How Are Engines De-rated? A Review of Basic Factors and Formulas
Understanding de-rating a natural gas generator is a key component when it comes to employing of this type of unit at your work site. Many customers ask us, “What does De-Rating an engine mean?” Or “How Does De-rating a Generator Work?” Understanding this particular aspect of natural gas generators contributes to your decision-making skills when it comes to employing a natural gas generator. Most importantly, understanding de-rating will allow you to have an apples-to-apples comparison when considering multiple generators.
What Does De-rating a Natural Gas Generator mean?
Being an informed consumer is a useful bargaining chip since the more informed you are, the better decisions you can make, and the wiser you can be relative to expenditures and operational efficiency. All of the factors related to De-rating can be complicated, but for the purposes of this article, we will explain it in easily understandable terms.
The two major components of a generator (engine and generator end) will have a manufacturer’s power rating. The engine is typically the weakest link and is generally most affected by external factors such as elevation, gas quality, etc. Therefore, the engine is the starting point for determining the overall rating of the generator.
De-rating is then applied to account for variables that reduce the performance of the engine (elevation, gas quality, etc.) so as to end up with the useful power output. The de-rated figure helps consumers understand the power they will have available to serve their needs.
Note that some generator packages simply use the engine or generator-end rating as their system rating; in this case, be aware that this rating does not necessarily reflect the power available for use. Nobody benefits from a company getting an 11 liter engine when it really only needs an 8 liter, but we don’t want our customers getting an engine that won’t meet the needs of the site under full load either. If you were shopping for new work boots, this article might be the device (called a Brannock Device) that shows you how wide/long your new boots need to be so your feet won’t get blisters or pinched.
Factors for De-rating Engines: Altitude, Temperature, EPA
Environmental factors contributing to de-rating are altitude and temperature. Typically, generator manufacturers rate and the EPA certifies units at or near sea level. As altitude increases, the air becomes less dense and contains less oxygen, which results in less efficient combustion and less efficient heat transfer. As the elevation in which the unit will be placed increases, the amount of de-rating increases as well.
Formula for De-rating Engines
A standard formula for de-rating is 2-3% for every 1,000 ft. in altitude. However, natural gas generators should use 5%, which is necessary to accommodate the various BTU content of the field gas being used for fuel in order to prevent detonation or knock.
Some generator manufacturers de-rate for running on propane as well. Propane has a much higher energy density than natural gas and is more prone to knocking under heavy loads.
Loads on the engine when running on propane should be limited to keep cylinder pressures and temperatures down to a level where you won’t experience knock. Translation for detonation, or knock, is to use an engine large enough to provide power for the required load without running it over 75% of its capability. The process of de-rating will show you what size unit to use if you know what the load requirement is.
How does Mesa Natural Gas Solutions De-Rate Their Engines?
Given the experience we have in oil and gas plays across the world, we have standard de-rating that we apply to all of our units. The chart below lists how our common Doosan engines are rated for prime and standby power as well as the Mesa de-rated power. It is important to ignore standby rating numbers when you want to use the generator for prime or continuous power. Mesa also over-sizes the generator end so that they are never a limiting factor in the overall generator rating.
We hope this has been useful in helping you understand what it means to de-rate a natural gas generator. Understanding this process will help you be a more powerful consumer. It’s really a matter of sorting through all the information to better make an educated decision that will best meet your needs. Mesa Solutions can answer any question you might have about natural gas power generation.