POWER November 2012 - 63

PLANT DESIGN
3. Probe-style SU desuperheater. This design is similar to that shown in Figure 2, but
the water flow to the desuperheater is controlled via a separate flow control valve. Source: Tyco
Valves & Controls
DCS
rect spraywater control valve stem position
setting. Thermal probes are often placed incorrectly
during unit construction, resulting
in probe wetting or water droplet impingement,
which will result in inaccurate steam
temperature measurement.
TIC
Steam flow
Whether an attemperation system is circumferential
or probe style in design, it must
be supported by robust integrated control
components and control functionality. The
placement, design, and function of temperature
probes are critical. A spraywater control
valve or valves must enable " bubble tight "
shutoff, and manual valves required for component
and system isolation should be routinely
inspected.
Most current HRSG steam attemperator
systems are designed for minimal to zero
water flow at maximum steam flow. CC
plants that are dispatched through automated
load-following management systems or automatic
generation controls will see constant
superheated steam attemperation as load is
increased or decreased to meet fluctuating
megawatt demand. This mode of operational
dispatch will stress existing design limitations
of the attemperation system.
Common system and component failure
issues associated with extreme cycling conditions
are:
■ Spraywater control valve packing leaks or
packing blowout.
■ Wetting or droplet impingement of downstream
thermal probes.
■ Nozzle spring failure.
■ Nozzle cracking or erosion.
■ Linear weld attachment (pin) cracking or
complete line failure.
■ Main steam pipe cracking.
■ Foreign object damage to the steam
turbine.
Engineering and Design
Considerations
Attemperator system components are designed
and engineered to an expected life
span, based on detailed 3-D finite analysis
computer models, operational case histories,
material composition, and expected thermal
cycles associated with each component.
Some shortened component life in the steam
November 2012 | POWER
attemperator system can be attributed to supporting
operational systems, such as feedwater
or condensate supply conditions, water
chemistry, distributed control system (DCS)
settings, or response times. These support
systems are usually designed for no or minimum
spray conditions at design or baseload
conditions for maximum efficiency.
The attemperator system installed at a
plant designed for baseload may exhibit much
different operation when cycled. A functional
field test often proves prior factory test settings
to be inaccurate. The following is a
minimum list of supporting systems and parameters
associated with the attemperator that
should be reviewed and/or inspected to minimize
the chance of downstream damage:
■ Feedwater or condensate supply pressure,
flow rate, and temperature at the spraywater
control valve during various load conditions,
or at the attemperator probe if an
integrated design is present.
■ Thermal probes, operational temperature,
and location specifications should be verified
and/or tested.
■ DCS logic settings should be consistent
with plant operation. The dead band of
the control signal should be within the required
tolerance.
■ Water chemistry should be known throughout
the steam and condensate systems under
various load conditions.
This equipment, if not originally designed
for cyclical operation, can be redesigned or
modified to better suit current operational
conditions. Often, a presumed shortage of
feedwater or condensate spray capacity can
be attributed to a logic setting in the DCS for
valve position, or for response at a predetermined
load condition.
Additionally, if the plant infrastructure has
been in service for a period of years and has
experienced a series of routine control valve
preventative and corrective maintenance actions,
operators may observe a minor, incorwww.powermag.com
Locating
the Attemperator
In addition to mechanical design and field
operations, accurately predicting water droplet
atomization is very important. However,
measuring droplet atomization in the field is
difficult. If atomization of spraywater into
the steam system is negatively affecting the
ability of the temperature probe to measure
downstream steam temperature correctly,
then severe overspray and underspray conditions
can produce increased thermal cycles
and component damage.
Thermal probe location is a first step in
verifying or eliminating probes as a possible
contributor to poor steam attemperator system
performance. Here are two general rules
for measuring and verifying the proper location
of upstream and downstream attemperator
thermal probes (in a straight pipe):
■ The upstream thermal probe should be a
minimum of five pipe diameters from the
attemperator location.
■ The downstream thermal probe should be
a minimum of 20 pipe diameters from the
attemperator location.
These rules of thumb should be used as
a quick check of an existing installation in
straight pipe and are useful in determining
if a gross error was made in thermal probe
placement. Droplet atomization calculations
can be used to determine the exact requirements
and dimensions for the piping arrangement
and thermocouple locations.
Advances in Steam
Temperature Control
Precise steam temperature control has been a
challenge for steam plant operators since coal
was first shoveled into a furnace. Today's superheat
temperatures and daily plant cycling
place extraordinary stresses on critical components.
Effective steam temperature control
is needed to protect expensive downstream
equipment, such as the steam turbine.
In a typical CC plant, precise steam temperature
control often conflicts with compact
steam piping design. That makes it difficult
to select an attemperator that can operate in
the shortest possible straight length of pipe
yet with an effective evaporation rate. This is
particularly difficult when short pipe length
is coupled with a high turndown ratio and
the desire for a flat temperature distribution
across the steam pipe.
63

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POWER November 2012

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