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Boiler Water Parameters

Hydrazine is a colourless liquid which is highly soluble in water. 
It is a powerful reducing agent and will reduce oxygen to form nitrogen and water, with no resulting dissolved solids. At high temperatures and pressures, the reaction between hydrazine and dissolved oxygen also forms very small quantities of ammonia. This ammonia carries over into the steam and helps to reduce the acidity of the resulting condensate as it is returned to the feedwater system. Hydrazine also reacts with soft haematite layers on the boiler tubes, forming a hard magnetite layer which then protects the tubes from further corrosion.

The alkalinity of water is a measurement of its buffering capacity. 
Alkalinity of natural waters is typically a combination of bicarbonate, carbonate, and hydroxide ions. Sewage and wastewaters usually exhibit higher alkalinities due to the presence of silicates and phosphates. Alkalinity inhibits corrosion in boiler and cooling waters. Hydrate alkalinity is a component of total alkalinity. Boiler operators must maintain relatively high hydrate alkalinity levels when phosphate cycle treatments are used to ensure the formation of softer, more easily removable deposits.

Chloride: When water is converted to steam, the dissolved solids do not travel with the steam, but are left behind in the boiler water. 
Water enters the boiler to replace the amount lost through steam 
evaporation. When this new water is converted to steam, more solids are left behind. As steam is continually produced, evaporated, and replaced with new water, the amount of solids in the boiler continues to increase. For every pound of steam generated, a pound of water must be replaced. The amount of solids in the water will have doubled when the amount of new water that has entered the boiler is equal to the amount of water that was used to originally fill the boiler. When the amount of solids has doubled, there are 2 cycles of concentration in the water; when the amount of solids has tripled, there are 3 cycles of concentration. Cycles of concentration is an indicator of the amount of solids buildup in the water. 
Chloride is chosen as the indicator for cycles of concentration because, 
1) it is always present in the makeup water, 
2) it does not change character when heated, 
3) it is not affected by chemical treatment, and 
4) like the other dissolved solids, it does not leave the water in the boiler when steam is produced. 
If the Chloride in the water doubles, all the solids have doubled. The Chloride Test is run on a sample of the raw water and on a sample of the water from the boiler sight glass. When the Chloride reading of the boiler water is 6 times the Chloride reading of the raw water, there are 6 cycles of concentration.
The chloride test is used (often in conjunction with the conductivity test) to regulate boiler blowdown. Blowdown is necessary to keep boiler solids (both dissolved and precipitated) from building up to the level where they might cause scale and carryover. Boiler water chlorides limits are set on the ideas that: The boiler should get good fuel efficiency , and, Based on both makeup and treatment dissolved solids, the total boiler water dissolved solids should not go so high that carryover, scaling and corrosion are a potential problem. The chloride test is also useful in determining the percentage of condensate return and in finding out if the condensate is contaminated by process water in-leakage or carryover.

Conductivity is a measure of the ability of water to conduct 
electric current. The ability to conduct electricity is related to the 
amount of dissolved (ionizable) solids in the water. Conductivity can also be used as a quick test for condensate contamination. For this purpose a multiple range conductivity meter (or one designed to accurately read conductivity under 100 micromhos) should be used. The conductivity test can be used as a simple and accurate method of blowdown control; but, it should be used for this purpose after a firm correlation has been established between conductivity, total dissolved solids, and chlorides.

Water hardness is composed primarily of dissolved calcium and 
magnesium compounds. Hardness is the main source of boiler scale. Feed water hardness is one of the main factors in making treatment recommendations. Where feed water is softened, the amount of hardness in the softener effluent is a measure of the softener performance and an indication of when the softener needs to be regenerated. The presence of hardness in returned condensate water is an almost positive indication of in-leakage and contamination of the condensate.

pH is a measure of the acidity or basicity (alkalinity) of water on 
a scale running from 0 to 14. The neutral point on this scale is pH 7; values below 7 indicate increasing acidity and those from 7 to 14 indicate increasing basicity. Since the pH scale is logarithmic, each change in pH value by a whole unit indicates a 10-fold increase in acidity or basicity. For example, at pH 6 there is 10 times more acidity than at 7. pH 5 is 100 times more acid than pH 7. Similarly, pH 8 is 10 times more basic (alkaline) than pH 7 and pH 9 is 100 times more basic than pH 7, and so on. pH can be measured either with an electric pH meter or colorimetrically using indicators that give different colors at different pH values. 

Boiler pH should be kept between 10.5 and 12.5. Below 10.5, there is insufficient protection from corrosion and hardness compounds are not precipitated in the desired, free-flowing form. Above 
12.5 there is the possibility of caustic embrittlement and carryover. 

Condensate pH should be kept between 8.0 and 8.6. This pH is necessary to assure complete protection from carbonic acid corrosion.

Phosphate: Phosphates are used in boilers to precipitate calcium 
hardness in a form which is readily removed by blowdown. This can be accomplished only if the phosphate levels are correct and alkalinity/pH levels are correct. To assure that the calcium + phosphate reaction goes to completion, there must be at least 20 ppm excess phosphate in the boiler. If phosphate rises much over 60 ppm, it can contribute to boiler carryover. To a small extent, phosphate also helps prevent boiler corrosion by forming a protective iron phosphate coating on the boiler metal.


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