Previous in Forum: Stamford Alternator Rectifier Diode   Next in Forum: TMD Breakers
Close
Close
Close
10 comments
Power-User

Join Date: Aug 2007
Location: Lahore
Posts: 369

Operating of Generator < 40% of Rated Load. The Harm?

05/31/2014 4:16 AM

Hello;

The rental generator companies quotes that their generator should not operate below 40% of rated load. If genset is say 1000KVA then the applied load must be > 400KVA. If I ask them the technical reason they just reply that it is harmful for engine. I want to know the technical reason behind it. So please help me to understand it.

__________________
Don't assume any thing, always check/ask and clear yourself
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
3
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20985
Good Answers: 781
#1

Re: Operating of generator < 40% of rated load. The harm?

05/31/2014 7:06 AM

1. Fuel efficiency is less.
2. Spray pattern from injectors decreases, and can result in "dribbling" or "blubbering".
3. Exhaust temperature is lower, possibly leading to condensation ("wet stacking"), which in turn can lead to corrosion of the exhaust system.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply Good Answer (Score 3)
Guru

Join Date: Sep 2012
Location: Iowa, USA
Posts: 577
Good Answers: 50
#2

Re: Operating of generator < 40% of rated load. The harm?

05/31/2014 9:52 AM

I'm remembering this from a similar problem back in the 60's (told to me 20 years ago). The company had a backup genset that used locomotive diesel engines. They ran them at low load. The oil would cause carbon build up at the exhaust valves. This then caused the cast iron cylinder head to crack at the valve seat, from a hot gas leak. Which caused coolant to flood the cylinder. The root cause was detergent oils, that produced the carbon (and additive) build up. At least that's my memory of the story from the long ago retired guy who figured out root cause.

__________________
ignator -
Register to Reply Score 1 for Good Answer
Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 4659
Good Answers: 796
#3

Re: Operating of generator < 40% of rated load. The harm?

05/31/2014 10:22 AM

What most people do it that situation is to have a load bank available to be able to perform a full load "exercise" routine at regularly scheduled intervals.

This excercise routine is often referred to as "blowing the carbon out" for the reasons stated above.

__________________
** All I every really wanted to be, was... A LUMBERJACK!.**
Register to Reply
Guru
United Kingdom - Member - New Member

Join Date: Aug 2010
Location: Resting under the Major Oak
Posts: 4348
Good Answers: 180
#4

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/01/2014 2:19 AM

↑↑↑↑ Again as above ↑↑↑↑

Carbon build up will ruin the engine eventually.

Think of it as your car. If its main use is just pottering around town, a good "blow out" on the motorway will clear the engine out.

Funnily enough I'm in the middle of a similar conversation on another forum.

__________________
The reports of my death are greatly exaggerated.
Register to Reply
Guru
Engineering Fields - Marine Engineering - New Member

Join Date: Aug 2007
Location: Australia
Posts: 1630
Good Answers: 19
#5

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/01/2014 3:36 AM

It is a fact that diesel engines do not like being run under light load conditions, some manufacturers recommend not to run under 66% of full load for extended periods. Where this is likely to happen, dummy loads are used, to load up a gen set up to over the 66% load requirement.

As pointed out previously, running under light loads cause problems with complete combustion and injectors start to dribble..........this leads to lubricating oil dilution fron the fuel and generally once 5% dilution occurs the nett result is excessive engine wear because the lubricating oil cannot effectively perform its role in protecting mating surfaces against wear.

The detergent additive in the oil scours carbon from the engine and due to excessive carbon, the dispercency additive in the oil can also become choked with carbon, again causing problems. Some may say that's OK I have a regular scheduled oil sampling program in place...........however, problems with oil can occur quite quickly and I would recommend that one finds out how to carry out simple oil test procedures that you can do say on a weekly basis or less depending on circumstances.

I recall once getting a 15% dilution reading in slightly over 48 hours of operation on a lightly loaded gen set........there were extenuating circumstances in this example that involved useless Royal Australian Naval marine engineering officers, but what I am saying is that just relying on scheduled oil sampling may not be enough if we are running an engine outside of the manufacturer's recommended parameters.

__________________
TO BE. or NOT TO BE. That is the question!! The Bard
Register to Reply Score 1 for Good Answer
2
Commentator

Join Date: Oct 2010
Location: Quantum Dimension
Posts: 85
Good Answers: 1
#6

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/01/2014 4:00 AM

Remembering the Wall Street Journal Europe Jan 1985. @the new installation of the 4 color presses towards the Harris computers and the PLC's

They had a backup generator of 250 KvA. This was coupled on an Energy PLC.

They asked me to take a look (I was only assigned for the project there , but they had heard some stories via Siemens & Iraq)

The generator ran every fortnight for a couple of hours for some lighting at 20% capacity. When a in weekend push came to shove when she was actually needed it broke down miserably.

Totally carbonized. It would give a max output of around 40% of spec.after a few years.

It took me 2 weekends to get it conform the output again. When the newspaper went to bed we took over, slowly switching more and more output until the turbo and the exhaust manifold glowed.

Later they asked me to take a look as well for the 'Ghost of the Wall Street Journal' I caught that one with specially build connections toward a multi light board from the 'lifeline' (ask the maintenance dept. in Heerlen it was quite a story)

This is what one of the biggest failures is in Emergency Situations.

I even know an Oil company with this problem on their emergency fire water pumps. Only 50% of the rated capacity and when I told that in the management meeting I got SACKED !!

__________________
Humans use 10% of their mind , WE take the rest
Register to Reply Good Answer (Score 2)
3
Power-User

Join Date: Jul 2013
Location: Faridabad Near New Delhi India
Posts: 240
Good Answers: 34
#7

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/01/2014 10:05 AM

Running a Diesel Engine below 33% results in Excessive wear and tear of Piston and liner (called Glazing of Liner or cylinder bore).

Reason being, The fuel pressure pump is designed to cater for:

1. Flow required at maximum load

2. Cooling of injector (fuel returning keeps injector at desired temperature).

The flow of diesel is thus practically constant. At low load as a result slightly excess fuel gets in to cylinders (one of the cause of poor fuel efficiency).

Also when load is less than 33%, engine runs continuously relatively cool - Thermostat By pass opened.

The excess fuel with cold engine, results in partially increased carbon deposit (due to poor combustion in cold engine) and partially remains unburnt.

This unburnt fuel and carbon (residual mixture) form a thick paste.

Now remember the piston rings do not run flat relatively to cylinder or liner walls as piston moves up and down in reciprocating movement.

The ring profile is designed in such a way that during compression and power state it is flat (almost parallel with cylinder wall) for maximum sealing but during exhaust and suction stage almost on edge (just a line contact) - this is to reduce wear.

Thus for minimum wear of cylinder walls/liner, the ring must be free to twist...

At low load the carbon + Diesel Paste formed above restricts above movement of rings and thus rings always run flat relative to walls. This results in excessive wear.

Now on walls of cylinder/ liners, there are very fine cross lines (looks like scratches) called cross honing. These are purposely formed to retain lube oil for lubrication of cylinders.

Above excessive wear clears off Cross Honing of Cylinder liners. Hence we loose retained lube oil and the wear and tear further increases.

I have seen that liner of a NEW 90kW DG Set, which were installed at near 10,000 feet in Oman for a cold store to stock farm produce by the Government (PAMAP) shining like mirror with in 3 months of its Commissioning (because in off season - no farm produce, but operators staying will be running the DG Sets for power at very low load). DG Sets were only source of power and no body dare to tell local tribes to not not to use DG Sets for personnel use.

Thus in 3 months the Engine lost power and started throwing lot of oil - and Manager asked me the reason. When explained, he asked for Load banks.

Next I calculated and explained him that the cost of diesel consumed at 50% load in those days (in 1987) in one month will be equal to cost of new engine block.

So being an Engineer - differs from Scientist that solutions shall be economical suggested him to install load bank with cyclic On/OFF .

That means let engine run at low load but once for in every 30 minuets, increase the load to 80 to 90% for 5 minuets to heat up the engine and burn excess diesel in cylinders. PAMAP (Public Authority for Marketing Agriculture Produce) got the Engines overhauled once and purchased above solution from me.

I was in Muscat for next 4 years (left in 1991) but never came back with this problem.

The rule is that if the Diesel engine generator has to run at low load continuously, load it to maximum capacity for 5 min in every half hour to burn the excess diesel to avoid above explained damage.

Other effect is slobbering: After some hours of low load run, the mixture of excess diesel and carbon residual starts entering exhaust components with engine exhaust and starts leaking from exhaust joints which do not have gaskets (just sliding seals). This black thick paste flowing out is called Slobbering of Engines.

Register to Reply Good Answer (Score 3)
Guru

Join Date: Sep 2009
Posts: 1047
Good Answers: 88
#8

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/02/2014 9:45 AM

There is nothing to understand. Nobody can understand stupidity, so don't be that ambitious. Other than the expected absolute fuel efficiency deterioration and marginaly higher exaust and intake pipes carburization mostly on engines with exaust gas refeeding, there is NO technical truth in that less engine load will give more engine problems. H*ll the RPM IS the same, it's just the engine torque required that changes. The guy that told you that, wouldn't know science, if it kicked him in the b*tt. S.M.

__________________
Life is complex. It has a real part and an imaginary part.
Register to Reply Score 1 for Off Topic
Guru

Join Date: Jun 2013
Location: Central Canada
Posts: 677
Good Answers: 28
#9

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/02/2014 10:45 AM

Very interesting answers, which lead me to an off-topic question -How does any of the information given relate to a vehicle diesel engine?

I dont want to pollute this thread, so if you are willing to inform me you could message me.

__________________
Smart as a post and twice as fast.
Register to Reply
Guru
Engineering Fields - Marine Engineering - New Member

Join Date: Aug 2007
Location: Australia
Posts: 1630
Good Answers: 19
#10
In reply to #9

Re: Operating of Generator < 40% of Rated Load. The Harm?

06/03/2014 9:52 AM

With prime movers this problem does not exist.........it exists where you have a diesel engine operating at constant rpm and the load is variable

__________________
TO BE. or NOT TO BE. That is the question!! The Bard
Register to Reply Score 1 for Good Answer
Register to Reply 10 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

ignator (1); JRaef (1); LongintheTooth (1); MOBI (2); powersolutionsFBD (1); SimpleMind (1); timelord (1); TonyS (1); Tornado (1)

Previous in Forum: Stamford Alternator Rectifier Diode   Next in Forum: TMD Breakers

Advertisement