20 minutes of Motoring Diesel Generator

[sparky1976]

Hi,

I've been waiting to write this for 8 months just to proof that the engine is OK.
About 8 months ago we have an incident that generator breaker was fail to open, resulting approx about 20 minutes we have reverse power about -1 MW. At that time the operator donot realize that breaker fail to open so they shut the fuel valve. After calling one of the Senior operator he found out that breaker fail to open and he open the "downstream" breaker in swicth yard to shut the electricity to the plant.

We change the failed breaker, check every part of diesel engine closely, Nothing found damage. I think one of the mech eng do NDT test to find cracks. No cracks. coupling in good condition.
Test the engien full load for several hours, OK.
The engine is 5 MW, and 6 years old.
The reverse power protection for genset is set at 5% 10 seconds delay. (recommended by genset mfr)

During the investigation meeting one of analogy come up from one of mech eng, that motoring the diesel engine is same as if you riding your truck down hill and put to low gear and let the engine to do the breaking job.

The question
Is any one experince prolong motoring on diesel engine cause engine to cath fire or explode (as per IEEE buff book) ?

Now we change the delay to 30 secs to avoid reverse power trips since the diesel engine we use to chase erratic load, and steam plant as base load.

 

[itsmoked]

It really shouldn't hurt anything since the oil pump is generally turning if the crankshaft is turning. The only fire potential I can think of would be if all cooling is electrical and power was missing. Compression heat might build up. I haven't seen it though.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

There are a couple of possible problems.
1> Lube oil carry over. Worn engines tend to lose lube oil out the exhaust. The pressure of combustion helps to seal the oil control rings, and the engines will often lose more oil when lightly loaded than when heavily loaded. (I've seen this a couple of times).
Motoring would be a worst case for such an engine. Lacking the hot exhaust gasses to burn the lube oil, it will probably accumulate in the muffler, or low points in the exhaust piping. If so it will be a fire/explosion hazard the next time the engine is started.

2> Mechanical injectors and some fuel metering/control pumps are lubricated by the fuel and may be damaged if the engine is motored with no fuel.
Damage would probably show up as low exhaust temperature on the affected cylinders.
I was called in after an almost identical motoring incident at a small plant.(350 kw and 600 kw sets.}
No damage was done except to the feelings of the operator who forgot to open the breaker when he took the set off line.
I made some nice money installing 5 reverse power relays to avoid future incidents.
In my younger adventures, I ran out of fuel in a large diesel logging truck just before the start of a 6 mile hill. I was cold and late at night, and there was a warm bed at the bottom of the hill. I had to keep the engine turning in gear to mauntain air pressure for the brakes. No damage was done to the engine coasting down the hill.
respectfully

 

[dpc]

If you were absorbing 1 MW on a 5 MW unit, you were at 20%. Last time I checked, typical recommended settings for diesel generators would be a pickup at a motoring level of around 25%. So probably you didn't cause much problem.

I've never had any problems with diesel generators and motoring.

Steam turbines are a much bigger problem because of potential overheating in the LP section of the turbine.

 

[rmw]

Some trucks spend half their time being motored as they transport their loads downhill. The up hill journey is not a heavy load on the engine as the rig is empty on the way up hill.

Before the advent of engine (compression) brakes, the engines would cool off on an extended down hill run so much that the term "iceboxing" was used to describe it. Without an engine brake, the work of compression is returned to the crankshaft as spring work so except for friction losses they balance. The compression brake releases the compressed charge and another cylinder has to do the work to replace the lost spring work. That heats the engine.

In your case, except for the oil blow by issus mentined, I don't think you would have a problem. If your injection sysem is not designed for a "no fuel" condition, then you might have issues with the non lubricated fuel injector parts.

rmw

 

[sparky1976]

Everyone, thanks for the input, the report showed that operator should not cut the fuel to keep lubrication for the injector, and we install generator breaker failure protection scheme for with time delayed 1 minutes for diesel and 30 seconds for steam turbine to be on safe side.