TECHNICAL POINTS
By clearly explaining only three key technical points a greater understanding of water treatment can be obtained and enlightenment possibly attained.
OXIDATION OF IRON
With regards to corrosion the oxidation of iron is the most important reaction in water treatment. Understanding this reaction is critical for operating a treatment program. The formation of rust falls into the category of oxidation/reduction reactions. The common characteristic of these reactions is the transfer of electrons. The oxidizing component donates electrons while the reducing component accepts electrons. Consequently there are two reactions: one oxidation and the other reduction. Both reactions occur simultaneously and not independently – they are a couplet.
The first reaction is iron being oxidized at the anodic site. The anodic site is where metal loss occurs. The chemical equation is written below followed by a simple diagram of what occurs. As the reaction proceeds a base metal iron atom, Fe+0, releases two electrons, 2e-, which are driven to the cathodic site. After electron loss the iron atom, Fe+2, becomes soluble in the water – metal loss. This soluble iron is what we are testing for to determine corrosion rates. High levels of soluble iron is a strong indication of excessive corrosion rates.
Oxidation Reaction
Diagram
Reduction Reaction
The reduction side of the reaction combines 1) dissolved oxygen -½ O2 ,2) water (H2O) and 3) the electrons from the oxidation reaction to form hydroxyl alkalinity.Diagram
Normally the soluble iron, Fe+2, reacts with the hydroxyl ion, OH- to form iron oxide – rust.
APPLICATION OF EQUATIONS
The principles of these equations can be applied to common water treatment applications. Listed below are the insights which can be gleaned from these reactions:
Theory - Removal of dissolved oxygen can effectively stop the oxidation/reduction reaction
Application – Removal of dissolved oxygen in boilerwater is critical in preventing corrosionTheory - Maintaining a high hydroxyl alkalinity concentration inhibits the oxidation/reduction reaction
Application - High pH levels inhibit corrosion in closed loop systemsTheory – soluble iron is released during the corrosion reaction.
Application – testing for soluble iron provides a direct correlation to corrosion rates
MASS BALANCE STUDY
A mass balance study is a simple analytical tool that employs testing and deductive reasoning to provide critical information concerning your system. These studies track key items as they pass thru the systems. Basically the test results are used to calculate both the theoretical and actual value. The comparison of these values is quite revealing.
A good example is tracking iron as it passes thru a boiler system. Iron tests are performed on condensate, softeners, feedwater, and boilerwater. This provides a picture of where the iron is and how it moves around the system. Even under optimal conditions there will be some minor corrosion in the condensate system and soluble iron will be released into the condensate. When this iron returns to the boiler it will promptly deposit on the tubes unless properly addressed.
A mass balance study takes these results and can make the following deductions. Typical testing shows 0.02 ppm of iron in both the feedwater and condensate receiver. Operating at 40 feedwater cycles the boilerwater should have a theoretical iron level of 0.8 ppm. If our testing results are under 0.8 ppm the iron is baking out onto the tubes – a negative balance. If iron results are over 0.8 ppm iron is being removed from the boiler – a positive mass balance.
Another good example would be calcium in cooling towers. Operating at four cycles on Lake Michigan water calcium levels should be 340 ppm. If calcium levels in the tower water are at 340 ppm then all our incoming calcium has been accounted – a neutral balance. Over 340 ppm indicates we are removing some existing calcium deposit – a positive balance. Conversely the lower the calcium levels are under 340 the greater amount of deposition is likely occurring – a negative balance.
The mass balance studies provide us with critical information on the effectiveness of the treatment programs but more importantly assures optimal results for the customer – our ultimate goal.
PH
Understanding the concept of pH and more importantly its ramifications is critical to all water treatment programs. Listed below is a summary of the key theoretical points:
These concepts provide the theory however the bottom line is the practical results. Listed above are standard applications along with their pH implications.
Boilers
Lowering carbonate/bicarbonate alkalinity in feedwater directly reduces consumption of condensate treatment.
High boilerwater pH passivates metal surfaces and drives phosphate/hardness reaction.
Excessive hydroxyl alkalinity levels can generate carryover critical in turbine plants.
Correct condensate pH not only prevents corrosion of the condensate system but also reduces the amount of iron returning to the boiler. Condensate corrosion is the major source of iron in boiler deposits. Preventing iron transport is key.
Cooling Towers
As pH levels increase over 9.0 scale deposition will result. Calcium mass balance studies will indicate scaling conditions.
pH levels below 8.0 normally have ferrous metal corrosion – that can be checked.
pH levels over 8.3 preclude the use of sodium hypochlorite – the higher pH degrades the hypochlorite into chlorite – an ineffective biocide. This is a common problem.
Closed Loop
Higher pH levels passivate metal surfaces reducing corrosion.
Molybdate based inhibitors function more effectively at higher pH levels
Elevated pH inhibits microbio growth.