Measuring Flow - The Transit-Time Difference Method

Transit-time difference Method
(courtesy of FLEXIM)

The Transit-Time Difference method exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium.

Similar to a swimmer swimming against the current, an ultrasonic signal moves slower against the flow direction of the medium than when in flow direction.

The Measurement Principle

Diagram of FLEXIM transit-time
difference flow meter design.

For the measurement, two ultrasonic pulses are sent through the medium, one in the flow direction, and a second one against it. The transducers are alternatively working as an emitter and a receiver.

The transit-time of the ultrasonic signal propagating in the flow direction is shorter than the transit-time of the signal propagating against the flow direction. A transit-time difference, Δt, can thus be measured and allows the determination of the average flow velocity based on the propagation path of the ultrasonic signals.

An additional profile correction is performed by proprietary FLEXIM algorithms, to obtain an exceptional accuracy on the average flow velocity on the cross-section of the pipe - which is proportional to the volume flow.

Since ultrasounds propagate in solids, the transducers can be mounted onto the pipe.

The measurement is therefore non-intrusive, and thus no cutting or welding of pipes is required for the installation of the transducers.

Clamp-on Ultrasonic Flow Measurement Advantages in Pipes and Storage Tanks

Clamp-on ultrasonic flow meter
(courtesy of Flexim)

In order to accurately pump and store fluids, reliable and accurate flow measurement is essential. Intrusive flow meters such as turbine meters and orifice plates are problematic when it comes to maintenance and long-term accuracy.

A better solution is a clamp-on, externally mounted, ultrasonic flow meter. These types of meters simply mount right to the outside of the process piping and do not expose the sensor to the process media. Nor do they require the pipe of vessel to be open for maintenance. Furthermore, they provide high accuracy and repeatability needed for leak detection and minimization of product losses.

Another advantage of using clamp-on flow meters is to use sonic velocity to determine what hydrocarbons are flowing through the pipe. Sonic velocity, when corrected for temperature change, is a good way to distinguish most hydrocarbons. By comparing the actual sonic velocity in a given application, and comparing it to known sonic velocity data, hydrocarbons at given temperatures can be identified.

This comes in particularly helpful in detecting interface changes. Different product batches are easily recognized by the meter. As interface changes are detected with a high degree of sensitivity, product mixups are significantly reduced.

Additional advantages of clamp-on, ultrasonic flow meters:

• No wear and tear - no clogging effects
• Quick and accurate detection of Interface changes - reducing amount of product mixing
• Accurate measurement on thick walled pipes and exotic pipe materials