Anti-parallel tag flow measurement system

Abstract

A tag flow measurement system wherein a first and a second measurement path are provided across a flowing fluid, and a receiver in each path receives signals modulated by scatterers in the fluid. The direction of signal propagation in one path faces in an opposite sense to, e.g., is anti-parallel to, the direction of propagation in the other path, and the two receiver outputs are correlated to determine a time interval representative of flow velocity. In one embodiment each path is defined by a transmitter on one side of the conduit and a receiver on the other side of the conduit, and the positions or orientations of transmitter and receiver are reversed in the second pair. Thus, the first transmitter may lie on the same side of the conduit as the first receiver. Diametral or chordal paths may be used. A prototype clamp-on system detects flowing air at atmospheric pressure in a schedule 40 one inch steel pipe over an extended range, at flow rates as low as several meters per second. The coherent crosstalk between one transmitter and the receiver of the other pair is greatly reduced, and travels only through the pipe and not across the fluid, so the cross-correlation signal to noise ratio may be enhanced by a factor of ten or more over that of a conventional tag correlation system. This allows effective operation in small conduits, at small spacings, at low flows and in other difficult measurement situations. Each transmitter may operate at a different frequency, and the received signals may be demodulated in phase quadrature to further enhance channel separation and received signal power. Frequencies or frequency pairs in the range of approximately one to four megahertz may be useful for one inch pipe, while lower frequencies in the range of 0.1 to 0.5 megahertz are advantageously employed for larger conduits. Spacings may be {fraction (1/4 )} to {fraction (1/1)} of a pipe diameter, and a common spacing, e.g., two inches may be employed for conduits over a diameter range of one to ten inches with high accuracy.

What is claimed is:  
     
         1 . An ultrasonic measurement system for determining flow of a fluid in a conduit, such system comprising: 
 a first transmitter receiver pair configured to define a first ultrasonic signal chordal path across fluid flowing in the conduit;    a second transmitter receiver pair configured to define a second ultrasonic signal chordal path across fluid flowing in the conduit;    said second transmitter receiver pair being mounted so that the second signal chordal path is antiparallel to the first signal chordal path and spaced a fixed distance therefrom; and    a processor operative to correlate a tag-modulated output signal of said first pair with a tag-modulated output signal of said second pair to determine a time interval representative of flow.    
     
     
         2 . The system of  claim 1  wherein said first pair operates at a different frequency than said second pair.  
     
     
         3 . The system of  claim 1  operating in a frequency range above 100 kilohertz.  
     
     
         4 . The system of  claim 3  wherein said frequency range lies above approximately 900 kilohertz.  
     
     
         5 . The system of  claim 1  wherein said first pair operates at a frequency different than frequency of operation of said second pair, and received signals are demodulated at their transmission frequency.  
     
     
         6 . The system of  claim 5  wherein said first pair operates at a frequency within approximately ten percent of said frequency of operation of said second pair.  
     
     
         7 . The system of  claim 5  wherein said first pair and said second pair operate in a continuous mode.  
     
     
         8 . The system of  claim 1  wherein the fluid is a liquid.  
     
     
         9 . The system of  claim 8  wherein the liquid contains scatterers.  
     
     
         10 . The system of  claim 8  wherein the liquid is turbulent.  
     
     
         11 . The system of  claim 1  wherein the fluid is a gas.  
     
     
         12 . The system of  claim 9  wherein the gas is a low density gas.  
     
     
         13 . An ultrasonic measurement system for measuring flow of a fluid in a conduit, such system comprising: 
 first and second transmitter receiver pairs defining first and second transit paths across a conduit, the second transit path being anti-parallel to the first transit path and wherein said paths are chordal paths;    a signal processor for processing signals received along said first and second chordal paths; and    a correlator for determining a time interval between correlated tag modulated signals on said first and second chordal paths.    
     
     
         14 . The system of  claim 13  wherein said transducers are coupled to a steam pipe of a building heating system.  
     
     
         15 . The system of  claim 13  wherein said transducers are attached to a process feed gas pipe of a chemical plant.  
     
     
         16 . The system of  claim 13  wherein said transducers are attached to a conduit having a nominal diameter under about two inches.  
     
     
         17 . The system of  claim 13  wherein at least one of said transducers is attached to a conduit by clamp-on.  
     
     
         18 . A method of measuring flow of a fluid in a conduit, such method comprising the steps of: 
 providing a first transmitter/receiver pair defining a first signal path through fluid in the pipe such that a first receiver output is modulated by tags in the fluid;    providing a second transmitter/receiver pair defining a second signal path through fluid in the pipe such that a second receiver output is modulated by tags in the fluid;    the second signal path being anti-parallel to the first path, the first and second signal paths being chordal paths; and    correlating the second receiver output with the first receiver output to determine flow rate.    
     
     
         19 . The method of  claim 18  further including the steps of operating the first transmitter/receiver pair at a first frequency and operating the second transmitter/receiver pair at a second frequency different from the first frequency, wherein the first frequency is sufficiently high to be well modulated by the tags, and the second frequency is close to the first frequency.  
     
     
         20 . The method of  claim 19  wherein the steps of providing transmitter/receiver pairs include providing clamp-on transducers.