THE USE OF COMPRESSED AIR is common practice on many construction sites, whether above or below ground. Modern compressors deliver higher operating pressures than was the case a few years ago. Dealing with such machinery and equipment has become routine in many places, but this can easily lead to dangerous situations or even accidents and injuries.
The past two years have seen accidents on European construction sites involving hose connections to high-pressure (HP) compressors – sometimes with serious injuries. This makes it imperative to highlight the potential danger from the sometimes very unskilled handling of such hoses. While some of the problems can only be tackled by introducing appropriate on-site measures, three companies – AST Bochum, Gollmer & Hummel and Lüdecke – have come together to develop lay-flat hoses with coupling systems that can be safely connected to the latest types of HP air compressor.
HIGH TEMPERATURES WITH HP COMPRESSORS
Problems have mostly occurred connecting two-inch (50mm) hoses to modern high-pressure compressors which can operate with pressures of up to 40bar. In these cases, the standard solution using hose clamps, which can deal with operating pressures of up to 25bar without problems, is no longer feasible. With such high pressures, the temperature at the compressor outlets sometimes rises to over 100°C. At this point, even proven hoses fail (especially flat hoses), which are normally suitable for such high pressures. The use of a temperature compensation hose placed between the compressor and the regular hose is, therefore, to be recommended. A hose securing cable is also necessary to soften the strong whip effect from the explosive and lasting pressure release from a failed hose (Figure 2).
Tapered threaded fittings are generally suitable for this application, although the connections must be exactly adapted for the hose used. Furthermore, the contour of the hose clip must be adapted exactly to the hose so that it can resist high pressures and temperatures at all times. Germany-based Lüdecke, a leading supplier of coupling systems for compressed air applications worldwide, recommends hydraulically-crimped ferrules instead of the usually used clamps (Figure 3).
Due to the high potential for danger from high-pressure compressed air, it is recommended not to repair hoses on site. As is usual in hydraulics, non-dismountable coupling elements should be used, so that in the event of damage, the entire hose has to be replaced or crimped again. This procedure ensures the necessary working safety.
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By GlobalDataEVERYDAY WORK ON SITE
Modern compressors can now supply an operating pressure of 40bar instead of the former 10-25bar. In the development of hose materials, however, the emphasis has often been on optimising the material (design, cross-section and wall thicknesses) and on adaptation for various areas of application. The great challenge however is everyday work on site: the material is improvised and repaired, hoses are extended or even kinked, the wrong size of clamps are used, they are faulty or not adequately fastened, even including the use of completely unsuitable couplings, hoses and connections. Often, the hose used is not coordinated with the hose fittings and clamps or clamp sockets, and the hose unit may not even be suitable for the application. In addition, low-quality, cheap materials are often found in ‘safe’ products, thereby endangering operatives and machines.
COUPLING SYSTEMS FOR HOSES AND CLAMPS
A safe hose for compressed air work consists of a flexible hose and fittings (e.g. hose couplings) which are connected at both ends with connecting elements (e.g. clamps, ferrules or clips).
Due to the lack of standards, some hose manufacturers and suppliers frequently offer diverse hoses for the same system sizes and identical purposes. However, significant differences exist, such as the inner or outer diameter, i.e. the wall thickness. The composition and material of the hose can also vary, which can heavily limit its resistance to external effects. Some manufacturers of fittings offer various couplings and connection processes for standard hose diameters. As with hoses, these fittings are sometimes subject to considerable dimensional tolerances. Due to this, for example, the barb contours of different manufacturers can differ in both form and size.
SECURITY AND QUALITY DIFFICULT TO ASSESS
Each connection is only as strong as its weakest link. Information on maximum operating pressures and temperatures in use for the individual parts of a hose can mostly be found in manufacturers’ datasheets. Despite this, the numerous influential parameters mean that no simple statements can be made about the performance of a hose connection based on its components. This applies to the hose as well as to the fittings.
PRE-ASSEMBLED HOSES
Prefabricated hoses often show greatly variable pressure and temperature behaviour, which, depending on the application, can lead to problems. Also, the requirements for hose lines when it comes to resistance to operating pressures, ambient and operating temperatures, chemical substances and external mechanical loads are becoming increasingly more stringent.
LOW-QUALITY FITTINGS
Poorly cast and processed fittings are frequently found on the market. This applies to claw couplings and tapered screw fittings as well as hose clamps. The use of such fittings and hose clamps poses an enormous safety risk. With imprecise barb contours and sometimes wide dimensional deviations, a reliable connection to the hose cannot be guaranteed. Many cast parts also show wide tolerances, which frequently makes exact coupling impossible or causes leaks. Furthermore, copies of standardised compressed air fittings pose a danger of early material breakage (Figure 4).
The geometrical quality defects of these fittings are only part of the overall risk potential. It is more difficult to recognise that such copies often use inferior and unapproved materials, such as hard cast iron (white cast iron). The components can break quickly under high loads (e.g. when attached to strongly vibrating machines, such as compressors). Inferior fittings and hose clamps represent a considerable safety risk, the use of which must be discouraged. These components do not comply with the relevant standard regulations and usually do not have any manufacturer or labelling to suit the parameter in question.
DIFFERENCES IN HARD AND MALLEABLE CAST IRON
Chilled cast iron and other inferior materials are very hard and brittle due to their high cementite content. This makes them unsuitable as construction materials for use in heavy-duty applications.
Malleable iron is more complex to manufacture and therefore can be more expensive because it is subjected to an additional annealing process.
Consequently, it has greatly improved mechanical properties (e.g. a high level of toughness) and is suitable for applications in which components are exposed to strong dynamic loads, as well as high mechanical forces.
In principle, the distributor of any defective hose assemby can be held accountable for possible recourse claims due to personal injury and/or property damage, as well as a loss of production.
DEVELOPMENT OF A NEW HOSE AND COUPLING SYSTEM
Three companies operating in this field, namely AST Bochum (supplier of hoses, fittings and tunnelling equipment), hose manufacturer Gollmer & Hummel and hose coupling maker Lüdecke recognised this problem early on and pooled their expertise to develop a safer hose system. As it was not the pressure- and heatresistant fittings that were the problem but rather the actual hose lines, which tended to become softer when exposed to heat, the development team had to focus their efforts on producing a new type of hose system. It was decided that pressing the hose on to the fitting by force was still the most reliable and effective solution and therefore the team set about identifying a better type of hose material.
Silicon hoses exhibit excellent heat resistance, however, they have a very low-pressure rating and this latter quality cannot easily be improved. Metal hoses (such as exhaust gas hoses) could be made pressure-resistant, though these would inevitably become very heavy and stiff. Add to this the fact that each length of hose would have to be braided, i.e. manufactured, individually. The time expended on this operation and the high cost of such an individualised production process would quickly become quite unacceptable.
The work of developing a lightweight, heat and pressureresistant lay-flat hose initially focused on the composition of the hose material, the aim being to ensure good interaction between the fitting, the hose and the pressed sleeve. The experienced design team also paid more attention to developing a practicable press-fitting process that would save both time and money. After several trial runs with different hose materials and coupling connectors, they came up with a hose system that met the requirements for pressure and heat resistance.
NEW INTEGRAL COUPLINGS WITH LAY-FLAT HOSE SYSTEM
The new system is suitable for both compressed air and fluids and has been designed for maximum operating pressures of up to 40bar and temperatures of between –30 and +80°C. The integral couplings are made from steel and malleable iron, and the high-pressure lay-flat hose system comes fully-fitted with hose-tails and sleeves.
Initial applications in the field have confirmed that the new hose system is robust and durable and has a high safety factor. The hose itself is very light and compact to handle, yet at the same time has shown itself to be extremely resistant to abrasion and quite capable of withstanding the toughest of site conditions. The hoses have now been pressurised in numerous operations and have performed flawlessly, with the entire system promising to deliver a long operational life.
SAFE FITTING AND APPLICATION
When the pressed sleeves are being fitted during the assembly process, it is important not to exceed the maximum operating pressures specified for the sleeves, couplings and hose. The process of fitting the sleeves to the hose and spigot must always be performed by qualified personnel using a suitable hydraulic press (Figure 6). The entire hose assembly must be inspected for signs of damage before each use. It is recommended that a protective hose stocking or cable be employed as an additional safety measure (Figure 2).
CONCLUSIONS
A clear distinction has to be made when using compressed-air equipment on site. When working at operating pressures of up to 25bar, it is quite acceptable to continue to use tried-and-tested systems with hose-clamp fittings. But when working at pressures of up to 40bar, it is advisable to fit the hoses with hydraulically-pressed steel sleeves and matching hosetails, while at the same time respecting the specified operating temperatures. Hoses with a certain plastic content can be especially prone to significant changes in their material properties at high working temperatures and may begin to ‘flow’. This consequence should be avoided at all costs.
Such a situation does not require the drafting of new standards or guidelines. What is important, however, is that users have a basic awareness of the fact that flexible hoses must be used carefully and competently when operating compressedair equipment. This means that only experienced personnel should be employed to undertake work of this kind. Furthermore, compressed-air lines should always be fitted with parts from trusted and reputable suppliers who not only operate an enquiry helpline but can also provide the necessary safety data documentation, assembly instructions and manufacturers’ warranties.
