July/August 2010

Special Section: Flow/Level

Portable ultrasonic flowmeters

Portable aspect expands scope of this traditionally fixed technology

Fast Forward

  • Rechargeable battery used to power transmitter and sensors, eliminating need for an external fixed source of power.
  • One common application is for temporary flow measurement if the fixed flow device fails.
 
By Kevin Lavelle

Clamp-on ultrasonic flowmeters are widely used throughout the process and other industries because their non-invasive nature confers inherent advantages over other flowmeter technologies. Material compatibility, contamination, and corrosion risk factors are eliminated. Process integrity is unaffected, and installation costs are much lower.

aug17A portable ultrasonic flowmeter contains the same fundamental hardware as a fixed ultrasonic meter, as both use a process transmitter connected to a pair of sensors that are attached to the process piping. But a portable ultrasonic flowmeter has unique capabilities that differentiate it from a standard fixed instrument.

First, a rechargeable battery is used to power the transmitter and the sensors. Second, portable ultrasonic transmitters contain built-in data acquisition capabilities that allow the user to locally record flow velocity, signal strength, sound velocity, totalized flow, and other process variables. Some portable transmitters also feature analog input channels that can be used to receive and record data from other local instruments and various data sources.

All of this data can be exported from the transmitter, commonly via a USB memory stick. After export, the data can be transferred to any number of data analysis packages such as Microsoft Excel. This built-in data acquisition and storage capability further enhances portability by obviating the need for a fixed and permanent connection to a higher level control system.

As opposed to a fixed flow measurement instrument, a portable ultrasonic flowmeter can be quickly installed and configured without any special installation tools. Most portable meters are provided with step-by-step configuration guides that show the user how to align the system for different process parameters such as pipe diameter and pipe liner material.

With any ultrasonic flowmeter, the alignment and spacing of the sensors is critical to the performance of the system. The clamp-on ultrasonic sensors are held in place via a mounting bracket and straps. The mounting bracket provides defined sensor positioning to ensure optimal flow measurement. The sensors use a sonic coupling gel to form a continuous signal path between the pipe wall and the sensor face.

A portable ultrasonic flowmeter requires two sensors in addition to the standard measurement sensors to measure fluid velocity and wall thickness. In addition to measurement information, the additional sensors provide diagnostic data that aid in installation and troubleshooting.

The installed accuracy for a portable ultrasonic system varies, with the total measured error being the sum of two independent factors-the error of the sensor/transmitter plus the error associated with the installation. It is important to make a distinction between the two different sources of error.

The sensors and transmitters generally feature accuracies of 0.5% of the measured value. The more significant source of error arises from the site specific installation and operating conditions.

Factors such as build-up, scaling, and fluid variations all account for increased measurement error. Due to the varying nature of the site conditions, the accepted practice is to assume a 1.5% error associated with the installation, and thus a total measurement error of 2%.

The major source of errors and installation issues are usually associated with unknown or changing pipe conditions. Before installation, it is critical to have a complete knowledge of pipe characteristics such as inner and outer diameter, types and thickness of pipe liner, and pipe material. The installation of the sensor should be done on a clean surface free of rust or dirt, and it may also be necessary to remove any paint on the pipe.

Another common installation problem is the placement of the sensor upstream or downstream of flow obstructions. Vortex, turbine, and pressure differential flowmeters installed in the same pipe run can create serious downstream flow disturbances, so proper installation practices must be followed with respect to recommended upstream and downstream straight runs.

The portable aspect of clamp-on ultrasonic flowmeters opens up applications that expand the scope of this technology, with the most important being flow verification.

Flow verification is often used to assist in start-up or troubleshooting of fixed flowmeters. With two independent meters measuring the same parameters, it becomes very clear whether there is a process problem or a meter problem.

Note a difference between flow verification and calibration. Portable ultrasonic meters are ideal for verification, but should not be used for calibration as they are not NIST traceable and usually have less accuracy than the device being calibrated.

Another common application is for temporary flow measurement if the fixed flow device fails. This can allow repair or replacement of the fixed flow device on a planned as opposed to a reactive basis, often preventing a costly and lengthy emergency shutdown.

Portable ultrasonic flowmeters can also be used to ascertain the best type and location for permanent flow device sensors. Different sensor types such as low versus high frequency, and varying alignments such as single as opposed to double traverse can be evaluated.

ABOUT THE AUTHOR

Kevin Lavelle is product business manager-Pressure at Endress+Hauser. His e-mail is Kevin.Lavelle@us.endress.com.

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