Process Control Improvements at USS/Kobe

1.  INTRODUCTION

Noncontact laser Doppler velocimetry has been utilized for more than 10 years as a proven technique to measure the velocity and length of various shapes and sizes of metals products.  The technique eliminates process errors caused by roll slippage incurred with contact methods.  Over two hundred and fifty (250) TSI LaserSpeedÒ systems have been sold worldwide for a variety of applications in the metals industries.  This paper will discuss the theory of operation, mounting considerations, and improvement in mill performance at USS/Kobe's large diameter pipe mill using this device.

Figure 1.  LaserSpeed® 2000 System

 

2.  Theory of Operation

 A LaserSpeed measurement system consists of an optics head that contains transmitting and receiving optics, a connecting cable, and a signal processor (Figure 1).  It can be used for both hot and cold applications since the technology is not affected by temperature.  Two equal intensity beams are split from a single laser beam inside the optics head and crossed outside to form a measurement region (Figure 2).  The bisector of the crossing beams must be perpendicular to the direction of motion of the surface.  When a light scattering surface passes through the measurement region, light from each beam is simultaneously scattered.  The mixing or heterodyning of the scattered light from the two beams onto the aperture of the photodetector inside the optics head provides the Doppler signal which contains the surface speed information.

Figure 2 

 

The signal contains an AC component (Doppler frequency) and a DC component.  The DC component is used to determine when material is in the measurement region and the Doppler frequency of the AC component is directly proportional to the speed of the material.  A patented TSI DSP signal processor calculates the speed of the moving surface and integrates the speed over time to obtain the length.

           

Zero speed measurement and direction tracking can be accomplished by shifting the frequency of one laser beam by a reference frequency in an accoustic-optic modulator.  The bias caused by the frequency difference between the beams is subtracted by the microprocessor to obtain the true relationship between Doppler frequency and speed.

 

3.  Mounting Considerations for Tube Mills

Proper mounting of the optics head is very important to obtain optimum accuracy from the system since the tube surface needs to be perpendicular to the bisector of the crossing laser beams.  It is also necessary that the edge surface be always in the crossing region of the beams (typically 100 mm. or 200 mm.).  The optics head can be mounted from above the conveyor looking down, from underneath looking up at the tube, or from the side.  There are advantages and disadvantages to each position, as discussed below:

 

3.1      Optics head mounted above

 

Fig. 3  Optics Head Mounted Above Conveyor

In order to mount the optics head above the tube line, it is necessary that the tubes are always centered in the middle of the conveyor.  Otherwise, the edge of the tube will not always stay in the measurement region of the laser sensor as it passes underneath.  Depending on the range of diameters to be measured, the optics head may need to be moved up and down between size changes to keep the tube edge in the measurement region.

 

The advantages of this configuration are three:

(1)    Safety concerns are minimized since the optics head is pointed down¾making it more difficult for someone to look up into the optics head and damage that person's eyes (the only major safety concern with this technology).

(2)    The above position makes it unnecessary to protect the optics head from falling debris.

The adjustment of the optics head between large diameter changes needs only to be in the vertical plane. 

 

The disadvantages of this mounting position are:

(1)    The tubing must be centered on the conveyor.

(2)     With large diameter variations, it is necessary to adjust the vertical position of the optics head to keep the edge of the tube within the measurement region.

 

Fig. 4  Optics Head Mounted Below  the Conveyor

3.2      Optics head mounted underneath the conveyor

When mounting underneath the conveyor with the optics head pointed upward, it is also necessary (as with the mounting above) that the tube be centered on the conveyor.  However, since the lower edge of the tube is supported by the conveyor, it is always the same distance from the optics head¾whatever the diameter.  Therefore, it is not necessary to move the optics head up or down for large diameter size changes. 

The major advantage of mounting beneath the conveyor is that the optics head does not need to be moved up or down for diameter changes.

 

The disadvantages of this location are: 

(1)    An air purge may be necessary to deflect debris and particles from falling on the optics head and (over a period of time) blocking the optical window

(2)    There may be increased safety concerns since it is easier to look down into the optics head from above.

(3)    The tube must be centered on the conveyor.

 

3.3      Optics head mounted from the side

The optics head can also be mounted from the side looking horizontally at the tube (the configuration used by USS/Kobe).  This arrangement may require the adjustment of the optics head both in and out and up and down for different diameters.  A milling table or more precise traverse arrangement may be necessary.

Figure 5  Optics Head Mounted From Side of Conveyor

 

Two advantages of this location are: 

(1)    There is easy access to the optics head for maintenance.

(2)    The tubes do not have to be exactly centered on the conveyor. 

The disadvantages of this location are: 

(1)    The optics head may have to be traversed both in and out and up and down with large diameter changes to keep the edge of the tube located in the measurement region.

(2)    Safety concerns may necessitate a containment tube or box to prevent persons from looking into the optics head.

 

3.4      Summary of mounting options

Each of the mounting positions has application depending on the situation of the particular mill for which this technology is being considered.  If a limited range of diameters is being produced so that the optics head does not need to be traversed, the above conveyor mounting might be preferred.  For situations where a large range of diameters is produced and traversing is not desired, the underneath mounting may be the best.  For applications where the center line of the tube is fixed (as in the case of USS/Kobe described below), the side mounting should be considered.  The choice of mounting position should be carefully considered for each application.

 

4.  USS/Kobe Steel Installation

4.1      Facility description

USS/Kobe's No. 3 Seamless Pipe Mill is the only tubular facility in North America capable of manufacturing seamless pipe over 16 inches in diameter.  The mill was commissioned in 1930 and has been upgraded both mechanically and electrically over the years.  The present range of this facility is 10-5/8" O.D. to 26" O.D., with a product wall range from 0.312" to 2.250".  The mill equipment consists of a rotary hearth billet heating furnace, a Mannesmann double piercing operation, reheat furnaces, a plug rolling mill, rotary expanding mill, two reeling machines, and a three stand in-line sizing mill.  The hot mill product flow chart for No. 3 Seamless Mill is shown in Figure 6.  Products from this mill are used for gas and oil drilling and transmission, pressurized gas storage, process piping, fittings, and structural components.

Fig. 6  No. 3 Seamless Mill Product Flow Chart

 

4.2       Installation at USS/Kobe's No. 3 Seamless Pipe Mill

Prior to this installation, the product was rolled to a predetermined finished length based on billet weight and an estimated scale loss factor.  Pipe samples were periodically removed from the mill line, after final hot sizing, and evaluated at an out-of-line inspection area equipped with a weigh scale in order to determine product weight/foot.  Decisions concerning mill adjustments were made from information gathered from these random samples.  Due to the time involved to manually perform these measurements, the number of samples evaluated was quite small.  In general, control of product weight/foot variation was very difficult.

Fig. 7  Equipment Layout

           

The TSI Model 2000 LaserSpeed System was installed in the No. 3 Seamless hot mill processing line in 1993.  The system was installed at exit side of the sizing mill, in conjunction with an in-line pipe weighing scale.  The equipment layout sketch for the TSI LaserSpeed installation is shown in Figure 7.  The TSI LaserSpeed unit was mounted horizontally so that the elevation of the beam path was at the fixed center line of the sizing mill.  The in-line weigh scale was installed downstream of the sizer exit conveyor.  This installation provided the mill with the ability to gather data for every piece of pipe processed.  The block diagram for the TSI LaserSpeed length system and the pipe weigh scale is shown in Figure 8. 

Fig. 8  Block Diagram

          

The purpose of this installation was to minimize product variability by measuring product length and weight  in order to determine weight/foot.  Data gathered from the system is displayed on a video terminal at key operators' workstations, using a Real Time SPC control chart.  Data can also be printed in various hard copy formats, or archived for later data analysis.  A typical video screen display is shown in Figure 9.

 

Fig. 9  Screen Display

           

In order to utilize this system effectively, all operating employees were provided SPC training in order to understand control charting and the significance of the data displayed, in order to make timely mill adjustments.  This training process required approximately four months and was completed in 1994. 

4.3      Mill performance improvement

A round to finished pipe yield performance improvement of 1% has been realized from this installation because of the following factors:

A.     The data collected from the system allowed the mill to reduce the ordered billet weight because they were able to determine actual average scale loss by product size, during the manufacturing process.

B.     The use of real time SPC X-bar and Range charts displayed on video screens at key operators' stations for product weight/foot control has reduced the amount of non-conforming product.

 

5.  Other Applications on Tube Mills

In addition to the described application at USS/Kobe, TSI LaserSpeed systems have been used for other tube mill applications.  For instance, length measurement of hot tubes (Figure 10) has been used in conjunction with an O.D. Gage and a scale to calculate wall thickness "on the fly".  This allows the mill to make adjustments after diameter changes during production--without having to stop the mill, cut off a sample, cool the sample, and physically use a micrometer to measure the thickness.  Throughput is increased substantially.

 

Fig. 10  A TSI LaserSpeed gauge installed on sizing mill for wall thickness control

           

Another application (figure 11) has been to measure the cold tube lengths prior to shipment to the customer.  In one case, an automated marking system was used in conjunction with the LaserSpeed to automate a previously manual process.  Prior to installation, a worker used a tape measure to measure the length and a marking pen to tag each tube.  Automation of the process increased accuracy of the length measurement and reduced labor costs.  

Fig. 11  A TSI LaserSpeed gauge measuring length of cold tubes for final length verification

 

6.  Summary and Conclusions

Noncontact TSI LaserSpeed systems have been used to improve the productivity of tube mills.  It is important to choose the proper mounting position so that the best combination of access, convenience, accuracy, and safety factors is achieved.  The USS/Kobe installation on No. 3 Seamless Mill improved yield performance by 1% with much improved control of product weight/foot variation.  Other applications have enabled customers to monitor wall thickness of hot tubes without having to stop the mill and to provide accurate lengths of cold tube to customers.

 

The nature of a tube's round cross section makes a noncontact measurement of speed and length very attractive because it is very difficult to use a contact device without incurring slippage.  As customer requirements continue to become more stringent, the use of LaserSpeed systems will become more prevalent throughout the industry.

 

This application note was written and provided by TSI. TSI Incorporated designs and manufactures precision instruments used to measure flow, particulate, and other key parameters in environments the world over. TSI® serves the needs of industry, governments, research institutions, and universities, with applications ranging from pure research to primary manufacturing. Every TSI instrument is backed by unique technical expertise and outstanding quality. For more information about TSI, please visit their website at: www.tsi.com

References

1.       Gallob, R. and Taschner, W.  "Modernization of the Hot Strip Mill at Voest-Alpine Stahl Linz Ges. m.b.H."  Metallurgical Plant and Technology, 3/89.

 

2.       Jenson, L., Dahlerup, K., and Carter, K.  "A Noncontact Velocity and Length Sensor", TSI Incorporated Technical Bulletin, TSI Incorporated, St. Paul, MN, 1990.

 

3.       Kimball, V. and Buttler, A.  "Use of Noncontact Speed Sensors to Improve Tempering and Gage Control Processes", TSI Incorporated Technical Bulletin, TSI Incorporated, St. Paul, MN, 1988.

 

Kimball, V. and Gehring, W.  "Noncontact Speed Sensors Improve Gage Control Processes", The Proceedings of the 7th International Aluminum Sheet & Plate Conference, The Aluminum Association, Washington, D.C., 1992