TM 5-810-15
(3) Fans will have turning vanes or inlet
(4) Boilers should not be operated at capaci-
boxes, or four to five diameters of straight duct at
ties or pressure and temperature conditions not
the inlet. The FD, PA, or Overfire Air (OFA) fan
anticipated by the manufacturer.
inlets located too close to building walls will have
(5) As previously stated, boilers with super-
splitters.
heaters are guaranteed to meet a 1 ppm steam
(4) Where the duct arrangement imparts a
purity condition leaving the steam drum. Boilers
swirl to the inlet of the air or gas, the swirl will be
without superheaters can be guaranteed to meet a
in the same direction as the fan rotation.
3 ppm steam purity limit; or in the case of some
(5) All fans will use inlet bells to insure a
low pressure, 150 psig saturated and lower, boilers
smooth air or gas flow at the fan inlet.
used for heating or similar conditions may only be
k. Fan outlet. For a minimum pressure drop,
required to meet a 0.5 percent moisture condition.
there will be three to six diameters of straight duct
These limits are not. stringent if the proper feed-
at the fan outlet.
water treatment is used and the proper equipment
incorporated in the plant design. In addition, the
3-17.
Boiler water circulation and chemical
operators must make proper use of the boiler water
treatment.
blowdown and the addition of the chemical
a. Water circulation. A description of the inter-
treatment. Steam drum internals are revised when
nal or water/steam-circulation features of water-
lower guarantee limits are stated. In fact at times,
tube boilers is listed below:
manufacturers may rely on natural separation of
(1) The limits of the capability of a boiler is
steam and water within the steam drum. In this case
determined by water circulation and the feedwater
and boiler water treatment. Boilers that do not
they may eliminate all steam drum internals except
circulate properly will rupture tubes in a very short
a dry pipe or other such collecting device.
period of time when operated at or near rated load.
(6) In referring to the proper feedwater
Such items as superheat and tube metal
treatment and operation of the boiler blowdown
temperatures as well as fire side design
and chemical treatment, attention to these items
considerations, physical limits of firing equipment
will pay off in the long run in reduced maintenance,
and air and gas fan and their physical limits are not
retention of design efficiency and minimum cost of
being overlooked.
feedwater treatment chemicals through elimination
(2) The basic design of boilers and the size
of tube deposits and steam carryover problems.
pressure and temperature range of this manual are
(7) Figure 3-16 graphically describes the dif-
at the lower end of the technology scale.
ference in the specific weights per cubic foot of
(3) The American Boilers Manufacturers
water and saturated steam at various pressures up
Association has set certain standards for boiler
to approximately 3200 psig. This chart illustrates
water limits. Table 3-8 shows the allowable
their ratio which may be considered a margin of
concentrations for boiler water. These conditions
safety. For boilers operating in the range of 150 to
are normally stated in proposals submitted by those
400 psig, depending on the boiler design, location
manufacturers. They should be considered
of the tube in the furnace or boiler area, slope, and
minimums for feedwater and boiler water
treatment. All reputable feedwater treatment
other similar conditions, the ratio of pounds of
consultants or vendors will be able to meet and
water circulated to the pounds of steam entrained
improve on the conditions required for the
and then released from the steam drum is very
operating conditions of boilers covered by this
approximately 30 to 15 to 1. This ratio decreases
manual.
quite rapidly as the operating pressure rises.
Table 3-8. Boiler Water Concentrations.
Circulation is assisted by the height of the boiler as
well as the burner heat input located at the bottom
Operating
Total
Total
Suspended
of the U-tube which acts as a thermal pump.
Pressure
Solids
Alkalinity
Solids
b. Chemical treatment.
psig
ppm
ppm
ppm
(1) Oxygen (O2) is one of the more trouble-
0-300
3500
700
300
some components of feedwater. It is readily re-
301-450
3000
600
250
moved by proper operation of the deaerating heater
451-600
2500
500
150
together with a minimum water temperature of
601-750
2000
400
100
approximately 220 degrees F leaving that heater.
751-900
1500
300
60
Frequently a chemical O2 scavenger such as sodium
901-1000
1250
250
40
American Boiler Manufacturers Association
sulfite or hydrazine is used in the boiler feedwater
Stipulation in Standard Guarantees on Steam Purity
3-25
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