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Gas turbine meter
Gas turbine meters are velocity meters, and the upper velocity limit is essentially unchanged by pressure.
Turbine meter overview
A basic turbine meter consists of:
- A pressure-containing meter housing with end flanges
- A set of internals, incorporating the turbine wheel and gearing mechanisms
- A means of counting the turbine wheel revolutions
A typical turbine meter has additional components such as:
- Flow conditioning devices
- Bearing lubrication mechanisms
- Sophisticated mechanical and electrical counter systems
An exploded view of a turbine meter is given in Fig. 1. For additional information about turbine meters and their use in liquid measurement, see Inference liquid meters.
Like orifice meters, turbine meters should be mounted within a meter tube (Fig. 2). Most modern turbine meters have integral flow conditioners. These conditioners help to remove swirl and much of the distortion from the flow profile, and hence, the overall straight length requirement upstream of the meter can be relatively small. A typical requirement is 5 diameters.
Theory of operation
The operation of a turbine meter is based on the measurement of the velocity of gas. The flowing gas is accelerated and conditioned by the meter’s straightening section. The integrated straightening vanes prepare the gas flow profile by removing undesirable swirl and asymmetry before the gas flows over the freely rotating turbine wheel. The dynamic forces of the flowing gas cause the rotor to rotate. The turbine wheel is mounted on the main shaft, with high-precision, low-friction ball bearings. The turbine wheel has helical blades that have a known angle relative to the gas flow. The gas flow drives the turbine wheel at an angular velocity, which, in the linear range of a well-designed meter, is proportional with the gas velocity. Using a gearing mechanism, the rotating turbine wheel drives the mechanical counter. In addition, the rotating blade can also be used to generate pulses via a proximity sensor. Each pulse detected is equivalent to a discrete volume of gas at actual conditions (i.e., the total number of pulses collected in any period of time represents the gross observed volume during that period). For each meter, a calibration characteristic (K factor) is required. This factor is expressed in pulses per volume and is given by the manufacturer.
The K factor is determined by means of a flow calibration. This flow calibration should be carried out over the entire operating range of the meter because the K factor may vary with flow. This variation with flow is the turbine meter’s linearity. Once the K factor has been defined, the flow through the meter can be calculated because the two quantities are proportional.
Advantages and disadvantages
The advantages and disadvantages for turbine meters are given in Table 1.
Gas turbine meter sizing varies from one manufacturer to the next; however, the variables to be considered are consistent. Gas turbine meters are velocity meters, and the upper velocity limit is essentially unchanged by pressure. Thus, a given size turbine meter will have an associated upper uncorrected flow rate limit. The range of the measurement is affected at the low end by the amount of mass flow through the meter. Thus, the range of a turbine meter is enhanced by increasing line pressure. For example, a 3-in. gas turbine meter might have a range of 30:1 at 200 psi but more than 60:1 at 500 psi.
- ISO Standard 9951, Measurement of Gas Flow in Closed Conduits: Turbine Meters. 1994. Geneva, Switzerland: ISO.
- OIML R 32, Rotary Piston Gas Meters and Turbine Gas Meters. 1989. Paris: OIML.
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