CHAPTER 14 - MEASUREMENTS IN PRESSURE CONDUITS
4. Propeller Meters
Propeller meters are commercial flow measuring devices used at the ends of pipes and in conduits flowing full and under pressure (figure 14-6). The uses of propeller meters at the end of pipes (open flow propeller meters) are discussed in chapter 13. Propeller meters use multiple blades made of rubber, plastic, or metal. The propeller rotates on a horizontal axle geared to a totalizer that displays total volume that has passed the meter. The propellers are sometimes hung from a sealing plate with a gasket to seal around a saddle opening on the top of the pipeline. Others have propellers supported by spiders in short, permanent tubes for connection into pipeline flow. Some meters also display instantaneous discharge rate with indicator hands on dials.
The meters are available for a range of pipe diameters from 2 to 72 in. They are normally designed for water flow velocities up to 17 feet per second (ft/s). The accuracy of most propeller meters varies from +/-2 to +/-5 percent of the actual flow. Greater accuracy is possible, and minimum driving velocities as low as 0.5 ft/s are sometimes claimed for certain meters. These claims may, at times, be justified; however, they are sometimes difficult to verify or reproduce, even in carefully controlled laboratory tests. Small changes of frictional resistance of bearings and other mechanical parts caused by wear can cause large deviations from calibration, especially at the low discharge end of measurement range. With wear, error increases greatly for velocities below 1 to 1.5 ft/s.
Propeller meters should be selected to operate near the middle of their design discharge range. This equipment can be a problem in existing irrigation systems with oversized pipes relative to delivery needs. Sections of the oversized pipe may need to be replaced with smaller pipes to provide enough velocity and approach pipeline length to allow development of velocity profiles.
Any condition that makes the approach flow different from calibration conditions affects the accuracy of the meter registration. Insufficient driving velocity relative to friction, unusual velocity distributions, or undeveloped velocity profiles and spiral flow can cause considerable errors.
If the propeller diameter measures less than half of the pipe diameter, the meter will be more sensitive to velocity profile differences. Changes in velocity distribution or velocity profile also influence registration. If the conduit from the canal to the meter is less than about six diameters long, typically, flow does not have sufficient time to develop a normal velocity distribution profile. This condition results in a blunt, evenly distributed velocity pattern (figure 14-7, case A). However, a conduit length of 20 to 30 diameters or longer will allow a typical, fully developed velocity profile (figure 14-7, case B).
With a fully developed velocity profile (case B), the velocity of flow near the center of the pipe is high, compared to the velocity near the walls. Thus, a meter with a propeller diameter of only half that of the pipe diameter would read 3 to 4 percent higher in this flow distribution than it would in the flat velocity profile (case A). Larger propellers up to 0.8 of the pipe diameter sample more of the flow velocity, producing greater potential accuracy. Laboratory tests show this statement to be true; and when the propeller diameter is 75 percent or more of the pipe diameter, the variation in registration caused by these velocity profile changes are minor.
Spiral flow is caused by poor entrance conditions and combinations of bends and fittings such as valves. Measurement errors caused by spiral flow can be large and, depending on spiral rotational direction of the flow, are either positive or negative.
Normally, the manufacturer provides detailed installation instructions which should be followed carefully. The same straight pipe approach and flow straightening vanes to prevent spiral flow that the manufacturer uses during calibration must be reproduced in field installations. Straightening vanes, at least several pipe diameters long, should be placed in the straight pipeline an appreciable distance upstream from the meter as specified by the manufacturer.
Propeller shafts are usually designed to rotate in one or more bearings. Bearings are contained in a hub, where they are protected from direct contact with objects in the flow. However, water often can and does enter the bearing. Some hubs trap sediment or other foreign particles. After these particles work into the bearing, a definite added resistance to turning becomes apparent. Some propellers are, therefore, designed for flow-through cleaning action so that particles do not permanently lodge and partially consolidate in the bearings. However, some of these bearing flushing systems have been plugged when the bearings have become fully packed with sediment. Newer propeller meters generally have sealed or ceramic bearings to minimize sediment wear problems.
Although propellers are designed to pass (to some degree) weeds, moss, and other debris, only a limited amount of foreign material that can be tolerated in the flow. Even moderate amounts of floating moss or weeds can foul a propeller unless it is protected by screens. With larger amounts or certain kinds of foreign material in the water, even screens may not solve the problem. Heavy objects in the water can damage propellers. Where rodents, such as muskrats, can get to plastic propellers, they have been known to cause chewing damage.
Propeller meters require continuous inspection and maintenance, which may amount to very little to very much, depending on local conditions and brands selected. Potential users should seek information from other local users before selection. In some cases of high maintenance costs and expensive water, these meters have paid for themselves in as little as 2 months on the basis of water conserved. However, in other areas where water is relatively plentiful, they have never repaid their original cost. Propeller bearing troubles are the most expensive and common problem and may be difficult to overcome except by means of a well-planned maintenance program. Maintenance costs can be excessive if meters are used for water with sediment. Propeller meters require a maintenance routine where bearings are replaced based on time of operation.