Effect of Underload to generators
Effect of Underload to generators
(OP)
Hello Everyone,
I have 2 Cummins generator NT 855 at 320KVA capacity each with ATS. Only one generator is being used and at the moment is at about 10% or below load capacity. Each of the generator runs for 2 weeks 24/7 before we do the change over. Two questions:
1. Does the generator, even at low running capacity, has to be maintained after it reaches the 250 running hours?
2. Does under loading has effects/create problems to the generators?
Hope somebody there can help. Seems that our company has lots of money to purchase big gennies without evaluating the required power supply. Thanks.
Mech
I have 2 Cummins generator NT 855 at 320KVA capacity each with ATS. Only one generator is being used and at the moment is at about 10% or below load capacity. Each of the generator runs for 2 weeks 24/7 before we do the change over. Two questions:
1. Does the generator, even at low running capacity, has to be maintained after it reaches the 250 running hours?
2. Does under loading has effects/create problems to the generators?
Hope somebody there can help. Seems that our company has lots of money to purchase big gennies without evaluating the required power supply. Thanks.
Mech
Mechathan






RE: Effect of Underload to generators
BUT, when the one large motor started, it took everything the engine had to keep the generator turning.
Diesel engines do not like light loads, it leads to crankcase dilution and slobbering or wet stacking.
Sometimes, however, the load profile makes light running unavoidable.
As to oil change frequency, yes and no. Some of the computer controlled engines set their own oil change interval based on an algorithm which allows extended intervals when the engine is less than fully loaded.
Due to the possibility of crankcase dilution at light loads it may not be wise to extend change intervals on very lightly loaded sets.
The bottom line is to contact the manufacturer for advice before changing service intervals.
Bill
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"Why not the best?"
Jimmy Carter
RE: Effect of Underload to generators
I agree with you Diesel Engines does not want light loads.
Our load profile is really only up to 10% max. I just can't believe these people only buy but don't compute???
Thanks again,
Mechathan
Mechathan
RE: Effect of Underload to generators
Regardless of the load you should keep the same maintenance intervals for the most part. Especially if you are in in the Middle East. I would assume there is a lot of dust, dirt and sand blowing around that place.
RE: Effect of Underload to generators
Bill
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"Why not the best?"
Jimmy Carter
RE: Effect of Underload to generators
You do not need to do it but if you expect to get the maximum amount of hours from your gen then the maintenance at regular intervals would be desired.
RE: Effect of Underload to generators
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If we learn from our mistakes I'm getting a great education!
RE: Effect of Underload to generators
Lube oil sampling would be a help if you can get reasonable turn around from the labs, both wear metals and IR (IR requires a new oil sample to most accurate), should be dome, as well as a TAN or TBN test to determine how well the additive package is holding up in the current service, you can use that info to adjust your oil change intervals if desired. Other maintenance associated with the 250 hour interval, like fuel filters, coolant conditioner checks, and air filters should also be done, even with the lower loads, your environment plays a big factor in how often all that really needs to be done. By adding some instrumentation like fuel filter restriction gauges and oil filter differential pressure gauges you can get an idea of how well these systems are doing their jobs and if you room to move the intervals out.
On overall engine life I believe you'll find it decreased by running extended periods with light loads, I recently was asked to do some failure analysis on two Cummins KT series engines, older units with relatively low hours, but found based on hours and fuel usage records the load factor was quite low, around 15%, the cylinders were badly polished, the top piston land and ring grooves were heavily carbon packed and the rings badly worn. Also noted that most of the valves appeared to be leaking due to heavy deposits in the seat areas. I have had similar experience with many brands of engines, newer engines seem to be more sensitive to load factor, turbocharged engines with keystone ring packs do very poor with light loads as compared to NA engines with rectangular rings. Poor cooling system maintenance and temperature regulators not working properly compound the problems.
A portable load bank on a few hours a week would go a long way in helping the engines meet their target service life in my opinion. At one site we added some large electric hot water heaters and ran them for equipment washing and laundry, look around there might be a relatively painless way to intermittantly increase load once in a while to help burn some of the crud out.
As to Bill's question, I have seen a few different types of calculations done to help extend some maint intervals, either with a calculated fuel consumption or a load or duty factor. Sometimes they seem to help a bit, sometimes the results from oil samples and filter inspections drive a change opposite from what the controls are trying to say you can get away with. Best reults I've seen is in units extending intervals going from 80-100% load factor down to 50-60%, below 50% I have found maintenance usually starts to trend upward toward the full load numbers, and very light loads often cost more in maintenance and repairs than do units operating at high loads, at least in my experience.
Most manufacturers advise maintenance intervals based on "normal" conditions, some don't do a very good job explaining what normal is. Both CAT and Cummins have some pretty good application and maintenance literature, but many factors influence things like oil and filter life, fuel filter change intervals and major services like valve and fuel system adjustments and overhauls.
Monitor fuel and oil consumption rates and trend them, when they start to deviate from the trend things are going bad, the more the deviate from the trend the worse things are getting. Oil smaple reports work the same way. I am also big on opening up a used oil filter and peeking inside, can tell you alot about the health of an engine.
Hope that helps,
RE: Effect of Underload to generators
Regards,
Mech
Mechathan
RE: Effect of Underload to generators
Thanks for the great answer! About as I expected, below about 50% load, things get worse instead of better. I have had to boil quite a bit of water with a couple of new sets when there wasn't enough load to seat the rings and severe wet stacking resulted. One of the sets was a Cummins 855 very like the one under discussion. My load bank was several plastic barrels filled with salted water and some discarded truck leaf springs.
Bill
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"Why not the best?"
Jimmy Carter
RE: Effect of Underload to generators
Amazing what we have to do in the field sometimes to do our jobs. My worst load test experience was with a salt box on a Pacific Island, the tank was rotted out, so we ended up hanging the plates into the bay and about 5 small boats to circulate the water, locals picked up quite a few fish that day!
I have recently fielded a large number of calls on this issue. Most people are not getting it that newer engines really have a hard time with light loading. Diesel engines like to run at near rated load factors, just the way they work. Add turbo's, aftercoolers and aftertreatment and things just get worse. Most of this is managable, just need to assure that when the units go in regular testing at proper load levels can be achieved.
A couple years ago had a bank building in downtown San Diego that the standby had run at no load once a week for 4 years, the bank's insurance company insisted on a load test. Load bank got hooked up (with over 200 feet run of cable, not a very good design)about 30 minutes into test the silencer caught fire, smoke pouring out. The Fire department sent four engine and two ladder companies on what turned out to be a "false alarm", the descision was made to keep running it and burn it out since shutting it off would have made a bigger mess. After about three hours the smoke cleared, engine actually ran pretty good, and test was deemed "succesful". End cost, a hefty APCD fine, callout charge for fire and police, and a new exhaust system, rumored to be near $350,000 when all said and done for a 500 ekW set.
And don't think that gen set gets a real good test at the factory, unless you specify a witness test and a defined duration, most units are out of the cell in less than 30 minutes, know most manufacturers are doing this.
Actually Mechathan has one of the best engines in my experience for long term low duty factor running, and I worked a long time for the competition.
Mike
RE: Effect of Underload to generators
While we are on the topic of low loads on ICE, what is your view on Cummins 6BTA 5.9 - G2 I engine on low loads ? I am planning to replace my 25-year old Cummins NTC 495 G engines.
My load cycle is typically low at 10 to 15% most of the time but would require 100% at a very short notice.
In both cases, the generators is 415 V, 125 KVA, 1500 RPM.
Should I buy the load bank to keep the engine loaded ? If so, what should be the minimum running hours per month at such full load ?
RE: Effect of Underload to generators
I would go with the load bank, and provide at least 50% of the rating. How much to run is a tougher question, mainly because between cost of fuel and if you have air quality issues, the "extra" run time can become a real cost issue.
I have quite a few customers who basically go by visual prompting, when the stack gases get hazy they run with load until the stack clears. Doesn't sound very scientific but it actually works pretty good.
If you're running prime, I'd see if 4 hours a week will keep slobber down. If you're using lube oil sampling, try increased sample intervals for awhile and get an idea of soot loading. IF you have a load bank also record some transient loads, say 25 and 50% if possible, because one of the first engine performance parameters affected by light load issues is usually transient response, especially on turbocharged engines. Wish I had a good canned answer for you, but every site and engine combination seems to have quirks all their own, what has worked well on several sites sometimes doesn't quite seem to do it on others.
You might see if you have any peak loads, like water pumping or heating, either at your site or on a neighbors site to get a higher load once in a while. On one of the smaller Pacific Islands the power plant made a kava dryer out of a load bank, so at least they got some benefit out of the "waste" heat.
Hope that helps.
RE: Effect of Underload to generators
RE: Effect of Underload to generators
Bill
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"Why not the best?"
Jimmy Carter