The application of water treatment chemicals to the boiler feedwater or directly to the boiler constitutes the internal boiler water conditioning process. The goal of this procedure is to control scale formation, mitigate corrosion, and maintain steam quality and purity. The required chemical balances that are selected for a given steam plant are based on a unique set of feedwater characteristics and operating conditions. These balances are maintained by means of various water quality control tests and appropriate adjustments in chemical feed and blowdown control.
Regardless of the pretreatment methods which may be used such as filtration, ion exchange softening and deaeration, the final adjustment of the boiler water chemistry must be accomplished by the use of internal treatment. When properly applied and controlled, the internal chemical treatment program will extend the useful life of plant equipment and avoid costly maintenance and repair procedures in the water and steam cycle.
IMPURITIES IN WATER
Water impurities govern, to a considerable extent, the choice and proper use of boiler water treatment chemicals. It is, therefore, important to understand the role these impurities play in the selection and application of water treatment programs.
Natural water supplies contain dissolved solids such as calcium, magnesium and sodium salts; dissolved gases such as oxygen, carbon dioxide and hydrogen sulfide; plus suspended solids like clay, silt and organic matter; all of which are detrimental to the boiler. The calcium and magnesium compounds (hardness compounds) are of particular interest since they are responsible for scale and sludge formation in the boiler. Calcium and magnesium ions must either be removed by external treatment or precipitated in the boiler to avoid direct deposition of scale on heat transfer surfaces.
Sodium salts do not cause scale or sludge deposits due to the solubility of these compounds under practically all conditions. Sodium salts, however, add to the total dissolved solids and increase the specific conductance of the boiler water.
Raw waters may be classified based in the relationship between hardness and alkalinity. Calcium and magnesium ions are responsible for hardness and, therefore, any compounds containing these ions are classified as contributing to total hardness (TH). Calcium and magnesium ions combined with bicarbonate (HCO3) alkalinity are termed carbonate or temporary hardness because these compounds breakdown upon boiling resulting on the precipitation of some of the hardness as carbonate salts (CaCO3).
Other compounds that contribute to total alkalinity (M alkalinity) such as sodium salts of bicarbonate and carbonate increase the alkalinity, but not the total hardness.
With regard to dissolved gases, oxygen is the most objectionable from the standpoint of boiler feedwater supply, since this gas is the principal cause of internal pitting-type corrosion. Carbon dioxide may be present to a considerable extent in ground waters. CO2 is a volatile gas that leaves the boiler with the steam only later to condense in the condensate to form carbonic acid; a causative agent for condensate line corrosion.
It is preferable to remove impurities ahead of the boiler by effective pretreatment rather than use the boiler as a chemical reaction vessel. The essential features of pretreatment selection include the following considerations.
1. If the boiler is intolerant of suspended sludge such as many of the modern watertube designs, the pretreatment method must reduce the feedwater hardness to zero.
2. Under boiler operating conditions where silica may be a problem such as with steam turbines, the pretreatment method should remove essentially all of this impurity.
3. If the carbon dioxide content of the steam is an important factor such as in the case of controlling steam line corrosion, the pretreatment method must reduce the bicarbonate alkalinity.
4. Where it is necessary to promote steam purity or avoid concentrating film deposits, the pretreatment method should reduce the total dissolved solids to a low level.
5. Recovering as much good quality steam condensate for recycle as boiler feedwater is always a good practice and should be encouraged in the pretreatment system design.
BOILER OPERATING PRESSURE
The selection and application of an internal treatment method is influenced by the boiler operating pressure. It is difficult to establish definite values by means of which low and high pressure boiler operation can be classified. In general, however, the following guidelines may suffice:
Low pressure: Up to 150 psi. The load is generally space heating and the percentage of condensate return is high.
Medium pressure: From 150 to 650 psi. These are power generating plants where process steam may be used and considerable makeup water is required.
High pressure: Over 650 psi. These installations are typically power generating stations operating as closed systems with a high percentage of condensate return and a small amount of makeup.
LOW AND MEDIUM PRESSURE BOILERS
In reviewing internal treatment requirements for any boiler system, it should be recalled that reducing hardness by the application of treatment chemicals within the boiler produces an equivalent amount of suspended solids (boiler sludge). Boilers operated at low pressure will tolerate more sludge than is the case at higher pressures due to the higher rates of heat transfer and ratings associated with higher pressures. Relatively thin localized deposits in high pressure boilers may cause damage or failure, whereas much heavier deposits can be tolerated at lower pressures, notwithstanding the loss of efficiency.
The use of phosphate type internal treatment chemicals for reducing the concentration of calcium hardness is common practice. This treatment method is based on the concept that calcium phosphate has a very low solubility under boiler conditions. Caustic alkalinity is maintained in the boiler water so that the phosphate reaction goes to completion and magnesium is precipitated as insoluble magnesium hydroxide. Both calcium phosphate and magnesium hydroxide show little tendency to attach to heat transfer surfaces especially in the presence of a polymeric boiler dispersant and sludge conditioner.
In recent years, important advances have been made in the development of polymeric sludge conditioners, sequestering agents and chelants. These treatments are designed to assist in the formation of a fluid, non-adherent type of sludge that shows little tendency to adhere to heat transfer surfaces. In some cases, anti-foaming agents are blended into the treatment formulation to increase steam purity by minimizing the tendency of the boiler to foam and carry over boiler solids with the steam.
In many low pressure boiler plants where most of the steam is used for space heating, a high percentage of returned condensate is available for recycle as boiler feedwater. Industrial boilers in the low or medium pressure category may furnish steam for processes where condensate is lost or contaminated. The percent of makeup required frequently determines the degree of softening and perhaps alkalinity reduction necessary.
Internal treatment in low and medium pressure boiler plants often includes the use of neutralizing amines for the control of condensate return line corrosion caused by carbon dioxide and oxygen. These include both neutralizing and filming amines. Either may be applied directly to the boiler water since the chemicals are volatile and distill off with the steam where they are carried to all parts of the steam and condensate return system. Some plants are restricted on the type and amount of amines that can be used, so the engineer is advised to check first to verify the suitability of amines for a specific application.
Dissolved oxygen is the principal corrosion accelerator in steam generating equipment. Therefore, steps must be taken to remove oxygen by mechanical deaeration, by oxygen scavengers, or both in order to prevent pitting-type attack and general corrosion within the boiler. The first line of defense is a properly functioning mechanical deaerator. Modern units are capable of reducing the dissolved oxygen level to 7 parts per billion (ppb). It is also common practice to use a chemical oxygen scavenger such as sodium sulfite or hydrazine to remove the last traces of oxygen that remain in the feedwater plus produce a slight excess residual in the boiler water.
HIGH PRESSURE BOILERS
Boiler plants operating at pressures greater than 650 psi are designed with high-purity pretreatment systems that produce makeup of demineralized water quality. In addition, these systems recover a high percentage of condensate so the makeup requirements are low.
Deposits may sometimes form on heat transfer surfaces, however, even though all indications are that the chemical balances within the boiler are being properly maintained. Such deposits may occur in locations where the rate of heat transfer is high resulting in overheating and tube failure. In this case, if a deposit is formed on the water side of a heat transfer surface, the temperature of the metal under the deposit will rise. Such deposits usually have high porosity. Boiler water that finds its way through the film will have zero circulation and, therefore, will be evaporated to dryness. This leaves a residue of all the solids present in the boiler water. Here heat is transferred to the boiler water through a film of saturated steam resulting in a condition known as film boiling, which is a gross deviation from the desired nucleate boiling condition.
Deposits that form on the heat transfer surfaces cause an increase in metal temperature and eventually lead to failure. Even excess sodium phosphate can concentrate in these films giving rise to a condition known as phosphate “hide out”.
This process of film formation is reversible in that the deposits (especially the sodium salts) may be rinsed away by the circulating boiler water. Alternatively, if this rinsing action does not follow the deposit formation, the deposit may grow in thickness. Usually, when the failure occurs the localized deposit is scoured away from the metal and may not be in evidence when the tube is examined.
Deposits or surface films in high pressure boilers may contain any constituent that is present in the boiler water. Since all boiler water contains sodium (Na) and hydroxide (OH) ions, highly concentrated solutions of sodium hydroxide (NaOH) may form. High concentrations of sodium hydroxide tend to dissolve the normal oxide protective coating on the tube surface. This exposes the metal to caustic corrosion with the corresponding liberation of hydrogen, subsequent embrittlement of the metal and ultimately failure.
The selection and application of the optimum boiler water treatment program for any given power plant is based on careful evaluation of raw water quality, pretreatment options, boiler operating pressures, process loads and steam purity/quality requirements. Success in this endeavor is manifested by extending the useful life of plant equipment, avoiding unscheduled outages and eliminating costly maintenance and repairs.