| You’re generally not compressing water or hydraulic fluid because they’re not very compressible. (Although I think many hydraulic fluids are actually more compressible than water, but still barely compressible. They’re used for other reasons. The ideal hydraulic fluid for most applications is entirely incompressible.) But systems using hydraulic fluid to carry power have something that keeps it pressurized as it moves: that is, something that can supply power. Often very large amounts of power. (So do water pipes that have some form of supply, of course, but the pressures are much lower.) So if you pressurize fluid in a pipe, you are storing some energy in the compressed fluid. You may also have a lot of energy stored or power available in some other component of the system. In any event, the type of injury you’re describing seems like more of a velocity issue. When you allow fluid to escape from a high pressure environment to a low pressure environment through an opening that doesn’t have a whole lot of friction, the stored energy converts to kinetic energy. Some basic physics says that a small volume V of fluid has potential energy PV and mass rho * V. Conservation of energy says that, when that potential energy is gone (the fluid escapes), it has the same energy, but now it’s 1/2 m v^2. You can solve for velocity, which scales like sqrt(P). (The real story is more complicated. Pressure in fast-moving things is a bit messy due to Bernoulli’s Law, and different disciplines use annoyingly different terminology. Static pressure, total pressure, stagnation pressure, just plain old pressure, oh my!) Water (or air) jetting out of a hose through a tiny hole at even 100 psi is not likely to inject itself into a person. |