May I ask whether the pressure difference of the flow restriction orifice plate refers to the pressure difference between the upstream side and the section with the smallest cross-sectional area (where the flow velocity is highest)?

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As the pressure difference Δp across the orifice plate increases, the flow rate qm also increases. When Δp reaches a certain value, the pressure pvc at the throat drops below the saturated vapor pressure pv of the fluid; as a result, part of the fluid vaporizes, and the pipe flow rate no longer increases with rising pressure difference – this is what is known as the choked flow phenomenon. At this point, the pressure difference across the orifice plate is called the choked flow pressure difference Δps. When the actual pressure difference Δp across the throttle orifice is less than its corresponding Δps, flashing or cavitation can be avoided. Does the pressure difference for a flow-limiting orifice refer to the pressure difference between the upstream side and the section with the smallest cross-section (where the flow velocity is highest)?