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Characteristics of boiler feed water

Water absorbs more heat for a given temperature rise than any other common inorganic substance. It expands 1600 times as it evaporates to form steam at atmospheric pressure. The steam is capable of carrying large quantities of heat. These unique properties of water make it an ideal raw material for heating and power generating processes.
All natural waters contain varying amounts of dissolved and suspended matter and dissolved gases the amount of minerals dissolved in water varies from 30 g/l in sea water to anything from 0.005 to 1500 mg/l in fresh water supplies. Since water impurities cause boiler problems, careful consideration must be given to the quality of the water used for generating steam.
The composition of boiler feed water must be such that the impurities in it can be concentrated a reasonable number of times inside the boiler, without exceeding the tolerance limits of the particular boiler design. If the feed water does not meet these requirements it must be pretreated to remove impurities. The impurities need not be completely removed in all cases, however, since chemical treatment inside the boiler can effectively and economically counteract them.

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 fro the boiler manufacturer, who will base them on the characteristics of the boiler in question.

The following tables are extracts of recommended levels from APAVE (Association of electrical and steam unit owners), up to pressures of 100 bar for medium steaming rates and for volumes of water in the chambers sufficient to properly control the blow down rates, and from ABMA (American Boiler Manufacturers Association) in its standard guarantee of steam purity.

Working Pressure (Bar)

 

0 - 20.7

20.8 - 31.0

31.1 - 41.4

41.5 - 51.7

51.8 - 62.1

62.2 - 68.9

69.0 - 103.4

103.5 - 137.9

Feed water

Dissolved oxygen (measured before oxygen scavenger addition)

 

0.04

0.04

0.007

0.007

0.007

0.007

0.007

0.007

Total Iron

mg/l

0.1

0.05

0.03

0.025

0.02

0.02

0.01

0.01

Total copper

0.05

0.025

0.02

0.02

0.015

0.015

0.01

0.01

Total hardness (CaCO3)

0.3

0.3

0.2

0.2

0.1

0.05

not detectable

Non volatile TOC

1

1

0.5

0.5

0.5

0.2

0.2

0.2

Oily matter

1

1

0.5

0.5

0.5

0.2

0.2

0.2

pH at 25

 

7.5 - 10.0

7.5 - 10.0

7.5 - 10.0

7.5 - 10.0

7.5 - 10.0

8.5 - 9.5

9.0 - 9.6

9.0 - 9.6

Boiler Water

 

 

 

 

 

 

 

 

Silica

mg/l

150

90

40

30

20

8

2

1

Total alkalinity CaCO3

350

300

250

200

150

100

not specified

Free hydroxide alkalinity CaCO3

not specified

not detectable

Specific conductance at 25 without neutralization

mS/cm

3500

3000

2500

2000

1500

1000

150

100

Working Pressure (Bar)

 

0 - 15

15 - 25

25 - 35

35 - 45

40 - 60

60 - 75

75 - 100

Feed water

Dissolved oxygen (measured before oxygen scavenger addition)

mg/l

0.02 (Physical removal of dissolved oxygen)

Total hardness

French degrees

0.5

0.3

0.2

0.1

0.05

0.05

0.05

Oily matter

mg/l

absence

0.05

0.05

0.05

pH

 

> 8.5

Total Iron

mg/l

not specified

0.05

0.05

0.03

Total copper

not specified

0.03

0.03

0.01

Boiler water

M alkalinity

French degrees

100

80

60

40

15

10

5

P alkalinity

0.07 M

0.07 M

0.07 M

0.07 M

> 0.5 M

> 0.5 M

> 0.5 M

SiO2

mg/l

200

150

90

40

15

10

5

TDS

4000

3000

2000

1500

500

300

100

Phosphates

30 to 100

31 to 100

20 to 80

21 to 80

10 to 60

10 to 40

5 to 20

pH

 

10.5 to 12

10 to 11

Make up water

 

Softened or softened and carbonate free

Demineralized


Find information about the main problems occurring in boilers: scaling, foaming and priming, and corrosion.
Check also our general web page about boiler feed water.
Click here for more information about boiler water treatment, in particular through deaeration (deaerating heaters or membrane contractors)

References
Water treatment handbook Vol. 1-2, Degremont, 1991
Industrial water conditioning’, BeltsDearborn, 1991
http://www.thermidaire.on.ca/boiler-feed.html







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