Current trends:

The galvanizers have been confronted since many years to a growing demand for the reduction of the fixed production costs such as the cost of manpower, tighter specifications, requirements regarding the zinc coating and so on. Easy and reliable operations are also requested.
Within this framework, the trend during these last years has clearly been oriented towards galvanizing lines that are highly effective in production in the field of the tonnage to be produced by hour (this is the case for the low carbon market) as well as in the field of the flexibility of the line (this is the case for the high carbon market). An important stage has appeared in the 70’s and 80’s with the introduction of the zinc wiping system with H2S.
This technique, which led to the success of many galvanizers, was however obliging the operators to make a compromise on the tolerances of the diameters to be treated as well as on the zinc coatings to be obtained. Added to the safety factor, H2S is a lethal gas; the market was open for a new wiping control.

This intrinsic link between the diameter and the speed and wiping material, which is resulting from the applied technology, has been broken approximately 15 years later with the discovery of the nitrogen wiping technology.
The principle is simple as it consists in the passing of the wire through a nitrogen knife surrounding the wire where the pressure modulation of N2 is immediately influencing the quantity of wiped zinc.

 

 

 

Up to now, this pressure adjustment was done manually on the basis of charts or of a little expert software program.

I: Transition and drive due to the wire in movement II: Transport by viscous effect III: Laminating by N2 jet IV: Relaxation profile V: Increase of viscosity due to solidification VI: Solidified layer

This technology is already presenting the enormous advantage of giving the possibility not to work with similar diameters for the whole wire field. The system allows the production of different coatings for each wire diameter.
Furthermore, once the manual adjustments have been correctly carried out, the system is limiting the intervention of the operator to a frequency of approximately every 4 hours to clean the nozzle.
Due to this phenomenon, except if there is a manual compensation of the nitrogen pressure done by the operator, the coating will vary with time.

  

So, the next step was to have a system that would be able to: - Control in a continuous way the zinc coating - Automatically pilot the nitrogen pressure according to a wished zinc set point - Warn the operator anticipatively of any major drift compared to the wished zinc set point and give the possibility to correct it immediately - Avoid some laboratory measures that would demonstrate late that during hours, the zinc coating was not correct.

A solution based on the use of sensors working with Eddy Current has been chosen.

Concept of the new way of control:

- Anticipating any deviation of the zinc coating weight during the production - Correcting automatically any deviation in the zinc coating weight - Let the operator concentrate on the handling of the wires - Requiring the intervention of the operator only when necessary

Working principle:

- Nitrogen nozzle wiper controlled via:   - PLC with a man/machine interface   - Special micro valves tuning the flow of N2 - Monitoring wire per wire

Comparison between various wiping systems

Economic impact about the zinc monitoring

The impact of the zinc savings are potentially very significant with this type of system, particularly when taking into account the current price of raw materials.

As an example, let us consider the case of a galvanizing line with 20 wires.

The calculations are based on the following prices:
- Zinc: 2,3 €/kg (March 2008 value)
- Nitrogen: 0,06 €/Nm³ (March 2008 value)

Let us consider a zinc coating target corresponding with ASTM A 475 class A on wires having a diameter of 2,5 mm to 3,56 mm.
Three cases are studied respectively at DV 72, 90 and 120 for 8160 hours/year.

1 2 3 4 5 6 DV Diameter mm Zn coating target g/m² Zn coating with charcoal wiping g/m² Yearly saving in zinc T/year Net yearly saving EUROS 72 2,5 3,56 230 290 475 436 540,44 323,38 1.220.000 720.000 90 2,5 3,56 230 290 553 487 893,16 545,42 2.031.000 1.230.000 120 2,5 3,56 230 290 690 582 1700 1077 3.881.000 2.451.000

Of course, these figures are theoretical as we should also consider the breaks and maintenance periods but we can easily notice that the zinc saving is very significant and is getting higher when the speed is increasing.

Lastly, the constancy of the zinc charges is allowing a very easy further drawing of the wires.

What is Eddy Current?

Working principle:
- Inducing a magnetic field in the wire and looking for its response
- System needs preliminary calibration with reference wire samples
- After calibration : ready to start

The Eddy Current principle is regularly applied in surface controls. When a magnetic field is induced in a conductive material, a current is resulting from it. This current is inducing a magnetic field, the presence of which will induce a perturbation of the induction field.
This field variation will be analyzed and will give a signal that will be treated.
So, there is a previous condition to the functioning of this principle: the body on which we want to do the measure must have a magnetic permeability. Furthermore, it must go through an inductor, which is making the passage of the wire “in a tube” obligatory. The general properties of magnetic permeability (variable according to the zinc quantity) will give the wire a more or less strong answer to the magnetic field that it has to undergo. So can the zinc quantity on the wire be measured.
This same technology with specific sensors would also allow, for instance, the measure of the brass quantity on a steel cord wire after the process of thermal diffusion. Due to this priority, we cannot measure the quantity of iron/zinc alloy on the wire. However, this point can easily be compensated either with a particular calibrating of the sensor, while authorizing a “shift” of the total measured zinc quantity taking account of the alloy quantity, or while adding to the measured zinc quantity, the quantity of alloy corresponding with the staying time of the wire and easily foreseen by mathematical modeling.
So, the system cannot measure the quantity of zinc on a pad wiped wire.

The sensor:

The currently offered sensors are free from technical constraints that required, on some models, a very regular calibration of these or even requested the introduction by the operators of numerous and variable correction factors according to the circumstances. These faults were making the exploitation of these former models quite difficult.

Our sensors are offering the following specificities:
-	Insensibility of the sensor to the humidity of the wire. This gives the possibility to work either with waxed or non-waxed wires.
-	Automatic compensation of the temperature. So can the drift of the signal according to the ambient temperature of the sensor be avoided.
-	Linear answer of the signal. This allows a direct reading of the coating quantity.
-	Automatic compensation of the signal according to the diameter of the treated wires. This allows working with one and the same sensor on relatively large ranges of diameters (1,6 mm to 4 mm, 4 mm to 8 mm, and more than 8mm).

The threading-up of the wire is however requiring that the wire be introduced in a cylinder and this is requiring the use of welding at the change of coil. It is interesting to note that the same necessity is appearing as soon as a nitrogen wiping is used. The range of diameters that can go through a same sensor diameter is essentially linked with the accuracy level that we wish to obtain in the reading of the results. In fact, passing a wire that is “too thin for its sensor” is leading to a loss of reading accuracy but does not prevent the system from reacting.

- Linear output - No sensitivity to the temperature - No sensitivity to the humidity of the wire - Low sensitivity versus change of diameter

Distribution of diameter sensors versus the diameter:

Wire size Type of sensor 1-4 mm ZS-1 2-8 mm ZS-2

The regulation loop:

The sensor is located vertically on the take-ups bench. The wire is stabilized before its entrance in the sensor, for instance through the adding of some small tension pulleys.
> This one is then linked with the control central that:
  - Is measuring the signal sent by the sensor
  - Is transforming this signal in coating quantity
  - Is comparing this measure with the required set point
  - Is eventually modifying the quantity of nitrogen to be injected in order to obtain the requested zinc quantity

 

 

The operator, through a user-friendly man/machine interface, can follow the evolution of the coating, record it if needed in a full quality process and, in order to avoid any significant drift, know at which moment he has to intervene on his wiping system.

Conclusion:

Galvanizing, the last step which is leading to the fact that we get a good product or not, can today be controlled by the use of active control techniques of the zinc coating on the line. The consistency of the results obtained, as well as the possibility to anticipate a problem of coating quantity is giving the possibility to avoid the production of non-conform material and is therefore leading to some substantial savings on the line.
The operators can concentrate themselves even more on the threading-up operations and no longer have to care that much for the permanent running of the vertical draught. If the control of the constancy of the zinc level is a point that still has to be controlled, we can almost be sure that this tool will open some new exploitation possibilities of the lines at much better costs that are also better controlled than in the past because it ensures a permanent control of quality at the exit of the product from the line.

RM 1.3.2.1.1