1. Technical Definition
A Gas Roots Flow Meter (also called a Roots-type gas meter or rotary displacement gas meter) is a positive displacement flow measurement instrument used to determine the volume of non-corrosive gases—such as natural gas, city gas, propane, nitrogen, or carbon dioxide—passing through a pipeline. It belongs to the positive displacement (PD) category of flow meters, meaning it measures flow by mechanically isolating and counting discrete volumes of gas rather than inferring flow from velocity, pressure differential, or acoustic signals.
The working concept is straightforward: gas entering the meter body pushes against two synchronized, figure-eight-shaped rotors (the "Roots lobes"). Each rotation displaces a fixed, known volume of gas. By counting rotor revolutions, the meter calculates cumulative and instantaneous flow with high repeatability.
Compared to velocity-based meters (turbine, vortex, ultrasonic), a Gas Roots Flow Meter does not rely on the gas flow profile being fully developed. This is why it typically requires no upstream or downstream straight pipe sections for accurate measurement—a key structural differentiator from inferential flow technologies.
2. Working Principle Explanation
Positive Displacement Measurement Principle
The Gas Roots Flow Meter operates on the principle of volumetric displacement. Inside the meter body, a fixed measuring chamber volume is defined by the meter housing and the rotor geometry. Every time the rotor assembly completes one full rotation, it displaces this fixed chamber volume of gas from the inlet to the outlet.
Rotation of Roots Lobes
Two Roots-shaped rotors are mounted on parallel shafts inside the meter body and are mechanically synchronized through timing (synchronous) gears. As gas pressure differential builds across the meter, the rotors begin rotating in opposite directions without physical contact, maintaining a small, controlled clearance. This synchronized, non-contact rotation is what allows the meter to operate with minimal wear and low pressure loss.
Gas Volume Displacement
As the rotors turn, each lobe cavity seals off a defined pocket of gas near the inlet and releases it at the outlet. Because the geometry of the rotor pair and housing is fixed and calibrated, each revolution corresponds to a known, repeatable volume of gas—regardless of variations in flow velocity profile.
Flow Calculation Process
The number of rotor revolutions over time is converted into a volumetric flow rate at working conditions. In intelligent Roots meters, integrated temperature and pressure sensors feed real-time data into a microprocessor that applies a gas equation (commonly based on standards such as SY/T 6143-2004 or SGERG-88) to calculate a compression factor. This factor converts the working-condition volume into standard volume, compensating for temperature and pressure fluctuations that would otherwise distort commercial billing accuracy.
Signal Output Method
Depending on the meter configuration, output signals may include:
- Mechanical counter display (odometer-style, no external power required)
- Electronic LCD display showing cumulative and instantaneous standard flow
- Digital communication via RS-485 (MODBUS RTU protocol)
- Pulse output or 4-20mA analog signal for integration with SCADA or telemetry systems
3. Product Structure Analysis
A typical Gas Roots Flow Meter consists of the following core components:
- Meter Body: A precision-machined housing that forms the sealed measuring chamber and defines the fixed displacement volume per rotation. Bodies are typically designed for flange connection and support both horizontal and vertical installation orientations.
- Roots Rotor Pair: Two figure-eight-shaped rotors that displace gas volume with each synchronized rotation. Manufactured to tight tolerances for repeatable volumetric accuracy.
- Synchronous Timing Gears: Ensure the two rotors rotate in perfect coordination without mechanical contact, minimizing friction, wear, and pressure loss.
- Bearings: Support the rotor shafts and maintain precise clearances between rotors and housing walls over the meter’s operating life.
- Transmission Mechanism: Transfers rotor rotation to either a mechanical counter, an electronic pulse generator, or both, depending on the display configuration.
- Sensor System: In intelligent variants, integrated temperature and pressure sensors continuously monitor operating conditions for real-time compensation.
- Electronic Converter/Correction Instrument: A microprocessor-based unit that executes the volume correction algorithm (PV/T gas equation), manages data storage (typically several thousand status records), and drives the display and communication interfaces. Some models also include intrinsically safe circuit design (e.g., Exia II CT4Ga) for use in hazardous gas environments.
4. Measurement Characteristics
- Accuracy: Roots-type gas meters commonly achieve accuracy classes of 1.0 or 1.5, meeting national and international metrological standards such as GB/T 28848 and JJG 633.
- Repeatability: Because measurement depends on fixed mechanical displacement volume rather than inferred velocity, repeatability across varying flow conditions is a defining strength of this technology.
- Low Flow Measurement Capability: Roots meters are well suited to detecting very low flows—starting flows as low as 0.02 m³/h are achievable—making them effective for capturing pilot-light-level consumption in commercial gas applications.
- Wide Rangeability: A rangeability of 160:1 or greater allows a single meter to accurately measure both minimal and peak flow demands without requiring meter replacement or bypass systems.
- Pressure Loss Characteristics: The non-contact synchronized rotor design minimizes friction-related pressure drop across the meter, which is important for maintaining downstream gas pressure in distribution networks.
- Gas Compatibility: Suitable for non-corrosive gases including natural gas, city gas, liquefied petroleum gas (LPG), nitrogen, and carbon dioxide.
5. Comparison Knowledge
Gas Roots Flow Meter vs Turbine Gas Flow Meter
Turbine meters infer flow from the rotational speed of a bladed rotor driven by gas velocity, making them sensitive to flow profile distortion and typically requiring straight pipe runs upstream and downstream. Roots meters, being positive displacement devices, measure discrete gas volumes directly and are largely insensitive to velocity profile, allowing installation with minimal straight-pipe requirements. Turbine meters generally perform better at higher flow rates, while Roots meters excel at low-flow accuracy and wide rangeability.
Gas Roots Flow Meter vs Vortex Flow Meter
Vortex meters detect flow by measuring the frequency of vortices shed behind a bluff body, a method well suited to higher velocity, steady flows but less accurate at low flow rates. Roots meters maintain accuracy across a much broader flow range, including near-zero flow conditions, making them more suitable for applications with fluctuating demand, such as commercial and residential gas billing.
Gas Roots Flow Meter vs Ultrasonic Flow Meter
Ultrasonic meters use transit-time or Doppler effects to infer velocity and are non-invasive, with no moving parts and low pressure loss. However, their accuracy can be affected by gas composition, flow disturbances, and installation geometry. Roots meters provide direct volumetric measurement independent of acoustic properties, offering more predictable performance in environments with mechanical simplicity requirements or where electronic infrastructure is limited.
6. Application Knowledge
- Natural Gas Measurement: Used extensively in city gas distribution networks for accurate commercial and residential billing.
- LPG Measurement: Suitable for liquefied petroleum gas metering in industrial and commercial settings due to compatibility with non-corrosive gas types.
- Industrial Gas Measurement: Applied in industrial boiler systems where standard volume conversion from working-condition flow is essential for process accounting.
- Commercial Gas Distribution: Widely deployed in catering and hotel industries, where fluctuating peak and off-peak demand requires wide rangeability and low-flow sensitivity for accurate settlement.
7. Engineering Considerations
- Installation Requirements: Roots meters support both horizontal and vertical flange installation, typically across a DN20 to DN200 size range, offering flexible integration into existing pipeworks.
- Straight Pipe Requirements: Unlike velocity-based meters, Roots-type meters generally do not require dedicated straight pipe sections upstream or downstream, simplifying installation in constrained spaces.
- Gas Cleanliness: As with any mechanical rotor-based device, gas should be reasonably free of particulates or debris that could interfere with rotor clearances and long-term measurement stability.
- Pressure and Temperature Influence: Because gas volume varies with temperature and pressure, intelligent Roots meters incorporate real-time sensors and a correction instrument to convert working-condition flow to standard conditions, ensuring commercial settlement accuracy.
- Maintenance Considerations: Standardized lubrication systems support long-term rotor and bearing performance. Verification cycles (commonly every 2-3 years depending on accuracy class) help maintain metrological compliance. Electronic variants with battery-powered displays typically offer multi-year battery life, simplifying field maintenance.
8. Technical FAQ Section
What is the principle of a Roots gas flow meter?
A Roots gas flow meter operates on the positive displacement principle. Two synchronized Roots-shaped rotors trap and displace a fixed volume of gas with each rotation, and the meter counts these rotations to calculate total and instantaneous flow.
How accurate is a Roots gas meter?
Roots gas meters commonly achieve accuracy classes of 1.0 or 1.5, in compliance with standards such as GB/T 28848 and JJG 633, making them suitable for commercial and industrial settlement applications.
What gases can a Roots flow meter measure?
Roots flow meters are designed for non-corrosive gases, including natural gas, city gas, liquefied petroleum gas (LPG), nitrogen, and carbon dioxide.
What are the advantages of Roots gas flow meters?
Key advantages include high accuracy independent of flow profile, no requirement for straight pipe sections, wide rangeability (up to 160:1 or more), low starting flow detection (as low as 0.02 m³/h), low pressure loss, and the option for mechanical backup display for data continuity during power interruptions.
How long does a Roots gas flow meter last?
With proper lubrication maintenance and periodic metrological verification (typically every 2-3 years depending on accuracy class), Roots gas flow meters are engineered for long-term reliability, supported by high-precision machined rotor and bearing components designed for extended service life.
https://www.sytcflowmeter.com/
Kaifeng Xinya Instrument Co., Ltd.
