Improved coating range and uniformity via X-JET Air Knife combined with Strip Stabilizer, for lower OPEX production
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Improved coating range and uniformity via X-JET Air Knife combined with Strip
Stabilizer, for lower OPEX production
Authors Names and Affiliations
Enrico Vettori, Gianluca Caporal, Danieli & C. Officine Meccaniche S.p.A., Italy.
Contact data
Enrico Vettori, Danieli & C. Officine Meccaniche S.p.A., Via Nazionale 41, 33042 Buttrio - Italy;
Phone: +39 0432.1957824, E-mail: e.vettori@danieli.it
Summary
The Danieli X-JET: Maximum wiping capability with minimum flow and pressure resulting in the optimum Operating
Range. It is well known that for any jet wiping system there are substantial benefits to working as close as possible
to the coated strip where the jet is still substantially laminar. It is possible to make the lightest possible coating for a
given speed. Coating appearance is improved. Edge overcoating is minimized. Using minimum pressure and flow
diminishes splashing and the need for skimming. At the same time, strip shape deviations and vibrations limit the
ability to operate close to the strip. The unique character of the X-JET keeps the laminar region much farther away
from the nozzle exit. This means that the X-JET can obtain the benefits associated with operating close to the strip
while operating at a greater distance. This improved Operating Margin allows the operator to minimize the effects
of strip shape deviations and vibration. It is possible to minimize edge overcoating with all combinations of line
speed and coating weight, improve coating uniformity, minimize operating costs for blower power and/or nitrogen
usage and minimize splashing and skimming while maintaining superior high speed/light coating capability. The jet
lips require less cleaning. The reduced skimming and lip cleaning improve operator performance and safety. Use of
the Danieli electromagnetic strip stabilizer further enhances performance by providing a flatter, more stable strip.
The improved strip shape and position allows the operation of the X-JET to be optimized resulting in even better
coating uniformity and lightest possible minimum coating.
Key Words
Wiping Process, Technological Packages, Air Knives, Kohler, Danieli, X-JET, HDGL, coating.
Introduction markets lead the way, the need for higher quality is
The intense competition of the recent years in the universal.
galvanized strip market has led to a major reduction Tight tolerances on coating distribution along the
in production costs, by increasing plant productivity
and tightening process control. The tendency is
clearly in the direction of low coating weights to be
produced at the highest possible process speeds.
It is well-known that the physical limit of the wiping
process is the critical bottleneck for higher
production. Our research and experience have
shown that this can be affected by the specific design
of the wiping system itself.
Due to the international economic situation, hot dip
galvanizing production capacity is currently oversized
for several producers. Therefore a producer can Figure 1: Figure 1: Historical price graph for Zinc
achieve a significant competitive advantage if its hot (source LME)
dip galvanizing facilities are equipped with a high-
performing wiping system, permitting improved longitudinal and transverse profile of the coil are
production flexibility and optimized allocation of more and more critical to be competitive.
resources. The off-specification lengths during transitions in
As shown in Figure 1, over the last five years the production must be minimized as much as possible.
average price of zinc has been around 2,100 In response to these market needs, Danieli is
US$/ton. This saving in zinc is one of the most continuously striving to improve its technological
important parameters that can be used to reduce the capabilities in the field of hot dip galvanizing. A few
production costs of hot dip galvanized strip. years ago Danieli Wean United developed a new
In parallel with production cost reduction, quality generation of wiping nozzles and updated the
demands from customers are becoming increasingly associated wiping medium supply system, resulting in
stringent. While the automotive and white goods the X-JET system. This was possible only through
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significant efforts in research and development. Fluid- Table 1: Process data from the first month of
dynamics computational simulations, workshop production in one of the X-JET plants
prototype testing, installation at the Customer’s site
and technological performance tests during Strip Strip Process Knife-to- Total
production were conducted. The results confirmed Thickness Width Speed knife coating
that a technological breakthrough was achieved in [mm] [mm] [mpm] distance weight
terms of wiping nozzle design [1]. [mm]
2
[g/m ]
But this is not enough. In accordance with Danieli’s 0,48 1200 160 18 73
policy of continuous innovation and improvement, the 0,45 1200 180 17 80
X-JET wiping system is undergoing further 0,48 1005 170 18 80
development and enhancement resulting in a 0,46 1055 170 19 84
completely Integrated Technological Package,
0,48 1200 180 18 104
capable of improving the performance of coating
0,37 1200 180 16 103
weight control: in terms of both pure wiping efficiency,
and tight process control. The package comprises:
> Danieli Kohler X-JET wiping nozzles Transverse coating uniformity is achieved by
> Wiping medium supply system optimum pressure distribution across the nozzle
TM
> Q-Zinc CLC Coating Weight Closed-Loop width (Figure 3) and by the system’s ability to
Control independently set the end position of each nozzle to
TM
> DES Danieli Electromagnetic Strip Stabilizer better follow the strip (Figure 4). Superior coating
TM
> Q-Robot Zinc Robot for dross removal. uniformity along the coil length is achieved thanks to
TM
the Q-Zinc CLC Coating Weight Closed-Loop
Control (Figure 5).
Danieli Kohler X-JET air knives An advanced-type wiping medium supply system
The first and most important step in this program was provides fast transition times between two different
the development of a new air knife nozzle which led pressure set points and therefore quick system
to the introduction on the market of the X-JET air response following a coating change request from
knives. A complete wiping model (Figure 2) was one coil to another. The system consists of two
created to guide the development of the nozzle positive displacement blowers, each one feeding one
design. Comparison with actual data from plants X-JET nozzle and independently controlled through
confirmed the reliability and the accuracy of the variable speed drives, valves and pipes arranged to
model. Utilization of the wiping model is a precious ensure low pressure losses and fast reaction time.
guideline for fast and accurate setup of the air knives
from the process pulpit of the galvanizing plant.
Figure 3: Figure 3: Pressure distribution along X-JET
Figure 2: Example of wiping model printout width
Various installations have validated the X-JET’s In addition to the benefit of reaching the highest
capability of producing Z100, Z80 and Z70 at much speed with minimum coating weight, wiping is also
higher speeds than traditional air knives. Table 1 more efficient. Reduced flow rates and pressure can
shows some interesting points which are an extract of produce the same results, with positive
the first month of production of an X-JET installed in consequences on costs and operational working
an existing HDGL. The operational points are always points.
equal to or higher than a 16-mm knife-to-knife
distance, which can be considered a conservative
operational distance for a modern HDGL processing
normal good shape strip.
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element analysis was applied in the design of the X-
JET nozzle to create a more rigid structure,
especially on the lips. As shown in Figure 6, the
excess flow at lower pressures (mainly due to
unavoidable leakage) is significantly less in the X-
JET and the rate of growth in the excess flow with
pressure (due to the increase in nozzle opening) is
also minimized.
The final result, as shown in Figure 6, is a greatly
reduced actual flow rate to produce the same coating
weight under the same conditions, because of:
> greater wiping efficiency of X-JET knives, which
requires lower pressure at nozzle inlet
Figure 4: Coating uniformity along strip width
> reduced (actual) gap opening
> minimized leakage.
Figure 5: Coating uniformity along strip length
The graph in Figure 6 shows the trend of coating
weight and flow rate as a function of pressure at air
knife beam inlet. It compares the plant data from a
previous Kohler S-JET Dual plant and from two Figure 6: Trend of coating weight and flow rate as a
recent Danieli Kohler X-JET plants in terms of function of pressure at air knife beam inlet
pressure and flow rate to achieve a certain coating
weight. The curves for coating weight versus In addition to the benefits linked to the innovative
pressure compare the X-JET curve, obtained by concept of new X-JET nozzle design, the air knives
regression of the wiping model and validated with are equipped with automatic gap Width Adjustment
actual plant data, with the same curve for a previous which can provide further savings in flow rate (Figure
generation air knife, a Kohler S-JET Dual. 7). Assuming that the flow rate is roughly a linear
The graph also shows flow rate vs. pressure curves function of the actual nozzle width, the potential flow
which are directly associated with the previously
rate saving with the use of X-JET Width Adjustment
mentioned concept of cost optimization. These
can be estimated assuming a reference strip width
curves are derived from simulations and were
(e.g. 1361 mm) and considering as a first
validated with workshop measurements and actual
plant data. The theoretical curves of flow rate versus approximation to have air knife lips with a flat gap
pressure are very similar for the two designs. profile. With these preconditions we have taken into
However, the X-JET uses lower pressure and account a 100-mm safety distance between the edge
therefore lower flow to achieve the same coating of the strip and the edge position of the sealing strap
weight, just from a theoretical point of view. In actual of the gap Width Adjustment, in order to consider the
practice there are additional savings. For both border effect generated by the edge of the sealing
designs, the actual data shows excess flow for a strip and to allow for some offset of strip along the
given pressure compared to the CFD simulation. nozzle width. The results are shown in Table 2 where
Further investigation revealed two causes. Firstly, the an estimation of savings is given for the tapered lip
simulation does not account for leakage at the ends gap profile as well. In the latter case the saving
of the nozzle opening or elsewhere in the fluid circuit. percentage is higher and depends on the actual gap
Secondly, the simulation assumes that the nozzle profile set.
structure is absolutely rigid and that the opening
therefore does not change with pressure. During
laboratory testing of the X-JET prototype, particular
care was taken in identifying all gas leakages and
minimizing them for the industrial application. A finite
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Table 2: Flow rate saving with X-JET Width Skimming is the removal of top dross and scum from
Adjustment the pot surface. Dross is generated by intermetallic
reactions of the molten zinc bath with the iron which
reduced "out of dissolves from the steel substrates being coated.
nozzle lip nozzle reference strip" estimated Scum forms as a result of oxidation of the zinc at the
width lip width by strip width nozzle flow rate bath surface and in particular in the wiping area,
WA length saving where the excess zinc flows back downwards from
the knives to the pot surface. The main factor in the
amount of scum generated is the area of strip
[mm] [mm] [mm] [mm] [%] processed in a given time.
2150 / 1361 789 / However, the flow rate of the wiping medium
contributes to scum generation by disturbing the
Case (1) - Flat lip gap profile surface of the bath, which exposes more zinc to
oxygen. This phenomenon can be partially limited by
2150 1561 1361 200 ~ 27%
the use of nitrogen instead of air as a wiping medium,
Case (2) - Tapered lip gap profile
but with a considerable cost increase. The X-JET
nozzle reduces the quantity of skimming compared to
(1,4 x 762 mm center flat, 1,65 mm edges)
a traditional air knife system working at the same
2150 1561 1361 200 ~ 29%
production point, thanks to its improved efficiency
and the use of the width adjustment system as
described above. However, as mentioned before,
market trends force galvanizers to continuously move
to more aggressive operative points, with consequent
unavoidable increased skimming.
Traditional manual skimming has at least three major
limitations:
> This is one of the most dangerous jobs which
operators have to carry out on the line, due to the
proximity to the molten zinc;
> It is a physically demanding activity for operators
with a high risk of lower back injuries;
> It is a repetitive and labor-intensive task that is
performed in a harsh environment (hot, noisy and
dusty). For a Galvanizing line operating at high
average speed, hourly dross removal can be up to
300-400 kg/hr;
> Since this a very tiring activity, operators remove
the dross from the pot as quickly as possible,
without optimizing the recovery of the pure zinc.
Figure 7: Schematic representation of the gap width The human workforce working side by side with an
adjustment principle for both flat and taper lip gap automatic system for skimming and zinc dross
profiles removal can produce a great change in operator
health and safety protection while also providing
economic benefits. Danieli has developed its own
The benefits of reduced flow rate are: system (Figure 8), merging the most advanced
> Reduction of blower power consumption (for air) industrial robotics technology with process
or nitrogen consumption automation.
> Reduction in top scum generation The preferred configuration of the Q-Robot Zinc is to
> Reduced spitting of the coating at the edges place the robot so that it skims the area behind the
> Ability to work closer to the bath without splashing furnace snout. The operator’s task is reduced to
with consequent improvement in edge coating merely moving the top scum from the front of the pot
uniformity and overall coating appearance to the rear. Executing the same job performed by a
> Reduction in jet line defects and lip cleaning human operator, the robot continuously makes cyclic
passes across the surface of the zinc bath to collect
Q-Robot ZincTM for dross removal the scum. Since the robot is unaffected by the harsh
Increasing process speed and reducing average environment and never gets fatigued or bored, it
coating weights on Hot Dip Galvanizing lines also has faithfully follows the optimum procedure for
another effect: the increase in dross, which is the by- separating the scum from the pure zinc which then
product of the wiping and skimming operations. The returns to the pot. Q-Robot Zinc can be supervised
effect of the variation of these parameters on by a PLC integrated into level 1 automation to
skimming quantity is highly non-linear. establish the work cycles (Figure 9). While the
operator’s health and safety benefits are readily
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apparent, there are other positive effects. Firstly, the Therefore the focus has to be on reducing the
time saved, improved physical condition and better operating distance. Strip shape and strip vibrations
morale of the operators compared to manual are the constraints which typically affect the system’s
skimming, all contribute to better overall operation of ability to operate below a certain distance. This led
the line. Secondly, some customers report a Danieli to develop the DES (Danieli Electromagnetic
significant reduction in absenteeism of pot operators’. Strip Stabilizer).
Furthermore, from aStrip
direct width
economic standpoint the One of the unique features of the DES is the spatially
Strip width
use of the dross robot allows the skimming operation continuous distribution of force along the transverse
results to be optimized, leading to significant zinc section of the steel strip (patent publication:
savings. Feedback from one high-speed HDGL WO2012/114266A1).
shows a skimming reduction of up to 27%.
Strip width
Figure 8: Q-Robot ZincTM for dross removal
Figure 10: DES continuous force distribution concept
along the strip width (top) compared to the
conventional spot force distribution concept (bottom)
In addition to the obvious benefits in terms of better
TM strip shape control, thanks to continuous force
Figure 9: Q-Robot Zinc HMI interface distribution, the DES electromagnetic architecture
always permits application of force at the edges,
DESTM Danieli Electromagnetic Strip regardless of strip width. Reference figures are
Stabilizer shown in Figure 10.
The preceding paragraphs have already dealt with As an example of damping capability, Figure 11
the X-JET’s optimized nozzle design. With a view to shows the result of an impulsive test performed in a
further increasing productivity, one could easily think laboratory. Two consecutive and calibrated
of increasing the operating pressure. This is correct disturbances were applied to a steel strip. The first
up to a certain threshold, as shown by the wiping impulse was given to obtain the free vibration of the
model printout in Figure 2. This shows that by steel strip, without a stabilizer. The second one was
increasing the blowing pressure at the nozzle and given after switching on the DES. The damping effect
keeping all other parameters unchanged, the curve of resulted in a total energy dissipation of about 78%,
2
the coating weight (for instance 40 g/m per side) with reference to the free vibration of the strip. This
tends to a horizontal asymptote. Excessive pressure was obtained with a damping gain set to 50% of the
has no further benefit. On the contrary, the higher maximum value applicable for the stability of the
pressure can create operational difficulties. control loop.
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Conclusion
A complete, integrated technological package is the
key concept for cost-effective operation in the pot
area. Several installations have proved the X-JET’s
increased wiping capacity and wiping model
accuracy. X-JET combined with DES and Q-Robot
TM
Zinc de-drossing robot further improve profitability,
health & safety conditions and quality.
For instance for a line producing mostly light coating
at high speed the typical revamping includes X-Jet
wiping equipment, Q-Robot Zinc dross removal robot
and associated automation, while reusing existing roll
equipment with an installed cost of € 2,650,000.
Considering a reference production with average line
speed 130 m/min, average width 1100 mm, average
Zinc coating weight 140 g/m2. The saving in zinc,
power and skimming plus the profit from increased
Fig. 11: Damping effect of Danieli DES compared to production and yield are € 2,750,000. The payback
the free-vibration case is just under 12 months.
The electromagnetic strip stabilizer has undisputed
benefits; however its integration in a hot dip Abbreviations
galvanizing line is of key importance to maximize DES = Danieli Electro-magnetic Stabilizer
them. In particular, in the most likely case of a strip HDGL = Hot Dip Galvanizing Line
stabilizer installed in an existing plant, the integration CLC = Closed-Loop Control
of such a device into a typically crowded pot area is CFD = Computational Fluid Dynamics
not an easy task. Thus a maintenance-oriented
design has been developed by Danieli. The result is References
the DES supporting system, shown in Figure 12. [1] Vignolo L, Caporal G, Vecchiet F, Zorzut M.
The supporting system provides: Recent progress and continuous improvement in
> Minimum distance between nozzle bodies and wiping process and coating control by means of the
electromagnetic stabilizer; new generation Danieli Air Knives – Galvatech 2011.
> Synchronized vertical movement of DES and Air
Knives according to operational setup;
> Independent vertical movement of DES and Air
Knives for providing access to the nozzle bodies
during line running.
In addition the system does not impair the
maintenance and operation of the line, having an off-
line parking position with maintenance platform,
where easy accessibility is provided for maintenance.
In-line and off-line positioning can be executed while
the line is running. It is not necessary to disconnect
the power and signal cables when moving to the off-
line position.
Figure 12: DES installation arrangement in
conjunction with X-JET air knives
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