Cooling Tower Maintenance: Using the Langlier Saturation Index for Effective Water Treatment

The langlier saturation index (LSI) tells us whether the nature of water is corrosive and scale forming.

The langlier saturation index is given by:

I₀ = PH- PHs

PH= The actual PH

PHs= Pca +Palk + C

PHs= log factor of the calcium hardness expressed in ppm as CaCO3

Palk = Log factor of M-alkalinity expressed in ppm as CaCO3

C= Log factor of the total solids in ppm at the water temperature.

Case1:

When I₀  zero so at that case PH=PHs , A saturation equilibrium exists there is no scale formation, and corrosion attack is minimized.

Case 2:

When I₀ is positive, it means PH is greater than PHs then super-saturation of CaCO3 exists with alkalinity so scale formation takes place.

Case 3:

When I₀ is negative, PH is less than PHs, then fluid PH will be towards the acidic side can corrosion also the scale formed will get dissolved.

Common Problems in Cooling Tower:

Scale and its Control

Cooling water may contain carbonates, chlorides, and sulfates of Ca+ and Mg+ as well as salts of Na and dissolved gases like CO₂ and O_2. The tendency of cooling water to form scale or cause corrosion depends upon the balance of these constituents. Scale is formed by heating of carbonates hardness or by an increase in alkalinity.

Corrosion and its Control:

The contact of air with water passing over a cooling tower makes the cooling water rich in O2. This causes severe corrosion problems. The air may contain gases like SO2, H2S, CO2, NH3, etc. The presence of chlorides and sulfates conc arising from continuous recirculating cooling water. Hence the corrosion rate is increased with the formation of HCL and H2SO4 etc. So, it makes Corrosion control difficult by Corrosion inhibitors.

Slime and its Control:

• Application of slime control agents or chemical such as chlorine, phenolic and ammonium compounds.
• Side stream filtration.
• Improve the operating condition of the equipment