TPT July 2016

AR T I C L E

Dalian Field

Tube mill productivity and line speed are equally impor tant for the market competitiveness of a tube plant By Guowu Gao, chief engineer at FD Machinery

of the cut-off will not meet the performance of the welder in respect of speed. (There are some technologies for making long tubes and cutting them for desired length offline, which there is not enough room to discuss in this article.) Both 200kW and 300kW welders for 76mm/4mm tube mill will provide a “nominal” speed for this mill as fast as 120 or even 150m/min, as these high speeds are applicable only to smaller sizes of tube that this tube mill is capable of producing; the power of this welder is enough to provide enough speed for small tube production. As you start making tubes with larger gauge, it is clearly seen that by raising the capacity of the welder from 200kW to 300kW you can raise productivity by 30 per cent. While making a tube with thinner wall, for example 76mm/1mm, the speed can be up to 200m/min. But the recommendation from the equipment producer will not exceed 100m/min, otherwise the cost of the saw blades and the productivity loss due to line stop for blade changes will greatly exceed all the benefits of the speed, which of course no producer would want. We have already mentioned that in the case of using a 300kW welder for 76mm/1mm tube production, theoretically the line speed can reach 200m/min. If all other sections of the line (forming, cut-off, entry, packing, etc) could meet this speed requirement, this would be the line with the highest productivity in the world, with three to four times the productivity of an ordinary mill. We may also say that a line with this high productivity saves three to four times on labour,

xx For a tube mill, line speed is a very important indicator. Normally the line speed for small mills can be up to 120- 150m/min, middle mills up to 80-100m/min and large mills up to 40-60m/min. But in daily factory conditions tube mills seldom run at the highest speeds. In my 28 years of experience in tube machinery construction, I have only once witnessed a Turkish company run their line at 150m/min on regular basis. In a debate on what the best choice of tube mill characteristics is, many say that speed is most important, while others argue that speed is not important at all. This is because the most important characteristic of a tube mill is productivity. One may say that speed just means productivity, because the higher the speed the better the productivity. It is correct in theory, but it is not the same in reality: the highest quoted line speed is applied to the smallest tube size only, and as the tube size grows the speed is lowered dramatically. The more the speed falls, the lower the overall productivity. So, high nominal speed does not always mean low productivity and vice versa. Specialists in tube machinery design know very well that line speed depends on the following: 1) power capacity of the forming section 2) power capacity of the HF welder 3) power capacity of the flying saw (milling saw) There are some other elements that influence the line speed, such as entry section, pipe collecting section, labour intensity and so on, and the most important are the three mentioned above. The power of the forming section can be increased by strengthening the mill structure, and the HF welder capacity can be increased by raising power output. Also, the only item that influences the speed and can not be upgraded is the cut-off (flying saw). The cut-off is a real bottleneck that holds down the speed limit of the entire mill. It is possible to determine the possible line speed by the power capacity of an HF welder; for example for a 76mm tube mill, as you make a 76mm/4mm tube, the line speed with a 200kW welder will be 40m/min, and a 300kW welder will give you a speed of 60m/min. For tube mills that use a milling saw for cut-off, the choice of a 300kW welder is a mistake, only because the performance

Two-blade milling saw

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J ULY 2016