Orifice Plate / Restriction Orifice

Orifice plate or restriction orifice is a device used for measuring volumetric flow rate in a pipe. Orifice plate works based on Bernoulli's principle.
Bernoulli's equation states that pressure drop across the restriction orifice is directly proportional to the volumetric flow rate passing through the orifice plate.

A restriction orifice also works on the same principle, but serves a different purpose. The main difference between orifice plate and restriction orifice is their usage.
Restriction orifice is used for killing pressure in a pipe by increasing fluid velocity.
Restriction orifice and orifice plates are similar in structure and both are based on Bernoulli's equation.

Bernoulli's equation,

Orifice plate is a thin plate with an orifice in the middle. It is placed in a pipe where the volumetric flow measurement needs to be done.

When the fluid flows through the orifice in the plate, fluid velocity changes and according to Bernoulli's equation pressure also changes.
By measuring the difference between pressure values immediately before and after the orifice plate, volumetric flow calculation can be done.

In case of restriction orifice, the size of the orifice is chosen in order to have desired pressure drop across the restriction orifice.
This effect is known as choked flow across the orifice.

As the name suggests, function of a restriction orifice is to restrict the flow and kill pressure downstream of the orifice.