Optical flow metering is an emerging technology for natural gas and flare measurement. It uses optical laser technology to measure the actual speed of particles rather than the speed of sound through a medium (as in ultrasonic flowmeters) or conductivity (as in mass thermal flowmeters).
How it Works
The Laser-Two-Focus (L2F) technology uses two laser beams to detect the passage of any light-scattering particles carried along by the moving fluid.
As a particle passes through each laser beam, it redirects the light away from its normal straight-line path in such a way that an optical sensor (one per beam) detects the scattered light and generates a pulse signal.
As that same particle passes through the second beam, the scattered light excites a second optical sensor to generate a corresponding pulse signal.
The time delay between two successive pulses is inversely proportional to the velocity of that particle.
IMAGE TO FOLLOW
In order, to obtain a measurement of bulk (average) fluid velocity, the raw velocity measurement provided by the sensor must be corrected based on the expected Reynolds number for the process fluid, which is why insertion L2F flowmeters are equipped with pressure and temperature transmitters in addition to the optical probe.
When the three measurements (pressure, temperature, and velocity at the probe) are combined, the Reynolds number may be calculated which then predicts how “flat” (consistent) the velocity profile is for the fluid stream.
IMAGE TO FOLLOW
Advantages
Moderate sensitivity to entrained mist or liquid
Meter diagnostics can detect fouling prior to failure of the unit (does not require physical inspection which may require the line to be removed from service)
Good turndown ratio (1000:1 to 1500:1)
Good accuracy over a large measurement range (e.g., 1 to 100 m/s)
Limitations
High sensitivity to fouling
Sensitive to flow disturbances
Accuracy decreases within low measurement ranges (e.g., 0.1 to 1 m/s)
Limited industry experience with flare service
The meter may be difficult to extract for maintenance while line is in service
Case study
No case study available at this time.
Optical flow metering is an emerging technology for natural gas and flare measurement. It uses optical laser technology to measure the actual speed of a fluid flowing in a pipe to determine its flowrate.
