There are several types of matter which can form deposits in Steam Power Plants.
Corrosion products on metal surfaces are a form of deposits. They can be formed and remain in place on metal surfaces and eventually become quite thick.
Corrosion products can also be detached from the metal surface where they were formed and can be carried along by the condensate or feedwater as suspended solids. These corrosion products can redeposit in other locations. Iron oxide and copper oxides formed on and released from the surfaces in a condensate and feedwater system usually form deposits in a boiler. Corrosion products can also be released from steel surfaces within a boiler and can be transported to other locations in the boiler by the circulating boiler water. Almost all corrosion products are insoluble in boiler water.
Deposits of insoluble corrosion products can often be found lying in the bottom of drums and headers. In these locations they usually do not lead to any problems. Corrosion products can also deposit on boiler tubing surfaces where they can act as thermal insulation, retarding heat transfer and thereby causing the boiler tubing to become overheated. Corrosion product deposits lead to the formation of corrosion sites for the caustic or acid attack.
Corrosion product deposits are minimized by proper control of pH, oxygen and carbon dioxide.
Calcium and magnesium (hardness) from seawater can form deposits called scale in a number of locations in power plants. The scale is a deposit of minerals that come out of solution on a metal surface.
Scale deposits can form in makeup evaporators. Evaporator brine is made up of concentrated seawater at relatively high temperature.
A scale can form in boilers when makeup water is not pure or when leaks develop in the surface condenser and contamination of the feedwater with hardness occurs. All of the contaminants present in feedwater enter a boiler where without proper boiler water treatment the elevated temperature conditions in the boiler cause the hardness to precipitate as scale.
The types of scale deposition that are usually found aboard ships include the following:
- Calcium carbonate is formed by the reaction of calcium with bicarbonates and carbonates at high temperature and elevated pH in the boiler water. Ca(HCO3)2 Heat -----> CaCO3+CO2 +H2O
- Calcium sulfate is remarkable because the solubility decreases rapidly with increasing temperature. Although it is more soluble than calcium carbonate, it can be deposited as a scale if the sulfate concentration in the boiler water is high.
- Magnesium hydroxide is also produced by the decomposition of bicarbonates or carbonates. Magnesium carbonate is not usually found in boiler scale because it is much more soluble than calcium carbonate or magnesium hydroxide. MgCO3 + H2O Heat ----> Mg(OH)2 + CO2
- Calcium silicate and magnesium silicate can be formed when the boiler water contains dissolved silicate. Many different silicate minerals have been found in boiler scales. It is not practical to discuss all the conditions that cause a particular type of silicate scale to be formed. However, like other kinds of boiler scale, silicate scales can be prevented by properly controlled water treatment.
Sludge deposits are formed in boilers by any type of suspended solids that will adhere to a metal surface. This includes suspended corrosion products which we have already discussed. Sludge deposits can also be made up of suspended solids formed by the reaction of calcium and magnesium and boiler water treatment chemicals. If excessive amounts of calcium and magnesium find their way into a boiler or, if blowdown is not adequate to remove the suspended particles from a boiler, sludge deposits can form. The reaction that forms sludge in boilers are as follows:
3CaCl2 + 2Na3PO4 —> Ca3(PO4) + 6NaCl
3MgCl2 + 2Na3PO4 —> Mg3(PO4) + 6NaCl
MgCl2 + 2NaOH —> Mg(OH)2 + 2NaCl
5CaCl2 + 3Na2HPO4 —> 4NaOH Ca5(OH)(PO4)3 + 10NaCl + 3H2O
No boiler water treatment program can completely prevent the formation of sludge deposits if excessive amounts of hardness are permitted to enter a boiler over an extended period. Proper boiler water treatment will prevent the formation of scale, prevent corrosion and minimize the formation of sludge deposits. However, chemical treatment is not a complete solution. When contamination feedwater occurs, the problem must be corrected at its source at the earliest possible time.
The causes of makeup water contamination must be found and corrected. Condenser leaks must be stopped. It is as important to increase blowdown rates when contaminated feedwater enters a boiler (to prevent high concentrations of suspended solids from building up and sludge deposits from forming) as it is to maintain the proper chemical residual in a boiler.
Because deposits can lead to both overheating and corrosion problems, it is necessary to clean boilers. In older type boilers it was possible to mechanically clean (turbine) the tubes. In most modern boilers it is impossible to gain access to all of the tubes with mechanical cleaning equipment and chemical cleaning with acids or other solvents is required.
After grease and oil have been removed by an alkaline boilout, new high pressure boilers are usually cleaned with acid to remove mill scale (iron oxide). Boilers that have become dirty in operation require periodic chemical cleaning to prevent problems. The frequency of cleaning, the choice of solvents and the methods of cleaning are determined by the amount and types of deposits to be removed.
