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Performance
M AT T E R S ™
Application Discussion: Attemperator
Spray Valve
Reheater
Application:
During power plant load changes, dynamic
variations in steam flow and heat rate
occur. Allowing temperature to fluctuate
during these transitions can dramatically reduce
the life span of the high-temperature steam
components, piping, turbine, and superheat
tubing. Power plants use a range of methods to
control the steam temperature, such as varying the
firing rates, the position of the burners, the amount
of air injection, and the feedwater flow rates; however,
many of these modes are relatively slow. When
undergoing plant load changes, the most common
practice to control steam temperature is to overfire the boiler, heat the steam above the required
temperature, and then spray water into the steam to
cool it down to the required temperature. The process of injecting
water into steam is called steam attemperation, and typically
water is sprayed between the superheat or reheat sections of the
boiler tubing. Superheat spray valves and reheat spray valves are
used to control the amount of water entering the spray nozzles
to manage overall temperature. Once the plant reaches steady
state loads, the plant is trimmed and the levels of over-fire and
waterspray are decreased. The SH and RH spray valves spray a
relatively small amount of water during normal plant operation,
and unless special design features are adopted, this will require
valves to throttle at very low lift, near the valve seat.
The startup is an extremely dynamic stage of plant operation.
During startup the steam temperature after the superheat and
reheat systems tends to be much higher than that during steadystate operation, which means a large amount of waterspray is
required to cool the steam to the required temperature. Therefore,
the spray valves operate at high capacity during startup. Once
the plant reaches required loads, the heat input is primarily
controlled by the burners, baffles, etc, minimizing the need for
attemperation to control steam temperature.
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577 w 05/03 w ©2003 CCI w DRAG is a registered trademark of CCI.
Reheater
Spray Control
Valve
H.P.
Turbine
I.P.
Turbine
Secondary
Superheater
Primary
Superheater
Superheater
Spray Control
Valve
Economizer
Area of Boiler
Deaerator
Feedwater
Regulator
Valve
Boiler
Feedpump
Condensate
Pump
DA Level
Control Valve
Hot Well
Figure 1: Plant Schematic
The spraywater source is the discharge or intermediate stage
of the main feedpump. The spray valve reduces the high inlet
pressure to the outlet pressure and controls the water flow. The
reheat pressure is much lower than the superheat pressure, and
the pressure drop in an RH spray valve is higher than in a SH
spray valve. These valves operate continuously, modulating
close to the seat, and erode the seat and plug, causing poor
control of the steam temperature. The resulting fluctuations
in steam temperature in the main steam line and reheat line
impact the heat rate and the life of the steam turbine. Wide
variations in the amount of spraywater during startup and
during normal operation also demand high rangeability of
these valves. Therefore, it is important to apply the proper
control valve with the right technology to ensure excellent
control under all conditions. The impact of the RH & SH spray
valves on plant efficiency makes them some of the most severe
control valves in a power plant.
Another principal application for attemperation is in turbine
bypass systems. In these systems, steam from the main
steam line is bypassed to the reheat line during startup,
load fluctuation, or turbine trip. Since the reheat pipes are
designed for lower temperatures than the main steam pipes,
the steam must be attemperated. Spraywater valves are used
for this purpose. Because turbine bypass systems are typically
closed, another requirement for this application is repeatable
tight shutoff to protect the valve and piping components
from thermal fatigue damage.
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The best fuel to use is natural gas. This is most common in
today’s boilers because of the high efficiency associated with
the gas turbine, heat recovery boiler combination. Natural gas
burns without any ash, so there is no short-term variation in
heat absorption in the superheat and reheat sections. Thus the
attemperation control is more of a continuous water flow process
at constant load, and the spray is only needed to trim for any
excess heat transfer capacity built into the boiler as a design
margin.
Attemperator Spraywater Valve Requirements
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Provide a precise amount of spraywater in response to
the control circuit demand over the entire load range for
accurate steam temperature control
Handle high pressure drop (up to 3500 psi, or 240 bar)
without damaging the trim components
Have high rangeability to handle wide flow rate
requirements
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Consequences of Attemperator Spraywater Valve Problems
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Figure 2: Plant schematic with turbine bypass and auxiliary steam
systems
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The Influence of Fuel Type
The process of steam temperature control – attemperation – is also
related to the fuel used in the boiler. Coal is the worst type of fuel
to utilize since it causes a great deal of variation in attemperation
water flow. In coal burning applications, thermal absorption
continually fluctuates in the superheat and reheat sections due to
ash that accumulates on the superheat and reheat tubes, which is
periodically blown off by soot blowers. Because the number of
tubes is fixed, heat transfer declines over a period of several hours
during ash buildup, and the steam temperature drops. Therefore,
the quantity of attemperation spraywater required to control the
steam temperature emerging from these heat exchanger sections
also decreases. When the ash is blown off the tubes, the heat
transfer and steam temperature rise, demanding a quick increase
in the attemperator spraywater flow. Depending upon the fuel’s
ash content, this cycle could be repeated as many as four to six
times per day.
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Ensure tight shutoff to prevent damaging effects to the valve
or to the downstream equipment
577 w 05/03 w ©2003 CCI w DRAG is a registered trademark of CCI.
Heat Rate Losses – Leaking spraywater valves result in
overspray, lowering the throttle steam temperature. Thus
additional heat input is required in the boiler to keep the
throttle steam temperature at set point, which adversely
affects the plant’s heat rate. A change of 34°F to 40°F (19°C
to 22°C) in mainsteam temperature corresponds to roughly a
1% change in heat rate at pressures above 1800 psi (124 bar).
MW losses - steam temperature below set point due to poor
spray water control reduces power output. A 13 F (7 C)
drop in RH temperature from a set point of 1000 F (538 C)
reduces plant MW output by 1%.
Temperature Control Problems – Poor steam temperature
control significantly reduces the life of the main steam
piping, reheat piping, and steam turbine due to thermal
stress-induced fatigue.
Figure 3: Cavitation damage on plug
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High Maintenance Costs – Frequent trim replacements or
maintenance cycles due to trim erosion damage are costly.
Production loss – Failure of spray valves or other system
components can result in plant shutdown and production
losses.
Symptoms of Attemperator Spraywater Valve Problems
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Poor temperature control and excessive leakage – Caused
by erosion of trim components and body due to excessive
velocity inside the valve
Damage of valve or piping components – Caused by
overspray of cold spraywater on hot metal parts due to poor
shutoff capability
Cracking or fatigue failures of valve stems – Typically caused
by high trim velocities, rotating forces due to turbulence,
and poor design tolerances resulting in plug vibration and
fatigue failure
Eliminating Erosion
Erosion is a function of fluid velocity cubed (erosion = kV3);
therefore, limiting the fluid velocities to acceptable levels will
eliminate problems associated with body, trim, and seat erosion.
The damage due to high velocity on the seat ring is especially
apparent when the valve modulates very close to the seat during
normal operation. Consequently, the valve trim design should
incorporate features to control the velocity of water impinging
on the seat ring, plug, and body parts. CCI’s design incorporates
three features to address trim and body erosion.
The first feature is keeping velocity inside the trim below 100
ft/s (30 m/s), as recommended by ISA, using DRAG® velocity
control disk stack technology. Velocity is controlled by passing
water through multiple torturous paths, which have several
right-angle turns. The kinetic energy of water jets exiting these
torturous paths is not sufficient to cause damage to the seat or
plug material. In addition, CCI uses Inconel 718 as disk stack
and plug material due to its high erosion resistance.
Use of isolation valves to block the control valve leakage –
Caused by the spray valve’s inability to provide tight shutoff
lead to manually closing the upstream isolation valve
Key Features and benefits of the DRAG® Attemperator
Spray Valve Solution
The combination of velocity control, equal percentage
characteristic, long stroke, and high seating force ensures that
the valve is protected from trim erosion, achieves repeatable
Class V shutoff, and maintains excellent controllability at all
flows. Following are the key features and benefits of DRAG® spray
control valves.
Features
Benefits
DRAG trim controls trim exit
velocity below 100ft/s (30m/s)
Prevents erosion of plug, seat and
valve body
Equal percentage
characteristic disk stack
provided
Reduces risk of seat and plug
erosion damage since plug
modulates farther away from seat
at low flow
High grade Inconel 718
material used for plug and
disk stack
Excellent erosion resistance
Labyrinth grooves provided
in the disk stack end plate
just above the seat
Protects seat from erosion by
reducing velocity of clearance flow
between plug and disk stack
Long stroke length
Outstanding flow control
High seating force (>500PLI)
and softer SS 316 seat ring
material provided
Ensures repeatable Class V tight
shut off
Spring energized balance
seal designed for 500F
(260 C)
Longer life of seals
Tight tolerances provided
between trim components
Allows very high rangeability and
better control on low flows
Over the plug flow
configuration provided
Eliminates chances of seat and plug
damage due to entrapped metal
particles
PER grooves provided in the
disk stack
Figure 4: DRAG® disk stack featuring
equal percentage trim
The second feature is the equal percentage characteristic of the
disk stack. The individual Cv of disks near the seat is kept very
low so that the valve plug has to open more to get the low flow
requirements for normal plant operation. This means that the
valve plug throttles away from the seat, which reduces the impact
of a high-velocity water stream on the seat ring.
% Flow
100
90
80
70
60
50
Eliminates plug vibration by
balancing pressure around the plug
40
30
20
Modified Equal %
10
0
0
10
20
30
40
50
60
70
80
90
100
% Stroke
Figure 5: CCI DRAG® valves are custom characterized to provide
premium valve performance
A conventional valve solution using a top-guided plug or cage
allows very high trim exit velocities to impinge on the cage, plug,
and seat. These high fluid velocities will cause trim erosion, thereby
degrading the controllability of the trim. Overall velocity control
below 100 ft/s (30m/s) inside the valve and the other features
incorporated in the valve trim design make CCI’s spray valve the
best for this application.
Figure 6: Labyrinth groove are provided to protect the plug and
seat
Excellent Controllability
The startup and load transition conditions in the plant may demand
a large amount of flow through the SH and RH spray, but during
normal plant operation the amount of water required can be quite
small. These varying demands for flow capacity result in a need
for better controllability in the spray valve. Since the disk stack is
composed of several disks, it is possible to use disks with different
capacities and numbers of stages. This flexibility allows CCI to
custom design a valve to fit specific process conditions at different
plant operating modes. The disk stack with relatively long stroke
length, an equal percentage flow characteristic (see Figure 5), and
very close clearance between valve plug and disk stack provides
excellent controllability to the DRAG® spray valves. In sharp contrast,
the conventional top-guided or cage valve solutions offer poor
control since these designs cannot easily accommodate the features
incorporated into DRAG® spray valves.
contact force in conventional valves
is frequently the result of a supplier
sacrificing performance for a lower
cost. CCI’s DRAG® spray valve
actuators are sized to provide at
least 500 PLI (pounds per linear
inch of seat ring/plug interface, 9
kg/mm) force on the plug to get
repeatable Class V shutoff and
to protect the seat and plug from
damage.
Elimination of Vibration
Uncontrolled velocity and the
resulting turbulence within the trim
can cause significant vibration of the
components. Vibration can lead to
the failure of components such as
the valve plug stem, valve plug, cage,
actuator accessories, pipe hangers, and weld joints. In power
plant design, pipe size is determined based on the velocity of
the water flowing through it. In most plant designs, this velocity
is limited to less than 15 ft/s (5m/s). Since valves use superior
technology and better materials, ISA recommends limiting the
velocity exiting the trim to less than 100 ft/s (30 m/s), which is
roughly six times larger than the pipe velocity. CCI’s long-term
experience in solving valve problems proves that controlling
velocity can prevent vibration and erosion. DRAG® trim is
selected following the ISA guideline of 100 ft/s trim exit velocity
to eliminate any chance of vibration and erosion.
Figure 8: PER grooves balance pressure around the plug to
eliminate vibration
Repeatable Class V Shutoff
Conclusion
One of the most important features in a spray valve is to have tight
shutoff when the valve is closed. This helps prevent a high-velocity
water stream from damaging the seat-plug interface. A conventional
top-guided plug or cage valve solution uses very low force between
the plug and the seat, which allows fluid to pass between the plug
and the seat when the valve is subjected to high shutoff pressure
differentials, causing wire drawing. The leakage will increase over
time until the unit’s efficiency is noticeably degraded. The low
CCI’s DRAG® valve provides the best attemperator spraywater
control solution by addressing the failure mechanisms associated
with this application and incorporating features to address each
of these failure mechanisms. There are thousands of satisfied
customers around the globe who take advantage of the CCI
DRAG® attemperator spray control valve to improve their plant’s
efficiency and reduce component damage and maintenance.
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The third feature is the labyrinth grooves provided in the disk stack
end plate, just above the seat ring. These grooves help to slow down
the water stream flowing through the clearance between the plug
and the seat ring and protect seating surface from any potential
erosion. In many cases these labyrinth grooves also help increase
the controllability at the minimum flow.