UFC 3-240-13FN
25 May 2005
3-1.4.2
of minerals dissolved in the feedwater, causing the formation of one or more types of
scale on boiler system components. Deposition can occur by other mechanisms when
corrosion products (rust) enter the boiler after being created in either the after-boiler or
pre-boiler sections. These corrosion products can form iron-based scales. Using certain
water treatment chemicals can also result in the formation of scale when the chemicals
are not properly applied. All scales provide insulation to the transfer of heat between the
water and the heated metal tubes (see Figure 3-5). Scale can occur in any section of
the boiler system, resulting in a reduced capacity for heat transfer. Due to the resulting
reduced heat transfer from the fireside of the tube to the water, the metal tubes operate
at a higher temperature than if the scale were not providing insulation. Consequently,
steam is produced inefficiently due to greater fuel demands, and fuel costs are
increased. The greater the thickness of scale, the greater the insulating effect, and the
higher the temperature of the tubes (see Figure 3-6). At sufficiently high metal
temperatures, the tube can lose its tensile strength and rupture (see Figures 3-7 and 3-
8).
FIGURE 3-5. Scaled Boiler Tube
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