The venturi tube has certain practical disadvantages for ordinary plant practice.
It is expensive, it occupies considerable space, and its ratio of throat diameter to pipe diameter cannot be changed.
For a given meter and definite manometer system, the maximun measurable flow rate is fixed, so if the flow range is changed, the throat diameter may be too large to give an accurate reading or
too small to accommodate the new maximum flow rate.
The orifice flange overcomes these shortcomings of the venturi but at the price of a larger power consumption.
An orifice flange assembly is the combination of an orifice plate, flanges with taps, bolts and nuts and gaskets.