Is Hydraulic Fluid Compressible?

Hydraulic fluid, a type of oil, is not necessarily a static substance that will always react in the same way. Like most other substances, hydraulic fluids can change and shift, have impurities mixed in, and be exposed to a wide range of external temperatures and in pressures. So, while in practice and in most considerations hydraulic fluid is incompressible, in theory it is possible to compress in extreme temperatures and pressures.

Is Hydraulic Fluid Compressible?

Compressibility is a measure of fluid based on the change in volume with respect to a change in pressure, provided temperature remains constant. It is a quantifying of how much a fluid expands with lowered pressure and shrinks with higher pressure (at least, that’s the way fluid should work). As this measure requires temperature stay unchanged, it can also be called an isothermal change.

The way to determine compressibility begins with determining the bulk modulus, also known the “elasticity” of volume as it changes under varying pressures. This is defined as:

hydraulic fluid bulk modulus

hydraulic oil bulk modulus

or put in terms of fluid density:

bulk modulus vs density

bulk modulus as a function of density

where:

  • K = Bulk modulus of fluid (Pa)
  • V = initial volume of hydraulic fluid (m3)
  • dP/dV = derivative of pressure with respect to volume (Pa/m3)
  • ΔP = change in pressure (Pa)
  • ΔV = change in volume (m3)
  • ρ = initial density (kg/m3)
  • dP/dρ = derivative of pressure with respect to density (Pa-kg/m3)
  • Δρ = change in density (kg/m3)

The inverse of this bulk modulus is then said to be the compressibility of a fluid. The higher the bulk modulus is, the lower the compressibility coefficient, and the harder a substance will be to compress.

hydraulic fluid compressibility

where:

  • β = Compressibility (1/Pa)

This is how compressibility is calculated. In general, most gases will have much higher compressibility coefficients than liquid, which are often low enough as to be negligible. As oil is a liquid, compressibility ability is quite low.

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So, to reiterate, the higher a fluid’s bulk modulus, the lower the compressibility coefficient, and the less compressible a fluid will be. Oil (in the case of hydraulic fluid) and other liquids will have low bulk moduli, low compressibility coefficients, and low overall compressibility.

Hydraulic Fluid Explained

is hydraulic fluid compressible

Hydraulic fluid is used to transfer mechanical energy (also referred to as “power” or “work”) from one part of a system to another. It is widely used in the world of machines, from brake systems and power steering in automobiles, to lubrication and heat dissipation. The less compressible hydraulic fluid is, the more efficient it is.

Hydraulic fluid these days is largely made up of various mineral oils, or synthetic man-made chemicals. Water is used in certain applications, but is much less common due to being more susceptible to freezing or evaporating, and being a much worse lubricant.

Are Liquids Compressible?

As discussed above, liquids are compressible, but not very much. How much is quite dependent on the external conditions, like pressure and temperature. But it can be compressed, as demonstrated by the fact that ocean water at the bottom of the sea is in fact more dense than that of water floating on the surface at a little over 1 KPa (though this is in part due to colder ocean temperatures at lower depths).

But this is quite an extreme scenario. Under normal conditions and temperatures, water is incredibly resistant to compression. Liquid water will compress by only 0.024% under a pressure increase of 500KPa. That’s about 500 times atmospheric pressure just for that little reduction in volume!

So in practice, liquids are not considered to be compressible, but in theory they are. This is due largely to their molecular makeup; molecules of liquids are packed much tighter together than that of gases. However, when compared to solids, the molecules of liquids are relatively “loose”, and compressibility is relatively higher.

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Is Hydraulic Fluid Less Compressible Than Water?

Both hydraulic fluid (which is an oil) and water are quite incompressible under normal pressures, but actually water is the less compressible fluid here. The bulk modulus of water is around 2.15 GPa, whereas it’s slightly under 1.38 GPa for hydraulic fluid. But this is gigapascals we’re talking about here, a massive amount of pressure!

So yes, hydraulic fluid is in fact more compressible than water. But both liquids do not compress easily, especially under the circumstances (ie pressure and temperature) for normal, every day use. But hydraulic fluid in the form of oil has other advantages that water cannot match.

Benefits of Using Incompressible Fluids in Hydraulic Systems

Incompressible fluids are necessary to use in hydraulic systems precisely because they do not compress. If a compressible fluid were exposed to a pressure force in a hydraulic system, it would simply shrink, rather than be forced through whatever pipes were used. Let’s go through an example.

Say we have a brake system where the opening of a tube is smaller than the end. Pascal’s law states that if there is a pressure increase at one point of a closed space, the same increase will happen at every point.

pascal's law

If the force on the smaller end is increased, to push hydraulic fluid through the tube, then that pressure will increase on the larger end as well (which is now applied over a larger area, to do some mechanical work). If the hydraulic fluid was compressible, the pressure on the other end we need to push or mechanical system will never arrive! Instead our hydraulic fluid would shrink, the volume wouldn’t be big enough to push as much as we need to.

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There are some concerns about hydraulic fluid compressing as temperatures get high. In general hydraulic fluid and other liquids become more compressible as temperatures increase. One way that some machines deal with this is have the hydraulic fluid run through a heat transfer device to cool the fluid before circling back through the system, to keep it cool and to help cool everything else.

Hydraulic Fluid Properties

hydraulic fluid properties

There are a number of qualities a good hydraulic fluid should have to work effectively besides being incompressible. Let’s look at a few of them.

“Just Right” Viscosity

If the viscosity of a hydraulic fluid is too high, the fluid will be too thick and difficult to pump through the hydraulic system. However, if the fluid is too thin, and has a low viscosity, it will not seal enough, and be more prone to leaking. It is important to note that temperature, both internal and external, can have a big effect on viscosity.

Stability

Stability means a lot of things when it comes to hydraulic fluid. Foremost it means it does not degrade when exposed to things like temperature change, mixing with other fluids, and high usage over time.

Thermal stability is important because of the viscosity reasons discussed above, but also because high temperatures break down hydraulic fluid more quickly. If the fluid breaks down it can degrade further and form acids and sludge that clog and corrode. Note that hydraulic fluid does naturally degrade over time, but high temperatures can accelerate this process.

Oxidation stability in hydraulic fluid helps to slow the chemical reactions it has with oxygen. Oxidizing hydraulic fluid degrades quickly, causing issues within the system. It’s also important for hydraulic fluid to have hydrolytic stability, resistant to mixing with water. Water in the system can cause blockages, as well as the formation of corrosive acids.

 

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