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	µS/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	mg/l	not specified				0.03	0.03	0.01
Boiler water
M alkalinity	French degrees	100	80	60	40	15	10	5
P alkalinity	French degrees	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