Treating your Cooling Water Tower with GeoSIL

Cooling Tower operators are under increasing pressure to meet ever more stringent

standards of management and control. There are four problems generally associated with cooling towers and their ancillary systems: Corrosion, Scale, Deposition and Microbiological growth. All these must be dealt with effectively if the system is to remain clean, efficient and problem free.

Conventional wisdom suggests using an inhibitor, alternating biocide and a dispersant, together with a clean and disinfection using sodium hypochlorite twice per year. Recent work has shown that this standard treatment is not effective in removing Biofilm, a bacterial matrix that forms on the surfaces of the cooling water system. Biofilm is present in 90% of water systems and can harbour Legionella. This problem is aggravated by the presence of corrosion deposit, scale or other debris in the system.

GeoSIL is a new approach to treating cooling water in industrial and commercial premises, where the biocidal effect of the GeoSILcoupled with the strong dispersing properties of the flocculant removes biofilm and other debris (corrosion deposits, scale and silt). The removal of this surface deposit allows a cost efficient, environmentally friendly polysilicate based inhibitor to work in soft water systems, or a threshold treatment to work in hard water systems.

The Eternal Quadrangle - The Individual Problems

q       Microbiological

q       Corrosion

q       Scale

q       Deposition

This section looks at the individual problems experienced in a cooling water system always bearing in mind that none of these problems occur in isolation but that any of the individual problems can be the starting point for the rest.

Microbiological

The understanding of the microbiology of a cooling water system has increased dramatically over the past twenty years.

Free bacteria rarely present any real problems to conventional biocide treatments and the concern today is what is happening on the various surfaces in the system. The development and maturing of biofilm on the surfaces of a cooling water system holds the key to Bacterial and Legionella control in cooling systems.

It has been proved that Chlorine and bromine are not capable of penetrating biofilm. Systems with biofilm containing Legionella as part of the sessile phase, and which are disinfected using these biocides, are liable to rapid reinfection.

Biofilms develop rapidly on surfaces which provide a food source. This means that elastomers and plastics will promote Biofilm formation before metals, particularly copper. Obviously metal with a film of organic materials will promote biofilm formation.

Modern understanding of biofilm shows that it consists of a Basal Layer and a Raised Layer. The Basal Layer is only 5µm thick whereas the Raised Layer will extend into the water flow and interact with materials dissolved or suspended in the water flow.

Legionella in water systems is now known to be closely associated with adherent biofilm, which comprises numerous other bacterial species, protozoa and ciliates. Together these form a complex balanced ecosystem in which the Legionella are able to express several physiological states; as planktonic cells, as free living components of the biofilm ecosystem, and in association with amoebae, which may become parasitized by the organism. It has been shown that the presence of iron and other nutrients will influence the type of Legionella. These factors viz. the host, the food source, and the development of the biofilm will all have an influence on the efficiency of any biocide treatment used to control Legionella.

It is again obvious that what goes on at the metal surface is vital to the success of any treatment used for microbiological control in general and Legionella in particular.

GeoSIL is a next generation biocide capable of penetrating biofilm and killing amoebae.

Corrosion

Cooling water systems can be complex, comprising an assortment of metals in a variety of configurations subjected to a wide range of different conditions (temperature, flow rate, chemical concentration). It is very probable that almost every type of corrosion mechanism will be found in a very large cooling water system (eg. in a Refinery or Petrochemical Works during the system lifetime). A cross section of small systems will also exhibit the full range of corrosion problems.

There are essentially two classes of corrosion namely general wastage where the whole metal surface is affected and localized corrosion where only a small area of the metal is affected. The first class is easier to deal with; the second is more clandestine and can appear in a variety of different guises.

Metal surface condition is key to corrosion protection and this is something which has rarely been taken into account by the bulk of the water treatment industry.

The main aim of any cooling water treatment programme must be to give the inhibitor the maximum chance of performing well i.e. the metal surface must be kept as clean as possible to maximize the ability of the inhibitor to reach the surface and protect the metal. If clean surfaces can be achieved a relatively inefficient inhibitor can offer better protection than a very efficient inhibitor will give in a system where surface deposits and biofouling obstruct the transport of the inhibitor to the surface of the metal.

In the 60`s Petrey proved that, given deposit free surfaces a polysilicate based inhibitor could perform as well as a zinc chromate based formulation.   

Scale

Scale in cooling water systems consists almost entirely of calcium carbonate. Its presence can generally be predicted from the chemical analysis of the circulating water using Langelier and Ryznar Indices. Once again any predictions based on these indices are general and many unexpected scaling problems have occurred in systems operating on soft water which have experienced a small alkaline process leak in a critical exchanger. The indices also do not take into account the roughness or smoothness of the metal surface or the presence of other surface contaminants, all of which can be critical to the initial formation and keying of the scale to the metal surface.

Scale poses a number of problems in cooling water systems, viz.:

Loss of heat transfer: This is obvious and can be critical from a process viewpoint

as in general the hotter the process the greater the tendency for scale to form on the water side leading to higher process temperatures etc. This cycle leads to condenser blockage and process shutdown on high temperature.

Resistance to flow : In the 1960's and 1970's a lot of time and attention was given to the cost of operating cooling water systems with and without surface deposits and scale. Scale effectively reduces the diameter of the pipework increasing friction losses and pumping costs. 8% to 15% of the power costs could be saved if metal surfaces were kept clean.

Poor Distribution: Scale can cause blockages and partial blockages resulting in insufficient water flowing to certain parts of the system. This will tend to reduce the overall efficiency of the system as there may be preferential cooling in certain areas. Linked to this there can also be scale deposits in the cooling tower itself which can block channels leading to tower inefficiency. Ultimately scale in the tower packing can lead to packing collapse

Treatment Absorption: One feature of Scale is its ability to absorb other treatment chemicals. This can increase the cost of a particular treatment. and render certain biocide treatments ineffective.

Micro environments: Scale in cooling water systems can be associated with corrosion deposits, adventitious deposits and biofilm. It can therefore be responsible for protecting certain bacteria from biocide treatment. It can also in certain situations lead to under deposit corrosion.

It can be seen from the above that it is not possible to control scaling in a cooling system by controlling the Langelier or Ryznar Index. The most cost efficient method of controlling scale is to use threshold treatment chemicals which prevent the regular build-up of crystals and the formation of scale on a metal surface. The main advantage of threshold chemicals is that they are very cost efficient. Threshold chemicals backed up with a chemical treatment which would keep the metal surface clean would provide the ultimate scale control programme. In general when a scale control programme is being used a corrosion inhibitor will not be required. Once again control of scale depends to a large extent on controlling surface conditions and the key to successful scale and corrosion control must be to keep the surface of the metal clean

Deposition

We have already seen the importance of keeping metal surfaces clean from corrosion products and scale. It is fairly obvious that every attempt should be made to keep surfaces free from adventitious solids.

Suspended Matter can get into the cooling water system in a number of ways.

q       The cooling tower acts as an air scrubber in which any solids present in the air will be transferred into aqueous solution

q       Debris left behind during the construction phase can be picked up by the water flow and perhaps transferred to a more critical part of the system.

q       Process leaks can produce solid material on the water side. This would be true in situations where there is an oil or hydrocarbon leak.

q       Airborne material can enter the tower sump.

q       Algae which can form in the upper well lit areas of some large cooling water towers can fall down under its own weight contributing suspended solids to the circulating water.

The composition of material found on the metal surface of any cooling system will be extremely variable. In addition to the rust/corrosion/scale deposits likely to be found there may also be a mélange of silt/and an assortment of organic and inorganic debris. It almost certain that there will be some microbiological activity associated with any such deposits and this will be discussed in more detail in the next section.

There are a number of problems associated with deposition. Severe deposition will ultimately lead to blockage or poor distribution and as it is likely to take place in low flow areas it is important that such areas do not coincide with situations where design heat transfer conditions are critical to the process.

In general most solids in the water end up in the tower sump which effectively acts as a settlement tank.

Deposits on the metal surface can promote under deposit attack by causing differential aeration conditions on the metal surface.

It is this type of attack coupled with biofouling which can create complex conditions on the metal surfaces in cooling water systems. Deposits can provide the ideal habitat for microbiological growth in that they can often provide the food as well as the cover from biocides.

Once again we have arrived at a situation where the metal surface and the complex interactions which take place there are critical to the integrity of the system from a corrosion/scaling/deposition/microbiological viewpoint. If cooling water surfaces could be cleaned and maintained in a clean condition most of the problems associated with industrial water cooling systems would disappear.