Vibration compensators tested in Martinique

Danish company Maag Gears has tested a method of reducing mechanical vibrations in gensets. In the test conducted at an EDF diesel power plant in Martinique, electrically driven FLS vibration compensators were fitted to two Wärtsilä NSD 12RTA 84C(F) engines.

The ability to run gensets with decreased vibrations allows increased output and reduces operation and maintenance costs. Traditionally, mechanical vibrations have been reduced by the creation of counterforces within the engine frames. This typically involves bracing two engines and running them in counter rotation. The units also have to be synchronized and loaded uniformly. The use of compensators allows gensets to be run individually.

In the Martinique trial, balancers were mounted on each engine frame to create sinusoidally varying counterforces. Sixth order vibrations were introduced by applying X-type moments through lateral guide force.

Compensator arrangement

X-moments tend to cause twisting in engine frames. Maritime experience shows that one compensator at each end of the engine frame mounted above the crankshaft – in anti-phase with the upper guide forces and in phase with the lower guide forces – will reduce X-vibrations in the top and bottom of the frame.

To obtain acceptable vibration levels C-40H compensators optimized at 85 per cent were used. Synchro- nization and monitoring of the compensators was via a computer and synchronizing module located in a control room.

Although the deviation between calculated and measured vibrations can be significant due to uncertainties regarding frame rigidity and distribution of forces, setting the compensators was straightforward. Natural frequencies were estimated and the risk of resonance through excitation sources evaluated. The type and size of compensator was then selected to balance the excitation source while allowing sufficient margin.

Peak values of vibrations recorded over a period of time were found to be well below ISO 8528-9 guidelines. Reduced peak velocity of 2.4 mm/s was measured compared to ISO guidelines of 28.3 – 63.6 mm/s (see Figures 2 and 3).

The compensators consume little power, just 2-3 kW, and are easy to install but the benefits are significant. In engines where clearances are small, increased vibrations can cause reciprocating parts to hit contact faces. The resulting scratches then deteriorate lube oil film. The use of compensators reduces this and thus increases engine life.