Paddlewheel Flow Meters - Instrumart

Turbine or paddlewheel flow meters are mechanical meters that have a freely rotating turbine set in the path of a fluid stream. The flowing liquid or gas causes the turbine to spin upon its axis. The rate of spin will be proportional to the velocity of the flow. The simple and reliable design of turbine meters makes them popular choices for large commercial and industrial users such as gas companies and municipal water districts.

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Turbine meters are less accurate than some other types of flow meters but since the measuring element does not severely restrict the path of flow, they are able to measure high flow rates with low pressure loss. Though versatile, turbine meters do best in applications with constant conditions in liquids such as water or lower viscosity fluids. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the flow system.

Selecting a Flow Meter

The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation.

1. What is the fluid being measured by the flow meter(s) (air, water, etc…)?
2. Do you require rate measurement and/or totalization from the flow meter?
3. If the liquid is not water, what viscosity is the liquid?
4. Is the fluid clean?
5. Do you require a local display on the flow meter or do you need an electronic signal output?
6. What is the minimum and maximum flow rate for the flow meter?
7. What is the minimum and maximum process pressure?
8. What is the minimum and maximum process temperature?
9. Is the fluid chemically compatible with the flow meter wetted parts?
10. If this is a process application, what is the size of the pipe?

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Flow meters measure the volume or mass of liquid, gas, or steam moving through a piping system. There are many different flow meter technologies, and each type delivers different accuracies than other technology types. The accuracy requirements for a flow meter depends a great deal on the exact application. While it may seem like it may be advantageous to gravitate towards the technology types that deliver extremely high accuracy, those meters may have technological principles or other limitations that do not work with your needs.

Ultra-high accuracy flow meters, like Coriolis flow meters, typically are more expensive than any other flow technologies. A flow meter with an accuracy of 5%, which costs significantly less than another flowmeter with 0.2%, may deliver adequate results for your process to run correctly and deliver a much lower cost. Accuracy versus budget considerations and understanding your application’s exact accuracy needs can sometimes be confusing. Our sales engineers are available to help you find the best solution for your application for free.

Accuracy is the most common term and is sometimes used incorrectly. Accuracy is how close your instrument comes to giving you the exact value that exists in the process at that moment. It is commonly expressed as a value, or margin of error, above or below the reading that the instrument is showing.

For example, let’s say that your magnetic flow meter is showing a result of 1 GPM, with an accuracy of ± 10%. The exact value of the flow in the meter is more than likely not exactly 1 GPM because of the inherent deviation. More than likely, the actual flow rate is somewhere in between 0.9 GPM and 1.1 GPM. This is accuracy. When accounted for, in relation to the value being expressed by the meter, it gives you the range that the actual value falls between.

Repeatability is when close to identical results are produced after multiple measurements and there is no change in the conditions for all the results. In essence, it is the ability of the instrument to “group” the results, as in target shooting or darts. A highly repeatable instrument doesn’t necessarily mean that it is then accurate. For example, a temperature sensor could consistently be reading 5 degrees off every single time a measurement is taken. But if it is 5 degrees off every single time, calibration can come into play and turn a highly repeatable instrument into one that is highly accurate after the identified and consistent degree of separation from the actual temperature is accounted for.

Resolution is the smallest increment that can be measured by an instrument. In a sense, it is the smallest part of whatever scale is being used. For example, the resolution of a pressure transmitter could be 0.1 PSI or 1.0 PSI. How does this play into accuracy? While the importance of resolution may not seem as obvious as accuracy and repeatability, it does come into play.

Imagine that you have a process that demands that you know down to the tenth of a PSI to operate correctly. If you install an instrument that only can only give you a reading to the nearest 1 PSI, then that instrument will not deliver enough resolution for you to accurately know what the true reading is, as the instrument is, in essence, rounding up or down. In a sense, the instrument delivering a resolution of 1 PSI will not be accurate or finite enough for your process, even though it may be accurate in its actual reading.

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Flow meter accuracy can be stated in many ways and sometimes the way a specific instrument’s accuracy is stated is driven by the geographical area where it was produced and how accuracy is commonly stated or classified there. Certain flow meter technology types lend themselves to the accuracy being stated in a particular way that may be different from other flow meter technologies.

The ways that flow meter accuracy is stated are not always an apples-to-apples comparison, where one could essentially be converted into another. The essence of the way that the accuracy is stated may be telling you about a different element of the inherent accuracy. When choosing a flow meter, it is helpful to understand exactly what level of accuracy the flow meter will deliver.

Sometimes the accuracy will be stated specifically as an “accuracy class”. For example, a variable area flow meter may be listed as having an “accuracy class of 4 according to VDI”. VDI specifically applies to variable area flow meters and is assigned by the VDE/VDI Guideline , where a range of accuracy is designated to each accuracy class. VDI Class 4 would more typically be stated in the US as 2.5% to 4% of Full Scale (FS), as this is the actual accuracy range assigned to Class 4. For reference, the accuracy ranges for the VDE/VDI classes are below.

The most accurate flow meters are Coriolis mass flow meters. However, these are not appropriate for many applications because they are extremely expensive, usually large, and are complete overkill for most applications.

Magnetic flow meters, ultrasonic flow meters, and positive displacement flow meters generally deliver higher accuracy than flow meters that employ a more mechanical means of measurement like variable area flow meters.

However, for your exact application needs, a simple variable area flow meter may deliver sufficient accuracy at a significant cost savings. Magnetic and ultrasonic flow meters are generally more expensive than variable area flow meters but deliver many features that variable area flow meters cannot. Their technology types do not contain moving parts that experience wear or tear which can equate to lower maintenance and a longer service life.

Choose the correct flow meter that will meet all the exact needs of your application profile.

There are many elements of an application that can affect whether or not a flow meter delivers the factory stated accuracy. For example, if you choose a flow meter that requires full pipes and no bubbles to operate correctly and you run the pipe half full and it has bubbles and foam, it will not deliver the accuracy that it is built to. It may not even work at all. Running flows much lower than the stated minimum flow range for the meter can also cause the meter to suffer accuracy or can cause the meter to not work at all. To ensure full pipes for correct operation, install the flow meter vertically, with the flow running upwards.

Install the flow meter correctly.

Certain flow meters require that the flow profile in the pipe be uniform and non-turbulent. Not accommodating for those needs can cost you significant accuracy. For example, some flow meters require straight, uninterrupted pipeline with no impediments, bends, or valves so much distance before and after the flow meter. Not adhering to these requirements will cause your accuracy to suffer as the meter cannot properly function under those flow conditions.

Make sure nothing is broken.

For flow meters that measure by mechanical means, if the accuracy begins to suffer, verify that the functioning elements of the flow meter have not been compromised. Some meters are simple enough that they can be easily repaired by the end user while others must be sent back to the factory to be repaired.

Calibrate your flow meters in line with manufacturer recommendations.

Certain flow meter technologies require calibration more often than others and some may not require any calibration at all during their service life. Make sure that you are aware of the calibration needs of your meter and adhere to the maintenance schedule. Some meters are simple and can essentially be calibrated in the field and some require removal from the system and are then sent to a company that can perform the necessary calibration and return it to you.

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