【2026 Flow Measurement】Flow meters available for partial-filled pipe conditions
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The vast majority of traditional flow meters (such as electromagnetic, vortex, turbine, etc.) require the pipeline to be filled with fluid. In the case of non-full pipes (or open channels/partially filled pipes), there is gas – usually air – above the fluid, which can significantly affect the accuracy of measurements and even damage the instruments. The following are several flow meters designed specifically for partial-filled pipes or open channels, which can be selected according to specific conditions.
1. Ultrasonic Level/Flow Meter: This is currently the most commonly used and mainstream solution for measuring flow rate in systems where there is no full pipe volume. ➡Measurement principle: The instrument is installed at the top of the pipeline, and it measures the liquid level by emitting ultrasonic pulses. The microprocessor inside the instrument automatically calculates the cross-sectional area of the fluid based on the preset pipe dimensions (circular, rectangular, etc.) and the liquid level height. By combining the preset flow rate (for fixed weirs/channels) or the actual flow rate measured through multiple channels, the instantaneous flow rate and cumulative flow rate are finally calculated.
➡ Advantages: ① Contactless measurement: The sensor does not come into contact with the fluid, so it is not affected by fluid corrosion, wear, or contamination, resulting in minimal maintenance needs. ②Wide range of applications: suitable for pipes of various shapes (circular, square, trapezoidal) and open channels. ③Easy to install: No need to cut the pipes; it can be installed by drilling holes at the top of the pipes or directly above the channel. ④The measurement accuracy is high. ➡ Disadvantages: ① Highly affected by steam, foam, suspended solids in the pipeline, or scaling on the inner wall of the pipeline. ②For pipes with irregular shapes, precise calibration is required.
2. Electromagnetic Flow Meter – With partial-batch measurement capability. Ordinary electromagnetic flow meters must not be used when the pipe is not completely filled, as this can lead to inaccurate measurements and may damage the electrodes. However, there are electromagnetic flowmeters specifically designed for measurement in partially filled pipes. ➡ Measurement principle: It follows the same principle as ordinary electromagnetic flowmeters (Faraday’s law of electromagnetic induction), but it utilizes a special electrode design and signal processing techniques. For example, by using a three-electrode or four-electrode design, an electrode is always in contact with the liquid even when the liquid level changes, thereby enabling stable measurements.
➡ Advantages: ① Very high precision and good repeatability. ②Not affected by fluid density, viscosity, temperature, or pressure. ③It can measure corrosive liquids and sludges containing solids and liquids. ➡Disadvantages: ① High cost, much higher than that of ultrasonic integrators. ②Installation requires cutting the pipes, making the process complex. ③The fluid is required to have a certain level of electrical conductivity.
3. Flume/Venturi Flume + Level Meter: This is a classic measurement method, particularly suitable for open channels and large-diameter pipes. ➡ Measurement principle: A throttling device of a specific shape (Bashel groove or triangular weir) is installed in the channel or pipe. When fluid passes through this device, throttling occurs, and the liquid level rises at a specific position in the tank (upstream of the throat). There is a fixed mathematical relationship between this increased liquid level and the flow rate. By measuring this liquid level, the flow rate can be calculated accurately. Level gauges usually use ultrasonic level gauges or pressure-type level gauges.
➡Advantages: ① Simple structure, durable, and requires minimal maintenance. ②Fluids containing a large amount of suspended solids (such as mud) are less prone to clogging. ③Wide measurement range. ➡ Disadvantages: ① Requires significant installation space, with strict requirements for the straight sections ahead and behind. ②There will be a certain head loss. ③The accuracy depends on the machining precision of the groove and the accuracy of the liquid level measurement.
4. Doppler Ultrasonic Flow Meter: Although it is mainly used for full-pipe conditions, it can also be employed in certain specific partial-pipe situations. ➡ Measurement principle: By utilizing the Doppler shift effect generated when ultrasonic waves encounter particles or bubbles in a fluid, the movement speed of these suspended particles or bubbles is measured, thereby allowing the flow velocity of the fluid to be determined. However, this method cannot directly obtain the cross-sectional area; it needs to be used in conjunction with a level gauge to calculate the flow rate.
➡Applicable situations: A Doppler ultrasonic flow meter can be used when the pipe is not completely filled, but the liquid contains a sufficient amount of reflectors (such as bubbles and solid particles). Since the Doppler method operates by using sound signals reflected by bubbles and particles, it cannot function if the fluid does not contain such bubbles and particles. Fluids containing at least 30% suspended particles are used as the objects of measurement.
➡Advantages: ①It can measure complex media such as solid-liquid mixtures, slurries, and oil-water mixtures. ②There is no zero drift. When the fluid is at rest, no Doppler shift is produced. Therefore, the controller display unit will not experience zero drift. ➡Disadvantages: ① The measurement accuracy is greatly affected by the number and distribution of reflectors in the fluid, and its stability is inferior to that of other methods. ②A straight pipe section is required to maintain a stable flow regime (with a length of at least 15d upstream and over 5d downstream), and the installation location has strict requirements.