Robust plasma control with unrivalled precision
The US company Linde Advanced Material Technologies Inc (Linde AMT) is one of the leading suppliers of surface coating systems. The unconditional safety of the systems takes centre stage. Bürkert has supported Linde AMT to develop one of the most compact and reliable plasma spraying systems on the market.
Plasma spraying is a thermal spraying process for surface coating. Materials such as ceramics, metals, cermets or polymers are melted and sprayed at high speed onto surfaces, usually metal substrates. Applications are mainly found in the aerospace industry, in automotive and turbine construction and in electronics. The coating typically improves the wear resistance, corrosion resistance or thermal protection of a surface.
Gases such as argon, nitrogen, helium or hydrogen are dissociated and ionised by an electric arc in a torch during plasma spraying. The atomic components recombine behind the nozzle and emit an enormous amount of heat in the process. Plasma core temperatures of up to 20,000 K are used. The powdered coating material is then injected into this plasma jet, melted at the high temperature and propelled onto the workpiece by the plasma stream.
Plasma spraying requires stable, reproducible processes
Coating systems are usually in operation around the clock and in a harsh industrial environment. This is why it is not only the robustness of the components that counts, but also a repeatable process: coating thicknesses and densities are determined solely by setting and maintaining the spray parameters.
The fuel gas control of the TAFA® 6600XL plasma spray system controller from Linde AMT has a compact design.
The gas flows supplied to the burner can be controlled via mass flow controllers (MFC), for example. To determine the flow rate, the differential pressure or temperature is measured above and below the valve. Although this process is elegant, it is very sensitive to the influence of dirt and therefore requires a great deal of maintenance. In addition, MFCs are comparatively expensive.
Low-wear plasma spray system
On the other hand, the TAFA® plasma spray system control from Linde AMT uses a mechanical system to measure the gases. Each gas is fed via a manifold with four valves through high-precision ruby diffusers of different diameters. This utilises a physical phenomenon: if a pressure is built up above the opening that is roughly twice the pressure below the opening, the gas flow reaches the speed of sound. For physical reasons, the speed cannot increase due to the aperture. However, since the parameter pressure, flow velocity and flow area are related by a physical law, only the density of the gas changes if the velocity is kept constant and the pressure increases. This makes it very easy to adjust the mass flow of gas into the torch.
The individually selectable and easily exchangeable diffusers are made of ruby and are therefore very durable.
“Our system has no moving parts and therefore no mechanically stressed wear components,” explains Andrew J. Bolduc, Product Manager Plasma at Linde AMT. “The only wear and tear is on the ruby, which can easily last a decade before it needs to be replaced.”
Precisely regulate the flow rate
Another advantage of the Linde AMT solution is the precise closed-loop control of the flow rate. When using an MFC, the tolerance is based on its specified maximum flow rate (full scale). This is particularly critical in sensitive hydrogen applications when the controller has to determine both large and very small volume flows. The absolute flow tolerance can lead to intolerable deviations that make precise closed-loop control impossible.
The TAFA® control system from Linde AMT is different: the passages of the four valves mounted in the carrier can have four different diameters, so that one or more can be opened depending on the desired flow rate and thus 15 different combinations optimised for the desired flow rate are available. The orifice plates can also be easily exchanged for other diameters. “There is no other device today that can do this. On the same distributor, we can control mass flows of more than 1,000 SCFH (Standard Cubic Feet per Hour) down to 5 SCFH with an accuracy of better than 1 SCFH,” says the product manager enthusiastically.
Modular concept allows up to four burners
In order to achieve and maintain the desired quality of the coating process, the overall combination of gas control and plasma torch is validated. Replacing the burner would therefore mean that the entire system would have to be requalified. The Linde AMT system therefore has up to four connection points so that four burners can be connected in parallel and switched at the touch of a button. The overall concept also means that each system can be customised by Linde according to the desired variability.
Energy-saving and powerful Kick and Drop
Bürkert valves with kick and drop technology can be installed in potentially explosive environments and extremely tightly sealed. They also withstand high switching pressures. Four of these solenoids are positioned close together in the valve block.
One challenge of using the TAFA® system design is that the back pressure acting on the valve from the burner side can be up to 0.7 MPa (100 psi). A task that was only possible with Bürkert Kick and Drop technology given the required compactness of the valve carrier: a short current pulse generates the high tightening force required to open the valve, and only about half a second later the required power is already reduced by up to 80%. This enabled Linde AMT to use extremely strong springs to close the valves, which can withstand a significantly higher back pressure compared to normal valves. “It is of the utmost importance for the process that the exact amount of gas set flows. If one of the valves were to fail and a leakage current were to occur as a result, this would completely change the dynamics in the burner and adversely affect the coating properties,” Andrew J. Bolduc emphasises this important aspect.
Safer due to lower heat load
The reduced power in holding mode of the Kick and Drop coils also reduces the heat load. “Four of these coils are positioned close together in the valve block. In the event of thermal overload, a valve could switch off unnoticed. This must be prevented at all costs,” says the expert when describing a key requirement. “We loaded the coils with large weights in continuous operation and tried to cause an overload. We didn’t succeed; the Bürkert coils never failed,” confirms Andrew J. Bolduc. However, the overall system also benefits from the low inrush currents of the coils. This allows the Linde AMT control system to operate more valves simultaneously without overloading the power supply.
“Safety, reliability and ease of maintenance are our top priorities, and Bürkert was the only partner that helped us fulfil these three criteria.”
High explosion protection requirements met
Handling hydrogen as a fuel gas increases the demands on the system design. Hydrogen is the element with the lowest density, and it is colourless, odourless and very volatile. In combination with air, a very explosive mixture is easily created. Linde AMT therefore invested considerable effort in its own valve seat design. On the one hand, this made the narrow design of the valve carrier possible and, on the other hand, ensured a gap-free seal to the diaphragm. “We received a lot of support from Bürkert on this. They showed us how to manufacture flawless sealing surfaces,” said Bolduc gratefully. The successful qualification of Linde AMT’s TAFA® system also requires certified components that can be operated in potentially explosive environments, and Bürkert products fulfil this requirement.
Cooperation facilitates unique solution
“Nobody has such a solution as sophisticated as Bürkert’s. There is nothing that even comes close to these coils,” enthuses the Linde AMT product manager. “With our systems, we endeavour to focus on safety, reliability and ease of maintenance, and Bürkert was the only partner that helped us fulfil these three criteria. This means our system is unrivalled on the market.”
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