Part 2 of The Perfect Hydraulic Fluid
You may have done this experiment at school: your science teacher gives you a plastic syringe. With the plunger retracted she tells you to block the outlet with your finger and then attempt to close the plunger. You discover you are able to compress the air in the syringe by a significant amount.
She then tells you to repeat the experiment with the syringe full of water. This time the result is very different. No matter how hard you try, you discover you can’t compress the water in the syringe.
Seeing is believing. And to a 12 year old this experiment demonstrates, quite dramatically, that gases are highly compressible and liquids are apparently incompressible. By the way – if you didn’t do this experiment at school - you can try it at home!
The effectiveness of this simple physics experiment is illustrated by the fact that in my work as a fluid power engineer I regularly meet people who believe liquids, including hydraulic oil, are incompressible. But it’s not their fault. I certainly don’t remember my science teacher qualifying the obvious results of this experiment by explaining, compared to gases; liquids are highly incompressible – but compressible nonetheless!
In my previous column in H&P, I talked about the perfect hydraulic fluid with respect to viscosity. This ideal fluid would have a constant viscosity of 25 centistokes — regardless of its temperature. Another property of this ideal but non-existent hydraulic fluid would be perfect stiffness — just like the apparent stiffness of the water in the syringe in our school science experiment.
A fluid’s compressibility is defined by its bulk modulus of elasticity — which is the reciprocal of compressibility. The bulk modulus of a fluid is non-linear — meaning when the change in volume with pressure is plotted on a graph, the result is a curve rather than a straight line.
Bulk modulus is further defined as isothermal — where the heat associated with compression is dissipated (constant temperature) or isentropic — where the heat associated with compression is not dissipated and so both pressure and thermal expansion are considered. Isentropic can be thought of as dynamic bulk modulus and isothermal as static bulk modulus. The former is most pertinent to modern, high-response hydraulic systems.
The negatives of compression
Bulk modulus is an inherent property of the oil and therefore an inherent inefficiency of the hydraulic system. The fluid in the pipeline and actuator must be pressurized — and therefore compressed — before it will move a load. Because this compression of the fluid requires work at the input — which cannot be converted to useful work at the output, it is lost work and therefore a contributing factor to the overall inefficiency of the hydraulic system. The larger the actuator and the faster its required response time the higher the inefficiency attributable to bulk modulus.
And in high-performance, closed-loop electro-hydraulic systems, deforming oil volumes affect dynamic response and can cause stability problems such as self-oscillation.
Minimizing the losses
Bulk modulus varies with base stock, for example naphthenic oils have a higher bulk modulus than paraffinics. And unlike viscosity index, bulk modulus cannot be improved with additives. But there are things we can do to minimize the inefficiencies and potential control problems associated with compression of the fluid.
The first is to ensure the hydraulic machine doesn’t run hot. Compressibility of the fluid increases with temperature. Mineral hydraulic oil is around 30 percent more compressible at 100°C than it is at 20°C. Of course, there are many good reasons why you should never allow hydraulic equipment to run hot – many of which I have discussed in previous MRO articles. Reduced bulk modulus is another one.
The second is to prevent conditions that cause aeration. From our school science experiment, we understand that air is 10,000 times more compressible than oil. One percent of entrained air by volume can reduce the isothermal bulk modulus of the oil to as low as 25 percent of its normal value.
It is important at this point to distinguish between entrained air – bubbles typically with a diameter of less than one millimeter dispersed through out the fluid – and dissolved air. Hydraulic oil typically contains between 6 and 12 percent of dissolved air by volume. This dissolved air has no measurable effect on bulk modulus (or viscosity) – provided it stays in solution.
While controlling aeration is in no small part a design issue – for example, the amount of dwell time the fluid has in the tank, proper maintenance also plays an important role. Dissolved air comes out of solution as temperature increases. This is another reason to maintain appropriate and stable operating temperatures. Oxidative degradation and water contamination inhibit the oil’s ability to release air, often resulting in an increase in entrained air volume.
Given the perfect hydraulic fluid – one with infinite stiffness, does not exist, and in view of the current trend towards hydraulic equipment with higher operating pressures, higher power density and faster response, it’s more important than ever to consider the operational effects of fluid compressibility on the hydraulic equipment you design, maintain, repair or operate.
Brendan Casey is the founder of HydraulicSupermarket.com and the author of Insider Secrets to Hydraulics, Preventing Hydraulic Failures, Hydraulics Made Easy, and Advanced Hydraulic Control. A fluid power specialist with an MBA, he has more than 20 years experience in the design, maintenance, and repair of mobile and industrial hydraulic equipment.