BOILER WATER TREATMENT

Of the many uses for energy in the United States today—in industry, in transportation,

in homes and commercial buildings—the largest portion of total use is

directed toward producing steam through the combustion of fossil fuels. Utilities

account for the greatest share of this, but industrial plants also produce enormous

quantities of steam for process uses, often generating electric power through turbines

as a by-product (Cogeneration).

The treatment of water for steam generation is one of the most sophisticated

branches of water chemistry. An understanding of the fundamentals of boiler

water chemistry is essential to the power engineer who continually strives to

increase the efficiency of the boilers and steam-using equipment.

The pressure and design of a boiler determine the quality of water it requires

for steam generation. Municipal or plant water of good quality for domestic use

is seldom good enough for boiler feed water. These sources of makeup are nearly

always treated to reduce contaminants to acceptable levels; in addition, corrective

chemicals are added to the treated water to counteract any adverse effects of the

remaining trace contaminants. The sequence of treatment depends on the type

and concentration of contaminants found in the water supply and the desired

quality of the finished water to avoid the three major boiler system problems—

deposits, corrosion, and carryover.

DEPOSITS

Deposits, particularly scale, can form on any water-washed equipment surface—

especially on boiler tubes—as the equilibrium conditions in the water contacting

these surfaces are upset by an external force, such as heat. Each contaminant has

an established solubility in water and will precipitate when it has been exceeded.

If the water is in contact with a hot surface and the solubility of the contaminant

is lower at higher temperatures, the precipitate will form on the surface, causing

scale. The most common components of boiler deposits are calcium phosphate,

calcium carbonate (in low-pressure boilers), magnesium hydroxide, magnesium

silicate, various forms of iron oxide, silica adsorbed on the previously mentioned

precipitates, and alumina (see Table 39.1). If phosphate salts are used to treat the

boiler water, calcium will preferentially precipitate as the phosphate before precipitating

as the carbonate, and calcium phosphate becomes the most prominent

feature of the deposit.

At the high temperatures found in a boiler, deposits are a serious problem,

causing poor heat transfer and a potential for boiler tube failure. In low-pressure

boilers with low heat transfer rates, deposits may build up to a point where they

completely occlude the boiler tube.

In modern intermediate and higher pressure boilers with heat transfer rates in

excess of 200,000 Btu/ft

deposits will cause a serious elevation in the temperature of tube metal. The

deposit coating retards the flow of heat from the furnace gases into the boiler

water. This heat resistance results in a rapid rise in metal temperature to the point

at which failure can occur. The action that takes place in the blistering of a tube

by deposit buildup2/n (5000 cal/m2/hr), the presence of even extremely thin

ture drops through gas or water films have been shown. Section A shows a cross

section of the tube metal with a completely deposit-free heating surface. There is

a temperature drop across the tube metal from the outside metal

in contact with boiler water

development of a heat-insulating deposit layer. In addition to the temperature

drop from

deposit layer from

boiler water temperature

operating pressure, and operating conditions require that the same boiler water

temperature be maintained as before the development of the deposit layer. Section

C illustrates the condition that actually develops. Starting at the base boiler

water temperature of T

line from

by the line from T

higher than the temperature

prior to the formation of deposit on the tube surfaces. If continued deposition

takes place, increasing the thickness of the heat-insulating deposits, further

increases will take place in the tube metal temperature until the safe maximum

temperature of the tube metal is exceeded. Usually this maximum temperature is

900 to 100O

boilers, the problem is more severe: at temperatures in the 900 to 135O

732

structure of carbon steel boiler tubes, and Figure 39.3 illustrates the spheroidization

of carbon and successive changes in structure, which begin to take place

above 80O

are considerably above this critical temperature range. Water circulating(T2) to the metal(T1). Section B illustrates this same tube after theT2 to T1, there would be an additional temperature drop through theT1 to T0. This condition would, of course, result in a lowerT0. However, boiler water temperature is fixed by the0, the increase through the scale layer is represented by theT0 to T3. The further temperature increase through the tube wall is represented3 to T4. The outside metal temperature T4 is now considerablyT2, which was the outside metal temperature0F (480 to 54O0C). At higher heat transfer rates, and in high-pressure0F (482 to0C) range, carbon steel begins to deteriorate. Figure 39.2 shows the normal0F (4270C), weakening the metal. Temperatures within the boiler furnace

Deposits may be scale, precipitated in situ on a heated surface, or previously

precipitated chemicals, often in the form of sludge. These drop out of water in

low-velocity areas, compacting to form a dense agglomerate similar to scale, but

retaining the features of the original precipitates. In the operation of most industrial

boilers, it is seldom possible to avoid formation of some type of precipitate

at some time. There are almost always some particulates in the circulating boiler

water which can deposit in low-velocity sections, such as the mud drum. The

exception would be high-purity systems, such as utility boilers, which remain relatively

free of particulates except under conditions where the system may become

temporarily upse

The second major water-related boiler problem is corrosion, the most common

example being the attack of steel by oxygen. This occurs in water supply systems,

preboiler systems, boilers, condensate return lines, and in virtually any portion

of the steam cycle where oxygen is present. Oxygen attack is accelerated by high

temperature and by low pH. A less prevalent type of corrosion is alkali attack,

which may occur in high-pressure boilers where caustic can concentrate in a local

area of steam bubble formation because of the presence of porous deposits.

Some feed water treatment chemicals, such as chelants, if not properly applied,

can corrode feed water piping, control valves, and even the boiler internals.

While the elimination of oxygen from boiler feed water is the major step in

controlling boiler corrosion, corrosion can still occur. An example is the direct

attack by steam of the boiler steel surface at elevated temperatures, according to

the following reaction:

4H

This attack can occur at steam-blanketed boiler surfaces where restricted boiler

water flow causes overheating. It may also occur in superheater tubes subjected2O + 3Fe - Fe3O4 + 4H2t (1)

water salts, such as silica and sodium compounds; or it may be caused by foaming.

Carryover is most often a mechanical problem, and the chemicals found in

the steam are those originally present in the boiler water, plus the volatile components

that distill from the boiler even in the absence of spray.

There are three basic means for keeping these major problems under control.

1.

before it enters the boiler, to reduce or eliminate chemicals (such as hardness

or silica), gases or solids.

2.

or condensate with corrective chemicals.

3.

bleeding off a portion of the water from the boiler.External treatment: Treatment of water—makeup, condensate, or both,Internal treatment: Treatment of the boiler feed water, boiler water, steam,Slowdown: Control of the concentration of chemicals in the boiler water by