According to this Wikipedia article on orifice plates:
… the flow of real gases through thin-plate orifices never becomes fully choked.
Further to this, there is a reference given, which states the following:
Cunningham (1951) first drew attention to the fact that choked flow will not occur across a standard, thin, square-edged orifice.[10] The mass flow rate through the orifice continues to increase as the downstream pressure is lowered to a perfect vacuum, though the mass flow rate increases slowly as the downstream pressure is reduced below the critical pressure.
Is this true? If so, how/why is this the case? Surely an orifice provides a minimum flow area, which should impose a limit on the mass flow rate, beyond a certain critical pressure drop?
If anyone thinks that the Wikipedia article or the reference are incorrect, then I would be very interested in a reliable reference that contradicts it.
Best Answer
There are practical considerations involved in sizing an orifice plate and the associated piping. For the piping that is carrying flow to the orifice plate, you want a maximum gas or vapor velocity of approximately 200-300 ft/s, and a maximum liquid velocity of approximately 10 ft/s, because higher velocities lead to too much pressure drop per foot of pipe, meaning that too much energy is lost due to friction effects. For the associated orifice plate, you want a reasonable estimate of the flow rate through the pipe, but you also want to somewhat minimize the pressure drop across the orifice plate, because this also represents lost energy. This means that the hole diameter of the orifice plate is "substantial" when compared to the diameter of the piping that contains it. Thus, to answer the question, there is never choked flow through an orifice plate because the orifice plate and associated piping system are deliberately designed to avoid those flow conditions.