3.3.1: Mechanical Meters

Propeller meters (impeller meters) and turbine meters are common methods for measuring pipeline flow in agricultural irrigation and in municipal water distribution systems. The force of the flowing water turns the propeller. The propeller is sensing the velocity in the pipeline. A propeller meter is illustrated in Figure 3.5. The rotations of the propeller are converted to flow rate by proper gear ratios in the meter head. The diameter of the propeller is usually slightly smaller than the inside diameter of the pipeline. This gives a good estimate of the mean velocity in the pipeline and allows the meter to operate over a wide range of flows.

Figure 3.5 Typical irrigation propeller meter (image supplied courtesy of Sparling Instruments LLC, El Monte, CA).

The register in the meter head of these devices comes in various configurations. Four common ones are shown in Figure 3.6. The one on the left has the volumetric totalizer combined with a sweep hand, which can be used for timing the rate of flow. Each revolution of the sweep hand corresponds to a known volume of water that has passed through the meter. The second register from the left contains two components: the totalizer and the flow rate indicator. The third register from the left has three components on one meter head: the totalizer, the flow rate indicator, and an index hand or sweep hand for timing. The register on the right represents a digital display which contains both a flow rate indicator display and a volume totalizer. One key advantage of electronic register heads is that they lend themselves to remote monitoring through cellular or satellite communication.

Usually the accuracy of the meter is based upon what is registered on the totalizer. Since the totalizer and the sweep hands are directly connected, the true flow rate is best obtained by either timing the sweep hand or timing the rate that the numbers are changing on the odometer. The least accurate, or the poorest representation of the meter's accuracy, is the flow rate indicator. The flow rate indicator is helpful to observe changes in flow rate and as an indicator of excessive spiraling or disturbed flow. The latter condition is noticed by significant needle movement or bounce.

Proper selection and installation of flow measuring devices are very important. Propeller meters should be located away from pipeline fittings that cause spiraling or disturbance of water flowing in the pipe such as pumps, elbows, valves, etc. A flow disturbance refers to the disruption or distortion of the parabolic velocity distribution an example of flow caused by a pipe elbow is shown in Figure 3.7.

Figure 3.6. Options available for registers on a propeller meter.

Between any of these devices there should be adequate, straight, and unobstructed pipe ahead of the propeller so that the flow can be straightened before reaching the meter section. It is best to have a distance of at least 10 pipe diameters of straight pipe upstream of the propeller and at least 1 pipe diameter distance downstream between the propeller and a flow disturbance. Sometimes there is not adequate room available to allow for 10 pipe diameters. If not, a shorter distance can be used if straightening vanes are placed in the pipeline ahead of the propeller (Figure 3.5). A typical field installation of a propeller meter is shown in Figure 3.8.

It is also important that the meter section always flow full of water. This is to guarantee that the flow area is equal to the cross-sectional area of the pipeline. If the pipe discharges into the air and the pipeline is not flowing full, an upward turned elbow or a horseshoe-shaped fitting, as shown in Figure 3.9, is useful to guarantee full pipe flow.

Another approach to measuring flow in pipelines is paddlewheel meters. These mechanical meters usually have a magnetic pickup to measure the number of revolutions of the paddle wheel. The paddlewheel movement then is converted to flow rate by the velocity area relationships. Like the propeller meters, the paddlewheel should be installed with adequate piping ahead of the meter so that the velocity profile can be established before the water reaches the meter.

Figure 3.7. Flow disturbance of velocity profile caused by a pipe elbow.

Figure 3.8. Field installation of a propeller meter. Figure 3.9. U-shaped fitting installed to guarantee full pipe flow in the meter section.