Paddlewheel Flowmeters Get the Job Done

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Paddlewheel Flowmeters Get the Job Done

Paddlewheel flowmeters are easy to install and operate, resulting in a very low overall cost of ownership. Relatively low-cost paddlewheel style electronic flowmeters are proof that high cost doesn’t always mean better value. 

The components of a system must be able to perform the required task — get the job done — and meet the other physical requirements of the application. Excess capability, features and accuracy are a waste of money. You will get the most value for your money by purchasing system components that meet the demands of the system without being overkill. While expensive, high technology solutions may be available for your application. Low-cost paddlewheel flowmeters offer high accuracy solutions to many flow system applications — not just displaying the flow rate and the total accumulated flow amount. 


Dispensing systems
Water dispensing systems commonly use preset cycle timers to dispense water. When the timer is activated, the system begins dispensing water until the preset time cycle times out. These preset (fixed) cycle timers can result in accuracy problems because they do not actually measure the flow rate. Any problem in the system that results in a change in the flow rate will result in an error in the amount of water dispensed. Some examples include worn pump components and changes in the system pressure, either of which can result in changes in the pump output. The cycle timer control cannot compensate for various flow rates because it is not measuring the flow rate.

Paddlewheel sensors actually measure the amount of water dispensed. When the dispensing system is activated, the electronic flow controller starts the pump and opens the correct dispensing valve. The sensor begins to output electrical pulses. These pulses are counted by the electronic flow controller. Changes in the output flow rate of the pump will not affect the sensor count. When the correct amount of water has been dispensed, the dispensing valve is closed and the pump stopped.

Water treatment systems
Chemical metering pumps are used to inject chemicals such as chlorine and acids into water systems. The chemical must be injected into the system at the proper rate to achieve the correct water/chemical proportions. Depending on the application, too much or too little chemical can result in series problems. In a system that has changing water flow rates, a fixed feed rate chemical injection metering pump alone is not capable of reacting to changes in the flow rate of the system. Paddlewheel flowmeters can be used to start and stop inexpensive, fixed feed rate metering pumps resulting in the proper amount of chemical injection. When the meter has measured a specified volume of flow that has passed through the system, the chemical pump is turned on for a preprogrammed amount of time. This simple system results in a pump on time (chemical) per flow volume (water) ratio.

Variable-speed pumps are used when near continuous injection of chemical is required. These pumps can be controlled directly by the sensor’s output signal. The high-speed sine wave signal can be input directly into the pump’s electronic speed control. The pump speed, and therefore the amount of chemical injected, is programmed to react to changes in the frequency output by the flow sensor. A minimum pump speed is programmed for a frequency and a maximum pump speed is programmed for another frequency resulting in a speed-frequency ratio (pump output rate per flow rate).

Flow verification
Paddlewheel sensors are capable of measuring chemical metering pump output rates as low as 1 ounce per minute. When installed on the metering pump, the flow sensor can be used to alert the system operator that an error exists in the system. Some metering pumps include electronics that react to the paddlewheel output signal. If the metering pump should fail to inject chemical due to a pump malfunction, clogged fitting, exhausted chemical container, etc., an alarm output is triggered.

Flow rate and range alarms
When a system’s flow rate is critical, a paddlewheel flowmeter can be used to alert the system operator if the rate increases or decreases out of a programmed range. The electronic display can be programmed with a high and low rate amount that will trigger an alarm output signal if reached. The alarm can automatically reset or latch. Trigger and release values can be set, with hysteresis, which will eliminate “flickering” that can occur when the flow rate is at the alarm value. 

Installation requirements

Paddlewheel flowmeters work best with clean fluids. Particles and debris can prevent the paddle from spinning properly.

Install the pipe fitting in a location that includes a proper length of straight pipe before and after the meter. Because the paddle is inserted only a small distance into the flow stream, the flow stream must be a consistent velocity across the entire inside pipe diameter to obtain an accurate reading. The straight length of pipe will allow any swirl patterns in the flow stream to dissipate before contacting the paddlewheel. Swirl patterns can be caused by obstructions such as an elbow, tee, pump, etc. The minimum straight length of pipe required will depend on the type of obstruction before the paddlewheel. The absolute minimum is typically 10 times the nominal pipe size before the meter and five times after. Thus, a 4-inch pipe would require a minimum of 40 inches of straight pipe before the paddlewheel and 20 inches after. Refer to the manufacturers instructions for specific requirements.

Flow disturbances

Paddlewheel flowmeters may not function properly with high viscosity fluids. High viscosity fluids will tend to produce a laminar type flow profile. In a laminar flow profile, the center of the flowing fluid is moving faster than the outer edge. A turbulent flow profile, where the fluid velocity is the same across the entire pipe diameter, is required for accuracy. The fluid’s Reynolds number must be greater then 4,000 to ensure a fully developed turbulent flow profile. The Reynolds number is a dimensionless number that combines the effects of viscosity, density and flow velocity to identify either a turbulent or laminar flow profile.

Reynolds number equation

The pipe must be full of water at all times. When the system starts and stops, any air in the line may lead to an erroneous reading. 

Size the meter to work within the published operating range. Although the meter may read at flow rates other than published, the meter may not be accurate at these rates.

Be sure the saddle is properly installed. Saddle installation, pipe size, alignment and adjustment are critical to an accurate reading.

How paddlewheel flowmeters work

Paddlewheel flowmeters consist of three primary components: the pipefitting, the paddlewheel sensor and the display/controller. These components can be purchased separately or as a package to meet the particular requirements of the application. The paddlewheel sensor is designed to be inserted into the pipefitting. Approximately one-half of the paddle protrudes into the flow stream. Fluid flowing through the pipe causes the paddlewheel to spin. As the magnets that are imbedded in the paddle spin past the sensor, electrical pulses are produced that are proportional to the rate of flow. The manufacturer publishes the number of output pulses produced, per volume of flow, for each specific pipefitting. This number is called the K-factor.

Pipefittings – Various pipefittings styles are available. Some fitting styles are designed to install directly into the pipeline using various connection methods such as male or female threads, socket weld, socket fusion and butt fusion joints. These “in-line” fittings are available in a variety of materials such as PVDF, polypropylene and stainless steel. They are available with and without union connections. Because the manufacturer can control the inside diameter of the fitting, in-line fittings are available in a variety of operating flow ranges to accommodate various applications.

Saddle style fittings are designed to mount directly on an existing pipe. The saddle is installed by simply drilling a hole in the pipe and clamping the saddle onto the pipe. Cutting the pipe and installing special adapters is not necessary. Saddles are available in a variety of materials. 

Paddlewheel sensors – The paddlewheel sensor consists of the paddlewheel with its imbedded magnets and the electronic sensor. Manufacturers offer sensors in a variety of materials to meet most applications. Two types of sensor outputs are available, AC coil and Hall effect.

AC coil sensors generate an AC sine wave that is proportional to the flow rate. Because they generate their own power, these sensors do not require external input power. The signal range for an AC coil type sensor is limited to approximately 200 feet due to possible noise interference and voltage drop.

Hall effect type sensors output a digital, current sinking, DC square wave that is proportional to the flow rate. Circuitry that is sensitive to magnetic fields is triggered by the spinning paddle. This circuitry requires external input voltage to operate. The signal range for a Hall effect type sensor is approximately 1 mile.

Displays/controller – Flow displays and controllers are used to receive the signal from the paddlewheel sensor, convert the signal into an actual flow rate or flow total value, and display the values. The processed signal can now be used to open and close valves, start and stop pumps, indicate high or low flow rate alarms in the system, or transmit 4-20mA and TTL level pulse signals to external equipment such as a PLC, chart recorder, metering pump, etc. 

Paddlewheel flow sensors and display meters/controllers offer low-cost solutions to a variety of water system applications.

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