08-21-2011 09:38 PM
I was surprised to read the Cat diesel generator test recommendation to run the set weekly off-load. According to Cummins: "Exercise the generator set at least once a month for a minimum of 30 minutes loaded to no less than one-third of the nameplate rating. Periods of no-load operation should be held to a minimum, because unburned fuel tends to accumulate in the exhaust system."
The advice I have always worked to is that a set should never be run off load and wherever possible should always be tested against a live load. If it must be run off load the engine should be allowed to reach full working temperature before shutting it down.
Is weekly off-load testing the right solution?
08-21-2011 11:18 PM - edited 08-21-2011 11:25 PM
The best answer is, it depends. Ideally no modern diesel engine, regardless of who built it, likes to run at no or low loads for extended periods of time. Add emissions after treatment and it gets even less desirable.
All of the manufacturers provide testing guidelines based on VERY general criteria. How often you test your equipment will depend on many factors, such as how critical is your facility/process, are there regulatory, insurance or contractual test run requirements, are there air pollution, noise abatement or other regulatory restrictions? Can you interrupt your process or power to perform an on load test, can you load test in parallel to grid, or do you have a test load, like a load bank or non-critical facility load to use for testing?
In general terms, you need to regularly start the unit to asure the ability to crank, start and come to rated speed and voltage, then run it long enough to assure it is stable, has no leaks or abnormal operating conditions. You don't want to run extended periods of time during an unloaded test, just long enough to warm up the coolant a bit and assure the engine runs ok and the generator puts out the right voltage. Ideally operating the unit under a load where the engine operates efficiently is ideal, just not always practical.
There are large numbers of units (probably the majority of standby units) that start and run weekly for a short time unloaded, and hopefully at least once a year get an appropriate full load run. Some sites can be comfortable and comply with all requirements performing a monthly test with available load, but these sites still need to regularly perform a full load test as well to assure the component parts and the entire system can operate in a full load condition. Some units only run once a quarter, an increasing number of air quality districts are limiting "test run" run hours, so you may fall into that situation.
Now if you take even a Cummins unit, add a DPF and run it no load for 30 minutes, likely in a very short time the catalyst will be plugged, its turbos coked up and slobber running down the side of the block. Take a look thru all the majors players recommended maintenance guidelines, and find older literature from them as well, for many years the once a week no load run was pretty much a US standard, newer engines, different type facilities with standby gens, and regulatory pressure caused some of those recommendations to change, in some cases there are still directions to start and run a unit no load for 10 minutes once a week.
So like I started this reply, it depends on how the owner wants to operate his equipment to assure ALL of his captial investments are protected and his needs for reliable power are met.
Hope that helps, Mike L.
08-23-2011 12:15 PM
I have a similar question on minimum load and posted it in the site design forum, though probably should have put it here....anyway have not been able to find a spec in my manual or after much web surfing, though I also fond the Cummins specs you mentioned. If I do eventually find a definitive answer/reference for Cat recomended minimum load I'll post here though