The SZFG internal pneumatic tube conveyor transports samples and speeds metallurgical analysis

06.07.2022 | Article of Salzgitter Flachstahl GmbH

The pneumatic tube conveyor system that serves the Salzgitter Flachstahl works offers the means to transport samples swiftly using air as its power source. It comprises a network of around 12 kilometers of tubes laid above and below ground through which each day around 1000 - 1200 pig iron, crude and processed steel and slag samples are dispatched from various operational departments for analysis in the central laboratory. Analyzing the chemical composition in the central laboratory ensures that the highest quality standards are guaranteed. At the same time, a rapid response time is essential for the production department. So the ability to rapidly transport samples via the tube conveyor is enormously important for steelworks that are often spread across huge sites.

Internal records show that the history of the pneumatic tube conveyor at Salzgitter Flachstahl can be traced back to the 1940s. In contrast to the present day, at that time the conveyor was used not only to transport samples, but also as a postal system carrying letters, telegrams and documents both within and between various buildings. 

A pneumatic tube conveyor is comprised of several principal components: rotary piston blowers, transport containers to carry the goods to be dispatched, and a network of tubes.

Rotary piston blowers

Transporting a sample via the tube conveyor relies on air as a power source. The rotary piston blowers, also referred to as air compressors, generate air pressure and/or suction within the tube, which sets the transport container complete with sample in motion. 

At Salzgitter Flachstahl, samples are conveyed through the tubes at speeds of up to 70 km/h.

Transport containers

The transport container, also referred to as a capsule, is used to carry the material to be transported (Figure 1). The capsule is 21 cm long and consists of a cylindrical aluminum casing with ventilation holes to cool hot samples. At the point of dispatch, these samples can have a temperature of up to 900 °C. The capsule is supported by brass and plastic slide rings as it passes through the tube. With a diameter of 7.2 cm, it is slightly smaller than the internal diameter of the tube (⌀ = 7.5 cm), allowing air to flow past it.

To reduce slippage and increase speed, the slide ring itself is fitted with a drive ring. 


The capsule is opened by staff at the operational department via the cap opener and loaded with the sample. The inlet and the color of the capsule vary dependent on the type of sample (pig iron sample = blue, (crude) steel = red, slag = green). Each tube conveyor capsule is fitted with a transponder chip integrated into the plastic slide ring, so the type of capsule is automatically detected by the system. In order to further increase the throughput of samples, capsule stacking facilities have been installed at each receiving station, so it is possible to send and receive multiple capsules simultaneously.

Tube network

The 12 kilometer long network on the works site comprises four lines with outbound and return tubes (see Figure 2). These connect the following production areas with the central laboratory: blast furnaces, basic oxygen steelmaking plant with pig iron desulphurization and converters, secondary metallurgy with vacuum ladle furnace alloying plants (VPL) and Ruhrstahl-Heraeus (RH) plants, along with continuous casting lines. Dividing the network into four lines means that throughput of tube conveyor capsules can be increased as multiple capsules are dispatched to the laboratory simultaneously. The capsules loaded with samples are inserted into the tube conveyor by staff at the 13 so-called dispatch stations and sent on their way to the central laboratory. Once they have arrived at one of the four receiving stations at the central laboratory, the capsules are unloaded by a robot before being sent back to the operating department via the return tube. Because the type of capsule is detected via the transponder chip, the preparation and analysis (optical spark emission spectrometry, X-ray fluorescence spectrometry or combustion analysis) specified for this type of sample is then initiated.


To summarize, transporting samples via the tube conveyor not only saves time as it avoids the need for messengers constantly coming and going, it is also a hugely important element in the quality assurance process at Salzgitter Flachstahl. In a matter of moments, the department dispatching the sample has access to a very high quality laboratory analysis of a melt. Among other uses, this can then be compared with nominal values and the alloy composition if necessary adjusted. Despite having existed for decades, the pneumatic tube conveyor remains a modern instrument that is indispensible to the day to day operation of the steelworks.