SLUDGE OR FOULING IN COOLING WATER

SLUDGE OR FOULING

Under this heading are included dirt, mud, sand, silt, clay, scale salts, and other particulates of airborne origin or entering the system with the makeup water. Very often these suspended solids are tightly bound and cemented by corrosion products and organic matter.

1.     What is 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.

2.     What influences fouling in a cooling system?

The most important factors influencing fouling are:

§  Water characteristics

§  Temperature

§  Flow velocity

§  Microbial growths

§  Corrosion

§  Contamination

3.     How do water characteristics affect fouling?

Distilled water will not foul. However, most waters contain the dissolved and suspended materials that can cause a significant fouling problem under certain conditions.

4.     How does temperature affect fouling?

Increasing temperature increases the fouling tendency. Because heat transfer surfaces are hotter than the cooling water, they accelerate fouling.

5.     How does flow rate affect fouling?

At low flow rates typically 1 fps or less, fouling occurs due to natural settings of suspended material. At higher flow rates, 3 fps or more fouling can still occur but usually is less sensitive.

6.     How does microbial growth affect fouling?

Micro-organisms can form deposits on any surface. In addition corrosive or iron depositing bacteria 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.

7.     How does corrosion affect fouling?

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

8.     How does process contamination affect fouling?

Materials often leak from the process side of heat exchange equipment and can cause serious fouling problems in several ways.

§  Depositing as insoluble products

§  Providing nutrient for micro-organisms and causing severe microbial growth

§  Reacting with scale or corrosion inhibitors to form insoluble foulants

9.     How can fouling be controlled?

Fouling can be controlled mechanically or by the use of chemical treatments. The best method of control depends upon the type of fouling. Control of fouling in the cooling system involves three major tactics:

ü  Prevention: Whatever can be done to prevent foulants from entering the cooling system, this may require mechanical changes or addition of chemicals to clarify make-up water.

ü  Reduction: Steps taken to remove or reduce the volume of foulants that unavoidably enter the system. This may involve side stream filtering or periodic tower basin cleaning.

ü  Ongoing Control: Taking regular action to minimize deposition of the foulants in the system. This can include adding chemical dispersants and air rumbling or back-flushing exchangers.

10.  How do chemical inhibitors work?

Charge-reinforcement and wetting agent dispersants act to keep foulants in suspension, preventing them from setting on metal surfaces or helping to remove fouling deposits that have already formed. The charge reinforcement dispersants cause the foulants to repel one another by increasing the electrical charges they carry. The wetting agents make the water wetter (reduce surface tension), inhibiting new deposit formation and possibly removing existing deposits. This action keeps the particles in the bulk water flow, where they are more likely to be removed from the system, either through blow-down or filtration.

11.  What kinds of chemical are normally used?

ü  Charge reinforces – Anionic polymers

ü  Wetting agents – Surfactants

12.  What is the most important factor in reducing fouling?

Continuous control of both the chemical and mechanical programs is the only way to reduce fouling.

13.  What is Silt Density Index?

Silt Density Index is a measure of the fouling tendency of water based on the timed flow of a liquid through a membrane filter at a constant pressure.

14.  What could be the affects of Fouling on cooling water system?

Where abrasive, sludge deposits can damage pump seals and in addition to their insulating nature can also promote "under-deposit" corrosion.

The answer to the aforementioned problems created by scale, corrosion, bio-fouling and sludge is, of course, a comprehensive water treatment program comprising scale and corrosion inhibitors, micro biocides and dispersants coupled with adequate bleed off and appropriate equipment.