The Canary™ vs. Ultrasonic Flare Gas Meters
Do you need to measure flare gas in your drilling or production operation?
Is your flare line open to the atmosphere?
Do you have CO2 or H2S as part of the gas composition?
Do you expect low flow rates?
If you answered yes to any of these questions, then you need to turn away from Ultrasonic Flare Gas Meters and fire up The Canary™ !!
The Canary™ is the state-of-the-art flare gas meter that has been developed to provide accurate flare gas measurements in situations where ultrasonic measurement technology fails to perform adequately. Ultrasonic flare gas meters have known, documented performance limitations due to gas conditions and composition. Ultrasonic flare gas meters experience accuracy erosion when CO2 levels are above 5% with a potential total loss of signal at 15-20% due to signal attenuation. Ultrasonic flare gas meters also lose signal accuracy when high H2S levels are present in the gas being measured due to attenuation. Finally, it is difficult to transmit ultrasonic energy into a gas because of the mismatch of acoustic impedance between solid matter (the transducer) and the flare gas. This mismatch decreases as the gas pressure is increased – gas at atmospheric pressure is a worst-case scenario. These three conditions are the primary elements that weaken the ultrasonic signal and are the fundamental hurdles for this technology. These three conditions exist and are seen in some combination in the flare gas flowing out of the wellbore during drilling and production operations in the upstream oil and gas market.
Ultrasonic flare gas meters work on the “time of flight” principal. The prevalent design consists of two ultrasonic transducers mounted to a flowcell at a 45˚ angle to gas flow. The ultrasonic signal is transmitted through the flare gas with one signal moving with the flow of gas and the opposing signal moving against the flow as shown.
By measuring the two signal’s time of flight, a difference is noticed since one signal is moving with the gas flow and one signal is moving against the gas flow. Due to the fact that the ultrasonic transit time measurement is dependent upon the speed of sound in the flare gas, the gas density must be calculated, the temperature and pressure must be determined, as well as the flow velocity measurement. The time of flight being measured across the flow is relatively small and therefore extremely accurate transducer separation distances are required and transducer angle alignment tolerances are critical. In addition, as flare gas flow rates approach zero, the transit time measurement difference between the two signals becomes negligible and creates additional difficulties in obtaining a valuable measurement. Additional calibration, other than factory settings is often required to “tune” an ultrasonic flow meter to handle these difficult applications.
In refineries and chemical plants where flare gas composition is known and flare gas conditions are controlled, ultrasonic flare gas meters have had relative success. While drilling or producing a well in the oil and gas market, the gas flow coming from the formation and subsequently the wellbore is of constantly varying conditions, flowrate, and composition - in no way consistent. The Canary™ optical flare gas meter was developed to overcome these shortcomings by utilizing laser particulate velocity technology. The three ultrasonic killers: CO2, H2S, and ambient pressures – have absolutely no effect on the functionality or accuracy of The Canary™ . CO2 as a component of flare gas coming from the wellbore has a major impact on the reliability of the flow measurements produced by an ultrasonic flare gas meter. The following is a comparison of the impact of attenuation on ultrasonic signals at varying frequencies between air, methane, and CO2. The attenuation coefficient is defined by the Lambert-Beer-Law:
Comparison of attenuation coefficients gases at typical operational ultrasonic meter frequencies and loss[1]
The Canary™ is an optical flow meter that measures particulate velocity. The air all around us has more than enough particulates for The Canary™ to operate with great accuracy and repeatability. When utilized in the application where the gas velocity is measured just after the gas has exited the wellbore, the particulate count is extremely high due to downhole contaminants and mist carryover. The Canary™ operates best in dirty, wet conditions – those same conditions that present such difficulties to other gas velocity measurement devices.
The Canary™ utilizes an insertion probe to obtain a velocity measurement. This means that The Canary™ is easily installed on existing flare lines or through the utilization of a much simpler flowcell that does not have the critical manufacturing tolerances of an ultrasonic flare gas meter flowcell. The Canary™ insertion probe is designed with an orifice that is placed into the flare line parallel to gas flow. The orifice has a center target comprised of two sheets of laser light separated by 1mm. Particulates break each sheet of laser light scattering the light. The time between these two disruptions is measured and the velocity calculated based on the 1mm separation.
This measurement is taken independent of gas composition, temperature, pressure, or calibration. The Canary™ is only measuring the distance between peaks and not a magnitude.
Since there is no magnitude measurement the Canary™ never needs calibration. This is a significant cost savings over ultrasonic flare gas meters that may need to be recalibrated after every rig move. Low flow rates are also easier to measure as the time increases between breaks at lower particulate velocities as opposed to decreasing measurement differences experienced by ultrasonic flare gas meters.
The Canary™ flare gas meter is the fit for purpose technology for all gas flow applications in the oil and gas industry. Simply put, The Canary™ measures the time between particles carried in the gas flow. There are no negative effects from gas composition, pressure, temperature, or calibration with The Canary™ as they are not part of the velocity measurement process.
If you are going to be drilling or producing gas formations that have the possibility of experiencing low gas flow rates, CO2, or H2S, and you are using a flare line operating at atmospheric pressure, then The Canary™ is the only choice for you.
References:
[1]Koos van Helden et al., Examination of ultrasonic flow meter in CO2-rich applications, South East Asia Hydrocarbon Flow Measurement Workshop, 2009
