Load variation effects on bearings

[21121956]

Hello everybody:

I am trying to get to know the harmful effects on connecting rod journal bearings caused by load variations (hunting) on an alternating current generator driven by a medium speed Diesel engine.

Even when I know how do the engines work and can figure out the damages on main and con rod bearings provoked by load rejection, I would like to know about this subject in more details.

Do somebody in the forum know a site or document dealing with this subject?

Thanks in advance

 

[Fabrico]

With a proper functioning governor, no measurable damage should occur.

 

[jbthiel]

What good information are you going to get from a dyno that oscillates enough that it kills your bearings?

 

[21121956]

Hello everybody:

We have several Diesel generators operating in parallel.
The engines speed control (kept the frequency constant) is achieved by the governor in speed droop regulating mode.

When power demand fluctuates (including transients or load hunting for any reason) the generator will vary its output via fuel regulation to its governor. The synchronous generator must control its output voltage by controlling the excitation current. Thus, the Diesel generating system, as a unit, must be able to control its frequency and its output voltage.

The ability of the engine to respond to frequency (speed) changes is affected by the inertia of the Diesel gen-set, the sensibility of the governor and the power capability of the Diesel engine.

And here comes up the question, with all these frequency variations, provoking load hunting in the order of 3 to 5 MW, how is the performance of the crankshaft (main) bearings and the big end (connecting rod) bearings? Can this situation of sudden speed changes destroy these bearings?

Thanks

 

[Fabrico]

...with all these frequency variations, provoking load hunting in the order of 3 to 5 MW, how is the performance of the crankshaft (main) bearings and the big end (connecting rod) bearings? Can this situation of sudden speed changes destroy these bearings?

Between the governor and the sizable mass of the rotating parts, there should be no wild RPM variations. It's never good to run any engine at higer speeds with no load, but unless the unit runs at an unusually high speed, wear from that should be insignificant.

I'm not a generator expert, but to ease things on your engine, your best bet would be to minimize governor response time but limit fuel to the least amout possible that will provide an acceptable frequency range. In other words don't try to accelerate the engine any faster than it needs to.

The main and rod bearings are always ready for full load.

 

[jbthiel]

Sorry everyone, my bad. I obviously misread generator as dyno - too late at night

As far as bearing damage goes, as long as the engine always has a load on it and it doesn't oscillate too fast I don't think it will harm the bearings. In general a slowly varying load is better for components so that they don't spend inordinate amounts of time at some resonant frequency. Even a big rig with the cruise control on will not stay at exactly the same speed.

I would be more worried about the turbo. If it is constantly loading and unloading you might see some accelerated bearing wear. That also goes for the fan, belt, governer linkage, etc.