Bernoulli's Theorem in Valves

[Future_Engineer]

**I WORK IN A POWER PLANT WITH LOTS OF VALVES SO I AM TRYING TO UNDERSTAND ALL THE THEORY**

We say that according to Bernoulli's, there is an increase in velocity of process fluid in a valve which results in a pressure drop as potential energy is converted to kinetic energy due to the constriction.

1) That makes sense theoretically, but I don't get how there is low pressure at the vena contracta, if its the fastest velocity shouldn't it be high pressure?

2) And then how do waterjet cutters work? Don't they concentrate water such that it is both high pressure and velocity?

 

[RVAmeche]

Bernoulli's theorem basically says the sum of elevation, potential energy (pressure), and kinetic energy (velocity) at one point equals the sum at another point. By forcing the fluid to have a large increase in velocity (kinetic energy) the pressure (potential energy) term has to drop to compensate. This is ignoring the permanent loss associated with the fitting due to inefficiencies and friction. Note the velocity term is squared so the pressure term has to drop significantly to maintain the same total energy (assuming constant elevation).

 

[Compositepro]

A water jet converts high pressure water to a high velocity, low-pressure jet. The jet is at ambient pressure. When it hits something the velocity gets turned back into high pressure at the impact surface. Water jets also usually inject abrasive particles into the jet.