The most common fans in power plants are ID-,FD- and PA-fans of axial as well as centrifugal type
The most common fans in power plants are ID-,FD- and PA-fans of axial as well as centrifugal type. Their imbalances caused by wear and tear during operation result in periodical dynamic forces, which are fully transmitted into the foundation and beyond. Therefore, fan foundation design must consider not only static but also these dynamic loads resulting ultimately in a foundation concept which is practical as well as acceptable in both the technical and economical way. Spring support of fans can conveniently meet these requirements. Worldwide acceptance of this concept as a state of the art technology is of course, our best evidence.
Conventional fan foundations at power plants typically require very massive RCC-blocks but this huge mass alone cannot stop the transmission of vibration into the soil/surrounding at all. It is mainly meant to suppress the bad effects of unintentional system resonances, which cannot be ruled out in this case,with very little knowledge of the exact dynamic properties of the soil. Even rigorous dynamic analysis has to be based on estimation and assessment even if the design is done by an expert.
There is also the risk of settlements, especially uneven settlements due to these vibrations. It is, therefore, already common practice to support the conventional foundation at several meters below ground level where the soil bearing capacity is much more than adequate for just the static loads. Alternatively the foundation will have to be supported on piles. This may reduce the danger of settlements but it is on the other hand expensive and time-consuming.
Such a conventional foundation should therefore be replaced by a better alternative. GERB recommends dynamic uncoupling of the fan foundation from the soil by means of springunits equipped with high quality, fatigue free coil springs and non-aging Viscodampers.
GERB Vibration Control Systems, Inc., 1950 Ohio Street, Lisle, IL 60532. Tel: 630-724-1660; Fax: 630-724-1664.