Feed-water purity is a matter both of quantity of impurities and nature of impurities: some impurities such as hardness, iron and silica are of more concern, for example, than sodium salts. The purity requirements for any feed-water depend on how much feed water is used as well as what the particular boiler design (pressure, heat transfer rate, etc.) can tolerate. Feed-water purity requirements therefore can vary widely. A low-pressure fire-tube boiler can usually tolerate high feed-water hardness with proper treatment while virtually all impurities must be removed from water used in some modern, high-pressure boilers.
Only relatively wide ranges can be given as to maximum levels of alkalis, salt, silica, phosphates etc, in relation to working pressure. The actual maximum levels must be obtained from the boiler manufacturer, who will base them on the characteristics of the boiler in question.
Dissolved impurities
Mainly the dissolved solids found in water are mineral salts. These contaminants in water exist as salts of calcium, magnesium and sodium, predominantly in the form of bi-carbonates, carbonates, chlorides and sulphates. To a lesser extent, potassium and iron salts are also present. Nitrates and silicates of such substances are also found to a small degree. Very rarely, Phosphates and a few heavy metals are also found in natural water. The quantity and composition of the solids present depend upon the soil and strata details and the origin of water.
Various gases, mainly Oxygen, and others like Carbon dioxide and Hydrogen Sulfide are normally present in dissolved form and the presence of such dissolved gases alter the composition and concentration of certain salts.
Chlorides
Presence of chloride in feed water is harmful to the system as whole. Hence it is very advisable to limit chloride concentration in feed water, keeping in mind, limit prescribed for boiler water and restriction as per drum pressure ratings. If the chloride concentration in the feed water is high enough to measure accurately, it can be used to control blow down and to calculate the rate of blow down. Because chlorides do not precipitate in the boiler water, the relative chloride concentrations in feed water and boiler water provide an accurate basis for calculating the rate of blow down. The chloride test is unsuitable for this calculation when the feed water chloride is too low for accurate determination. A slight analytical error in determining feed water chloride content will cause an appreciable error in calculating blow down rate. Leak proof condenser and efficient demineralization are essential prerequisites to avoid contamination due to chloride. Dosing chemicals fed to the drum can contribute in sizable proportion to chloride contamination if the chemicals are not of adequate quality. Very low level chloride should be tested by selective ion electrode for accuracy.
Dissolved Oxygen
Dissolved oxygen plays significant role in boiler feed water. The oxygen accelerates corrosion of water tube material. Mainly oxygen is removed only in Dearetor. Hence performance of Dearetor is very important. Deaerator performance has to be maintained limiting dissolved oxygen to less than 0.01 ppm level.
Without proper mechanical and chemical deaeration, oxygen in the feed water enters the boiler. Much is flashed off with the steam; the remainder can attack boiler metal. Oxygen in water produces pitting that is very severe because of its localized nature. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper bearing alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability.
Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems: iron oxide generated by the corrosion can produce iron deposits in the boiler. Oxygen corrosion may be highly localized or may cover an extensive area.
Organic Matter
It is very difficult to eliminate organic matter totally from water. The presence of the ionic and non ionic organic matter in the boiler feed circuit influences upon its reliable operation. The organic matter can alter the feed water and process steam quality in terms of their electrical conductivity increase and of their pH reduction, this being responsible for corrosion development and deposit formation. The organic matter presence is due to the leakage from the surface water demineralization in plants that was chemically polluted with organic products. Presence of organic matter is due to poor pretreatment practice, ineffective D.M. plant performance, resin leaking or condenser leakage. Quality of raw water at intake point is a deciding factor. Seasonal changes play a vital role.
Total Solids
TDS plays a very vital role in preventing super heater and turbine blades fouling. As steam is raised, pure water is evaporated, leaving the TDS behind in the boiler water which becomes increasingly concentrated. Initially this is beneficial since it allows the treatment chemicals to become more concentrated and offer better protection to the boiler but eventually, if unchecked, the water would become so concentrated that its surface tension increases and steam bubbles can no longer escape freely, resulting in foaming and carryover.
In order to prevent foaming and carryover, the build-up of the boiler TDS needs to be controlled by removing concentrated boiler water via blow down and replacing it with fresh feed water with a lower TDS. For most shell boilers that the TDS should be limited to a maximum 3500mg/l. However, in certain circumstances it may be necessary to maintain a lower TDS if carryover is to be avoided.
In order to control the TDS effectively, the boiler should be fitted with an efficient auto-blow down system which continuously monitors the boiler TDS and adjusts the blow down accordingly. Wherever possible, the auto-blow down system should incorporate a correctly sized flash vessel and heat recovery system which can potentially recover up to 80% of the energy in the blow down.
Suspended and colloidal impurities
Clay and sand particles constitute a major portion of the suspended matter. Very fine clay remains in colloidal state. Colloidal suspension of dye material and certain organic contents, give colour to water, in most cases.
Specific Gravity
The specific gravity of a boiler water is proportional to the dissolved solids. However, determination of dissolved solids by hydrometer measurement of specific gravity is so inaccurate that it cannot be recommended for proper blow down control.