POWER November 2012 - 64

PLANT DESIGN
Primary atomization of the feedwater
used to attemperate the steam is produced by
the nozzle design and geometry within the
desuperheater and the pressure differential
between the cooling water and the steam.
Together with the University of Eindhoven
in The Netherlands, Tyco Valves & Controls
commissioned a joint research project
to develop theoretical modeling of primary
atomization using computational fluid dynamics
(CFD) analysis and laboratory laser
diffraction to analyze water droplet size upon
discharge from the desuperheater.
The study examined two nozzle designs,
spring-loaded and swirl nozzles. Initial results
have identified that when operating at
25 bar (263 psi) with a 0.05 mm lift and Kv
0.047 (Kv is a function of the nozzle design,
=
and it relates the flow through a nozzle as a
function of the fluid properties and the pressure
drop across the nozzle), spring-loaded
nozzles produce droplet sizes of 87 µm (the
diameter of a human hair averages 30 µm).
The same calculation for swirl nozzles at 25
bar, Kv
The results of the Tyco desuperheater research
study demonstrate that swirl nozzle
designs offer enhanced performance and maximum
use of water pressure drop for atomization
in the shortest possible length. Optimized
spray injection angles of swirl nozzles allow
equal temperature distribution within the pipe
and provide the highest turndown ratio using
mass flow control, rather than pressure control.
Having no springs or moving parts within the
nozzle, and no pressure drop and cavitation in
the control valve, maximizes the operational
life cycle of the swirl nozzle design compared
to spring-loaded nozzles.
= 0.043 resulted in droplet sizes of
27 µm-a factor of two to four times smaller
than spring-loaded nozzles, depending on the
operational pressure range. Basic engineering
guidelines indicate that the smaller droplet
sizes will evaporate faster and provide
better desuperheater controllability.
Using this data, Tyco analyzed the secondary
atomization characteristics evident
when the speed differential and drag forces
between the cooling water and pipeline media
cause the droplets to split into smaller
sizes. By measuring the speed differential
of the two nozzle designs, Tyco is able to
define which nozzle achieves higher speeds
and therefore faster secondary atomization.
Optimum atomization will result in frictional
forces breaking the droplet size, which results
in complete mixing and true temperature
control and measurement.
4. Small droplets desired. This is a
laboratory image of droplet size analysis. The
size of the water droplets flowing into the
steam determines the rate of water atomization
and steam temperature. Source: Tyco
Valves & Controls
Improving Desuperheater Design
The next generation of combustion turbines,
HRSGs, and steam turbines will be able to
operate at final steam temperatures that are
projected to reach 1,150F. As steam temperatures
rise, the need for tight steam control
also increases, and multiple design solutions
are required, depending on the specific
modes of operation expected from the plant.
At these higher steam temperatures it is imperative
that potential weak points in a steam
temperature control system are identified before
it is installed in the field. That means relying
on modern engineering techniques, 3-D
design simulations, and the use of CFD and
finite element analysis (FEA) tools.
For example, using CFD and FEA allows
effective spacing of the nozzle openings to
prevent areas of high stress. These tools can
then be used to confirm whether the spray
water nozzles are designed at the optimum
angle for the shortest evaporation time and
the reduction of cold spots on the inner steam
line, which could lead to pipe cracking. Understanding
the stress of higher cycling on
the attemperator system and desuperheater
nozzle units helps engineers to avoid future
mechanical stress-related failures and maximize
system life.
CFD modeling techniques also enable
close examination of the droplet distribution
within the pipeline from both probe-style and
circumferential desuperheaters. This identifies
which design offers more equal droplet
distribution between the hot steam flow and
cooling water and therefore faster and more
effective evaporation. Using CFD analysis
provides greater understanding of attemperator
system design and how altering desuperheater
geometry and spray nozzle angle can
improve droplet evaporation and minimize
impingement on the pipe wall.
FEA offers particular advantages during
the design and engineering phase of the
project by analyzing the heating and cooling
cycles of critical desuperheater components.
Attemperator components in the " hot zone "
are at increased risk of thermal fatigue and
shock. Using an FEA program, Tyco can
identify where a crack may appear in, for example,
the desuperheater nozzle and predict
the potential failure point over the service life
of the product.
Taking its steam temperature control testing
further, Tyco has carried out thermal fatigue
cycle analysis on its desuperheater nozzle injection
units in two material types-F91 and
Inconnel 718-at steam temperatures up to
1,150F, water temperature of 307F, and up to
10,000 thermal cycles. These analysis tools
have allowed Tyco to improve the design geometry
and metallurgy of its severe service
desuperheaters, which then allow engineers
to produce a design that will minimize stress
points and optimize the design and engineering
characteristics of the attemperator system
(Figure 5).
5. TempLowHT flow path. TempLowHT incorporates a spraywater control valve located
outside the heat-affected zone, reducing the risk of thermal shock to critical components.
A single probe provides water droplet atomization through a series of nozzles located parallel to
the steam flow. Source: Tyco Valves & Controls
Control
shaft
Piston
Uninsulated
atmospheric
conditions
Steam flow
conditions
TempLowHT
64
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Piston
TempLow
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POWER November 2012

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