1. Introduction
Ultrasonic flowmeters measure liquid flow in a pipe using the velocity-difference (transit-time) method. They are available as clamp-on and inline (spool-piece) types. They are commonly combined with digital signal processing, multi-pulse techniques, and error-correction methods to improve environmental adaptability and reliability, and are widely used in chemical, power, petroleum, and metallurgical applications.
2. Structure
An ultrasonic flowmeter typically consists of an ultrasonic transmitter, an ultrasonic receiver, electronic circuitry, a flow display, and an accumulation system. The transmitter generates ultrasonic pulses and sends them into the fluid; the receiver detects the transmitted pulses after they pass through the fluid; the electronic circuitry amplifies and converts the received signals into electrical signals for display; the accumulation system performs totalization.
3. Working Principle
Ultrasonic flowmeters detect the effect of fluid flow on ultrasonic propagation using the transit-time difference method.
Probe 1 emits a pulse that travels through pipe wall 1, the fluid, and pipe wall 2 to be received by probe 2. Simultaneously, probe 2 emits the same pulse traveling in the opposite direction to be received by probe 1. Because the fluid velocity causes a difference in transit times, the time difference is used to calculate flow velocity and thereby the volumetric flow.
4. Advantages and Limitations
Because ultrasonic flowmeters use ultrasonic propagation through the fluid, they can measure flow in conventional pipes as well as in locations that are difficult to inspect or access. They can also measure fluids with corrosive, radioactive, flammable, or explosive properties when appropriate sensor types and materials are used.
However, ultrasonic flowmeters have limitations. They are constrained by the allowable fluid temperature range; currently ultrasonic flowmeters in China are only suitable for measuring fluids below 200 C. Measurement circuitry can be complex: to achieve 1% accuracy, sound speed must be measured to the 10^-5 to 10^-6 level, which places high demands on the measurement chain.
5. Installation Considerations
1. Full-bore condition
To ensure measurement accuracy and stability, the measured pipe section must be full. Otherwise the reading may be biased or measurement may fail. Sensors should be mounted on the horizontal plane of the pipe axis and within the recommended range shown in the figure to avoid locations where the upper part of the pipe is not full, where gas pockets can form, or where sediment accumulates.
Choose an installation point that ensures the pipe is full and avoids gas accumulation or sediment. For gravity-driven or unpressurized systems, consider the system high and low points when selecting the measurement location.
2. Flow stabilization
Stable flow improves measurement accuracy; disturbed or chaotic flow reduces it. Recommended upstream and downstream straight lengths are upstream 10D and downstream 5D (D is outer pipe diameter). If the meter is near a pump outlet or a partially open valve, keep 30D when possible.
If these conditions cannot be met, measurement may still be possible in some cases, for example when there are elbows or buffering devices between the pump outlet/partially open valve and the installation point, when the flow is from the pump inlet side, or when flow velocity is medium to low.
Flow speed classification: low < 1 m/s; medium 1–2 m/s; high > 2 m/s.
The following conditions make stable flow difficult and require caution when installing an ultrasonic flowmeter:
- Upstream straight length less than 10D without buffering devices such as elbows;
- Upstream straight length less than 10D combined with high flow velocity;
- Vertical downward or strongly downward-sloping flow;
- Downstream distance to an open pipe outlet less than 10D.
Note: If it is difficult to judge flow stability, use a portable ultrasonic flowmeter to check signal quality on site.
3. Fouling
Deposits on the inner pipe wall attenuate ultrasonic transmission and reduce the effective inner diameter, which can prevent proper measurement or degrade accuracy. Avoid installing at locations prone to heavy fouling. If unavoidable, consider the following measures to mitigate fouling effects:
- Replace the pipe section at the measurement point;
- Mechanically remove deposits at the measurement point (for example, by tapping the pipe) until signal improves;
- Use two-path measurement and treat the deposit layer as a lining in the configuration to improve accuracy.
4. Temperature
Exceeding the sensor's temperature rating can damage the sensor or significantly shorten its life. Ensure the fluid temperature at the installation point is within the sensor's rated range and, when possible, choose a lower-temperature location. Avoid boiler outlets and heat exchanger outlets; prefer the return line when feasible. If possible, measure the installation point temperature before sensor installation.
5. Pressure
The theoretical maximum pressure for insertion and spool-piece sensors is 1.6 MPa. Verify the installation point pressure before installation; installing where pressure exceeds this value poses safety risks and increases the likelihood of long-term leakage even if initial installation appears successful.
6. Electromagnetic and RF interference
The flowmeter electronics, sensors, and cable can be affected by interference sources such as variable-frequency drives, radio and television transmitters, microwave communication stations, mobile-phone base stations, and high-voltage lines. When selecting sensor and main unit locations, keep them away from these sources. Ensure the housings of the main unit, sensors, and the shielding layer of the ultrasonic cable are properly grounded (insertion sensors typically provide a grounding terminal). Do not share the same power supply line with a variable-frequency drive; use an isolated power source for the main unit when possible.
6. Commissioning
- Enter the pipe parameters required by the flowmeter and record them.
- Adjust the positions and spacing of the upstream and downstream sensors (probes) and the pipe jointing to maximize received signal strength in both directions.
7. Common Faults and Troubleshooting
Symptom: Instantaneous flow reading fluctuates widely.
Cause: Received signal strength fluctuates; the measured fluid flow itself is unstable.
Countermeasure: Adjust probe positions to increase and stabilize signal strength. If the fluid flow is inherently unstable, select a different measurement point that meets the upstream 10D and downstream 5D conditions.
Symptom: Clamp-on flowmeter shows weak signal.
Cause: Pipe diameter is too large, heavy fouling, or incorrect installation method.
Countermeasure: For excessive diameter or heavy fouling, consider insertion-type probes; re-evaluate the installation method and location.
Symptom: Insertion probe signal weakens after some time in service.
Cause: Probe drift or buildup of deposits on the probe face.
Countermeasure: Reposition the probe and clean the probe emitting face.
Symptom: No display after power-on.
Cause: Power supply characteristics do not match the instrument rating, or the fuse is blown.
Countermeasure: Verify power attributes match the instrument rating and check the fuse. If these are OK, contact the manufacturer or authorized service personnel.
Symptom: Display backlight is on but no characters are shown.
Cause: Typically program memory loss.
Countermeasure: Contact the manufacturer or authorized service personnel.
Symptom: Instrument fails under strong field interference at the site.
Cause: Large power supply fluctuations, nearby variable-frequency drives or strong magnetic fields, or improper grounding.
Countermeasure: Provide a stable power supply to the instrument; relocate the instrument away from variable-frequency drives and strong magnetic-field sources; and ensure proper grounding. Use shielded ultrasonic cables and separate power and signal cables, and consider adding ferrite cores or isolation transformers on the power input to reduce interference.
Overall, ultrasonic flowmeters' flow measurement accuracy is largely insensitive to fluid temperature, pressure, viscosity, and density. They can often be installed without shutting down the pipeline.
Supplement — Flowmeter Selection for Typical Media
- For turbid media such as sewage and pulp: ultrasonic flowmeters and electromagnetic flowmeters are options; avoid heavy entrained air or bubbles.
- For petroleum products such as crude oil and diesel: ultrasonic flowmeters are an option.
- For high-concentration slurries and media with large solid content, such as mortar and electrostatic powders: electromagnetic flowmeters are preferred.
- For large-volume potable water: electromagnetic flowmeters and ultrasonic flowmeters are suitable; other options include vortex meters and orifice plate meters.
- For gases: ultrasonic gas flowmeters and vortex meters are options. For gas temperatures above 300 C, consider differential-pressure-type flowmeters designed for high temperatures.
- For low-conductivity fluids such as deionized or desalinated water: ultrasonic flowmeters are well suited.
- For strongly corrosive fluids such as acids and alkalis: lined electromagnetic flowmeters or clamp-on ultrasonic flowmeters are options.
