Very low load on Generator 1

[AusLee]

Hello,

I intend to purchase a 1,000 kVA generator for an office building with a maximum expected demand of around 700 kVA.

My problem is that it is very possible that the public energy will cut out at night, and in this case the generator will enter, through the ATS switch, to supply a building with very low load, mainly some UPS and exterior lighting, summing up to not more than 50 kVA <<< 1,000 kVA.

Could you please tell me what damage this could cause, as i was told that the oil will not heat enough and the elevated friction in the generator will deteriorate the cylinders, plus i will have an oil leake from the join (i do not know why)?

Isn't there some kind of a heater available as an option one can buy for the generator in order to resolve this problem? I know there are heaters for the coolant water, used in cold countries to allow the generator to take the full load on startup, but what about the oil?

Thanks.

 

[davidbeach]

What you need is to find other load to put on the generator, through another transfer switch.

The generator itself doesn't care, but the engine needs to run at operating temperature. All the metal parts are designed (sized) for operation at rated temperature and extended running at less than normal temperature can leave things loose. Running at too low a temperature can also allow the exhaust to condense in the stack (wet stacking) and that will also cause trouble down the line.

Find more load, or add a 75kW generator that can carry your 50kVA of load and leave the large machine off.

 

[peebee]

I wouldn't be overly concerned about it, especially if this is a typical standby application. Check with the generator vendors if you really want to know.

How many hours a year do you expect this thing to run? And how many hours per year lightly loaded (say, less than 30% load). How many hours a year are you permitted to run before you run into emissions concerns? How many hours could the generator be expected to run reliably at low load levels? I have a sneaking suspicion that your low load level will have little or no impact on generator lifespan.

Yes, there are crankcase heaters available -- and I'd almost guarantee that your generator is coming with one. It's not clear to me if such a heater would bring the oil up to the temperature it needs to be.

I would think that proper oil selection might be part of the solution too.

But I still recommend you check with the vendor.

 

[ScottyUK]

The heaters are primarily to allow easier starting and allow the set to accept load pretty much as soon as it hits running speed. They won't come anywhere near getting the engine to running temperature. Your oil viscosity will be higher than designed and the lubrication will not be running at its optimum conditions so engine wear will be greater. Wet stacking - also known as 'slobbering' due to the oily crap which is ejected from the exhaust - is bad news if the set has to work hard after a long period of idling because the unburnt fuel and oil can start to burn in the exhaust.

If the likely condition is a few hours of low load followed by a period on load then it probably won't be a problem. The real problems are when someone oversizes a set by a huge margin and then runs it at a fraction of its rating for a long period. This is common with rental sets - that's why they are so cheap after only three or four years: the engines are knackered.

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Sometimes I only open my mouth to swap feet...

 

[waross]

I have installed a lot of smaller sets that typically run at 10% to 25% load.
I have had a few problems.
First, order a heater. I like the circulating block heaters the best. This keeps the engine coolant warm. When the engine starts, the warm coolant thru the oil cooler brings the oil temperature up fast. (Don't forget to spec. an oil cooler.)
The sets all need the capacity for motor starting loads and A/C loads, but many times on a cool night a power failure will leave the set with almost no load at all.
The problems that I have encountered have been with piston rings not seating well.
Most engines have run for years with no problems, but I have had a couple that pumped oil. The compression rings are so designed that combustion pressure forces them onto the cylinders for a tight seal. At low loading the combustion pressure and the force of the rings against the cylinder is much less. An occasional engine has a less than perfect fit to the rings and will pump oil badly if run initially on a light load. I had one that in a few hours pumped the crankcase dry and shut down on low oil pressure.
When this happens I head for a scrap yard and buy rent or borrow some discarded leaf springs from a heavy truck.
Then I locate some 50 or 60 gallon plastic barrels and a sack of salt. I fill the barrels with water, cable the leaf springs to the generator and stand them in the barrel.
Start the generator and add salt until you have about 80% load and start to boil water. 10 hours or so is ussually enough to seat the rings and the sets have never again given trouble on light loads.
Another remedy that I have not used myself is the Bon Ami treatment. For those of you in other climes, Bon Ami is the brand name of a pumice based kitchen cleaning powder. One or two spoon fulls into the intake manifold of a fast running diesel is said to seat the rings and stop excess smoke from a new engine. I prefer to boil water.
Given that at low load levels there is not enough wear to properly seat the piston rings, I don't worry about a diesel wearing at light loads.
BTW, The folks with the set that lost all its oil, found that if they pushed to little button, the set would restart and run for a few seconds. They admitted to pushing the button 3 times before calling me. I refilled the crankcase and the engine has never pumped an ounce of oil since.
Note: This was an anecdote, not a recommendation.
Back to boiling water, the man who put me onto this method described renting a large construction site trash container.
Maybe 30 or 40 cubic yards. He would seal the endgate with two by fours and duct tape, fill it with water and throw in 50 or 100 lbs. of salt. He would rent a small crane or forklift to lower his scrap iron electrodes into the water.
As I remember this method was good for up to about 1000 KVA.
Don't tell the rental firm the truth if they ask what you intend to put in the container. Bring lots of water. You will probably boil off a lot.
I had 6 60 gallon barrels (up on a roof for security) on a 280 KW set and I was refilling the barrels 6 times a day. You can boil a lot of water in a hurry.
This was at the demand of the vendor. He was threatening to void the warranty on a new set on the basis of light loading, but agreed to honor the wearranty if we gave the set 5 days of heavy loading to seat the rings. That was back in '93 and this set is still running fine with no oil pumping. it usually runs at 10% to 25% load, but there is one motor that demands all the set has when it starts. Then back to light loading. It's on standby and the hours are quite low.
respectfully

 

[genhead]

Extremely light loading will eventually cause engine problems as described above. However, if the genset is only running for a few hours a year (say 50-100?), then it is a good idea to load it up as mentioned by Waross. This could be done annually. Most generator rental/service companies will have resistive load banks available specifically for this purpose. I too have used the salt water method, but in today's safety-conscious world, a purpose built load bank is probably more acceptable (but less fun!). Access to the generator is sometimes a problem.
Another solution (if the genset is likely to run for prolonged periods) is to incorporate a resistive load bank into your system. It can be automatically switched in/out as required, to maintain a reasonable load on the genset.

 

[edison123]

waross

That was an interesting bit of anecdote.

I still use those barrels with water and salt as a LRS for testing many wrim motors / dc motors (as generators) at my shop.

 

[JBinCA]

The thumb-rule I have always heard with diesel generators is that if you are operating at less than 25% load you are contributing to the wetstacking problem.

This causes a build-up of creosote which can lead to fires in the exhaust system / ducting.

JBinCA

 

[waross]

Based on my experience with chronically lightly loaded sets, I don't think that AusLee will have any problems if his set is occasionally lightly loaded.
respectfully

 

[davidbeach]

I read the OP as though the light loading would be a regular occurrence, perhaps most nights, resulting in the potential for hundreds of hours per year of very light load - that would be bad. If on the other hand it is only 20 - 30 hours per year with that light load and it sometimes gets to power a load worthy of a generator of that size then I don't see much of an issue.

 

[AusLee]

Hello,

Thank you for your replies.

Indeed there would be hundreds of hours. There is not enough room for a small generator. All AC are split units, so i do not have a large air handling unit to activate automatically. The load will go back to 1,000 kVA in the morning, so i think any residue in the exhaust system will be pushed out, is that correct?

I will see about the automatic load bank, and will check with the manufacturer for oil heaters, even though as you describe the lack of pressure will cause oil leakage, which is a different problem than friction.

 

[davidbeach]

I think that if you can get an hour or two of 50% or more load for each night of nearly no load you will probably be fine. It would be much better to bring it up with lots of load and then back off for the remainder of the run than to start with very light load and then go higher load. Ensuring a good load and high temperature on the exhaust should minimize any issues with wet stacking.

 

[waross]

To clarify, I have found that most sets have the rings well seated from day one and have no oil pumping problems at light loads. The very few that do pump oil can be cured by running at 75% or more load for 10 or 20 hours. Some of my customers disable the automatic start at night in their stores.
It is my impression that "Wet stacking" is more of an issue with large slow speed machines rather than the new high speed diesels. Typically, an old diesel generating plant may use an old tired, worn out machine at light load and high excitation to either adjust the voltage or supply VARs to the system. An old worn machine that already pumps a little oil will probably pump a lot at prolonged light load.
respectfully

 

[JBinCA]

I think I can add from my operational experience now that we have more details.

I was an operator in a plant that had 4 2.5 MVA diesel generators used for emergency power. The gens were new in 1995 to give some idea of vintage, and were 1800 RPM machines.

Whenever they were operated at less than 25% load for more than one half hour, there was a requirement to run them loaded to burn off the creosote. I wish I could remember the load levels and time requirements, but it's been a while.

I would say that if you mentioned your intended loading (very light load at night followed by significant load each day) to the manufacturers you had in mind to supply the generator, they could put the issue to be for you. I believe that the daily loaded runs will remedy the problem.

Regards -