Temporary Load for AC Generator

This is a bit of a guess, but.... Diesel engines are sometimes said "to like" running close to full load. At lighter loads, depending in the injector system, the spray pattern of the nozzles may degrade somewhat, perhaps even dripping or "blubbering" fuel into the cylinder. This off-pattern fuel will not burn well, leading to deposits on the cylinder wall [¿¿ I'm not sure of this--this is the guess part??]. While it won't affect the generator, it may reduce the engine performance.

I think you should ask the genset vendor for more explanation.

If your main process has periods of light load, there might be some way to use the extra energy available. To pick one, there is a method called "ice-building," used in HVAC. The basic idea is that during off-peak hours you run refrigeration to build a reservoir of ice. Then, when you need energy elsewhere, you shut off the refrigeration and let the ice bank do your HVAC. I realize this may be a too-brief explanation. If you think it might be useful, I will be glad to provide more detail.

Thank you , Guru, it makes perfect sense.

What you say is totally correct.

We are facing the same problem with our Caterpillar 1600 KVA pair.

They are estimated and installed for higher rating than the actual need.

Engines are working half of their rating during power outage.

They are synchronizing.

But shutting one of them and working with only one machine is not secure due the nature of the work. Here is a hospital,

Sometime we see some brown dripping on the injector pipes. The manufacturer's representative said, the reason is working under the low power conditions than estimated minimum value.

Kindest Regards

Hi murthyraman,

Your supplier is saying that you also need to consider what happens during the 'running-in period, as this also affects the performance of the engine for the rest of its life. You can't hurt a diesel by working it hard but you can by underloading it. The answer is to provide it with more load which could be as simple as a large heater battery either immersed in water or pre-heating air in an AHU.

You should also avoid very short 'test runs' as these also prevent the machine reaching full operating temperature, I include oil temperature in this statement as the temperature of the piston crowns plays important role in determining the oil film characteristic.

Good luck,

Massey.

Low loads equal low flowrates of fuel and air . . . fuel is a coolant and lubricator to the injector and injector tips, air is a coolant to blowers/supercharges . . . during low load conditions . . . insufficient coolant and lubrication . . . thermal damage results . . . blower clearances increase . . . fuel injection atomization impacted . . . thus the next time you operate the engine at high load . . . poor performance. Unique thing about diesel . . . combustion tempertures and pressures are same at any load (reasonable if considering mechanical condition of engine).

if you underload the diesel engine you will end up replacing the injectors &overhaul earlier then estimated recommendations of engine manufacturer i would suggest never run your diesel generators with less than 60% load and when you select and size the diesel engines take extra care to avoid the issues we are discussing i am writing this with over 40 years of working experience on board ships and commercial building with all most all engines manufactured in US and Europe

crm

Thanks for the info, will do accordingly,

Warm regards.

murthy

Hi Murthyraman,

Normally, a question mustn't have to be answered by another question. However I am wondering why and how did you come up buying and installed 1000 kVa genset where you should know your maximum and minimum load?

The generator supplier should tell you that when you are sourcing the unit and not now when you already installed it, also it is very essential that before buying the generator proper consultation or study must be taken into accounts as this is big investment involved.

If in your situation your maximum peak load of let say 600 kW and having only 280 kW certain period of time If I were you, I will rather buy and install 2 units of 500 kVa fitted with auto synchronization and load sharing control , it would be more ideal for your operation and in case one is being service or breakdown you still have one running.

Running your unit for only 35% load continuously is not good. you will encounter some excessive oil coming out from your breather pipe, the engine oil can becomes sticky as the maintaining engine required temperature can't be attained. then you will encounter premature breakdown of your engine.

To give you idea on what kind of temporary load should you can use should requires us more understanding on the nature of your establishment, if you are in hotel industry, factory etc. as the load will varies on needs, remember to use temporary load just to compensate the required generator rated capacity is "wastage" anyway to run oversize unit on your requirement is already considered as wastage. and to save money is first, terminate or get rid your consultant/designer who advised to put that unit, and pick up most of valid and useful information advised from the CR4 member (which is free of charge).

However if you are the one who made the recommendation to buy and installed the said unit, then you must learn from your mistake.

Good luck

Roman

It is sometimes the case, and becomes a crisis in rapidly deteriorating electrical power situations, that the demand to have a power source makes it imperative that any available machine be taken advantage of. A few years ago I was involved in the development of new, refurbishment of old, and privatization of facilities for electric power production in a country where planning caused a crisis and brown-outs/blackout conditions were a daily happening. In that country, generator sets were impossible to be found . . . thus any generator would do. It was a crisis beyond imagination. There is also the "good" deal . . . someone has a set that has been idle and they sell it at a price that is just to good to be true. In each of these conditions, gen sets that were far greater than needed capacity suddenly find application. People buy these gen sets that have no idea what they are doing . . . turn the units over to their operations or maintenance organizations and say . . . I bought it . . . you got to make it work and then you can keep your job!!!

You can rent a load bank. Rent a size where you can load one unit to full load. Run one unit for a time suggested by the vendor then switch to the other.

You may need to do this every year or two until your in-house load is sufficient to load up your units.

Bob R

You may construct a load test bank by yourself are as follows:

1. construct suffecient steel tank based on Gen set capacity,

2. add water and salt in the tank

3. fabricate bus bar that can be lowered to the tank and properly insulated

4. used your initiative to improved the desing and you can run the unit based on unit requirements.

For more info, please feel free to contact me: + 971502487606

Nelson

Essentially achieving the load tests and officially certificating the test results, with indicating engine and alternator serial numbers is the manufacturer's business

They test and certificate any generator they produce, in accordance with the pre determined standards, before the goods are leaving the factory.

Since the load tests are fuel and time consuming process, some of them do this upon the customer's request.

Kindest Regards

The first thing anybody would have done is to calculate yr maximum load wt some tolerance value and proceed to buy a sizable gen.set.

Generators perform better or at their best when working on full load capacity.By giving it 35% load,its like a hungry or thirsty man who could not satisfy his hunger or thirst,what do u think would happen to him.

Even if u are not running it on full load,u should run it at 75% of full load.Read yr textbooks on generators,u will find what i am saying and why it should be so.Every new Generator has a manual and its always stated in the manual.

Well i would advise that u find or creat a usefull load that would keep yr gen at good performance or sell it off and get a sizable genset.When there is need to increase yr load(expand),u may then think of increasing yr genset capacity as well.Alternatively,u may still keep yr 1000kva for future expansion,while u get a smaller one for yr loads presently.

Patrick Whowha

This is good information, along with several other of your posts I have noticed. Welcome to CR4!

Despite all the remarks, I feel, that there is a near future expansion on cards, that's why you/ your management has decided to go for larger set.

instead of using a load bank and wasting precious fuel, did you consider putting a UPS and storing the excess, that way you may go for intermittent full load operation of the Genset ?

Any other possibility of passing on the load to grid/ nearby other factories etc for some time, may be at a discount till you are ready to gobble up the necessary?

Provided the generator installation is correctly engineered, operating a diesel genset at less than 50% load will not harm the injectors or any other engine components. A lightly loaded diesel engine cylinder does not generate much combustion temperature, thus the installation must capture engine heat to prewarm the inlet air to the cylinders.

If the engine is operated in a cold mode it will produce white exhaust smoke as the first indication of poor combustion. This will lead to excess carbon on the piston rings, allow unburnt diesel fuel to mix with the lubricating oil, carbon up the valves and injectors nozzles which will gradually loose the spray pattern and cause even more combustion problems.

Keep the system temperatures within the design parameters regardless of amount of power requirements and you will be just fine.

From Wiki

Diesel engine damage due to misapplication or misuse of generating set

Diesel engines can suffer damage as a result of misapplication or misuse - namely internal glazing and carbon buildup. This is a common problem in generator sets caused by failure to follow application and operating guidelines. Ideally, diesel engines should be run at least 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon buildup is due to prolonged periods of running at low speeds and/or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).

Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.

This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings, which causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly-sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel-like glaze which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface which are there to hold oil and return it to the crankcase via the scraper ring.

Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures.

Unburnt fuel then leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.

The problem is increased further with the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.

This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.

Under-loaded running inevitably causes not only white smoke from unburnt fuel but over time will be joined by blue smoke of burnt lubricating oil leaking past the damaged piston rings, and black smoke caused by damaged injectors. This pollution is unacceptable to the authorities and neighbours.

Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures allows the piston rings to scrape glaze off the bores and allows carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves, this will not have any effect.

The situation can be prevented by carefully selecting the generator set in accordance with manufacturers printed guidelines.

Also look here in the section

Diesel engine damage due to misapplication or misuse of generating set