The standard primary flow sensors commonly used in differential pressure flow meters are the orifice plates, flow nozzles and venturi tubes. These flow meters are often called "head loss" meters because there is a permanent pressure loss downstream these meters. In other words, upstream pressure never recovers to its original value downstream these meters. Various designs of these flow sensors are available which can provide the optimal meter for the desired operating conditions and requirements of the user. A critical factor in choosing a differential pressure flow meter is the pressure loss of the flow sensor. As a rule, when applying differential pressure devices, pressure loss must be small. This is because pressure loss means energy loss and higher pumping/compression costs.
The different installation versions of the primary flow sensors (orifice plates, flow nozzles, venturis) of differential pressure flow meters commonly used in flow measurement are tabulated below:
|Primary Element||Type of Primary Element|
|Orifice Plates||Corner Pressure Taps|
|Flanged Pressure Taps|
|D and D/2 Pressure Taps|
|Flow Nozzles||ISA 1932|
The graph below shows the percentage pressure loss against beta ratio (beta = d/D) for orifice plates, flow nozzles and venturis:
From the permanent pressure loss graph above, we can deduce the following:
(a) Orifice plates are universal in their use in differential pressure flow meters but have the basic disadvantage of high pressure loss.
(b) Flow nozzles are characterized by lower pressure drops compared to orifice plates
(c) Venturis (Venturi tubes and Venturi nozzles) are characterized by much smaller pressure drops compared to orifice plates and flow nozzles.
Application Limits for Orifice Plates, Flow Nozzles and Venturi Flowmeters
The table below summarizes the various application limits with respect to bore or throat diameters(d), internal diameter of pipes (D), beta ratio (β =d/D) and pipe Reynold’s number (Re) for orifice plates, flow nozzles and Venturi flow meters:
|Flow meter parameters||Orifice Plates||Flow Nozzles||Venturis|
|Corner Pressure Taps||Flanged Pressure Taps||D and D/2 Pressure Taps||ISA 1932||Long Radius||Venturi Tube||Venturi Nozzle|
|Re, Dmin||5,000 - 20,000||2,500 - 540,000||2,500 - 540,000||20,000||10,000||200,000||150,000|