POWER February 2014 - 36

INSTRUMENTATION & CONTROL
Establishing Proper Pressure
Drop for Feedwater Flow Control
Valves
I
n power plants with drum-type boilers and
constant-speed main boiler feed pumps,
the feedwater control valve (also referred
to as the drum level control valve) provides
the means for controlling flow to the boiler.
On the other hand, in power plants equipped
with variable-speed turbine-driven main boiler
feed pumps, the feedwater control valves are
usually eliminated from the main circuit but
may still be used on the startup circuit with the
smaller motor-driven startup feed pump.
In either application, the feedwater control
valve is in critical and severe service. As
such, it must be sized and designed to provide
adequate drum level control and cope
with varying drum pressures expected over
the range of plant operating conditions. In
this regard, one of the important parameters
to be evaluated is the control valve pressure
drop at the rated condition, as well as during
off-design conditions.
The control valve pressure drop needs to
be established carefully, as it is a performance
debit resulting from increase in horsepower
associated with the pressure head of the boiler
feed pump. Use of variable-speed drives
or turbine drives on the boiler feed pump can
avoid this debit by eliminating the control
valve altogether. Boilers designed for sliding
pressure operation generally utilize variable36
speed
drives or turbine drives not only to
eliminate the control valve penalty but also
to take advantage of minimized performance
debits at part loads due to lower pump head.
The part-load advantage is not available with
fixed pressure operation, where boiler pressure
remains constant and the pump pressure
head must remain high, even at part loads.
This article highlights the various plant
operating scenarios that must be considered
while evaluating the control valve pressure
drop. It also points to the fact that the conventional
methods used in industry for establishing
control valve pressure drop cannot be
used in power plants without reviewing all
plant operating scenarios.
Note that the difference between the boiler
feed pump head-flow curve and the system resistance
curve (Figure 1) provides the basis for
the pressure drop available for the drum level
control valve. During startup and low-load operation,
when drum pressures are low, the valve
may experience severe service due to high
pressure drop. These conditions could lead to
valve cavitation and subsequent destruction of
the valve trim along with pipe hammer, which
could lead to piping and piping support damage.
It is, therefore, essential that the sizing
and design of the drum level control valve be
such that these problems are avoided. For this
www.powermag.com
Courtesy: Bechtel Power Corp.
Feedwater control valves play a critical role in boiler operation. One important
parameter of their design is the pressure drop at the rated condition as well as
off-design conditions. However, conventional methods used for establishing control
valve pressure drop cannot be used at face value without reviewing all plant
operating scenarios.
S. Zaheer Akhtar, PE
purpose, the entire range of service conditions
should be provided on the valve data sheet, as
this will enable the valve supplier to make the
correct valve/trim selection.
Conventional Methods for
Establishing Control Valve
Pressure Drop
In general industrial applications, control
valve pressure drop has commonly been
established by one of the three methods
discussed below. However, note that these
methods may not be adequate for feedwater
control valve applications, which require additional
evaluation taking into consideration
the high static pressure head involved in
pumping feedwater to the boiler.
Traditional Method. This method traces
back to the ISA Handbook of Control Valves
by J.W. Hutchison, which provides guidance
for control valves in a pumped circuit. According
to this method, the pressure drop should
be 33% of the dynamic loss in the system at
rated flow, or 15 psi, whichever is greater. In
this context, the dynamic loss in the system is
expected to include the pressure drop of the
100% open control valve. Other references allow
the control valve pressure drop to be 25%
to 50% of the dynamic loss in the system, exclusive
of the control valve pressure drop.
POWER | February 2014
http://www.powermag.com
POWER February 2014

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