At Fluid Power World’s recent Fluid Power Technology Conference in Cleveland, Tom Wanke of the Milwaukee School of Engineering discussed the importance of contamination control in hydraulic systems. One focus was on gaseous contamination. Here are some highlights of his presentation.
We can break physical contaminants in a hydraulic system into several categories. One is fluid contamination, in most cases it’s water, but we could have other incompatible liquids that get into the hydraulic system. And these can cause chemical reactions in hydraulic fluid, leading to oil degradation and other problems. And then, of course, we have particulate contaminants and they also come in several forms.
I’d like to focus on gaseous contaminants. An obvious source of gaseous contaminants is from a failed accumulator. You’ve got that big accumulator bladder filled with nitrogen and the piston seal fails or the bladder fails. Where does all that gas go? It doesn’t vent to the atmosphere. When it fails, it goes into the oil. So now, you got a huge slug of gas mixed in your hydraulic oil. But air and gas can get into the system through a variety of sources: suction lines, return lines past seals or fittings, a pump not primed, the reservoir, all of these things could be contributors to where air is coming from.
Three different forms of air can be in your system. All hydraulic fluids contain some amount of dissolved air. The question is, how much? Well, that depends upon the fluid chemistry, temperature, viscosity and other factors that affect how much dissolved air the oil can hold. We have entrained air, which may or not be visible. With our naked eye the smallest particle we can see is 40 microns. We could have bubbles that are a lot smaller than 40 microns in the oil, but if you hold up the sample to a light, you’re not going to see those.
And we have foaming, which usually occurs on top of the reservoir, often in mobile equipment. If you have motion of the vehicle going over rough terrain, the oil is sloshing back and forth in the reservoir. The reservoir might not have proper baffles or design. Lines might not be properly routed, permitting high velocity fluid in a return line coming back into the reservoir and causing a significant mixing action, which could cause aeration and foam to occur on the surface of the fluid. Hopefully you’d have a good antifoaming additive package in your hydraulic oil that allows this foam to dissipate.
Gas in hydraulic systems leads to problems like cavitation. We could have dissolved air that is coming out of solution because we’re at a low pressure point in the system. We have a vacuum and dissolved gas will come out of solution and form bubbles. And then we go over to the high pressure side of the pump and those bubbles implode back in the solution. Wherever a bubble collapsed, it looks like an ice pick started gouging out the metal. It’s very easy to see the results of cavitation damage. And by the way, pumps, motors and cylinders, every component in the system can experience cavitation. It all depends upon where that bubble implodes. Some people just think only the pump can cavitate. Or, only the motor can cavitate if I overspeed without enough oil coming from the pump. But we can have cavitation and dieseling in hydraulic cylinders as well.
There are a number of other effects of gaseous contamination in your hydraulic system. These include:
Fluid appearance. It could be cloudy or look like it has a whole lot of bubbles in it.
System noise. Your system’s going to run noisier because you’re expanding and compressing these bubbles as the pressure changes from one level to another in the hydraulic system.
Temperature. Operating temperature could go up anywhere from 10 to 20 degrees with an aerated oil in your system.
Fluid compressibility. If air entrains in our oil, it affects the compressibility and stiffness of the fluid. Fluid is slightly compressible. The bulk modulus probably is somewhere about 250,000 psi, which equates to probably about a half a percent compressibility per thousand psi pressure. Of course the more entrained air, the more the compressibility you’re going to have. An effect that you see with this aerated oil is sluggish response time.
Poor system response. System response can go in the toilet if you have aeration in your oil. If there’s a lot of air, actuator movement could become jerky. The actuator moves a little bit, then stops, and then it moves up again because of the compressibility of the fluid. And sluggish response with aeration goes hand-in-hand with temperature and noise effects, these things all kind of work together.
Oxidation. Oxygen in the air will cause the oil to chemically break down over time. There are additives to prevent that but, again, those are short term solutions. So the more aeration you have, the quicker the fluid is going to break down.
Degraded performance. Less acceptable viscosity, lubricity, component wear and filter life, and higher power consumption (because we are lowering efficiency of our components) are some other effects of gaseous contaminants. At MSOE we’re working with fluids manufacturers on different types of antifoaming additives in hydraulic fluid. We actually built a system where we can percolate air into the fluid up to about 5%, and then we measure changes in a pump’s hydraulic efficiency. For example, with no air in the system our volumetric efficiency is about 89% for one particular pump. Turning on the aerator puts in about 2% air. Volumetric efficiency dropped by almost three percentage points, and noise coming from the pump increased substantially.
Then we turned the aerator off and measured how long it takes for the fluid to return to its initial state. We do that by measuring the mass flow and oil density, and that’s how we know when we’re back to the starting point of no air in the system. Some fluids reverted to the starting point within about 20 seconds. Other fluids ran for hours with the aerator turned off and no decrease in the air. The only way to remove air from the system was to shut it down for 24 hours. So that’s not the hydraulic fluid that you want in your system. Different manufacturers offer different additives that can have a major effect on overall performance.
