TPT March 2022

Standard system – loose, porous application

DMK system – clean, tight, dry film

the flow rate. If the operator increases the flow, the operator risks flooding the mandrel bar with too much graphite. Another key issue with many MBL systems is the manner in which they are pumped. Some systems use air-driven diaphragm pumps, or piston pumps. Diaphragm pumps do not give a good even flow while piston pumps wear out, due to the diamond like hardness of graphite. Strong pulsation and uneven flow cause even further problems with the application of lubricant through airless nozzles. The DMK MBL consists of air-assisted nozzles driven by positive displacement-metering pumps with “pulse free” linear flow. Air-assisted nozzles allow the DMK MBL system to both atomise and adjust the fan width for each mandrel size without replacing nozzles and at different flow rates. This allows for exceptional lubricant adhesion and reduces drying time significantly, thus providing excellent lubrication and thermal properties. In addition, the positive displacement pump that DMK uses allows the operator to adjust the flow on the fly with high accuracy output. These key features place the DMK MBL system a large step ahead of current systems, in my opinion, and provides the customer with the reliability, control and high quality production required in today’s highly competitive markets.

system. This coated mandrel serves as a mould, allowing for the rolling mill to elongate the shell along the length of the mandrel setting the shell ID and forming a pipe. As the graphite lubricant layer is situated between the mandrel and shell ID, its role as a lubricating layer and insulation layer are critical. If there is significant variation in the graphite layer, risks arise. The graphite layer works as a form of insulation for the mandrel to reduce thermal cycling. Thermal cycling is the repetitive cycle of heating and cooling of a work piece, in this case the mandrel. If the surface of the mandrel suffers from significant thermal cycling, the mandrel will wear out faster, significantly increasing tooling costs for production. Furthermore, significant variation in the coating will result in variation of the elongation of the shell and the resulting wall thickness, thereby affecting pipe quality. Moreover, if there is not enough graphite on the mandrel, the risk is high that the shell will stick to the mandrel, causing production to stop. Additional risks include damage to the rolls, the rolling mill and to the mandrel itself. It is therefore critical that the graphite lubricant be properly applied to the mandrel with a proper MBL system. With many of the presently used MBL systems, the graphite is pumped through airless nozzles. The problem with airless nozzles is two things: first, the spray fan is fixed, and, second, atomisation is dependent on the flow rate. Neither of these options allow for good control of the spraying of the lubricant on the mandrel. With a fixed spray fan, the operator has no ability to optimise the spray pattern for different-size mandrels, or allowing for over or under application of the lubricant. With too small a spray fan, portions of the mandrel risk being left exposed and uncoated. With too large a spray fan, lubricant is lost as it is sprayed past the mandrel, creating variation in the lubricant layers. In both cases, the spray fan will not coat the mandrel properly. The second problem, and probably the more significant one, is the atomisation of the lubricant droplets. Atomisation of the lubricant is the transformation of the liquid graphite into a fine spray of tiny droplets or even a very fine mist. With airless nozzles the atomisation is controlled by the flow of lubricant through the nozzle. Therefore, if the operator wants to increase the atomisation, the operator needs to increase

DMK Oy – Finland sales@dmkoy.com www.dmkoy.fi

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TUBE PRODUCTS INTERNATIONAL March 2022

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