Reference Manual

18−2
Boiler Feedwater System
The BFW system begins at the DA and ends at
the inlet to the economizer. The main components
are the DA, the boiler feed pump, and the high
pressure feedwater heaters. The main purpose of
the BFW system is to condition the feedwater for
entry into the boiler. The DA removes unwanted
oxygen from the feedwater, which, in turn,
prevents corrosion in the entire piping system. The
boiler feed pump raises the pressure and the high
pressure feedwater heaters raise the temperature
of the feedwater. The critical valves within the
boiler feedwater system are the boiler feedwater
recirculation, feedwater startup, and feedwater
regulator valves.
Feedwater Recirculation Valve
In order to protect the feed pump, there must be a
recirculation system. The boiler feed pump
recirculation valve takes feedwater from the boiler
feed pump and recirculates it to the DA. It is there
to protect the pump from cavitation and excess
temperature rise. There are three basic methods
of providing feed pump recirculation. Two older
methods are continuous recirculation and on/off
recirculation. The current method is modulating
recirculation. This provides minimum recirculation
flow to protect the pump and optimize efficiency. It
requires a high technology recirculation valve. The
recirculation valve typically experiences cavitation
and if not properly taken into account with valve
selection, cavitation damage will result. Because
of the cavitation, tight shutoff is required. Any
fluids leaking past the valve will cavitate and
cause damage to the seat. A leaking recirculation
valve can cause decreased unit capacity, reduced
efficiency and repeated maintenance and repair
cost. Plugging can occur if feedwater is not clean.
A common issue with all feedwater applications is
corrosion due to materials chosen. Amine or
hydrazine treated feedwater is corrosive to the
cobalt binding in alloy 6. If the feedwater is
treated, use of this material should be avoided.
Design Considerations:
D Cavitation
D Tight shutoff (Class V)
D Typical process conditions are 800-1200 psig
and 200-400 °F
Typical Specifications:
D easy-e, HP, EH, or Cavitrol IV trim
D Cavitrol III Trim
D HTS1 option with improved pressure balance
seal
D FIELDVUE digital valve controller with low
travel cutoff
Optional:
D NotchFlo or Dirty Service Trim
D Protected inside seat
Feedwater Startup and Regulating
Valves
The feedwater startup valve is used to initially fill
the boiler. Depending upon the design, this can be
through the main feedwater pumps or the
condensate pumps. The valve transitions
operation to the feedwater regulator valve, or
variable speed drive, once drum pressure has
been built up. During drum fill operation, the boiler
is under minimal pressure. This causes the entire
pressure drop to be taken across the feedwater
startup valve. Because of this, the formation of
cavitation becomes a concern. Sizing of the
startup valve must be done in combination with
the feedwater regulator valve. This is to ensure
that the feedwater regulator valve does not
experience any service conditions that lead to
damaging cavitation. The most common split is
that 80% capacity in the startup valve is equal to
20% capacity in the regulator valve. Once the
transition to the regulator valve has begun, the
startup valve closes. Improper use is one of the
main issues surrounding two valve feedwater
systems. For example, the startup valve is not
being used at all and the regulator valve is being
used to perform both functions. This can be a
major problem if the boiler feedwater regulator
was not sized or selected to perform both
functions. There can also be an issue if switching
between the startup and the regulator valve is
happening too quickly. Because of the cavitation
concerns and taking the full pressure drop, the
startup valve should utilize some form of
anti-cavitation trim. Typically, in process plants,
since the pressures are not as high as power
plants, Cavitrol III trim is selected. 440C trim is
recommended for the case of treated feedwater.
For cases where one valve is performing the
startup and regulator duties, characterized Cavitrol