Turbine Gas Meters
Velocity Flow rate
Is the speed of fluid passing through a pipe relative to distance over time. Typical units are metres per second (m/s), Feet per minute (Ft/m).
Volume Flow rate
Is the quantity of fluid passing through a pipe over time. Typical units are Litres per second (l/s), Cubic Metres/hour (m3/h).
Gas Turbine Meters
Gas Turbine meters are gas velocity flow meters and thus indirect volume measuring instruments. Using gas velocity for measuring the volume of gas within the meter, these inferential style meters measure the flow rate based on the rotor revolutions representing a unit of volume within the meter parameters.
Gas Turbine flow meters use the mechanical energy of the gas to rotate a bladed rotor within the flow stream which is axially suspended in the pipe. The rotational speed is directly proportional to the velocity of the gas traveling through the meter. The rotor mechanism is coupled to a gear train driving a totaliser indicating the volume measured passed through the meter or a signal output provided by a magnetic pickup sensor, excited by the rotor blade proximity as it sweeps by.
Turbine meters operate most effectively in applications with steady, high-speed flows. Gas Turbine meters are perfect for secondary metering industrial applications where gas flow continuity is crucial.
Major applications are gas transport utilities, storage facilities and in industrial high-pressure applications. If turbine meters are not subject to calibration, these counting instruments are often alternatively referred to as quantometers (MZ) or volumeters. Turbine meters are suitable for measuring the flow rate of various gases, e.g. natural gas, butane, propane, hydrogen, nitrogen, noble gases and others.
Natural gas measurement with turbine meters
A typical example of the application of turbine meters is the measurement of natural gas quantities along the logistics chain. This supply chain has its origin in the gas production plant. The natural gas subsequently passes through several gas transport stations and finally reaches the local distribution and transfer stations. Turbine meters are used at every point in the supply chain to determine the exact quantities of gas. Further examples of the practical application of turbine meters are the use in power plants or for large industrial consumers (e.g. cement or glass industry), where exact gas measurement is also essential for billing and process control.
Turbine Gas Flow Meter Advantages
- Wide flow rangeability including low flow rates.
- Turndown ratio is up to 50:1.
- Good level of accuracy with conformity for Fiscal (Billing) use.
- Simple, durable construction.
- Easy to install and maintain.
- Flexible connection to flow instruments for flow indication and control.
- Turbine meters can operate over a wide range of temperatures and pressures.
- Low pressure drop across the turbine.
- Provides a convenient signal output.
Turbine Gas Flow Meter Limitations
- Generally, need a length of straight pipe before and after the meter.
- Rotor can be stalled by dirty deposit build up. Use of sealed rotor bearings.
- Gas must be dry and clean.
- Limitations mainly apply to liquid Turbine meters.
- Velocity flow profile, Viscosity, cavitation, back pressure
Turbine gas meter operation type TZ
Gas flows through the body (1) via the inlet duct into the turbine meter. The flow straightener (2) eliminates possible swirl in the gas flow. It equalizes and accelerates the gas flow by reducing the free cross-sectional area to a flow channel. In this flow channel the gas hits the freely rotating turbine wheel (3). The blades of the turbine wheel are at a precisely defined angle to the gas flow. The incoming gas drives the turbine wheel and sets it in rotation. As the flow rate increases, the speed of the turbine wheel increases. The speed of the wheel is proportional to the flow velocity (instantaneous value), the number of revolutions is proportional to the volume of gas flowing through (indirect volume measurement). Based on this law, the flow rate can be determined. The magnetic coupling (4) transmits the speed of the turbine wheel via a gear to the mechanical totalizer (5).
Pressure Loss
This is very critical for gas flows applications. Gas pressure is regulated at various stages to ensure equipment operates within specification. Pressure loss occurs along the pipe due to wall friction and fittings. The Turbine meter generally has a very low-pressure loss but needs calculating at the maximum and nominal flow rates to check if the meter size selected is a suitable fit with the available pressure. Pressure loss increases with flow velocity. If possible, the size of meter can be increased to reduce the velocity if the flow range allows. With most meters sizes the flow range can be shifted according to a G rating.