Paddlewheel Flow Meter Installation Guide

The Lakewood Instruments paddle wheel flow meter is a simple & reliable meter for make-up water applications. View our guide for tips for proper installation.

Paddlewheel flow meters are devices that use the mechanical energy contained in a moving liquid to rotate a paddlewheel suspended in the flow stream. It acts much like the paddlewheel on a riverboat. This movement of the paddles is converted into rotational energy. When the fluid moves faster, the paddlewheel spins proportionally faster. The rotation of the paddlewheel can be detected mechanically or more often, the paddles are detected electromagnetically. Each time the paddlewheel passes by the detector, an electronic pulse is generated. When fluid moves faster, more pulses are generated. The transmitter counts the pulses and calculates the flow of the fluid.

Paddlewheel flow meters essentially measure the velocity of the fluid (water, chemicals, and other liquids) as it moves through a pipe. Knowing the velocity and the cross-sectional area of the pipe inside diameter, one can easily calculate flow and/or volume. 

Precautions for Paddlewheel Flow Meters 

Accurate flow meter reading can be difficult to obtain. High accuracy can be achieved provided careful installation techniques are employed. These flowmeters measure the velocity at the edge of the flow profile. As such, accuracy is affected by viscosity changes, bubbles, impurities, temperature, and pressure.  

Tips for Proper Installation of a Paddlewheel Flow Sensor

One of the most important considerations in ensuring high accuracy with flow meters is the way that the sensor is installed. The flow specifications supplied by manufacturers are based on test data taken from laboratory installations. The accuracy and repeatability achieved with the sensor is dependent upon its installation.  

Insertion paddlewheel flow sensors are velocity devices. For a paddlewheel flow sensor to achieve accuracy and repeatability, it requires the pipe that it is installed in to be full and flowing at a minimum specified velocity. 

Upstream flow disturbances can have a greater effect on some styles of flow meters than others. The effect of these disturbances can be quite serious with any style of meter. The fluid dynamics of each installation is quite complex and varies from installation to installation. Therefore, predicting the effect a given pipe configuration will have on a given flow meter is virtually impossible. To minimize the effects of upstream disturbances, a minimum length of pipe equal to 10 pipe diameters upstream of the flow sensor is required. Use the pipe inside diameter when known. If the inside diameter is not known, use the outside diameter.

Many believe that once a fluid has gone through a flow meter, the discharge flow path is unimportant. This is not true. Disturbances caused by a bend, valve, tee, etc. in the discharge piping, can propagate backward against the current for several pipe diameters affecting a flow meter installed within range upstream. Therefore, a minimum pipe length equal to 5 pipe diameters downstream of the flow sensor is required.

Steps to Install a Paddlewheel Flow Sensor 

When installing paddlewheel flow sensors, these steps should be followed to obtain accurate results.

1. Carefully choose a suitable location. Stable fluid flow makes for the best accuracy of flow measurement.

2. The flow rate must be within the recommended range for the pipe size in which the meter is installed. The pipe must be full of fluid at all times.

3. Flow meters do not read down to zero. There will always be a minimum flow requirement for the meter. The fluid flow being measured MUST be greater than the flow meter minimum flow  rating.

4. If, at all possible, install the flow sensor with appropriate lengths of straight pipe of the correct diameter upstream and downstream of the flow sensor. The generally approved specification is 10 times the pipe diameter upstream and 5 times the pipe diameter downstream so long as swirl generating pipe configurations are avoided as stated above. In cases where turbulence inducing components such as elbows, valves, tees, etc. are present, lengths of 50 diameters upstream and 10 diameters downstream are sufficient.

5. Remember that swirl can destroy a meter’s accuracy. Avoid the typical swirl-generating configurations where the flow is forced to make a dramatic change in direction. Avoid piping schemes that force the fluid to turn two successive right angles in two perpendicular planes. The minimum recommended distance from these swirl generating bends is 100 pipe diameters upstream of the flow sensor.

6. If the available space does not allow for use of desirable lengths of straight pipe, there are two possible alternatives. One, have the flow meter calibrated after it has been installed in its pipe work complete with all bends, valves, etc. Secondly, install a flow straightener, which is designed to remove swirl. Keep in mind that flow straighteners can reduce the straight pipe requirement from 10 pipe diameters to about 4 pipe diameters. Some types of straighteners are designed to improve a badly shaped flow profile as well as reduce swirl. However, other types do not have a beneficial effect on a bad profile.  Often these devices may make a fairly good profile worse.

7. Ideally, rotate the paddlewheel flow meter 45° or 135° from vertical on a horizontal pipe run. Avoid top dead center as bubbles in the stream can get caught in the sensor and affect the paddlewheel rotation. Fluid velocity tends to be higher at 90 degrees creating additional wear and tear on the paddlewheel mechanism. Mounting the sensor at bottom dead center will allow impurities in the fluid to settle in the flow switch mechanism. 

8. Install the flow meter at the lowest point possible in the system, assuring the piping will always be filled.

9. If necessary, paddlewheel flowmeters can be installed in a vertical pipe run provided the flow is upwards, against gravity. Avoid fluid flowing in a free fall pattern. 

Paddlewheel flow meters are simple and reliable, but should only be used for make-up water applications. Lakewood Instruments offers a variety of  time-proven and user friendly water treatment controllers equipped with a water meter input for a paddle wheel flow meter. Find the right one for your application today!

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If you have questions on how to choose the right water flow meter type or controller for your application, we can help. Contact our experts for assistance.

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