A BRIEF DESCRIPTION OF THEIR MEANING AND PURPOSE
Expression of Test Results – PPM (parts per million)
In water treatment, results are most commonly expressed in parts per million (ppm). Other terms commonly encountered are milligrams per liter (mg/l) and grains per gallon (gpg).
For practical purposes, 1 ppm = 1 mg/l and 1 gpg = 17.1 ppm.
The term “ppm” is unitless; that is, as long as the same units are used on both sides of the relationship, any units can be used. For example, 1 ppm can be used to express all of the following; one ounce per million ounces, one pound per million pounds or one ton per million tons. However, one ppm is not one pound per million gallons because the units are not the same on both sides of the relationship.
1 pound per 1000 gallons = 120 ppm
An item that is 99.9% pure contains 1000 ppm of impurities. A boiler that makes up 4000 gal/day of water having 100 ppm of hardness, has a potential of accumulating over 1000 pounds of scale per year.
In natural waters, alkalinity is most commonly the result of bicarbonate and carbonate ions; in treated waters, alkalinity may also be contributed by hydroxide, phosphate, silicate, and other treatment ions.
The color changes of phenolphthalein indicator, which occurs at pH of 8.3 (P Alkalinity) and bromcresol green indicator, which occurs at pH 4.2 (Total Alkalinity) are the standard reference points for expressing alkalinity.
For boilers operating up to 300 psig, the accepted alkalinity range is 200 ppm to 700 ppm, with P being 60% to 80% of Total Alkalinity.
Balances alkalinities are no guarantee of clean, trouble-free boilers. However, this is one of the important factors, together with others, that must be properly controlled if the boiler is to be kept clean.
Chloride ions, unlike other ions that enter the boiler, are extremely soluble and do not precipitate or decompose when subjected to boiler conditions. Therefore, chlorides are used as a measure of boiler water concentrations (i.e. how many times the mineral content–which stays in the boiler when steam is produced–of the raw water has been concentrated or built up in the boiler.
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 Test (Boiler and Condensate)
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 is generally read in units called micromhos, using a sample which has been neutralized with gallic acid to the phenolphthalein (P) end point. The reason for neutralization is that ions which exist above the P end point contribute disproportionately to the conductivity. The reason for using gallic acid is that it is only slightly ionized and excess gallic acid does not contribute markedly to conductivity.
For boilers up to 300 psig, the accepted limit for total dissolved solids (measured gravimetrically, not conductimetrically) is 3500 ppm. About 85% of the time, this results in a conductivity (neutralized) of 4500 micromhos.
The limit is set to minimize carryover and excessive scaling/corrosive tendencies.
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 – Boiler and 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.
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.
Sulfite is added to boilers to remove dissolved oxygen, which causes severe pitting corrosion. Theoretically, it takes about 8 ppm of sodium sulfite to react with 1 ppm of oxygen. To assure that this reaction goes to completion and that all oxygen is removed, an excess of 30 to 70 ppm of sulfite should be carried in the boiler water.
To eliminate oxygen corrosion and assure that the sulfite/oxygen reaction is complete before the feedwater reaches the boiler, sulfite should be fed as early as possible in the boiler cycle – preferably to the deaerator storage tank, feedwater storage tank or feedwater line – rather than directly to the boiler drum.