Under what circumstances should a venturi flow meter be used?
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This post was last edited by ppandao on 2010-7-10 at 16:21. The principles behind venturi flow meters and orifice plate flow meters are the same; both are flow measurement methods based on the law of conservation of energy – Bernoulli’s equation – and the equation of continuity. Both rely on measuring pressure differences. Venturi tubes have a streamlining effect, do not require sharp edges like orifice plates, result in lower pressure losses, and offer higher measurement accuracy compared to orifice plates. However, in practical applications, orifice plates are more widely used. Apart from cost considerations, is it only for large-diameter applications that venturi flow meters are chosen? In what other situations would a venturi flow meter be chosen over an orifice plate flow meter?
There is essentially no significant difference between the two; the venturi tube is more competitive in processes with high requirements regarding pressure loss, as its pressure loss is only 5%-20% of that of orifice plates
Venturi tubes are generally used for measuring gases at relatively low pressures
It is generally used for measuring gas flow rates in applications with low pressure, large pipe diameters, and low flow velocities.
Putting aside its advantages, the main reason is economic; the manufacturing process for venturi plates is more complex than that for orifice plates, resulting in higher costs
I. Overview The venturi tube is a throttling flow sensor developed based on the Venturi effect; it is a standard throttling device. Based on their structure, venturi tubes are divided into standard venturi tubes and universal venturi tubes. Standard (classic) venturi tubes are classified into standard venturis with a rough-cast contraction section, standard venturis with a mechanically machined contraction section, and standard venturis with a roughly welded plate contraction section, depending on their manufacturing method. Standard venturis are designed and manufactured in accordance with the national standard GB/T2624-2006, and calibrated in accordance with the national standard JJG640-94. The General Venturi series of flow sensors not only inherit the advantages of standard venturi tubes, such as high accuracy, good repeatability, low pressure loss, and short required straight pipe lengths, but also feature a compact design and resistance to clogging. It can be used to measure complex flow problems such as two-way flow, mixed-phase flow, low flow rates, large pipe diameters, and irregular-shaped pipes. II. Measurement principle: When fluid fills a pipe and flows through a throttling element within it, the flow velocity increases due to a local constriction at the throat of the Venturi tube; as a result, the static pressure decreases. This creates a pressure difference before and after the throat of the Venturi tube. The greater the fluid flow rate, the greater the pressure difference generated; thus, the flow rate can be determined based on this pressure difference. This measurement method is based on the continuity equation of flow (the law of conservation of mass) and Bernoulli’s equation (the law of conservation of energy). III. Characteristics 1. The standard (classic) Venturi tube is a standard throttling device designed and manufactured in accordance with the national standard GB/T2624, and calibrated in compliance with the national standard JJG640; it requires no further calibration. 2. In a standard throttling device, it requires the shortest straight pipe sections upstream and downstream, resulting in the lowest permanent pressure loss. 3. Stable performance and high reliability. 4. Accurate calculations with low energy consumption. 5. It can be used with various dirty media such as liquids, gases, vapors, and two-phase flows. 6. Simple structure, easy to install, and convenient for maintenance. 7. The standard venturi body is relatively long, being about 2 to 5 times the pipe diameter. IV. Main Technical Parameters 1. Nominal diameter: 50mm ≤ DN ≤ 3000mm; Rough-cast contraction section: 100mm ≤ DN ≤ 1600mm; Machined contraction section: 20mm ≤ DN ≤ 250mm; Rough-welded steel plate contraction section: 100mm ≤ DN ≤ 3000mm 2. Throttle orifice ratio β: 0.3 ≤ β ≤ 0.75; Rough-cast contraction section: 0.3 ≤ β ≤ 0.75; Machined contraction section: 0.4 ≤ β ≤ 0.75; Rough-welded steel plate contraction section: 0.4 ≤ β ≤ 0.7 3. Reynolds number range: 2×105 ≤ ReD ≤ 2×106; Rough-cast contraction section: 2×105 ≤ ReD ≤ 2×106; Machined contraction section: 2×105 ≤ ReD ≤ 106; Rough-welded steel plate contraction section: 2×105 ≤ ReD ≤ 2×106 4. Accuracy grade: Grade 0.5, Grade 1, Grade 1.5, Grade 2 5. Operating pressure: Nominal pressure: PN ≤ 42 Mpa 6. Medium temperature: -50°C ≤ t ≤ 500°C 7. Referenced standards: GB/T2624-2006, JJG640-94, and ASME PTC 19.5-2004 8. Connection methods: Flange connection, welding, threading
Sometimes, a venturi is used when it is not possible to meet the requirements of straight pipe sections with orifice plates; moreover, a venturi can also measure media containing a small amount of particles, which is not feasible with orifice plates.