Roots Gas Flow Meters
A Roots meter is, in effect, a proprietary gear type positive displacement gas flow meter in which low frictional resistance is obtained by using lightweight meshing rotors of a figure of eight shape. Roots meters give good linearity over most of their range, and are suitable for handling most types of clean, dry, common gases.
Roots meters are made by Dresser Utility Solutions, a subsidiary of Baker Hughes.
Roots Meter Operating Principle
Two oppositely rotating impellers of figure of 8 shape are housed within a rigid measuring chamber with inlet and outlet connections on opposite sides. During impeller rotation, the precision machined measuring chamber traps a known volume of gas between the impeller and the adjacent chamber wall. The meter will measure and pass four equal gas volumes with each complete revolution of the impellers.
The sum total of the four volumes is the volume of gas flowing through the meter per revolution. Revolutions of the impellers are counted by a magnetically coupled gear reduction unit attached to the chamber. Temperature compensated units, and pulser units which allow remote volume data collection and monitoring are available.
A meter’s rated capacity is the maximum flow rate at which the meter may be operated and is determined by the dynamic loads acting on the moving parts of the meter. These loads are primarily related to meter RPM, and secondarily to the pressure of the gas being measured. The standard volume capacity of a rotary meter increases directly with changes in absolute line pressure and inversely with changes in absolute line temperature.
Roots Meter Charachteristics
The performance curve of the roots meter follows the familiar pattern of positive displacement meters curves, with slip at low flowrates causing a sudden fall in the curve.
You can expect rangeability of around 20:1, with good linearity over most of this range.
Accuracies of +/- 0.5% can be obtained.
Volumetric accuracy of the meter is dependent on the shape and the precise machining of non-wearing fixed and rotating parts. The volumetric accuracy is permanent and non adjustable. Measurement accuracy may be reduced by dirt or other types of foreign material in the gas stream.
Actual displaced volume measurement is completely independent of the gas specific gravity or gas pressure and can be easily converted to volume at Standard conditions for elevated pressure and varying temperature by application of the Gas Laws.
Roots Meter Installation
The performance of the Roots meter is little affected by upstrem flow disturbances or distorted velocity profiles, therefore there are no specific upstream or downstream straight length requirements.
They can be mounted both horizontally, and vertically. The preferred installation is vertical with gas flow downwards through the meter. This arrangement enhances the ability to pass dirt, pipe scale, or other debris through the meter. Having said that, it is usual to instal a Y-type strainer upstream of the meter.
Points to watch when buying and using
Roots meters can introduce cyclic pulsations into the discharge flow stream
Roots Meters are not suitable for measurement of hydrogen or oxygen
Life expectancy may be adversely affected by dirt or other types of foreign material in the gas stream
Roots meters can not be used in liquid service.
Technical Library
The following pages on Control and Instrumentation.com give more information, and explore in more detail topics associated with flow measurement basics:
