POWER February 2014 - 38

InstrumentatIon & Control
for the extra flow capability.
Consider a boiler drum with a 3-minute
storage between the normal level and the lowlow
level in a plant that is operating at full
load. If the drum level dropped to the low-low
level due to transient conditions, in order to
restore drum level in a short period of time
(for example, 15 minutes) without decreasing
load, the feedwater pump and the associated
control valve will be expected to handle
20% flow above normal load operation (since
3 minutes x 100% = 15 minutes x 20%). On
the same basis, if the drum level is to be restored
over a longer period (for example, 30
minutes), then the feedwater pump and the associated
control valve would need to handle
only 10% flow above normal load operation.
This additional surge capability is built into
the control valve normally operating at 80%
of full travel and the pump's design point being
above the 100% operating point.
Boiler Makeup During High Drum Pressure
Condition (Pressure Safety Valve
Discharging). The ASME Boiler & Pressure
Vessel Code, section I, paragraph PG-61.1 requires
that the source of feeding shall be capable
of supplying water to the boiler at a pressure
of 3% higher than the highest setting of any
safety valve. Under the conditions mentioned
in the code, the increase in drum pressure reduces
the pressure drop available to the drum
level control valve. This reduces the feed flow
to the drum, and the control valve is required to
open further to compensate, if possible.
If the control valve is already fully open
and unable to compensate for the flow reduction,
then care should be taken that the reduced
flow rate does not decrease to a point
where it falls below the pump's minimum
flow recirculation point; otherwise, all flow
will be diverted to recirculation, and no flow
will reach the boiler drum. Such a condition
is unacceptable, as it would violate the
ASME code requirement.
A Graphic Example
These design conditions and the control
valve pressure drop variation can be well
represented on a graph similar to the generic
one shown in Figure 1, which includes the
boiler feed pump (constant speed) head-flow
curve, system resistance curve, and the control
valve pressure drop (dPcv).
As flow increases, the available pressure
drop across the control valve decreases. As
a result, the control valve opening increases.
The increase in control valve opening is,
however, restricted to around 80% to 85%
(due to controllability considerations), and
the corresponding Cv establishes the maximum
flow capability of the control valve.
Figure 1 also shows that as the pressure
head increases due to high drum pressure
38
(highest set pressure of pressure safety valve
plus 3%), the system resistance line moves
upward and cuts back on the available pressure
drop across the control valve. At this
point, the control valve opening increases,
reducing the valve dP to compensate for the
increase in pressure head.
The result is that the operating point
moves to the left side of the head-flow curve.
At this point, it is important that the control
valve opening remains within its operating
range; otherwise, the drum level may not be
controllable. It is also critical that this operating
point falls on the right hand side of the
minimum recirculation flow line; otherwise,
the entire flow through the feedwater pump
will go toward recirculation, and no flow will
reach the drum.
Specific Design Considerations
Some of the specific operating cases and functionalities
of the drum level control valve that
also need to be considered are discussed below.
Combined Cycle (2 x 2 x 1) Plant Operating
in Single Train (1 x 1 x 1). In the case of
a 2 x 2 x 1 plant-assuming one high-pressure/
intermediate-pressure (HP/IP) boiler feedwater
pump per heat recovery steam generator
(HRSG)-the HP pressure could operate at 135
bar. However, this HP pressure could decrease
significantly to 85 bar during 1 x 1 x 1 operation
(50% steam turbine load). Under these conditions,
the pump continues to operate along the
pump characteristic curve, but due to lower
pressure in the drum, the control valve closes
and has to take a high-pressure drop.
The control valve pressure drop could be as
high as five to six times the normal pressure
drop. At this point, the question to be addressed
is whether the control valve remains within the
regulating range when it closes and experiences
the high dP during 1 x 1 x 1 operating conditions.
If at this high dP the control valve is out
of regulating range, then a different base dP for
the control valve must be selected.
Combined Cycle Bypass Spray Operation.
On a steam turbine trip, the HRSG
steam is bypassed (usually 70% to 100%) and
the bypass sprays are placed in service. The
HP-cold reheat (CRH) bypass valve spray
water is often taken from the IP section of the
HP/IP pump. However, in some cases, the IP
section spray water pressure may not be high
enough, and the spray water is taken from the
HP discharge section of the pump. In either
case, the spray water requirements enhance
the flow capacity of the HP or IP section of
the pump, depending on the location of the
takeoff spray water line.
Consider the case of added capacity when
spray water is taken from the HP discharge
section and assume that the HP section flow
has to be upgraded by 20% to include the bywww.powermag.com
pass
spray water flow. This 20% additional
capacity can be considered as spare during
normal operation and can be utilized, if required,
for drum level makeup (from low
level to normal level), for example, in a 15minute
time period.
Therefore, the drum level control valve
should be suitable for handling the 20% additional
flow with a significantly reduced
pressure drop as projected by the difference
between the pump curve and the system resistance
curve. In other words, under these
conditions the control valve will open wide
but must remain within the regulating range
of the control valve.
Combined Cycle Part-Load Operation
at 75% Load. In the case of part-load operation
with combustion turbine generators
at 75%, the HP feedwater flow can decrease
to around 60% of normal flow while the system
head could decrease by around 75% to
80% (due to lower HP steam outlet pressure).
With a constant speed feed pump, the lower
feedwater flow along with the drop in system
head requires the control valve to absorb
the additional pressure drop and, as a result,
the control valve tends to close. The extent
of valve closure should be verified to ensure
that the control valve remains within the controlling
range under these conditions.
Valve Pressure Drop and Cavitation.
High-pressure drop across the control valve
(especially that experienced during commissioning
and startup) can lead to cavitation,
which can destroy the valve trim in a short
period of time (within a few days of operation).
Therefore, power stations have typically
used a two-valve arrangement working in split
range operation. The smaller sized valve (with
anti-cavitation trim) is used for startup conditions,
while the other, larger valve (without
anti-cavitation trim or minimal anti-cavitation
trim) is used for higher-load operation.
Alternatively, a single valve with a characterized
disc stack can be used to handle the
wide range of operation. The bottom of the
trim provides low Cv values and provides
anti-cavitation features, but at higher valve
openings the anti-cavitation features decrease
and resemble a standard trim.
Pre-operational Procedure. Note that
before commissioning the boiler there are
several pre-operational procedures, such as
boiler fill, chemical cleaning, passivation,
steam blow, and startup. For each step, the
boiler drum must be supplied with feedwater,
and this procedure should be clearly established
up front.
Usually, the boiler is filled using the boiler
fill pump because low drum pressure and low
feedwater flow is outside the range of the
feedwater control valve. But some of the other
pre-operational steps may require the use of
POWER | February 2014
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POWER February 2014

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