POWER February 2021 - 29

FUNDAMENTALS
take a portion (about 10%) of the condensate
exiting the hotwell for cooling
the pump inlet. The minimum required
flow (QB
temperature (TFW, F) after mixing will be:
QB
= [1 - e^(-m / M)] x QFW
and
TFW
, lb/min) and the new pump inlet
= T1 + [(QB
/ QFW
) x (TB
- T1
)]
where TB is the temperature of the bypass
source and T1 is the temperature of
the saturated FW in the suction pipe before
the transient.
The control system can be designed
to monitor the deaerator and pump inlet
pressures and bypass enough cold
condensate, when needed, to lower
the pump vapor pressure sufficiently
to provide a margin over NPSHr. Other
control options are possible. For example,
a simpler but less accurate system
would be to use a bypass valve with a
full open/close actuator that responds
to the same turbine signals that control
the extraction steam flow to the FW
heaters. The bypass would be a lowtemperature
and
-pressure
system,
which
allows the use of carbon steel
standard wall pipe components.
The main disadvantage of this option
is that it increases the oxygen
content of the FW. To minimize this effect,
the bypass should be designed
for only the minimum flow necessary.
Another concern is to ensure the cold
condensate injection does not cause
high thermal stresses. If this is possible,
the injection point should be
moved a safe distance away from the
pump and a static mixer installed in
the suction pipe to insure complete
thermal mixing. The mixer must not
have a high head loss, as frictional
losses will reduce the NPSHa.
Deaerator Pressure Control. The initiating
cause of the transient is the deaerator
pressure decay that results from
the loss of extraction steam. Pump NPSHa
can be increased by supplying the
deaerator with additional steam to compensate
for the loss of extraction steam.
The steam drum or the cold reheat system
can be used as the supplemental
steam source.
The control system can be designed to
monitor the internal deaerator pressure
and pump inlet pressure. A pressure reducing
control valve on the supplemental
steam supply line will only admit enough
steam as needed to maintain deaeraFebruary
2021 | POWER
tor pressure. This option does not have
the oxygenated water, thermal shock,
and pressure drop disadvantages of the
pump inlet cooling option.
The option's disadvantages are that
robust components suitable for steam
service are required. There will also be
a very high pressure drop across the
control valve requiring a severe service
valve be used. The control system will
be more complex than that required for
the inlet cooling option. For example, in
the case of a load reduction, the deaerator
pressure must eventually be allowed
to drop to the level associated with the
new load. Also, the supplemental steam
source must also be in an operating state
flow resistance. Studies have shown,
however, that for a small decrease in
pipe size, the benefit is usually greater
than the penalty.
■ Increase the Quantity of FW Stored in
the Deaerator. The mass of FW in the
deaerator stores heat. Increasing the
mass will slow the pressure and enthalpy
decay rates. Increasing the deaerator
storage capacity usually gives
more benefit than decreasing the suction
pipe size. If the storage capacity
cannot be increased, try raising the
low-water level.
■ Raise the Deaerator Elevation or Lower
the Pump. It is common practice in
new plant design to place the deaeraThe
most commonly recommended
options for preventing FW pump transients
attempt to eliminate the vapor pressure
differential between the pump and
deaerator by either cooling the FW
at the pump inlet or increasing the
deaerator pressure.
whenever the FW system is operating.
Additional Options. The following
options are reliable because they are not
dependent on the operation of a control
system. However, they can be difficult
and costly to implement into an existing
installation, or offer only modest gains in
NPSHa. Regardless, each situation encountered
will be somewhat different,
so keep these alternatives in mind:
■ Pump Specifications. Confirm the basis
for the pump NPSHr curve. Ask
the manufacturer what cavitation
level basis was used and whether a
manufacturer margin was included.
Ask the manufacturer to determine
the feasibility of pump modifications
such as adding an inducer or making
impeller material changes to increase
its resistance to cavitation. Consider
reducing the pump speed to lower
NPSH requirements.
■ Decrease the Suction Pipe Diameter.
Decreasing the mass of FW stored
in the suction pipe will create a beneficial
decrease in the residence time;
unfortunately, it increases the pipe's
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tor above the pump as high as possible
to maximize the static head at the
pump suction. Unfortunately, relocating
an existing deaerator is unlikely to
be practical.
■ Reduce the Length of the Suction
Pipe. NPSHa is increased by reducing
pipe pressure drop. Also, reducing the
pipe length reduces the volume of FW
in the pipe, which gives the benefit
of reducing the transient residence
time. Try rerouting the suction pipe to
reduce its length, number of valves,
and sharp bends. Replace horizontal
pipe runs with 45-degree downwardsloping
runs, where possible.
■ Add a Booster Pump. A low-speed,
low-head, single-stage booster pump
installed upstream of the FW pump
will increase its NPSHa. However, the
booster pump will also be susceptible
to NPSH cavitation; therefore, it
will also need to undergo a transient
evaluation. ■
-Michael F. Czyszczewski, PE
(mczyszczewski@asme.org) is a
mechanical engineer with 44 years of
design experience in the power industry.
29
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POWER February 2021

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