What Is a Venturi Meter?
A Venturi meter is a differential pressure flow measurement device consisting of a converging inlet cone, a cylindrical throat section, and a diverging recovery cone; all machined or cast as a single body installed in a pipeline. The gradual geometry accelerates the fluid through the throat, creating a measurable pressure drop between the upstream pipe and the throat. Because the diverging cone recovers most of the pressure, the Venturi meter produces a significantly lower permanent pressure loss than an orifice plate.
Venturi meters are standardized under ISO 5167-4 and ASME MFC-3M. They are widely used in water treatment, power generation, and oil and gas applications where pumping energy costs or downstream pressure requirements make the high permanent loss of an orifice plate unacceptable.
Operating Principle
Fluid enters the converging section (typically a 21-degree included angle), accelerates through the throat, and decelerates in the diverging section (7 to 15-degree included angle). Pressure taps at the upstream pipe and at the throat measure the differential pressure (dP). The flow rate is then:
Q = Cd * A_throat * sqrt(2 * dP / rho)
Where Cd is the discharge coefficient (typically 0.984 for a machined Venturi per ISO 5167), A_throat is the throat cross-sectional area, and rho is the fluid density.
The Beta ratio (d_throat / D_pipe) typically ranges from 0.30 to 0.75 for standard Venturi tubes.
Venturi Meter Sections
| Section | Geometry | Function |
|---|---|---|
| Inlet cone | 21 +/- 1 degree included angle (ISO 5167) | Smoothly accelerates the fluid |
| Throat | Cylindrical, length = 1 x throat diameter | Zone of maximum velocity and minimum pressure; pressure tap location |
| Diverging cone | 7 to 15-degree included angle | Gradually decelerates fluid, recovering up to 90% of the differential pressure |
| Upstream tap | At pipe wall, 0.5D to 1D upstream of inlet cone | Measures undisturbed upstream pressure |
| Throat tap | At mid-point of throat section | Measures minimum pressure |
Types of Venturi Meters
ISO 5167-4 defines three types based on manufacturing method:
| Type | Construction | Discharge Coefficient (Cd) | Pipe Size Range |
|---|---|---|---|
| Classical (machined) | Fully machined from solid or casting | 0.984 +/- 0.7% | 50 mm to 250 mm |
| Rough-cast | Inlet cone is rough-cast, throat machined | 0.984 +/- 1.0% | 100 mm to 800 mm |
| Rough-welded | Fabricated from sheet metal, welded | 0.985 +/- 1.5% | 200 mm to 1200 mm |
Venturi Meter vs. Orifice Plate
| Parameter | Venturi Meter | Orifice Plate |
|---|---|---|
| Permanent pressure loss | 5-20% of measured dP | 40-80% of measured dP |
| Discharge coefficient | ~0.984 (stable) | ~0.60 (varies with Beta) |
| Accuracy | +/- 0.5 to 1.5% | +/- 1 to 2% |
| Turndown ratio | 3:1 to 4:1 | 3:1 to 5:1 |
| Straight pipe upstream | 5-10D (shorter than orifice) | 15-40D |
| Cost | High (machined body) | Low (thin plate between flanges) |
| Maintenance | Low; self-cleaning geometry | Moderate; edge wear degrades accuracy |
| Dirty fluids | Good; smooth profile resists buildup | Poor; sediment accumulates at plate |
Materials and Sizing
Venturi meters are fabricated from carbon steel (ASTM A234 WPB), stainless steel (316/316L), duplex stainless steel, or lined carbon steel depending on the process fluid. Standard sizes range from 2” (50 mm) to 48” (1200 mm) for ISO-compliant designs; custom fabricated tubes can exceed 72” for large water mains. Venturi meters typically use weld neck flanges and pressure taps connect to the DP transmitter via compression fittings.
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