Minimum Generator Loading 4

[Gene1203]

I am looking for some standards and/or publications regarding the recommended minimum load (%) of generators (diesel engine-driven) with respect to their rated output - particularly on prime duty generators. I appreciate the help. Thanks.

 

[rbulsara]

If I understood your question correctly, diesel engine manufacturers recommend minimum 30% to 35% load to avoid "wet stacking". This is also a good practice observed in the USA. For a prime duty this should not be a problem ( to get 30% load!!).

Wet stacking occurs in a diesel engine when the engine is run at too light of a load during the run-in (also known as load banking). This causes unburnt fuel and oil to collect in the exhaust stack. The exhaust stack will be coated with a black, sticky, tar-like substance. If you run a new unit at light loads and/or idle for too long, the rings will not seat properly. If it is a diesel, wet stacking may occur. Both of these conditions will cause poor engine life and performance.

I am not sure it is a code issue (it may be) but a performance issue. You may want to browse leading DG set manufacturer's web sites or call them.

 

[Gene1203]

rbulsara

Thanks so much for the quick reply. Actually, we have a project where we supplied and installed a prime rated generator. What we observed is it slobbers (wet stacking) when the load falls below 40% of its rated output. This was observed by the supplier themselves. Now, the supplier of this generator suggested not to allow usage of the unit when the minimum load falls below 40%. The load varies in time. Our client would like us to submit an official document from the supplier regarding this %. However, the supplier is reluctant to give us their official recommendation. Our client also insists that 30% is the allowable minimum load. In order to have a guide for the operation of this generator, we need to submit a standard or publication to support the 40% or 30% issuse and from actual conditions, 40% is applicable.

The supplier also refered to EGSA stating that 50% is the minimum allowed. The EGSA page refered to is as follows:

http://www.egsa.org/powerline/past/mj99plantbas.htm

Where did you get the 30-35% data? Please let me know.

Again, thanks very much.

 

[rbulsara]

gene1203:

I am fairly sure that this is not mandated by any 'standard' but a inherent characteristics of diesel engine and 30-35% is what is normally recommended by mfr.

Wet stacking at 40% load DOES sound out of ordinary and it may be related to inefficiency (of combustion) of the engine. I am attaching what is published in 'Reference and Installation Manual' of Caterpillar, a leader in engine manufacturing.
You will have better chance of claiming the engine performance being below par compared to the normal experience of the industry.

There may be many other variable such as length of stack, age of your engine (u did not mention if this is a new installation). If this is old engine may have deteriorated over time, especially with a prime duty.

Below is a cut and paste from the manual referenced above, you can make you own conclusions (read the first and the last paragraphs carefully):

"Slobber:

Extended engine operation at no load or
lightly loaded conditions (less than 15% load)
may result in exhaust manifold slobber.
Exhaust manifold slobber is the black oily
fluid than can leak from exhaust system
joints. The presence of exhaust manifold
slobber does not necessarily indicate an
engine problem. Engines are designed to
operate at loaded conditions.

At no load or lightly loaded conditions, the
sealing capability function of some integral
engine components may be adversely
affected. Exhaust manifold slobber is not
usually harmful to the engine; the results can
be unsightly and objectionable in some cases.

Exhaust manifold slobber consists of fuel
and/or oil mixed with soot from the inside of
the exhaust manifold. Common sources of oil
slobber are worn valve guides, worn piston
rings and worn turbocharger seals. Fuel slobber
usually occurs with combustion problems.

A normally operating engine should be
expected to run for at least one hour at light
loads with significant slobber. Some engines
may run for as long as three, four or more
hours before slobbering. However all engines
will eventually slobber if run at light loads.
External signs of slobber will be evident unless
the exhaust system is completely sealed.

If extended idle or slight load periods of engine
operation are mandatory, the objectionable
effect of the engine slobber can be avoided
by loading the engine to at least 30% load
for approximately ten minutes every four
hours. This will remove any fluids that
have accumulated in the exhaust manifold.
To minimize exhaust manifold slobber, it is
important that the engine is correctly sized
for each application.”

 

[alehman]

I concur. Several U.S. manufacturers have also told me 30 to 35% should be considered a minimum load to avoid wet-stacking, but the load doesn't have to be continuous as noted above.

 

[jbartos]

Suggestion: Visit

http://www.ecmweb.com/ar/electric_sizing_gensets_large/

for:
Extended light-load operation of a diesel engine can result in the accumulation of unburned fuel in the exhaust system, due to incomplete combustion from low combustion temperatures, called wet stacking. Light load operation can also result in engine damage from fuel and water contaminating lubricating oil.

 

[Gene1203]

rbulsara and alehman:

The load as I have mentioned reaches to more than 80% of the generator capacity - this depends on the demand of course. The minimum load usually occurs at night time - but not always. At daytime, the minimum load is around 50%. The generator was sized to handle the entire load of the facility. However, the load is not constant throughout the day. If we put it on a 24-hour operation, 80-90% load occurs for more than 14 hours, 50% up to just below 80% occurs for more than 6 hours and for the remaining 4 hours, the load is 30% to less than 50%. But the above operation is just a generalization - it varies on the daily demands of the facility too. The power plant has existing units with equal ratings of half the new unit.

What the consultants want to obtain from the supplier is their official recommendation so that the facility should operate the existing generators during night time for less than 6 hours a day. But they disapproved the supplier's recommendation as I pointed out above. The supplier seems to want to stay on the safe side - very safe in fact.

If you have any documentation of the following generators regarding the 30 - 35% you pointed out, please give me a link or if possible, e-mail it to me:
1. Wartsila
2. Caterpillar
3. Cummins
4. Perkins
5. Other popular manufacturers

Thank you very much.

 

[jbartos]

Suggestion: Visit

http://www.gti-aci.com/pdf/GTI IOM 3-03.pdf

for Bi-Fuel System solution to generator wetting conditions

 

[GerH]

You could solve the problem by adding a load bank ( say 10% of genset) controlled to switch in when plant load falls below 40%.

 

[rbulsara]

For website you can do a search on the web and you will find them.

Are you saying you have total of 3 generator units? (2 old and one new?) Can you develop a control system to shut off one of the units during light loads to keep the remaining units fairly loaded?

 

[Gene1203]

Yes, there are 3 units (2 old and one new). That is why we need to provide the user a recommendation that in the event that the load (when they are using the new unit), falls below 30-35% (but the supplier insists 50%) of the new unit's capacity, they have to operate one of the old units and and use it instead of the new (the old units are half in capacity of the new).

All generators are synchronizable for transfering loads. The operation is that they can use the new unit as one to supply the entire load; or synchronize the 2 old units to supply the entire load; or use any one of the old units when load is low.

The user actually prefers to use the new unit all the time because of its efficiency in terms of fuel consumption but because of the obvious reason that the new unit is twice in capacity, the load should be considered as a factor.

 

[alehman]

The load duty you stated seems to be within normal operating range of any diesel I am aware of. I don't understand the supplier's concern. I would go with the manufacturer's recommendations. You may have to take this up the "food chain" with the manufacturer to get them to reach an agreement with their supplier. In most circumstances, the manufacturer's recommendation is the basis for the warranty and is what should be followed.

Operating the unit below even 30-35% should be tolerable as long as it is not for an extended time. I would think 4 to 6 hours per day even at very minimal load would be fine as long as you load them up for a while as well.

Both Cummins and Caterpillar's web sites are very lacking in specific infomration on their products. I'll dig out some O&M manuals. What does yours say?

 

[Gene1203]

alehman:

Thank you very much. I think we have the same point of view. Can I e-mail you directly? I would like to ask you some items I am afraid I cannot post here. Also, I do not have any info as to who to contact directly for the manufacturer of the generator in question. I feel you can provide me.

The O&M manuals I have are also vague. However, it mentions in a short paragraph about some value to test the generator for slobbering - but this is only a test procedure.

I really appreciate the effort.