Pdo

PDO in general, is using the ‘Flange Tapping’. The upstream and downstream orifice tapping are taken from the flanges.
what is PDO stands for?

The only reference to PDO I can find in relation to any valves (valved systems) is Petroleum Development Oman which appears to be some kind of certification for valves used in the petroleum field

Other than that, no clue.

As a quick guess, I would suggest Pressure Differential Orifice, as flange tapping is one way of obtaining a differential pressure reading across an orifice.

Steve

Yes, PDO, Pressure Differential Orifice. High Side, Low Side, (High side faces the direction from which the flow is coming)

Orifice Plates are a cheaper and less invasive way of acquiring a relatively accurate flow measurement through pressure differential compared to the longer "smoother restriction" and increased accuracy but more expensive and invasive installation of a venture tube.

No flow = Zero pressure difference between the two orifices, they each reflect the static pressure of the pipe whatever that is. As the flow increases the pressure difference between the two orifices will climb as a function of the square of the flow rate, or conversely the flow rate is proportional to the square root of the pressure difference between the two orifices.

If I'm not mistaken