Ultrasonic Flow Meters: Clamp-on versus In-Line for Advanced Flow Measurement

Titan Enterprises discuss the advantages and disadvantages of clamp-on and in-line ultrasonic flowmeters and the process applications most suited for these types of measuring devices.

Ultrasonic flow meters represent a significant advancement in flow measurement technology, offering precision, versatility, and ease of use with no moving or internal components. Providing improved electronic processing, increasing diagnostics and excellent accuracy, ultrasonic flowmeters operate using ultrasound to measure the velocity of a fluid flowing through a pipe of known dimensions giving the volumetric flow. There are two main types:

1. Transit-Time or ‘time of flight’ Ultrasonic Flow Meters: Measure the time difference between ultrasonic pulses sent in the direction of the flow and against the flow. The difference in the transit time is directly proportional to the flow velocity.
2. Doppler Ultrasonic Flow Meters: Measure the frequency shift of an ultrasonic signal reflected by particles or bubbles in the fluid. This frequency shift is used to calculate flow velocity.

Ultrasonic flowmeters can be either integrated into a pipe to measure the stream of flow ‘in-line’ or clamped onto the outside of an existing pipe. In-line and clamp-on flowmeters are both commonly used for measuring the flow rate of liquids and gases in various applications. Each type has its own set of advantages and disadvantages, which makes them suitable for different industrial applications.

In-line Flowmeters

Advantages:

1. High Accuracy: In-line flowmeters are generally more accurate because they are directly inserted into the pipeline, sometimes measuring over the entire cross-section of the pipe using multiple sensor sets to produce more accurate data for precise measurement of flow rate.
2. Independently calibrated: As a separate unit they can be finely calibrated by the supplier or independent calibration house to specific standards which will be directly relatable to the final installation.
3. Variety of Measurement Technologies: Though we are focusing on ultrasonic flowmeters here, in-line flow devices can use various other technologies such as turbine, magnetic, vortex, or differential pressure, which can be chosen based on the specific requirements of the application but ultrasonic has the added benefit of minimal or no flow restriction.
4. Stable Performance: These flowmeters tend to provide stable and consistent performance over time, making them reliable for long-term use.
5. Wide Range of Applications: Suitable for both liquid and gas flow measurements, in-line ultrasonic flowmeters can handle high pressures and temperatures depending on the construction material and design.

Disadvantages:

1. Installation Complexity: Installing an in-line flowmeter typically requires cutting the pipe and sometimes halting the process to empty the pipe prior to installation, which can be time-consuming and costly. Additional fittings may also be required.
2. Maintenance: Maintenance can be more challenging as it might require shutting down the process and disassembling parts of the pipeline.
3. Permanent Installation: Once installed, they are not easily relocated, making them less flexible if the measurement points need to be changed frequently.

In-line flowmeters are preferred for applications requiring high accuracy and stable, long-term measurement. Despite their higher installation costs, being electronic devices, their lifetime costs are relatively low. They are often used in Custody Transfer industries, such as oil and gas, as the high accuracy over time and the low maintenance costs recoup the initial installation expense fairly quickly.

Clamp-on Flowmeters

Advantages:

1. Non-Invasive: Clamp-on flowmeters are installed externally on the existing pipe, eliminating the need to cut or alter the pipeline, or drain the liquid. This makes them ideal for applications where the process cannot be interrupted.
2. Easy Installation and Maintenance: They are easy to install and maintain as stopping the flow or cutting into the pipe is not required. They can also be installed in difficult locations or tight spaces. Although there will be serious accuracy implications of doing this, it may be assessed as an acceptable process risk/penalty overall.
3. Usability: They can be fitted to a variety of pipe sizes, including large pipe diameters, and can be used on a variety of fluids. They can also be used to validate the performance of existing in-line flowmeters.
4. Portability: These flowmeters are often portable and can be moved to different measurement points, providing flexibility for temporary installations or spot-check measurements.
5. Cost-Effective Installation: Because there is no need to alter the pipeline, the installation costs are generally lower compared to in-line flowmeters.

Disadvantages:

1. Accuracy Limitations: Clamp-on flowmeters typically have lower accuracy compared to in-line flowmeters. The actual dimension of the pipe wall thickness and pipe bore are very important when calculating flow measurement accuracy. Factors such as pipe material, wall thickness, internal surface deposits and flow profile, will affect flowmeter accuracy. 
2. Calibration: Supplier calibration can only be done on a representative piped system and not on the final installed device, compromising the accuracy attainable.
3. Limited Application Range: They might not be suitable for very small or very large pipes and can be less effective for measuring certain types of fluids, particularly those with low fluid velocity or high levels of aeration.
4. Dependency on Pipe Condition: The condition and material of the pipe can significantly impact the performance of clamp-on flowmeters. For example, pipes with heavy insulation or thick walls might pose challenges to acoustic conductivity, causing signal degradation.  Pipeline material may also be an issue.
5. Potential for Signal Interference: They can be susceptible to signal interference from nearby electronic equipment or from the pipe material itself, which can affect measurement reliability.
6. Location: The choice of site for locating the clamp-on device is critical as the usual installation caveats, such as away from values and bends, still apply. Turbulence can be reduced by ensuring at a minimum, 10 diameters of straight pipe upstream and 5 diameters downstream. These factors would be closely monitored in a permanent situation by default.

Clamp-on flowmeters are ideal for non-invasive, flexible, and portable applications where ease of installation and low installation costs are priorities, even though they may offer lower accuracy and are influenced by the pipe and fluid characteristics. They are used across all industries due to their inherent flexibility.

The choice between ultrasonic in-line and clamp-on flowmeters depends on the specific needs of the application, including factors such as required accuracy, ease of installation, maintenance considerations, and the nature of the fluid and pipeline.

Titan’s range of in-line ultrasonic flowmeters utilise our patented ‘time-of-flight’ ultrasonic technology, operating with excellent accuracy over wide liquid flow ranges from 2ml/min to 20l/min.