Paddle wheel flow sensors not only continue to remain strong in industrial applications, their use is growing. Because these devices offer the lowest cost per sensing point of any flow measurement technology, users can save significantly on purchase, installation and maintenance costs.
Simple Design
Paddlewheel flow sensing is extremely simple:
- Install a paddlewheel in a pipe. The faster the fluid flow, the faster the paddlewheel turns.
- Determine the rotational speed of the paddlewheel, and you'll determine the fluid velocity.
- Calculate the velocity and the area of the pipe, and you'll calculate the flow rate.
This simple design utilizes a plastic rotor and magnets embedded in each blade. As the magnets move past the sensing coil, they generate a sine wave output. The frequency of the sine wave indicates the rotational speed.
Paddlewheel accuracy is well-suited for industrial process applications. It offers 1.0 percent linearity with 0.5 percent repeatability - with no pressure drop.
Sensor Outputs
The paddlewheel sensor output can be used to drive a meter directly at distances of up to 200-ft, giving the user a completely self-contained remote-reading flow meter that does not require external power. If the distance is greater than 200-ft, or if the user wants to drive a valve or current-activated monitor, connect the sensor output to a powered transmitter. The transmitter allows a 4 to 20 mA current loop to PLCs and other control devices, and gives the user a direct digital local readout.
In addition to standard open collector outputs, some paddlewheel flow sensors offer multiple output choices. Any one of the following outputs can be selected: flow switch, pulse divider, pump pulser, digital (S3L), or 4 to 20 mA output. The latest generation of paddlewheel flow sensors is designed to satisfy various application needs with just one family of products.
Handling Low Flow Velocities
Magnetic paddlewheel flow sensors will handle fluid velocities from 20-fps down to about 0.3-fps. For low flow measurement, the technology must be modified slightly to accommodate lower paddlewheel rotational speeds.
The low-velocity sensors require a power source. The design still utilizes embedded magnets, but instead of passing by a sensing coil, the magnets trigger a circuit that is sensitive to magnetic fields. The magnets' fields cause the circuit to conduct current. The powered sensor output is a process-ready square wave pulse train. The pulse repetition rate linearly indicates paddlewheel speed, which indicates flow, and the output can be sent to PLCs, DCSs, current-loop transmitters or monitors.
Insertion Mounting
Paddle wheel flow sensors are typically mounted by inserting them into an existing pipe. Insertion mounting can save money over full-bore designs where 100 percent of the flow has to pass through the sensor.
Full-bore flow sensors require users to cut the pipe and add flanges or fittings. In large pipe sizes, the pipe fitting operations are expensive. In even larger sizes, the full-bore instruments themselves are very expensive.
Insertion mounting requires no pipe cutting. By simply drilling a hole in the pipe, then clamping, welding or bolting on a mounting adapter, the user is ready to install the sensor. This operation is especially easy with plastic piping such as polypropylene or polyvinyldine fluoride (PVDF) systems. This installation fitting controls the sensor insertion depth to ensure consistent accuracy.
After connecting wiring to a frequency measuring device such as a transmitter, a monitor or controller, the user is ready to measure.
Location, Location, Location
For the best performance, paddlewheel flow sensors should be properly located within the existing pipe system. In a horizontal pipe run, the user will normally mount a paddlewheel sensor vertically, between the 10 o'clock and 2 o'clock position (see Figure 1). The 6 o'clock position should be closely monitored since sediment can settle in the bottom of the pipe.
Figure 1. Flow sensor mounting.
If valves, elbows, or fittings are too close to the sensor, disturbances in the flow may cause the measured velocity to be inaccurate. For maximum linearity and accuracy, the user should position paddle wheel sensors with a straight run of pipe upstream and downstream of the sensor to give uniform velocity in the pipe. Technically, the user is looking for "fully developed turbulent flow," with a Reynolds number above 4,500 (see Figure 2).
Figure 2. Mount the sensor in a straight run of pipe.
With highly viscous fluids, or at extremely low flow rates, the user is likely to encounter a laminar flow condition where fluid velocity at the center of the pipe is higher than the velocity near the wall. That will throw off paddlewheel measurement accuracy if the user is measuring a varying flow rate (see Figure 3).
Since the flow isn't uniform, paddlewheel speed no longer has a linear relationship to fluid volume. In low-velocity, high-solids applications, the user may need to consider an alternate technology, such as an insertion magnetic flow sensor with ranges from 0.15-fps to 33-fps.
Figure 3. Flow profile.
Wide Range of Durability
With only one moving part, paddlewheel sensors generally have long lives. Some sensors last between 10 to 15 years of service. Paddlewheel sensors come in a variety of materials including polypropylene, PVDF, stainless steel and brass. All the wetted parts (even the pins upon which the paddlewheels rotate) are available in a variety of materials.
Pumps & Systems, January 2007
