Water Treatment

The objective of water treatment, combining the external treatment and internal conditioning, in one word is cleanliness—cleanliness of the wetted parts. This, in turn, facilitates the production of clean steam, which keeps the boiler, piping, and turbine protected.

External water treatment is done before water is fed into the boiler and is differentiated for a better clarity from the internal water conditioning within the boiler island.

• Water treatment is the process of making the raw water from any natural source, such as a river, well, or lake, suitable for feeding into a boiler. It converts raw water to FW.

• Water conditioning is the dosing of appropriate chemicals at proper places to the treated water to prevent damage to the internal surfaces of a boiler and make the steam suitable for a turbine or process.

Water treatment consists of the following stages:

1. Clarification (sedimentation followed by filtration)—to remove suspended solids

2. Softening or demineralization—to remove hardness and dissolved solids

3. Degasification—to eliminate CO2 and other dissolved gases

Depending on the level of impurities present in raw water and the FW quality required for the boiler, a suitable water treatment process is selected.

Deaeration. Removal of dissolved oxygen, which is a major cause of corrosion, is carried out in a deaerator before water enters the feed pumps. The deaerator is located in the turbine island in a power plant. But in a process plant or cogen plant, where there are no intervening FW heaters (FWH) between the deaerator and the boiler, a deaerator is part of boiler island and the water treatment package is not handled by the boilermakers but by the water treatment companies.

Impurities in Water

The main impurities in water are summarized in Table 4.1.

Water Terminology

At this stage it is necessary to define a few of the common terms of water technology.

Major Impurities in Water and Their Effects and Removal

подпись: major impurities in water and their effects and removalHardness is the soap lather-destroying property of water caused by soluble salts of Ca and Mg. Total hardness is the sum of concentrations of Ca and Mg salts or hard­ness, each expressed in terms of CaCO3.

Item

Description

Effects

Removal

1. Hardness

Ca and Mg salts as CaCO3

Formation of scales

S, DM, internal treatment, surface agents

2. Alkalinity

HCO3, CO3, OH as

Foaming, carryover,

S, DM, HX softening,

CaC3O3 3

Embrittlement, CO2 in steam causing corrosion in condensate lines

Dealkalization by AX

3. Free acids

HCl, H2SO4, and so on, as CaCO3

Corrosion

Neutralization with alkalies

4. CO2

Corrosion in steam and condensate lines

A, Da, neutralization with alkalies

5. SO — a

Formation of CaSO4 scales

DM

6. Cl — a

Adds to corrosive nature of water

DM

7. Na+ a

Corrosion by combining with OH

DM

8. SiO2

Scale in band insoluble deposits in T

Adsorption in high basic AX in DM

9. Iron and

Fe++ (ferrous),

Deposits in B and

A, F, lime S, CX, surface-

Mn

Fe+++ (ferric)

Water lines

Active agents

10. O2

Corrosion in B, HX, and water lines

Da, Na2SO3, corrosion inhibitors

11. DS

Foaming

S, CX by HX, DM

12. SS

Deposits in B, HX, and water lines

F

13. Oil

Excessive foaming and hence carryover

Dual media or activated carbon filtration

TABLE 4.1

Abbreviations: S, softener; DM, demineralizer; Z, zeolite; A, aeration; Da, deaeration; F, filtration; AX, anion exchanger; CX, cation exchanger; TS, total solids; DS, dissolved solids; SS, suspended solids; B, boiler; T, turbine; HX, heat exchangers. a Adds to solids.

Alkaline hardness. If the total alkalinity is greater than the total hardness, the hardness is alkaline hardness caused by HCO3, CO3, and OH. This is also known as temporary hardness or carbonate hardness.

Nonalkaline hardness. If the total alkalinity is less than the total hardness, the hardness alkalinity is nonalkaline hardness caused by chlorides, sulfates, and nitrates. This is also known as permanent hardness or noncarbonate hardness.

• Alkalinity is the concentration of alkaline salts present in water measured by titra­tion. Bicarbonate, carbonate, and caustic alkalinity are caused by HCO3, CO3, and OH, respectively.

Total alkalinity is the sum of the above-mentioned alkalinities in terms of CaCO3 and is known as M-reading.

Dealkalization is the removal of bicarbonates by ion exchange.

• Dissolved solids are the solids in water that would be left behind if all the water was evaporated.

• Suspended solids are removed by filtration, coagulation, and sedimentation.

Demineralization or deionization is the partial or virtually complete removal of the dissolved solids by treating water in both anion and cation exchangers.

• Amines are the organic derivatives of NH3 categorized as volatile and neutralizing types.

Volatile amines remain volatile in steam.

Neutralizing amines are cyclohexylamine and morphiline that are used for neutral­izing CO2 in condensate and raising pH of FW and boiler water.

Filming amines are octadecyclamines that protect condensate lines by forming a film.

• Low solids treatment for the makeup water is employing the demineralizer (DM) water so that the dissolved solids are removed entirely from the chemicals used in the boiler water conditioning.

Zero solids treatment of makeup water is employed only when volatile chemicals are used in boiler water conditioning.

The following three processes must be understood by boiler engineers in proper detail, along with the steam purity requirements of the downstream equipment and process:

1. Deaeration

2. Water and steam conditioning

3. Carryover

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