1. How do Venturi flow meters work?
A differential pressure Venturi flow meter is a type of differential pressure flow meter that utilizes the Venturi effect to measure the volume flow rate of a gas, liquid or slurry. A differential pressure flow meters is a type of flow meter, with no moving parts, that exploits the velocity dependence of static pressure to determine the volume flow rate of a fluid. Bernoulli’s principal requires that as the velocity of a fluid increases, its static pressure (i.e. the pressure detected by a pressure sensor) decrease due to the conservation of energy. The decrease in static pressure is equal to the increase in kinetic energy of the fluid, which is known as its dynamic pressure. This is described by the following equation:
The equation is the Bernoulli equation, with the potential energy term (which accounts for changes in hydrostatic pressure) omitted for simplicity. It enables us to determine the change in fluid velocity from the change in static pressure. The flow meter component that causes the velocity to increase is called a Venturi tube and it operates according to the Venturi principle (which is closely related to the Bernoulli Principal). The Venturi principal states that when a fluid is forced through a constriction (i.e. a section of reduced diameter) in a pipe, its velocity increases and its static pressure decreases. A Venturi tube is a pipe whose mid-section is of smaller diameter than its inlet and outlet. Crucially, the change in diameter is gradual, unlike an orifice which is a step change in diameter. The small diameter mid-section of the Venturi tube is known as a choke. As fluid enters the choke its velocity increases due to the reduction in cross sectional area. As the fluid leaves the choke, its velocity decreases back to its initial value. If the area of each section is known, then by measuring static pressure at the inlet and choke, volume flow rate can be determined as follows:
The differential pressure Venturi flow meter consists of a Venturi tube and a differential pressure sensor. A differential pressure sensor is a device that measures the pressure difference between two locations. The decrease in static pressure inside the choke depends on the ratio of the choke diameter to the inlet diameter which is known as the β ratio (e.g. a Venturi tube with a 10 cm diameter choke and a 30 cm diameter inlet has a β ratio of 0.33) of the Venturi tube. As the flow leaves the choke and enters the outlet (of equal diameter to the inlet), its static pressure increases. However, despite the pressure increase, not all of the pressure is recovered. The Venturi tube causes a pressure loss of about 3-10% of the change in static pressure between the inlet and the choke. The β ratio has a profound impact on the performance of the flow meter. A small β ratio (i.e. small diameter choke) results in a significant reduction in static pressure as the flow enters the choke and therefore high measurement sensitivity. However, it also increases the pressure loss across the Venturi tube.
2. General characteristics of Venturi flow meters
Differential pressure Venturi flow meters utilize the Venturi principal to measure the flow rate of gases, liquids and slurries. They are high accuracy flow meters, with no moving parts and do not result in a large pressure losses in the fluid. Furthermore, they can be used to measure extremely large flow rates and experience less wear than other types of differential pressure flow meters. Although they are suitable for use with slurries, they have a major limitation in that the Reynolds number of the flow must be over 100,000 and so they are only suitable for highly turbulent flow regimes. Differential pressure Venturi flow meters are normally manufactured from stainless steel or aluminum. However, Venturi tubes for demanding application including high temperatures and contact with corrosive materials are often made of exotic material including Hastelloy, Inconel, tantalum and Zirconia. The Venturi flow meter requires straight sections of pipe to be fitted at its inlet and outlet to straighten the flow. However, the sections can be of shorter lengths than those required by most other flow meters.
3. Input and output signals
The Venturi tube is a passive device. However, the differential pressure sensor requires a supply voltage of 10-30 Vdc to power its sensing element (piezoresistive device or capacitor) and built in electronics. Differential pressure sensors output either an unamplified mV scale voltage or an amplified voltage, converted into a standard output signal form such as 0-10 V or 4-20 mA.
4. Applications of Venturi meters
Differential pressure Venturi flow meters are high accuracy flow meters used for measuring the flow rate of gases, liquids and slurries in high flow rate applications with turbulent flow regimes. They are particular popular for demanding applications (e.g. high temperature, corrosive liquids) and those requiring small pressure losses. Typical applications include flow measurement within the chemical, waste treatment and oil & gas industries. Due to their high accuracy, they are often used for custody transfer.
5. Typical specification
|Measurement range||0.001-0.001 to 500-5,000 m3/min|
|Liquid temperature||up to 1,200 °C|
|Max. pressure||500 bar|
|Turndown ratio||1:10 typical|
|Viscosity||Re > 100,000|
|Supply voltage||10-30 Vdc|
|Output signal||0-10 V or 4-20 mA|
|Passive / active||Active*|
|Contact / non-contact||Contact|
*The differential pressure sensor requires a voltage supply
6. Purchasing tips
Differential pressure Venturi flow meters are normally semi-custom made. The manufacturer will design and manufacture the Venturi tube according to the customers application requirements and select the most appropriate differential pressure sensor. Important purchasing criteria include:
- Material compatibility: Venturi tubes are commonly made of Aluminum or stainless steel. However, they can be manufactured from a large range of materials to achieve compatibility with corrosive fluids or high temperatures. Materials used in challenging applications include Hastelloy, Inconel, Titanium, and Zirconium. Venturi tubes can also be manufactured from plastic to reduce cost.
- Manufacturing method: Metal Venturi tubes are commonly manufactured by either machining or welding. Machined Venturi tubes achieve higher accuracy and can operate at marginally lower Reynolds numbers than welded Venturi tubes.
- Uni/bidirectional: Differential pressure Venturi flow meters are available in both unidirectional and bidirectional flow versions.
- Sensitivity & pressure loss: The sensitivity of the flow meter, i.e. the decrease in static pressure per unit increase in flow rate, is highest when the β ratio is smallest. However, a small β ratio results in a large pressure loss. A trade off must therefore be made between sensitivity and pressure loss.
- Viscosity: Differential pressure Venturi flow meters are only suitable for flow regimes with Reynolds numbers above about 100,000. At lower Reynolds numbers, the Venturi tubes discharge coefficient becomes dependent on the value of the Reynolds number (see graph below), rendering the flow meter unusable. The minimum Reynolds number differs slightly depending on the β ratio (ratio of choke diameter to inlet diameter) and the manufacturing method. Machined Venturi tubes can be operated at marginally lower Reynolds numbers than welded Venturi tubes.
- Differential pressure sensor: An appropriate differential pressure sensor must be selected for the Venturi tube. The considerations include the pressure measurement range, material compatibility and output signal form.
- Turndown ratio: The turndown ratio is the ratio between the minimum and maximum flow rates that can be measured by the flow meter. Differential pressure Venturi flow meters have fairly modest turndown ratios of about 1:10, though turndown ratio varies slightly between flow meters of different geometries.
7. Advantages of Venturi flow meters
Differential pressure Venturi flow meters:
- Achieve high measurement accuracy, particularly if calibrated.
- Are suitable for use with liquids, gases and slurries.
- Have no moving parts, are highly durable and are not prone to clogging.
- Are available over a wide range of flow rates (though individual units have a limited turndown ratio), including extremely large flow rates.
- Cause only a small pressure loss in the fluid.
- Depending on the design and materials, they can have excellent chemical compatibility and withstand extreme temperatures and pressures.
- Can measure flow in both directions (not all are able to).
- Can be installed in any orientation.
8. Disadvantages of Venturi flow meters
Differential pressure Venturi flow meters:
- Are more expensive than many other types of differential pressure flow meters, including orifice flow meters.
- Are only suitable for highly turbulent flow regimes (Re > 100,000).
- Require straight pipe sections to be installed at their inlet and outlet for straightening the flow.
- Have a fairly low turndown ratio of about 1:10.
9. Application tips
- Calibration: The accuracy of the Venturi flow meter can be improved by calibrating it to correct for changes in operating conditions and discharge coefficients. Furthermore, calibration can offset long term sensor drift in the differential pressure sensor.
- Mounting: Differential pressure Venturi flow meters can be mounted in any orientation. If mounting horizontally and measuring the flow rate of a liquid containing large quantities of entrapped gas, orientate the flow meter with the pressure ports facing downwards. Conversely, for applications with slurries, orientate the flow meter with the pressure ports facing upwards to prevent blockage of the ports.
- Straight pipe: Like many flow meters, Venturi flow meters require sections of straight pipe to be fitted at the inlet and outlet of the Venturi tube in order to straighten the flow. Typical straight pipe lengths are 5 diameters at the inlet and 2 diameters at the outlet.
- High velocity gas flows: In gas applications, the velocity through the choke must not approach the speed of sound. If fluid velocity approaches the speed of sound a phenomenon known as chocked flow occurs. In a chocked flow, the flow rate depends only on the inlet conditions and becomes independent of the outlet pressure. This causes a breakdown in the relationship between differential pressure and flow velocity. The chocked flow phenomenon is irrelevant for liquids.