Patent Number: 6,293,156

Title: Coherent multi-path flow measurement system

Abstract: A clamp-on ultrasonic measurement system for a gas or fluid of low acoustic impedance, such as steam, two phase fluid or flare gas includes an ultrasonic signal transmitter which clamps to the steam or gas conduit, and first and second receiving transducers clamped in the shadow zone of the transmitter. The transmitter signal skips within the conduit wall with a characteristic skip distance L.sub.p so ultrasonic signal energy is launched across the flow along plural paths over a region, and at an angle to the direction of flow. The receivers are positioned to receive the signal energy along different ones of said paths, modulated by tags or inhomogeneities in the flowing fluid. Cross correlation of the received signals then yields a peak correlation time interval or time delay from which the flow velocity is accurately determined. The transducers preferably employ air-backed crystals to generate a continuous shear wave signal of about 100 kHz-1 MHz, which reflects within the pipe wall and transmits a portion of its energy into the fluid with each skip. A structure or material mounts with an acoustically-matched impedance coupling to the pipe between two receiver transducers to absorb or block quasi-coherent signals that have passed through the fluid and have entered and propagated along the wall, thus eliminating phantom correlation peaks in the signals received downstream. Preferably, the receiving transducers are coupled with wedges to the conduit along a length nL.sub.p, and have an actual diameter less than L.sub.p or are otherwise configured to receive energy along a spatially limited path spread with high sensitivity to small scatterers in the fluid path. In a second aspect of the invention, a system employs two transmitters at different frequencies, opposed to a corresponding pair of similarly-spaced receivers. The receiver outputs are demodulated at the separate frequencies and then correlated to determine the flow time between receiving stations.

Inventors: Shen; Chang (Attleboro, MA), Jacobson; Saul A. (Medford, MA)

Assignee: Panametrics, Inc.

International Classification: G01F 1/66 (20060101); G01F 001/66 (); G01F 005/22 ()

Expiration Date: 09/25/2018