Controlling generator set vibration to minimize

There are two major components to vibration isolation. The first involves the isolation of the engine and generator from the remainder of the generator set assembly. The second component involves isolating the entire generator set from its mounting platform and the way in which this base connects to the building structure. Any vibration-isolation design choices then should serve to reduce the dynamic loading imposed by the generator set on its foundation, the building and ultimately its occupants.

When a generator set is ordered with vibrationisolation mounts, an application engineer sizes the spring isolators based on the total weight of the unit and the type of location, such as ground floor, isolated concrete pad or rooftop. The spring isolators are sized so that the weight applied to each is not more than 60 to 70 percent of the isolator’s capacity. The standard spring isolation mount is not suitable as a seismic mount and, therefore, is not applicable in all geographic locations. Some geographic locations require mounts with a high shear rating, and these must employ a seismicqualified mount.

Typical spring isolator mounts, standard or seismic, will transmit only about 5 to 10 percent of the generator set vibration energy to the supporting surface. In all but the most sensitive environments, this level of vibration reduction is satisfactory. There are, however, unique applications that require a higher level of isolation.

Hospitals, data centers, rooftop installations and large, multi-genset installations are examples of applications that require higher-thannormal levels of vibration attenuation. If these requirements are communicated when ordering a generator set, the application engineer can assemble a vibration-isolation package that will eliminate 98 to 99 percent of the generator set vibration energy transmitted to the base.

In rare cases where virtually all vibration must be isolated from the supporting surface, it is possible for vibration transmission to be reduced to a fraction of a percent. This level of isolation requires a double-isolation system. The easiest way to achieve this double isolation is to mount a diesel fuel tank under the generator set frame rails. In this way, the generator set can be spring-isolated from the tank and the tank can be mounted to the base with elastomeric pads between the mating surfaces. The primary expense of this arrangement is the second set of isolators, but when vibration has to be all but eliminated this is a very successful solution.

Regardless of whether the generator installation will be outside a building or inside, the generator set mounting surface or concrete pad should be engineered locally. If the unit is to be mounted within a commercial or public building, local building codes will probably require the design of the supporting system be approved by a licensed professional engineer. In other locations, concrete pads may not need to be approved by a licensed professional engineer but should be built according to local codes with regard to soil density, seismic risk and wind loading requirements.

Some installations, either because of the building’s purpose (i.e., mission critical) or its geographic location within a known seismic zone, require generator sets to conform to IBC (International Building Code) seismic guidelines. When generator sets have been tested and qualified as meeting IBC seismic standards, they are equipped with spring-vibration isolators that also meet IBC seismic requirements.

In applications where generator sets are mounted on their fuel tanks, spring isolation should be installed between the generator set and the fuel tank rather than between the fuel tank and the supporting surface. With this type of installation, the size of the vibration mounts can be smaller. The effectiveness of the spring isolation system is not affected by fuel-level variations (and thus weight variation) in the tank.

When the base for a generator set must be designed to comply with local codes, the licensed professional engineer will need to know the “dead weight” and the “dynamic loading” of the unit. The engineer will also want to know the physical footprint of the unit as well as the number and location of the vibration isolators. Other values that might be relevant include the magnitudes and frequencies of the primary modes of vibration associated with the generator set. This information is available from the generator set manufacturer.

The supporting structure must support both the static loading and the dynamic loading of the generator set. Static loading is simply the “dead weight” of the object. Dynamic loading is the force applied to the floor or foundation by the operating engine-generator combination. The engineer designing or analyzing the substructure on which the generator set will rest must evaluate both the static and the dynamic loads. In general, the dynamic force is the static force multiplied by a factor (greater than 1) due to the operating vibration/motion of the object.

Ascribing a dynamic loading factor to a particular generator set is more complicated than simply providing its weight. The total weight of a generator set is the combination of all the component weights and can be approximated to within 1 or 2 percent even before any of the components are assembled. The dynamic loading factor, on the other hand, must be determined during operational testing and may be unit-specific. Dynamic loading is the total vertical force applied to the substructure when the unit is operating. The dynamic force represents the maximum magnitude of the mass of the unit times the combined accelerations of the vibration and gravity.

The masses of the concrete substructure and the generator set mathematically combine to become one when “hard-mounted” (no vibration isolation) directly to a concrete substructure. The issues at the interface of the two masses are friction and the shear-loading due to the different rates of thermal expansion and motion. These issues are mitigated by the insertion of quarter-inch-thick elastomeric pads at the mounting points. While the vibration energy of the entire assembly will remain the same as that of the generator set alone, the resulting motion of the combined masses will be significantly reduced by the addition of the mass of the concrete substructure. The soil below and alongside the concrete pad will become the “sink” for that energy.