In fluid mechanics, a flow primary element is a device used to restrict the flow of a fluid while measuring the pressure difference. The main application is is flow measurement using a pressure transmitter differential. This measures the pressure on both sides of the orifice to calculate the flow rate. Orifice plates, nozzles, venturis, and V-cones are flow primary element. The conditions for their installation are described in ISO 5167 norms.
The main field of application is the measurement of water in heating and cooling circuits and the measurement of steam and condensates at very high temperatures in secondary circuits.
The aim is to create a localized restriction of the section (or constriction), within the pipeline, which will generate a differential static pressure. Its measurement will allow the flow deduction.
| Depressant organs | |||||
| Orifice plate and corner taps | Integrated orifice place | Venturis and Nozzles | Average Pitot tubes | V-ConeTM flow meter | |
 |  |  |  |  | |
| Diameter mini. | DN50 | 1/2 NPT | DN50 | DN50 | DN15 |
| Diameter max. | DN1000 | 1/2 NPT | DN800 | DN1500 | DN3000 |
| Static pressure max. | 500 bar | 160 bar | All | 50 bar | 689 bar |
| Temperature max. | 200°C | 120°C | All | 350°C | 450°C |
| Reynolds numbers | >2 500 | >2 500 | >20 000 | >12 000 | >4 000 |
| Rangeability | 5:1 | 5:1 | 5:1 | 10:1 | 10:1 |
| Straight lengths (upstream/downstream) | 3D/5D | 3D/5D | 5D/3D | 7D/4D | 0D/3D |
| Accuracy | ± 1,5 % | ± 2 % | ± 1% | ± 1% | ± 0,5% |
| Load loss | Average | Average | Average | Low | Low |
| Applications | Gas, steam, corrosive and non-corrosive fluids | ||||
