Orifice Flow Measurement......? p ˜ Q²

Hi,

Orifice plates and similar flow elements generate a pressure drop by either accelerating or decelerating fluid molecules. When the velocity of a fluid changes, its kinetic energy changes by a factor of that velocity change squared (E_k = 0.5 mv^2). Since energy is (mostly) conserved in a flowing stream of fluid, changes in kinetic energy must be matched by changes in potential energy, either in the form of pressure or fluid height. Since the fluid moving through a pipe is not free to seek its own height, the only variable that can change in response is pressure. Thus, a pressure difference forms across an orifice plate that is proportional to the square of the fluid velocity.

This is a really easy concept for people to mis-understand. Many people think the pressure drop comes from friction, but it's mostly due to the *acceleration* of the fluid. That same book I referred to has an appendix section discussing this misconception called "Doctor Strangeflow" which you may find enlightening (and even entertaining).

- Tony