If liquid is not compressible, why do we need special hydraulic fluid? Why not just use water?
In: Engineering
When fluid is compressed, it gets hot. Water when hot turns to steam, steam is a gas and can be compressed.
Everything is compressible under enough pressure. Hydraulic fluid turns solid at high pressures. At work we need to uses ethylene glycol by 100ksi and white gas to get to 200ksi.
As other have said there are issues with water and temperature as well as the fact that it corrodes many materials
Liquids are compressible however they have fairly high bulk modulus (or K factor) which means that they don’t compress much.
And there are a lot of water based hydraulic fluids usually a mixture of water and glycol to lower it’s freezing temperature.
The main reason why water isn’t commonly used is because of it’s thermal stability in the temperature ranges that hydraulic systems tend to usually operate.
Water freezes at 0 degrees and boils at 100, this isn’t good for anything that operates anywhere close to it’s freezing point or boiling point. The latter can be solved with anti-freeze but the former can’t be and the former also can generate steam which can introduce a lot of problems in a hydraulic system that wasn’t designed to handle it.
Additionally a lot of things are water soluble which can impact the fluid and lead to corrosion. And more importantly gasses are fairly well soluble in water which can cause a problem when pressure changes cause air bubbles to form and water has a relatively slow air release in general compared to other hydraulic fluids.
And lastly the viscosity index of the fluid is quite important especially for a high pressure system, which again water can be not ideal.
Oil is a very good hydraulic fluid for most cases since oil formulation can be made that still flow at -30c and have a smoke point of well over 200c which makes it pretty much perfect for nearly any application.
On the other hand water based fluids are commonly used in places where fire hazard is a real problem, these are very common in the mining industry, gas and oil refining and metal production and pretty much anywhere where you wouldn’t want a hydraulic fluid that could catch fire.
TLDR: Water has a very narrow temperature range which means that it’s out of the question for most applications without a lot of compensation in the design which would make the system more complex and expensive. Water is only used when oil and other synthetic fluids cannot be due to them being flammable.
Water isn’t slippery, reacts with a lot, and expands when it freezes *and* when it boils. Makes it a poor choice if hydraulic fluid
Hydraulic pumps and motors. There are sliding surfaces that require a thin layer of oil to keep parts form touching. Because if these parts touch, they become very fond of each other and start exchanging parts of themselves. And then things get really hot and angry.
Also oils higher viscosity greatly increases volumetric efficiency without the need for rubber seals, but rather tight tolerances to form a seal. Except in the case of cylinders where it needs to be a 100% seal to avoid unintended movement, hence rubber seals.
Everything is compressible. It’s just that hydraulic fluid is even less compressible than water.
Water also boils at comparatively low temperatures/pressures. I’m sure there’s something relevant about phase transitions in hydraulic systems that makes the behaviour of oils more stable/predictable.
Minor benefits are also that it doesn’t freeze (as soon) and doesn’t explode into a gas if a line breaks – it’ll still spray highly toxic, high speed, terribly hot oil everywhere but it won’t be as much of an explosion. I think.
As an example, you try to use water as brake fluid it will boil. Steam is compressible, so you would lose all braking.
Brake systems almost always have some amount of water (from the atmosphere) so race cars have to use special brake fluid that raises the boiling point.
Something I haven’t seen here yet; hydraulic reservoirs need makeup air so that you don’t get vacuum when the fluid is moved back and forth.
Not only will the water evaporate into that air into a place where you DONT want humidity, you’ve now got a warm, moist environment far from any sort of regular cleaning. With water in most hydraulic systems you’d be inviting mold and all sorts of crap to grow in there.
an ideal liquid is not compressible, a real world liquid is always compressible to a certain degree. In regards to water/hydraulic fluid, it just so happens the special fluid is closer to an ideal fluid than water is .
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