role in localized corrosion, which can accelerate equipment failures.

Because corrosion is an electrochemical process, any condition that causes one point on a metal surface to differ from another will allow a localized corrosion cell to establish.   Localized corrosion is produced by crevices, deposits of any type and local stresses.  Low flow areas can allow suspended solids to settle out, and localized corrosion can then take place under the resulting deposit.  This is one of the most common causes of localized corrosion in cooling water systems.

Another form of localized corrosion is galvanic coupling corrosion caused by the joining of two different metals in the system.  Galvanic corrosion is well known and understood, particularly in the case of copper and steel.  The areas of these different metals which form the galvanic cell will also play a part in the damage caused by corrosion- A large anodic area (more corrodible) connected to a small area of cathodic (more corrosion resistant) metal is commonly acceptable because the total corrosion is spread over a large area.  When the reverse situation is present, a small anodic area connected to a large cathodic area, corrosion will be accelerated because the corrosion is concentrated in a small area which can quickly lead to the failure of that area.

When two metals from this galvanic series are in contact in solution, the corrosion rate of the more anodic (corrodible) metal increases and the corrosion rate of the more cathodic (corrosion resistant) metal decreases.

5-Microbiological control

Microbiological Control is equally important for a cooling water program to be successful.  Not only will microbiological fouling impede heat transfer, it will also set up a localized corrosion cell.  Biological slimes can allow anaerobic microorganisms to thrive, which creates a localized corrosion cell which will result in pit type corrosion.  Pitting corrosion is the most severe type of corrosion, with the ability to create holes in equipment in a short period of time.

In evaluating different treatment programs, one must assume that good biological control will be maintained via the use of appropriate biocides.