Web Exclusive — Air actuators take the heat in coal pulverizers

The ambient conditions inside a coal pulverizer are harsh. Coal dust and vibrations place a great strain on materials. Where steel cylinders break down quickly, air actuators from ContiTech Air Spring Systems, Hanover, Germany, can last much longer. The pneumatically operated actuators ensure the correct grinding pressure in coal pulverizers and reduce the toll on resources for years.

For FTL Seals Technology, using air actuators is the answer to the difficult ambient conditions found inside a coal pulverizer. The company, which is based in Leeds, England, manufactures pressure cylinders that press the grinding gears of the pulverizer against the material to be ground. The cylinders ensure that the set grinding pressure remains steady while balancing out the abrasion on the grinding balls. A certain degree of compliance is also required when the grinding balls roll over larger pieces of coal.

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Until recently, these pressure cylinders in coal pulverizers have been fitted with traditional metal cylinders with hydraulically actuated pistons. The manufacturer documented an average service life of three months because the cylinders could not stand up to the coal dust and the oily environment for any longer. ContiTech's nitrogen-operated air actuators for coal pulverizers, on the other hand, have proven themselves under adverse conditions: they function maintenance- and oil-free and without sliding gaskets.

The expected service life of the new pressure cylinders equipped with air actuators from ContiTech Air Spring Systems is ten times as long as the steel ones. In one of their first places of application, in a coal-fired power station in South Africa, they even worked for more than five and a half years. The much increased expected service life and oil-free operation mean less strain not only on the coal pulverizers, but also on the environment. Air actuators are now in use in coal pulverizers and other pulverizers in England, Mexico, Holland, India, and South Africa.

Simple operation with air
Air actuators are dimensionally stable elements made of elastomer with vulcanized textile tensile members. ContiTech essentially produces two basic designs: the convolution air spring and the rolling lobe air spring. Movement is generated by the flexing of a highly flexible shell on air actuators designed for pneumatic functions. Piston rods and guiding and sealing elements are therefore not required in an air actuator.

The load is raised using channeled air, just as it would be with a pneumatic cylinder. The stroke can be realized in a number of different ways — with oil-free compressed air, with compressed air containing oil, or also with other gaseous media, such as nitrogen. In low-pressure areas, hydraulic media such as water and glycol can also be used.

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The operating conditions in the coal pulverizer are harsh. Coal dust and vibrations cause high levels of wear to hydraulic components. Photo courtesy of FTL Seals Technology

Tough components for long life
For application in adverse ambient conditions, air actuators are practically irreplaceable — they are wear- and maintenance-free because they function without mechanically moving parts. Dirt, dust, granulated material, and sludge cannot damage the air actuator, and it is also completely resistant to many chemicals.

The standard actuators are designed for temperatures from –30° to 70°C. Designs also exist for other operating conditions, such as a low-temperature cylinder (–60°C) in the wood industry and a high-temperature cylinder for the glass and paper industry (up to 115° constant temperature and 130°C peak temperature). Special elastomers are also available for specialist requirements.

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Cross-section of a coal pulverizer pressure cylinder. The rolling lobe air spring (labeled) movs the piston rod attached below. Illustration courtesy of ContiTech

The maximum pressure of the standard air actuator is 8 bar. Reinforced designs are available for higher demands. Even with just 8 bar internal pressure, pressing and load-bearing capacities from 0.5 to 440 kN can be achieved — meaning standard products with a diameter of 1000 mm can lift masses of 44 ton. Depending on the type used, the stroke can reach up to 455 mm without a stick-slip effect: even with low changes in pressure, the stroke is performed with zero pressure. In addition to this, air actuators can be tilted. Angles from 5° to 30° and a lateral offset of up to 20 mm are possible. This means that certain connection elements and joint structures are not necessary.

ContiTech produces many air actuator designs. These can be split into two basic variants: convolution air springs and rolling lobe air springs. Single, double and triple convolution air springs have a very low minimum height and low return forces. Compared to convolution air springs, rolling lobe air springs have a smaller diameter but the same maximum force, and emit an almost constant level of force via the stroke. The standard range includes:

Single convolution air springs for strokes of up to 164 mm and stroke forces of up to 440 kN,
Double convolution air springs for strokes of up to 400 mm and stroke forces of up to 410 kN,
Triple convolution air springs for strokes of up to 455 mm and stroke forces of up to 440 kN,
Sleeve-type rolling lobe air springs for strokes of up to 105 mm and stroke forces of up to 11 kN, and
Rolling lobe air springs for strokes of up to 280 mm and maximum stroke forces of up to 55 kN.

For information, contact Dipl.-Ing. Bernd Stter, Development and Application Engineering, ContiTech Air Spring Systems at [email protected]