New domestic equipment can considerably raise competitiveness of hot-rolled flat products.
CHEAP, EFFICIENT, AND ECONOMICAL
New domestic equipment can considerably raise competitiveness of hot-rolled flat products.
In spite of the low cost compared to rolled products made in the developed countries, Ukrainian-made HR flats have been gradually losing their positions on the world markets lately. The statistics show that, despite the overall 13.61% growth in Ukrainian exports of semi-finished steel, export supplies of HR flat steel dropped 30.07% during the 11 months of 1999 against the respective period of 1998.
So, what are the reasons for this decline in demand for our hot-rolled products? Quite a few things can be explained with decision of Polish and German shipyards to discontinue purchases from Ukraine. Their claims for replacement point out surface flaws of Ukrainian HR articles. In particular, there are problems with the so-called dents, which deeply penetrate into rolled steel’s surface and can be detected during shotblasting prior to application of coats over shipbuilding steel. Such a defect appears in the rolling process when the surface contacting the rolls is damaged by remains of scale, slags, and other inclusions, which are being forced in the metal by rolls.
The reasons for these flaws are in the obsolete systems of water-jet scale removal applied at Ukrainian metallurgical mills.
An overwhelming majority of hot-rolling mills in CIS countries use the systems of water scale removal with the working pressure of up to 150-200 bars, equipped with dozens of flat-jet atomizers, which are located, for instance, at intervals of 135 millimeters across the blanks that move at high speeds. The total flow area of these atomizers is definitely big, thus causing large consumption of water and electric power. Besides, these systems fail to provide effective scale removal from the blanks being rolled because of insufficient water pressure, as well as some other factors. In a word, the quality of scale removal conducted with the traditional water-jet systems fails to meet the world standards, thus bringing down competitiveness of critical metal articles.
There are various hypotheses evolving around the mechanism of water-scaling and about the degree of influence of mechanical and thermal effects. Most researchers believe that scale is mainly removed owing to its purely mechanical exfoliation under the influence of hydrodynamic force generated when a high-speed water jet contacts the metal being rolled. Moreover, the decisive keys to the process are the pressure exerted by water jet on scale (proportionally to the squared jet speed) and the water pressure in atomizer.
According to another hypothesis, it is considered that rapid cooling makes the scale jacket warp and breaks it off the heated metal underneath. With this, the scale is destroyed not by mechanical stroke pressure of high-speed water jet, but more by the cooling effect. Moreover, the more sudden the cooling, the more efficient the scale removal. Additional effect is reached by steam formation under scale’s surface. Metallurgical mills in the developed countries usually remove scale from hot ingots and slabs by hydro-scaling at the working pressure of 600 bars with the help of water-jet rotor heads. A rotor head consists of two or more nozzles directed at the surface being cleaned. The nozzles revolve around a common axis, through which water is conveyed under the working pressure. When the object being cleaned passes underneath the nozzles, high angle velocity of rotation (e.g. 400 to 3,000 revolutions per minute) ensures thorough and efficient scale removal, thus saving on consumption of water and electric power.
Such a system of hydro-scaling most fully complies with the mentioned hypotheses of efficient scale removal.
The traditional system of hydro-scaling with flat jets of water, when water stream should completely overlap the width of the object being cleaned, consumes too much water. In turn, it disables the manufacturers from boosting pressure in the hydro-scaling system since increased pressure results in even greater water consumption. At the same time, it is inexpedient to lower water consumption at the expense of decreasing the thickness of flat nozzles’ slits.
The rotor head works over a larger cleaning area not owing to the width of liquid jet like in case of flat nozzles with fanlike water jets, but due to spaces among the nozzles in relation to their common axis of revolution. When the object being cleaned is fixed, the revolving jets leave a ring-shaped trail with the width determined by thickness of water jets, their slope, and shape, with diameter depending on nozzles’ spacing, which can reach 150-500 mm. Motion of the sheet steel being cleaned in relation to rotor head leaves a trail with the width equal to the ring’s diameter. The number of rotor heads is determined based on the ratio of the width of surface being scaled and the width of trail. Thus, there is no need in covering the whole sheet with water jets, which greatly reduces consumption of water and enables amplification of pressure.
To assess utility of this scale-removal method, we have performed a comparative calculation of power and water consumption of a 200-bar traditional hydro-scaling system with flat nozzles and of a 600-bar system with rotor heads. Flat nozzles that clean 1,000-mm-wide strips have the following parameters: the smallest cross section (b x h) of nozzle’s slit equals 12 x 0.8 square millimeters, while the space among nozzles at the header comes to 125 mm.
The latter value equals to width of a strip cleaned by one nozzle on the surface of sheet being rolled.
A single rotor head with a couple of rotating nozzles, 1 mm in diameter, forms a circle of water jets, which clean a 250-mm-wide strip.
Let’s find out the figures on comparative consumption of water and electric power.
When the traditional method is applied, 8 nozzles (1,000/25) are required to clean a 1,000-mm-wide strip. Consumption of water can be presented as an equation Q1 = 8 х m 1 х b x h x O p1 x k,
where m 1 is a consumption coefficient of a flat nozzle; р1 is the pressure of water entering flat nozzle, and k is the coefficient of liquid’s properties.
For the proposed system involving 4 rotor heads (1,000/250) with 2 nozzles on each head, the following quantity of water is consumed by all the 8 nozzles: Q2 = 8 x m 2 x p x d2/4 x O p2 x k,
where m 2 is a consumption coefficient of the head’s nozzle; and p2 is the pressure of water entering flat nozzle.
The ratio of water consumption equations is: Q1/Q2 = 8 х m 1 x b x h x O p1/8 х m 2 x p x (d2/4) x O p2.
To simplify the calculation, let’s take the same consumption coefficients in both cases, i.e. m 1 = m 2.
Substituting the values, we get the following figure:
Q1 /Q2 = 7 (1)
Therefore, rotor heads use 7(!) times as little water as the traditional method.
Now, let’s compare the values of power consumed.
When using the traditional flat-jet hydro-scaling system, nozzles consume power equal to N1 = Q1 x p1
Consumption of power during application of the hydro-scaling system with rotor heads is N2 = Q2 x p2.
Thus: N1/N2 = Q1/Q2 x p1/p2.
Keeping in mind the ratio (1) and pressure values p1 и p2, we get the following: N1/N2 = 2.33.
What we have is that the power of the 600-bar hydro-scaling system with rotor heads and, correspondingly, its electric energy consumption is 2.33 times lower than those of the traditional method.
The conducted comparison of the two systems shows that two factors can bring the quality of cleaning HR sheet steel at rolling mills to a new level, namely increase in the pressure of scale-removing water up to 600 bars and application of rotor heads as cleaning tools.
Simultaneously, this yields large economy of electric power and water.
Until recently, application of such hydro-scaling systems in the CIS was held back by the lack of water-jet equipment working under pressure of 600 bars and more, in particular, powerful 132-kilowatt water pumps, rotor heads, applicable distributing and controlling valves and devices, etc.
However, in the 1990es such equipment with the working pressure of 600-1,200 bars was developed and is now manufactured by high-pressure technologies scientific and production enterprise INDRIS located in Kiev city. This water-jet equipment is successfully applied in various branches of the national economy. Water-jet equipment is protected by the Ukrainian patent for invention No.3416 issued on June 15, 1994, and by the Russian patent No.2112609 of February 10, 1998. The equipment has various advantages over similar western devices, namely ease of operations, ability to work in extreme conditions, and low price.
The main components of water-jet hydro-scale-removal equipment are as follows:
complex water-pumping plant with the working pressure of up to 700 bars, which includes several working and backup pumps (see photo 1), the number of which depends on the width of sheet being cleaned and on the rolling speed;
hydro-scale-removal headers with water-jet rotor heads (see photo 2);
high pressure water-mains; water-conditioning block prior to entering the pumping plant;
high-pressure filters (see photo 3)
Photo 1. 700-bar high-pressure pump
Photo 2. Water-jet rotor heads
Photo 3. Filters
Water-jet equipment cleans the surface of HR articles with rotor heads rotating at the velocity of 1,000-2,500 revolutions per minute, the nozzles of which efficiently remove scale, slags, and other foreign inclusions.
When the water feed is cut off, the pressure in pumps is automatically lowered and pumps switch to the idle mode.
The equipment features high reliability while operating in extreme conditions. It is proved, particularly, by the long-term experience of using INDRIS water-jet devices during outdoor repairs of gas conveyance and storage structures, including gas mains. 700-bar devices with 132-kW water pumps are equipped with 4 rotor heads, which move along the gas pipeline (see photo 4).
Other devices with working pressure ranging from 250 to 1,200 bars have water-jet pistols with rotor heads and water-abrasive tools for manual cleaning (see photo 5), as well as water-abrasive cutters (photo 6).
It should be noted that metallurgical mills do not limit application of water-jet equipment only to scale removal systems owing to a unique combination of its advantages, such as: environmental safety; dust control; absence of wearables; no heating; absence of thermal and mechanical strain in the area being worked (e.g. in the cutting zone); high performance; universal nature (ability to clean (cut) different objects (materials) with a single water-jet tool).
High-pressure water-jet devices can successfully clean fire grates, fans, filters, finished castings, tubes of heat and power plants and heat exchangers, floors and equipment, as well as chop sprues and cut metal, clean rust, salts, coking products, concrete, petroleum products, and remove scale from finished metal products instead of sandblast, and perform many other tasks.
This 700-bar hydro-scale-removal system, on other equal conditions, costs a couple times as low as similar western systems.
Thus, application of the hydro-scale-removal system based on high-pressure water-jet equipment with working pressure up to 700 bars and with rotor heads removes scale and cleans other inclusions in compliance with international standards, enhances the quality of rolled steel, lowers consumption of electric power twice or thrice, water consumption threefold to eightfold, as well as requires five or six times as little outlay.
Besides, hydro-scale removal equipment automatically adjusts the pressure to optimal performance mode, saving even more electric power.
As a result, greater quality of cleaning, elimination of flaws on the surface of rolled steel, together with simultaneous reduction in production costs, boost competitiveness of HR steel on foreign markets and yield various price advantages.
Photo 4. Four rotor heads cleaning a gas-main (P=700 bars)
Photo 5. Cleaning a welding seam with water-jet pistol
Photo 6. Water-abrasive cleaning and cutting tools
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