TM 5-810-15
d. Boiler feed pump sizing. Boiler feed pumps
Table 7-2. Specified Deaerator Information.
will be sized to deliver the desired flow and
pressure to the boilers from the DA. A 10 percent
Item
Engineering Units
flow margin for wear allowance will be included
when sizing the pump. These conditions are deter-
Maximum plant capacity
pph
mined by first defining the items listed in table 7-3.
Maximum DA outlet capacity
pph
Make-up water temperature
degrees F
Table 7-3. Boiler Feed Pumps Capacity Criteria.
Condensate temperature
degrees F
Make-up water flow
pph
Item
Engineering Units
Condensate flow
pph
Steam temperature to DA
degrees F
Steam pressure to DA prior to control
psig
Boiler steam outlet pressure
psig
valve
Boiler water side pressure losses
psi
DA design pressure
psig
Water temperature entering pump
degrees F
DA outlet water temperature
degrees F
Piping losses
psi
DA outlet water flow
pph
DA operating pressure
psig
Pump elevation relative to boiler and DA ft
Net positive suction head required
ft
7-4. Boiler feed pumps.
(NPSHR)
a. General. Boiler feed pumps convey water
from the DA to the boiler.
(1) Calculation of net positive suction head
available (NPSHA). Determining the NPSHA is an
comply with the latest revisions of Hydraulics
important design consideration for boiler feed
Institute (HI) and ANSI. A minimum of one pump
pumps because they take water from the DA at
per boiler and one backup pump will be provided
saturated conditions. To prevent cavitation of a
for all cases. The ASME Boiler and Pressure
pump operating at elevated temperatures, the DA
Vessel Code requires that coal fired boiler plants in
is elevated to increase the static pressure at the
this size range be provided with at least two means
pump suction and overcome the vapor pressure.
of feeding water. For stoker fired boilers, one
The boiler feed pump vapor pressure is equal to the
source will supply sufficient water to prevent boiler
DA operating pressure and cancel out each other.
damage during an interruption. A steam turbine
Thus, boiler feed pump NPSHA is the head of
driven pump is one method that is frequently used
water from the DA to the pump inlet minus the
to meet this requirement. Multiple pumps permit
pipe friction loss. A safety margin of at least one
backup capacity for individual pump failures or
foot of head will be subtracted from the calculated
scheduled maintenance and increase efficiency of
NPSHA to obtain the net positive suction head
pump operations at reduced loads. Multiple pumps
required (NPSHR).
are usually more cost effective for boilers subjected
(2) Discharge head calculation. The boiler
to large daily load swings. This arrangement allows
feed pump discharge head will be designed to
the pumps to operate in a more efficient range and
overcome the boiler drum pressure, valve and
gives the system more flexibility. The use of
piping losses within the boiler and external to the
multiple pumps will provide for between 50 and
boiler as well as the head of the water column.
100 percent of additional capacity beyond the
e. Pump construction. The boiler feed pumps will
expected operating loads.
be constructed to provide continuous operation for
c. Steam turbine drives vs electric motor drives.
the expected plant life. Pump manufacturers should
be consulted regarding specific features of
Steam turbine drives provide a more thermally
construction for a particular application. In general,
efficient system, but in this size range they can be
lower pressures and flows could use vertical in-line
less economical on a LOCA than electric motor
pumps with stainless steel shaft, impellers, and
drives. However, as noted above, the ASME Boiler
impeller casings. Suction and discharge chambers
and Pressure Vessel Code requires that both steam
on vertical pumps will be cast iron. For higher
turbine and motor drives be used in stoker fired
pressure and flow applications casings will be 11 to
bailer plants with capacities of 35,000 pph and
13 percent chrome steel, split on the horizontal
above. Steam turbine drives will not be used
centerline with suction nozzles, discharge nozzles
exclusively. An electric motor drive makes it easier
and feet on the lower half of the casing so the top
to bring a boiler on line from a cold start.
half of the casings can be removed without
7-3
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