How hot will my oil run?
It is so dependent on the application itself. Typically in the environment that we’re operating in, our lubricating oils are well below the temperature limits of the product itself. When you’re purely talking about temperature, you talk about a temperature that’s elevated enough that it starts cracking or breaking the molecule. It breaks it down into its carbon components. The temperature, however, also accelerates other chemical processes, particularly oxidation. When you get oil to a moderately high level, maybe 170°F (77°C), then you start having a dramatic effect on the life of the oil because you’re having a great impact on how fast oxidation occurs. If you didn’t have oxygen involved, you may be able to run the product up to 500-600°F (260-315°C) and not really see a degradation in the product itself for an extended period of time. An example of that is that we can take those same chemistries and put them in heat transfer systems for extended periods of time. Heat transfer systems run a lot hotter, around the 550-600° F (288-315°C) range, and they are typically blanketed with nitrogen so there is no exposure to air and no source of oxygen for oxidation. At that level, the systems are running 550-600° F (288-315°C) and can run for extended periods of time, sometimes even multiple years. You’re testing it every once in a while and you don’t see the negative effects that you would typically see when oil is breaking down with the acid number going up, sludge and things like that. You may see a few things indicating some thermal breakdown, but you won’t really see the oil itself breaking down in the manner that you’re used to seeing. Whereas, if you take that same chemistry and put it into an air compressor with a lot of exposure to air, at around 200°F (93°C) , the oxidation is sped up so much that it is difficult to make the oil last even a year. Even with the best synthetics, in that environment, it is a challenge to last 8,000 hours or one year running time without the oil breaking down or the acid number increasing and the formation of sludge.
So you can run it hotter, but it just won’t last as long?
Yes. So when we get the question – How high of a temperature can this oil run at? – then you have to start putting a time element to it. Yes it will run up to 250-300°F (120-150°C), there’s really not an upper limit, you just have to say how long will it run at that temperature before it starts oxidizing. Air compressors have a great deal of air running through so we have a good handle of about how long that is, maybe a year at an elevated temperature. In a hydraulic system, you have less exposure to air so you can run it at a higher temperature for a longer period of time, but there is going to be a limit there and it’s much more difficult to say how long that is. You can’t really say how high of a temperature at which the oil will operate without talking about how long will it run too, unless you’re talking purely a thermal breakdown and not just an acceleration of oxidation.
It sounds like if you’re going to run it hot, you definitely should get your oil analyzed, because once it starts going bad it gets bad quick.
That’s the key. There are a lot of environments in which we have a long history. We can say that we know in that type compressor or in that type gearbox at that temperature we know what the exposure to air is and we can give a good educated guess about how long it’s going to be suitable for operation. The only way to truly know is to do your oil analysis at your prescribed intervals and to watch the indicators that the oil is breaking down. The worst thing you can do is wait too long after the oil has shown some of these indicators before taking corrective action. It gets to the point where you have a much bigger job of cleaning out the equipment and cleaning up the deposit formation if you let it go too far.
