【2026 Flow Meters】Selection of Differential Pressure Flow Meters – Choice of Orifices and Nozzles
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Both orifice plates and nozzles are throttling elements used in differential pressure flow meters, and their principles are similar. A comparison for selection is made from six aspects: structure, pressure loss, accuracy, durability, cost, and application scenarios.
1. Structure and Geometry: Orifice plate: Its structure is relatively simple; it is a thin metal plate with a circular hole in the center. The edge of the entrance is very sharp (right-angled or with small rounded corners), which is its most crucial feature. A sudden contraction occurs as the fluid passes by, resulting in a distinct pulse narrowing downstream of the orifice plate – where the minimum flow cross-section is found.
Nozzle: Its structure is more complex than that of a orifice plate; it is a streamlined, constricted pipe section. The inlet is a smooth curved surface (elliptical or 1/4 circle) that gradually narrows to a cylindrical throat, the length of which is about 0.3 to 0.5 times the diameter. The fluid undergoes a gradual, guided contraction in the nozzle, reaching its smallest cross-section only at the throat.
👉 Selection advice: For applications with large pipe diameters, clean fluids, and budget constraints, the orifice plate has a simple structure that makes it easy to manufacture； If the medium contains particles or is prone to scaling, the streamlined inlet of the nozzle is more resilient.
2. Hydrodynamic characteristics and pressure loss: Orifice plate: Due to sudden contraction and expansion, intense vortices and turbulence occur in the fluid downstream of the orifice plate. The energy loss is significant; approximately 40% to 80% of the pressure difference cannot be recovered and is ultimately converted into heat energy. Recovery pressure is low.
Nozzle: Its streamlined design facilitates a smooth contraction and (partial) expansion of the fluid, resulting in a significant reduction in turbulence and vortices. The energy loss is much smaller, with only about 10%–20% of the pressure difference being lost. The recovery pressure is high.
👉 Selection advice: In applications that require energy savings, such as high-flow steam measurement, the low pressure loss of nozzles offers clear advantages； If the requirements regarding pressure loss are not strict, such as in some circuits of a cooling water system, orifice plates can also be used.
3. Orifice plates with high precision and range: Precision is greatly affected by the sharpness of the inlet edge; wear or fouling of the edge can significantly alter the flow coefficient, resulting in a decrease in precision. Orifice plates are more sensitive to upstream flow disturbances and require a longer upstream straight pipe section to stabilize the flow field. The range ratio is usually 3:1 to 4:1, and the accuracy decreases rapidly at low Reynolds numbers.
Nozzle: Its streamlined design ensures a more stable flow coefficient and reduces susceptibility to wear. It is less sensitive to upstream flow disturbances than orifice plates, and the required upstream straight pipe section is usually shorter than that of orifice plates. Its accuracy is usually higher than that of orifice plates, with a range ratio of 4:1 to 5:1; it can maintain good accuracy and stability even at lower Reynolds numbers.
👉 Selection advice: For high-precision measurement and trade settlement, choose nozzles first； Another advantage of the nozzle is its suitability for applications where straight pipe sections are limited (as the nozzle requires a shorter straight pipe section).
4. Wear and corrosion resistance: Orifice plates: The sharp inlet edges are prone to wear (e.g., with fluids containing solid particles) or corrosion, which can cause the flow coefficient to change, requiring more frequent inspection and replacement. Dirt or impurities tend to accumulate at the right-angled edges.