18/8/2015
Caterpillar - The Impact of Generator Set Underloading - Online Community
Diesel engines should run at a minimum of 30 percent load for about 30 minutes for every four hours of light-load operation, and operators should measure exhaust temperature prior to the turbo for analysis. Natural gas engines are more sensitive to low loads, so there are specific underloading time limits required to maintain optimal performance, see the chart above. After the time limit has expired, the engine should run at a minimum of 70 percent load for at least two hours. Power system underloading impacts individual components as well as overall performance, so it should not be taken lightly. While the simple solution is to operate generator sets at a load that meets design requirements, the reality is that system needs sometimes change. This makes underloading common in the realm of power generation—especially in standby applications. However, the effects of underloading can be minimized with a thorough operation and maintenance plan to preserve system health and avoid extra costs down the road. We’d like to hear from you. Tell us how generator set underloading has impacted your operation. What strategies have you implemented to ensure optimal performance at low loads? In your experience, how has underloading diesel generator sets differed from natural gas-fired engines? What can Caterpillar do to improve system design and help reduce the impact of underloading for your application? For more information, visit www.catelectricpowerinfo.com or contact yourlocal Cat dealer .
Comments by Bamgles2 on03-19-2015 04:45 AM By underloading the gene rator it will affect the service of the generator and el ectrical system.
by mikel on03-22-2015 02:37 PM In general, at least based on my experience, diesel engines tend to be much more robust in dealing with underloading issues. In general you can run the diesels underloaded for a fairly long time period, and as long as you periodicall y run the engine at a higher load to "clean it out" you can minimize the affects of low load operation. Multiple factors do come into play, like original age and design of the engine. Older designs with rectangular piston rings, valve stem seals and lower fuel injection pressures tend to be much more tolerant of long periods of low load operation than newer design engines with keystone rings and high fuel injection pressures. Turbochargers are also negatively affected by low load operation, mostly due to uneven buildup of deposits causing imbalance and changes to the turbo's response. Gas engines usually see the negative impact of low load operation much sooner in the service cycle, mainly due to spark plug fouling, that causes hard starting, rough runing and poor acceleration. Once a plug is fouled it almost always requires the plug to be removed and clea ned. A run at higher load typically won't "cure" a fouled plug. Also deposit formation due to low load operation can lead to an increase in compression ratio, and with that increasing the risk of detonation. On engines without detonation protection system this can lead to mechanical damage, on engines with protections it will lead to unwanted shutdowns. Underloading affects the operation of the prime mover, it causes problems with starting, ramping to rated speed, ability to respond to transient conditions, and stability. It will also causes "slobbering" and deposits of incompletely burned fuel and oil in the exhaust systems that can lead to excessive smoke and in some cases, exhaust fires. I don't think underloading has a negative affect on the generator end or the electrical system as the electrical side of the system is more prone to problems due to overloading and heat. Exhaust aftertreatment if installed also can have many problems with extended periods of operation below recommended minimum loads due to fouling of catalysts, inability to regenerate partculate traps, visible emissions (smoke) and out of http://208.74.204.235/t5/blogs/blogarticleprintpage/blog-id/EP_Blog/article-id/656
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18/8/2015
Caterpillar - The Impact of Generator Set Underloading - Online Community
compliance on permitted exhaust gas constituents. My two cents worth, MikeL
by Progen904 on03-26-2015 04:39 PM How is it possible to have an EPA rated engine at any Tier level that produces excessive exhaust emissions at any load level???
by mikel on03-26-2015 11:15 PM By operating it outside of "standard conditions". Things such as a plugged air cleaner, excessive exhaust restriction, wrong grade/type of fuel, incorrect air inlet temperture. when you look at the TMI exhaust emissions data at the end you see a lot of definitions that define the normal operating characterisitics that are required to maintain emissions compliance. A tier rated engine still does not have any form of active feedback controls (such as CEMS system) and whil e it does has more extensive protections to limit deviation from desired emissions levels, they can still operate unders certain conditions out of compliance.
by Progen904 on03-27-2015 07:56 AM So then the question may be ammended to read - how is it possible to have a properly installed and maintained EPA rated engine at any Tier level that produces excessive exhaust emissions at any load level ???
by Brian (bjabeck) on03-30-2015 01:47 PM
Progen - This is a very good question and one we receive often and there are multiple scenarios where emissions may be measured above EPA published limits. 1) Mfr's that provide stationary engines certified to EPA emissions standards report emissions based on 5 engine operating loads and speeds and results are reported as weighted average test cycle emissions. Weighted average means that some of the test points count more than others in the average. Further, the weighted cycle means that an individual test condition may be above the EPA limits , but the weighted average of all 5 test points must be below the EPA emissions limits. 2) Measurement variability and or different test methods can result in emissions levels that differ from those provided by the manufacturer. 3) Local site conditions which include ambient temperature, humidity, barometric pressure can also result in emissions above the EPA published emissions limits. 4) The engine may be operating off the test cycle which may allow slightly higher emissions than the standard yet remain capped by EPA. These are called NTE or not-to exceed emissions lev els. 5) The equipment may be experiencing a component problem due to age, lack of maintenance or manufacturing defect which can be resolved by contacting the appropriate dealer to diagnose and provide a mechanical repair. . Below is the information that dealers provide with our U.S. EPA Tier 4 certified emiss ions data. Emissions data measurement procedures are consistent with those described in EPA CFR 40 Part 89, Subpart D & E and ISO8178-1 for measuring HC, CO, PM, NOx. Data shown is based on steady state operating conditions of 77°F , 28.42 in Hg and number 2 diesel fuel with 35° API and LHV of 18,390 btu/lb. The nominal emissi ons data shown is subject to instrumentation, measurement, facility and engine to engine variations. Emissions data is based on 100% load and thus cannot be used to compare to EPA regulations which use values based on a weighted cycle. Emissions values are tailpipe out with aftertreatment installed. Values shown as zero may be greater than zero but were below the detection level of the equipment used at the time of measurement.
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Caterpillar - The Impact of Generator Set Underloading - Online Community
Please let us know if you have any other questions.
by Progen904 on03-31-2015 10:20 AM Brian, Thank you for the summary on EPA limits and certification. I hope that we may continue this blog for many more discussions. Does Caterpillar have records of any documented cases where underloading caused eventual failure of the engine to power the connected load? These would be cases where adding additional connected load solved the problem without any mechanical or electrical repairs. For instance consider a single generator set with a 100kW rating. The connected load when it was installed was 25kW steady state with an occasional large motor start that might add 5kW for ten seconds. This is a typical scenario where the generator set was sized to meet voltage dip minimums during motor starting. If the voltage dip issue is laid aside for simplicity, any problems created by underloading would have to render inoperable the 75% reserve (unused capacity) of the engine. Some documented cases of this actually occuring would be very interesting.
by visit their website on05-12-2015 02:48 PM One of key considerations for generators is ability to refuel your generators easily. Often a generator is placed in underground parking lots in buil dings with nearly impossible to refill. Great at event of emergency not so good when you have to refuel after 8 hours?
by kz on05-19-2015 02:17 PM This is more of a question and a shout for advice than a comment. Please note * Spares are a major issue* Engines running at 1200RPM. I am on a Jack up with 5 x 3516B gen sets. I am new here but I have noticed that we frquently have 3 cats running on light loads, around 30%. There is evidence of slobbering and we have sooty smoke from our exhaust system. There have been regular injector issues but the replacement parts although stamped with Cat may not be OEM. Recent Cut out tests have come back with several of the injectors as NOT OK even after being changed for new, I have no other info on this. I recently changed a head where the exhaust valve was stuck about 10mm inside the head body which was coked up and when I inspected the Cylinder bore there was a build up of laquer and signs of detonation distress on the crown. The rocker arm hadalso sheared in the process. These engines have all done around the 22K mark with no overhauls. Is there any way back from this? Would it make any difference at this stage if I implemented a policy whereby the load share was decreased i.e. convinced the guys to run 2 engines at higher loads (50/60%) instead of 3. Any pointers welcome .
by 3512B G on08-18-2015 03:44 PM This article matches my experiences, though 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: 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.
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