Correlation flowmeters expand technology

Just when you thought there were more than enough flowmeter technologies, along comes a new kid on the block, presenting new options for gauging and measuring fluid velocity.

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Correlation FlowmetersBy David W. Spitzer, Principal, Spitzer and Boyes

CORRELATION flowmeters gauge fluid velocity by measuring specific parameters associated with a flowing stream at various locations in the piping. To illustrate correlation’s general operating principle, consider a flow stream that abruptly changes color from red to green.

The fluid’s color could be sensed at two points that are 1 meter apart in the piping. If the second sensor detects the green fluid 1 second after the first sensor detects the green fluid, the velocity in the pipe could be calculated to be 1 meter per second. In general, fluid velocity can be calculated when the distance between sensors is known, and the time that the fluid takes to move from one sensor to the other is measured using correlation techniques.

Note that correlation flowmeters inherently measure velocity of the flowing stream. These flowmeters generally are based on the principles of physics and physical dimensions, so no pressure or temperature compensation is needed to effect these measurements. However, compensation often is required when measuring the mass flow of a fluid. 

Correlation flowmeters typically are applied to fluids in the turbulent flow regime and other fluid flows with coherent disturbances, such as slurries.

At least one correlation flowmeter has been around for about 10 years, but most were introduced in approximately the past five years. Correlation flowmeters offer some distinct advantages, not just because of their own capabilities, but also due to integrating existing flowmeter technologies with correlation techniques. In this sense, correlation flowmeters are similar to insertion flowmeters that use existing flowmeter technologies to implement insertion flow-measurement techniques.

Sonar, Ultrasonics, Optics Emerging
There are various flowmeter designs that use different measurements to determine flow rates. For example, correlation flowmeters don’t disturb the fluid flow, but rather passively or actively sense its characteristics.

Ultrasonic Senors

This pressure sensor array senses vortices caused by fluid velocity.

One correlation flowmeter (See Figure 1) uses a multiple-pressure sensor array attached to the outside of the pipe to measure pressure gradients caused by fluid vortices traveling down the pipe as a natural consequence of the fluid flow. These vortices are passively sensed at each pressure sensor. Computational techniques derived from sonar technology are used to determine the fluid’s velocity fluid. This technology also can determine the speed of sound of the fluid in the pipe, and can be installed without stopping the flow or contacting the fluid.

Another correlation flowmeter technology (See Figure 2 below) uses an ultrasonic transmitter/receiver pair located at a fixed distance downstream of another ultrasonic ransmitter/receiver pair. Their respective ultrasonic beams are modulated by turbulent eddies in the pipe to create a series of electronic “pictures” of the flow. Cross-correlation computational techniques are used to determine the velocity of the fluid. An advantage of ultrasonic technology is that clamp-on sensing methods can be used, so these flowmeters often can be installed external to the pipe without stopping the flow or coming in contact with the fluid.

Senors mounted on a pipe

Here’s the sensor pair mounted on a pipe under the insulation.

Yet another correlation flowmeter (See Figure 3 below) uses an optical transmitter on one side of a pipe, duct, or stack in conjunction with a dual-optical receiver on the opposite side. Eddies in the flow stream modify optical energy in the beam path, so the first and second receivers “see” the same eddy at different times. Correlation techniques are used to compare received signals, and determine the stream’s flow velocity without compensating for temperature, pressure, humidity, or opacity. These flowmeters typically are applied to gas flows in pipes, ducts, stacks, and flares.

Dan Gysling, chief technology officer at CiDRA, says there are “large-diameter, multi-phase, aggressive service applications that are prime applications for clamp-on correlation flowmeters.” These applications generally involve fluids that can damage other flowmeters, such as abrasive slurries that tend to erode flowmeters, and chemically active fluids that can corrode them. However, clamp-on correlation flowmeters have no moving parts, and don’t even touch the fluid, so it can’t erode or corrode these flowmeter. Gysling reports seeing clamp-on, correlation flowmeters replace magnetic, Venturi, and wedge flowmeters in abrasive slurry applications, in which intrusive flowmeters often wore out after three to 24 months of service.

Optical scintillation sensors
This graphic shows how an optical scintillation sensor pair works.

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