Effect of Underload to generators 5

[Mechathan]

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

Mechathan

 

[waross]

I had a similar installation. A Cummins 855 running at about 10% to 25% loaded.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[Mechathan]

Thanks Bill, Its just that when you're in Middle East, the service group of the manufacturer is harder to cooperate with client. They dont even want to have it serviced all the time, but has not given any sensible answer from us why.

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

 

[YuriSarcas]

If they are not worried about the money then go buy a load bank. You can then load that up to 50% or so and you will not have to worry about the wet stacking.

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.

 

[waross]

I'm hoping that catserveng sees this thread and posts. He may be able to share with us some information on the algorithm used on the Marathon units to calculate oil change intervals. I understand that the computer will extend the oil change interval if the engine is less than fully loaded. I wonder if the computer shortens the interval for extremely light loading.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[YuriSarcas]

I don't think there would be much of a computer on this thing. The NT 855 is old technology. After speaking with my Cummins guy he said he would recommend to keep the same intervals that is stated by the OEM.

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.

 

[ScottyUK]

The NT855 (NT14) is getting old but is still a current product, or was until recently. Good rugged engine, simple and reliable. They were being offered with Cummins' PCCP controller over here - nothing as complex as lube oil diagnostics on there, just engine hours and the live data you'd expect from a mid-range controller.


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If we learn from our mistakes I'm getting a great education!

 

[catserveng]

Since it appears you're running prime, even with the low load factor, sticking to the recommended maint intervals is probably the best way to go. You're likely causing more rapid oil breakdown to the lube oil since the engine is likely generating more soot and crankcase gases. It's also likey based on where you stated the units are that you are running on fuel with higher sulphur and contaminate levels.

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,

 

[Mechathan]

Thank you for all the inputs. I can now convince management to do the next steps. All of you are really of great help. Regards to all and again thanks for the help.

Regards,

Mech

Mechathan

 

[waross]

Hi catserveng;
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
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Hi Bill,
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

 

[edison123]

catserveng

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 ?

 

[catserveng]

I believe the newer 6BTA will have quite a few problems on the low loads you are expecting compared to the older unit. Mostly due to slobber and fouling. It is usually offered as a lower power prime genset I believe, so it won't be quite as bad as a tier 2 or 3 or TA Luft engine, but internally the ring pack and valve interface will have a harder time with light loads than the older engines.

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.

 

[edison123]

Thank you so much catserveng. I am amazed how ignorant I have been about ICE's despite owning 3 of them for over 2 decades.

 

[waross]

Thanks catserveng.

Bill
--------------------
"Why not the best?"
Jimmy Carter