The Case for Clamp-on Flow Meters: Part 2
By Thomas Michalowski
Vortex Flow Meters
Vortex meters rely on the detection of the frequency at which vortices are created from an obstruction placed in the fluid flow. This frequency, directly proportional to the flow rate, is converted into an electrical signal and the flow rate is calculated by the flow meter electronics. Vortex meters are inserted directly into the flow via a spool piece or directly as an insertion meter. They are used predominantly in petrochemical
plants, especially for high-pressure steam lines, but also used for used natural gas allocation and for mass balancing of non-viscous fluids.
Coriolis Flow Meters
Coriolis meters use the Coriolis effect to produce a mass flow rate, rather than a volume flow rate. As two tubes oscillate, the mass flow of the fluid through the tubes will act to oppose the oscillation, causing a deflection of the tubes. This deflection is identified by the time difference between the output signals of two pick-up coils, and by processing this information the mass flow can be derived. With their wide flow, pressure and temperature range, Coriolis meters effectively measure mass flow throughout the industrial spectrum.
Ultrasonic Flow Meters
Ultrasonic flow meters typically use two measurement techniques: Doppler and transit-time. Doppler meters require the presence of bubbles and/or solids in the flow to reflect the transmitted ultrasonic pulses. The shift in frequency of reflected signal relative to the transmitted frequency is proportional to the velocity of the reflector, and hence, the carrying fluid. In the transit-time technique, two ultrasonic transducers act
as both signal generators and receivers and are in acoustic communication with each other. In operation, each transducer functions first as a transmitter, generating a number of acoustic pulses, and then as a receiver for an identical number of pulses. The time interval between transmission and reception of the ultrasonic signals is measured in both directions. When the fluid in the pipe is not flowing, the downstream transit time will be the same as the upstream transit time. When the fluid flow exists, the downstream transit time will be less than the upstream time, as the pulse travelling downstream will be accelerated by the flow and the pulses travelling upstream will be slowed down by the flow. The difference between the two transit times is proportional to the velocity of flow and its sign indicates the direction of flow. Transit-time ultrasonic flow meters enjoy much wider use. They can be used for liquids and gases and offer excellent accuracy, no drift, no pressure drop, no obstruction to the flow and no routine maintenance. They can also handle bi-directional flow and a wide range of operating temperatures and offer a very high turndown ratio. By means of a thermal buffer, such as GE’s Bundle Waveguide Technology, they can operate at extreme temperatures from -200°C to 600°C.
Ultrasonic transit-time flow meters offer a very important advantage over competitive technologies. Although used extensively as insertion meters with “wetted” transducers, they can also be clamped to the outside of a pipe, offering a number of advantages, including portability and reduced installation disruption.