Elastic Support of Power Plant Equipment

Elastic support of Coal Crushers and Coal Mills

Coal Crushers

Coal crushers of all types, such as ring granulators, pulverizers, hammer mills, rotary breakers, roll and jaw crushers, can often cause significant and unacceptable vibrations in their supporting structures.

The source of the vibration from a pulverizer, hammer mill or a ring granulator is its imbalance, which may result from normal wear and tear or from an emergency situation, for example, a broken hammer. Additionally these machines (with speeds ranging typically from 600 to 720 RPM) pass through resonance during start-up and shutdown, resulting in large amplitudes in the tall structures that support them.

Imbalance forces in rotary breakers are usually low. However, the crushing process itself and the drive system produce random vibrations over a range of frequencies. Part of the frequency contents can precisely or nearly match the frequencies of the supporting structure, resulting in large resonance-amplified deflections.
Coal crushers in power plants are normally elevated in the structure. A proper conventional foundation design demands a thorough dynamic analysis of the foundation, including the substructure, soil properties and, where applicable, piles. Such an approach usually produces a design with a concrete slab on tall columns, all supported on a heavy base mat.

There is a better way.

Decades ago GERB introduced a spring supported foundation system for the vibration isolation of coal crushers, which is now standard in many parts of the world. The support system consists of spring units equipped with fatigue-free coil springs and non- ageing Viscodampers®.
Resulting system natural frequencies are typically between 2 to 4 Hz – far below crusher speeds ensuring smooth operation.
The spring system allows transmission of no more than 10 percent of the dynamic loads from the crusher into the supporting structure – keeping it nearly vibration-free.
Velocity-proportional Viscodampers®, working with the spring system, absorb vibration energy and eliminate any major amplitude amplification when passing system resonance during machine start-up and shutdown.

Fig. 1

Fig. 1

The GERB system not only eliminates vibration problems but also offers some very important advantages compared to conventional foundations, such as simplified design and analysis, easy installation and cost savings. Other advantages are:

  • The crusher foundation is limited to a concrete deck supported by spring units and Viscodampers®. The dynamic behaviour of this system can be accurately calculated and predicted.
  • Consequently, the design of the supporting structure, RCC or steel, below the spring system can be based on static analysis only. This eliminates the need for base mat and sometimes even the piles, thus saving construction time and material, making the GERB system more economical than conventional crusher foundations.
  • With spring support, the crusher and its deck are usually supported on just two beams framed into the nearest building column. Only the dead load of the crusher and deck are added to the column load, as dynamic loads are eliminated by the vibration isolation system. The layout of the structure is thus greatly simplified. Separate support columns are no longer necessary, providing more space below the crushers for the chutes and other auxiliary equipment as well as more efficient working environment.
  • This layout becomes especially attractive when several crushers are housed in one building. Concrete crusher decks can be supported by a common structure integrated into the main building.
  • The system provides protection against earthquakes as well as soil settlements.
  • The whole system can be conveniently installed in existing crusher structures – RCC or steel – troubled by vibration. Several crushers have been successfully retrofitted with spring support systems.
  • The highly flexible spring support results in less stress on the machine itself, which in turn increases the life of the bearings, reduces maintenance and increases crusher availability.
  • Crushers used for materials other than coal can also be effectively spring supported in the same way.

Coal Mills

Coal mills of all types can cause significant vibrations in their vicinity, leading to disturbance, damage, and interruption of power plant operations. Switch cabinets, control rooms and other equipment installed adjacent to the mills are mainly affected.

Over the past 50 years, GERB has developed and introduced the elastic support system for effective vibration isolation of coal mills in power stations, which has been adopted and accepted by the industry through-out the world.

GERB’s elastic support system consists of a combination of helical steel springs and special velocity-proportional Viscodampers®.

The GERB system provides some important advantages:

  • The mass of mill foundations equipped with the GERB system is approximately 40% of that of conventional foundations. Smaller capacity mills with a higher rotating speed may be vibration isolated using direct spring support without any foundations.
  • With mill foundations separated from the boiler house building, the construction of the mill foundations is independent of the construction of the main building. Mills may be installed either before or after the construction of the main building.
  • The installation of the elastic support system prevents the transmission of significant vibrations to the boiler and boiler house. Therefore, possible damage to the boilers, boiler brickwork, switch gears and control instrumentation is eliminated.
  • The use of correctly designed elastic support systems will result in lower stresses in the mill components which will provide longer maintenance free periods and increase life of the coal mill.
  • The GERB system provides effective protection against seismic loads, preventing possible damage of the mills.