POWER November 2012 - 65

PLANT DESIGN
The Future of Steam
Attemperation Technology
Drawing on the results of the water droplet
study, CFD modeling, and FEA, Tyco has
developed a new circumferential steam attemperation
system for the power generation
industry. CircTemp has been designed and
engineered to improve desuperheater performance
in severe-service boiler system applications.
In developing the product, Tyco used
the advanced modeling techniques to establish
the performance characteristics and operational
parameters. CircTemp's design uses the
high-temperature, high-cycling experience
Tyco has gathered through its Narvik-Yarway
TempLowHT probe desuperheater and applies
it to the new product development.
In spring-loaded
nozzle designs, the
control valve is the
limiting factor.
Tyco started with a nozzle design that provided
good primary atomization, ensuring
that droplets would only become smaller during
secondary atomization with the shortest
possible evaporation time. This approach has
reduced water droplet sizes from 100 µ to 20
µ, delivering shorter evaporation times. The
key is the higher turndown ratio achieved by
the CircTemp desuperheater, compared with
typical spring-loaded nozzle circumferential
designs. Using mass flow, rather than a
pressure-reducing control valve, maintains a
constant pressure differential within the attemperator
system. This increases the velocity
of the water discharged from the nozzle injection
unit, producing smaller water droplets. In
spring-loaded nozzle designs, the control valve
is the limiting factor because the pressure after
the valve determines the discharge velocity.
The CircTemp design enables individual
nozzles to be shut off as the steam load fluctuates.
This means that a considerably higher
turndown ratio can be achieved by sequencing
nozzle open and closing. As the load
increases, smaller nozzles and then larger
nozzles can be opened one at a time, as the
flow requires. When less flow is needed, the
nozzles can be closed in the reverse sequence:
first the largest and then the smaller nozzles.
Tyco's study into optimum spraywater angles
also determined that cooling water entering
the pipe perpendicular to the steam flow could
impinge on the pipe wall. Optimized spray angles
and nozzle configuration creates different
cooling water spray patterns and ensures equal
November 2012 | POWER
distribution. This research into desuperheater
spraywater characteristics has influenced the
CircTemp design to prevent potential damage
to downstream pipe and liners by eliminating
cold spots on the inner pipe wall and maintaining
constant steam temperature.
Striving for higher CC plant efficiency
means higher steam temperatures and, probably,
high cycling duty over the lifetime of
a plant. Components in direct contact with
the higher-temperature steam must have the
best materials, be based on state-of-the-art
research, and integrate field operating experience
into their design and manufacture. One
of those components, the critical yet problematic
desuperheater valve, is ready today for the
next generation of combined cycle plants. ■
-Martin-Jan Strebe (mstrebe@
tyco-valves.com) is director for global
product management control valves and
Arvo Eilau is marketing manager, Tyco
Valves & Controls. As of October 1, 2012,
Tyco Valves & Controls will be officially
known as Pentair Valves & Controls.
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