There are basically four types of flowmeters available for Flare applications:
Each has strengths and weaknesses. Ultrasonic Gas Flowmeters are cross stack instruments that measure the transit-time with and against the flow to determine flow velocity. By also measuring the speed of sound in the gas, the meter is able to establish approximate gas density giving a calculated #/hr output. Ultrasonic sensors are in contact with the process gas and proper alignment is necessary. System hardware cost and installation cost is higher than the other available technologies. Thermal mass flowmeters are insertion meters that sense the differential temperature between a heated and reference temperature sensor. By calibrating the sensor in a known gas composition with a known thermal conductivity the meter is able to measure the gas velocity in a flare with a similar gas composition producing an output signal proportional to scfh. By knowing the gas density the output is converted to pph. Because most hydrocarbons have similar thermal conductivities per unit mass the meter can handle some changes in gas composition without affecting output. Hydrogen, however, with its very high thermal conductivity greatly affects the meter output if the concentration varies. Also, thermal meters are affected by moisture condensing on the sensor due to the high thermal conductivity of a liquid vs. a gas. Thermal mass flowmeters are less expensive and easy to install and produce very accurate results when the gas composition is known. Thermal meters should not be used in flares with varying hydrogen content or after wet scrubbers. Doppler Vortex Flare Flowmeters are vortex shedding flowmeters that sense the vortices sonically and can measure flows down to 1 ft/sec vs. the 18 ft/sec low end limitation of other vortex shedding flowmeters. The Doppler Vortex Flowmeter has a shedder bar that sheds vortices at 22 Hz/ft/sec. Being a velocity-measuring device it must be pressure and temperature compensated to produce a SCFM output. Both pressure and temperature are sensed at the same insertion point. Where required a mass flow computer is an option. These meters are more expensive than thermal mass flowmeters with an equivalent installation cost. The meter has been a good choice for refinery flares with varying H2 content. Annubars have been used for years on flare applications. An annubar is a differential pressure device with the signal increasing proportional to the square of the flow. Annubars are good for high flow rate applications. Annubars are not good for low flow applications. They do require pressure and temperature compensation for mass flow applications. |