What is Ultrasonic Flow Meter

Introduction

Ultrasonic flow meter refers to a flow meter developed based on the principle that the propagation speed of ultrasonic waves in a flowing medium is equal to the vector sum of the average flow velocity of the measured medium and the speed of the sound wave in a stationary medium. It is mainly composed of a transducer and a transmitter. That Flo-Instruments adopts is transit-time method.

Working Principle

Transit-time method: The time difference caused by the different propagation velocities during forward and reverse propagation is measured to calculate the velocity of the measured fluid.

It uses two sound wave transmitters and two sound wave receivers. Two groups of sound waves from the same sound source are transmitted between the two sound wave transmitters and the two sound wave receivers respectively. The position where they are installed along the pipeline is at an angle θ with the pipeline (generally θ=45°). Since the sound waves transmitted downstream are accelerated by the fluid, while the sound waves transmitted upstream are delayed, the time difference between them is proportional to the flow velocity. It is also possible to send a sinusoidal signal to measure the phase shift between the two sets of sound waves or send a frequency signal to measure the frequency difference to achieve flow velocity measurement.

Features

(1) Clamp-on Transducer: Clamp-on installation belongs to non-contact measurement. It allows users to install directly without stopping the flow and cutting off the pipe. This is a unique advantage of ultrasonic flow meters in industrial flow meters. Therefore, it can be used for mobile (i.e. non-fixed installation) measurement, which is suitable for pipe network flow condition assessment and measurement.
(2) Non-obstructive measurements and no additional pressure loss.
(3) Suitable for large circular and rectangular pipes, and in principle are not limited by pipe diameter. They can be considered as the preferred option when actual flow verification cannot be achieved.
(4) Measure non-conductive liquids, which is a supplement to electromagnetic flow meters in terms of non-obstructive flow measurement.
(5) Used for clean liquids and no particles.
(6) Clamp-on transducers cannot be used for pipes with thick lining or scaling, nor can they be used for pipes with lining peeling off from the inner pipe wall or serious corrosion. Thus, it can be replaced by insertion transducer.

Classification

There are many types of ultrasonic flow meters. However, Flo-Instruments mainly engages in time difference ultrasonic flow meters, so we classify them according to the following:
(1) Classification by transmitter installation
Mainly divided into wall-mounted, fixed, portable, handheld, integrated, and split types.
(2) Classification by transducer installation
Mainly divided into clamp-on type, insertion type, and pipe section type (not available yet).
(3) Classification by the measured medium(liquid)
Mainly water, seawater, vegetable oil, petroleum, chemical liquids, etc.
(4) Classification by the number of channels
The four commonly used classifications by the number of channels are single channel, dual channel, four channel and eight channel. In recent years, three channel, five channel and six channel have also appeared. The multi-channel configuration of four channels and above plays a great role in improving the measurement accuracy.

The clamp-on type ultrasonic flow meter is the earliest produced, most familiar to users and most widely used ultrasonic flow meter. The transducer does not need to be cut off in the pipeline for installation. It can be used immediately after it is attached. It fully reflects the characteristics of simple installation and easy use of ultrasonic flow meter.

Some pipelines are due to loose materials, poor sound conduction, or severe rust, and there is a gap between the lining and the space inside the pipeline, which leads to severe attenuation of ultrasonic signals. It is impossible to measure normally with clamp-on ultrasonic flow meter, so a pipe section ultrasonic flow meter is produced. The pipe section ultrasonic flow meter integrates the transducer and the measuring tube, which solves a difficulty in the measurement of the external flow meter, and the measurement accuracy is higher than other ultrasonic flow meters, but it also sacrifices the advantage of the clamp-on ultrasonic flow meter that it can be installed without flow, and requires cutting the pipe to install the transducer.

The insertion ultrasonic flow meter is between the above two. It can be installed without flow. Use special tools to drill holes in the water pipe, insert the transducer into the pipe, and complete the installation. Because the transducer is in the pipe, its signal transmission and reception only pass through the measured medium, not through the pipe wall and lining, so its measurement is not limited by the pipe quality and pipe lining material.

Application

Various ultrasonic flow meters have been widely used in industrial production, commercial measurement and water conservancy testing, such as:

In the measurement of raw water, tap water, recycled water and sewage in the municipal industry, ultrasonic flow meters have the characteristics of large range ratio and no pressure loss, which improves the water transmission efficiency of the pipeline network while ensuring measurement accuracy;

In the flow measurement of water pipelines, channels, pumping stations and power stations in the water conservancy and hydropower industry, ultrasonic flow meters have the characteristics of large diameter, on-site installation and online calibration, making accurate measurement possible. At the same time, the purpose of equipment optimization and economic operation can be achieved by measuring single pumps and single machines of water pumps and turbines;

In the measurement of industrial cooling circulating water, ultrasonic flow meters realize online continuous flow pressure installation and online calibration;

In commercial measurement, multi-channel ultrasonic flow meters have achieved an accuracy (indication error) better than 0.2%.