ENGINEERING SOLUTIONS / Post date: 1 March 2014
Would you consider 2,000 degrees F to be hot? At this temperature, aluminum, copper, gold and iron have already melted; stainless and carbon steels are glowing red; and your Thanksgiving turkey would turn into a charred mess in less than a second.
So what is so significant about 2,000 degrees? Many hydraulic systems can create temperatures in this range. Have you ever walked by a hydraulic pump that was cavitating? Once you hear it, you will never forget the signature sound it makes. It is sometimes described as a can of marbles being shaken.
What is actually happening is that the pressure acting on the fluid is below the saturation pressure of the dissolved gas (normally air) in the fluid. If the gas bubbles pass through a higher pressure zone (like that found on the discharge side of the pump), they will violently collapse. This can cause serious reliability issues with the machine component in terms of vibration, noise, surface damage and potentially failure.
When an air-ignitable mixture is present inside the bubble, ignition is almost inevitable at these incredible temperatures. This is the process known as micro dieseling. It will lead to the oxidative degradation of the oil, higher operating temperatures, pressure spikes and the cavitational erosion of the hydraulic pump and other components.
The sources of the bubble formation within a system include (but are not limited to):
Problems that result from the formation or presence of these bubbles include:
The four states of air-in-oil contamination:
Based on a recent survey at machinerylubrication.com, 37% of lubrication professionals have seen the effects of microdieseling.
In layman’s terms, microdieseling is a pressure-induced thermal degradation. An air bubble will transition from a low or negative pressure area to a high-pressure zone and through adiabatic compression get heated to very high temperatures. These temperatures are high enough to carbonize oil at the bubble interface, resulting in carbon byproducts (sludge and varnish), as well as increased oil degradation (oxidation).
In the best-case scenario, you would be able to stop the root cause of the problem – the bubbles. If you can control the bubble population, you can control microdieseling.