b) Scheme 2 (see Figure 27). This method may be used where there
is no feedwater heater between the combined condensate raw water makeup tank
and the boiler feedwater pump. The only feeding equipment needed is a
chemical mixing tank with a valved connection to the top of the combined
condensate raw water makeup tank. All chemicals may be fed by this scheme,
except phosphates. If phosphates are introduced into the combined condensate
raw water makeup tank, deposits of calcium phosphate will foul both the tank
and the feedwater piping.
c) Scheme 3 (see Figure 27). This method may be used if there is
a deaerating feedwater heater. The chemicals are injected into the suction
side of the boiler feedwater pump. To overcome suction pressure a low-
pressure chemical feed pump is used. All chemicals may be fed in this manner
d) Scheme 4 (see Figure 27). This method also may be used if
there is a deaerating feedwater heater. The feeding device is installed as a
by-pass around the boiler feedwater pump. It is a simple and convenient
method of feeding all chemicals, except orthophosphates. A chemical mixing
tank, combined with the pressure-pot feeder and valved piping, is used instead
of the chemical feed pump of Scheme 3. Polyphosphates should be fed
continuously unless an "on-off" boiler operation is used; then connect to
chemical feed pump.
e) Scheme 5 (see Figure 28). This method has the feed device
installed as a by-pass around the boiler feedwater regulator, it may be used
wherever it can be operated conveniently. The equipment is the same as for
f) Scheme 6 (see Figure 28). This method is similar to Scheme 4
(by-pass around the boiler feedwater pump), except that a water jet eductor is
installed to draw the solution from the che mical mixing tank and discharge it
into the suction side of the feedwater pump. All chemicals (except
orthophosphate) may be fed in this manner. Care must be taken to avoid
sucking air into the feedwater system, because air promotes oxygen corrosion.
The valve marked "A" must be shut off before the chemical mixing tank empties.
g) Scheme 7 (see Figure 28). This method is applicable to
multiboiler installations. Feedwater discharge pressure forces the chemicals
from the pressure-pot feeder to the boilers. By means of a shut-off valve in
each branch, the chemicals may be fed separately in accordance with individual
boiler dosage requirements. Phosphates should be shot-injected in this
manner, and the lines flushed out after each use.
h) Scheme 8 (see Figure 28). This method is similar to Scheme 7,
except that a high pressure chemical feed pump takes suction from the chemical
mixing tank and discharges to the boilers. All chemicals may feed
continuously. Lines should be flushed out periodically.