Impeller flowmeters at times referred to as paddlewheel meters are one of the frequently utilized flowmeters for industrial flow measurement applications. Impeller flowmeters are cost effective compared to turbine meters and can be used in applications that are challenging to handle with other types of flow metering instruments. They are related to turbine meters in that the impeller meter has its rotary mechanical element to produce the output signal. They differ from turbine meters in that the impeller meter has its rotary axis transverse to the flow stream, as opposed to the turbine meter axis, which is parallel to it.
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Impeller flowmeters are available in two types:
The insertion meters are the most common type. The sensor is usually installed into a hole in a pipe, with saddle or welded fitting installed at the entry to seal the sensor to the pipe. The sensor can be pre-installed in the proper tees in the pipeline. Most manufacturers have designs that can be installed in operating pipe systems, with little, if any loss of the fluid during the installation process.
The impeller design is also supplied in in-line (through-flow) sensors for those applications where such use is appropriate. The in-line meters are typically of rather higher accuracy than the insertion style.
In-line meters are linear, inferential, volumetric flowmeters for measuring liquid and gas flows and are more sensitive at lower flow rates compared with, for instance axial turbine flowmeters. This is because the flow blade incidence angle is much greater. Insertion impeller flowmeters, on the other hand measure the flow velocity in a small region within a flow conduit.
Impeller flowmeters are typically appropriate for much lower flow rate ranges than the same size axial turbines and thus often find applications where axial turbines cannot be used. This, compounded with the fact that they tend to have a lower cost and have high reliability makes them a better choice to turbine meters. Competition with in-line axial turbines can occur in overlapping design flow ranges.
Some in-line meters have interchangeable orifice sizes, allowing the same body to be used over different flow ranges. The orifice in-line meters are typically insensitive to flow condition. Insertion meters are very sensitive to flow condition if volumetric measurements are inferred. This is a key distinction and can extend to differences between requirements for specific in-line versions.
Impeller meters, in other words, paddlewheel flowmeters, are never used in liquid hydrocarbon custody transfer applications. The flow measurement capacity is insufficient to compete in monitoring large volume transfers and accuracy is not adequate for valuable fluids.
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Impeller flow sensors detect the rotation of the impeller, and transmit a pulse train, at a frequency related to the rotational velocity of the impeller. Since it is basically a digital output, impeller sensors can typically transmit signals over somewhat long distances, up to 1 km where necessary. The detection principles employed include:
Since the impeller can operate to rotational velocities of 4000 rpm or higher, output frequencies can be as high as 500 Hz. At low flow rates, the frequencies can be as low as 0.2 Hz. This is a key factor to keep in mind in the selection procedure, because the chosen output device must be capable of the frequency output range of the sensor.
Generally, these flowmeters are capable of displaying both flow rate and accumulated flow for the sensor to which they are connected. Furthermore, outputs are available, either stand-alone or in combination with the meter, giving periodic pulse outputs at definable flow increments or analog outputs scaled to flow rate. Also, certain control functions, alarms and other special features needed by the different markets served are often included in these meters.
When installing impeller flowmeters, important factors to keep in consideration include:
The pipe sizes in which these meters have been installed run the full range from small bore tubing to 2.3 m outside diameter.
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Most manufacturers require at least 10 pipe diameter lengths of straight pipe upstream of the installed meter and 5 pipe diameter lengths downstream. These conditions are necessary to minimize the asymmetry of the flow stream in the proximity of the impeller, in the installed piping, which can be caused by elbows, tees and valves. Installation with less diameter lengths than the recommended upstream or downstream of the meter can adversely affect the calibration of the sensor due to the local variations of velocity due to flow disturbances.
Manufacturers’ standard offerings are normally consistent with the pressure limitations of the materials of construction of the piping with plastic piping systems and are commonly as high as 2.7 MPa with steel or brass piping. Higher pressures are available however at slightly more cost. Pressure drop generated by the installed flowmeter is usually low. This information is usually given by the manufacturer.
Calibration of the sensors is normally specified by the manufacturer. For instance some manufacturers provide a calibration factor in terms of gallons per pulse or in pulses per gallon. Others provide data relating to frequency to flow rate in GPM or other volumetric rate units.
For insertion style sensors, any such instrument must be field calibrated to accommodate the variation in the relation of the impeller rotational velocity with average flow rate with pipe size variation.
The accuracies for in-line flowmeters vary considerably but can be high, ranging from ±0.2% reading for liquids for one manufacturer to several percent of full scale for several others. It is important to keep note of the difference between accuracies specified as % full reading. Manufacturers use both, A ±1% full-scale device is often far less accurate than, for instance, a ±%2 reading device.
Output accuracy is typically specified as ±1% of full rated flow. Some manufacturers will custom-calibrate the sensors to meet the special need; a task that can only be accomplished when the mating pipe entry and exit can be shipped to, and accommodated by the manufacturer. Alternatively, the manufacturer can provide new calibration values if the meter used is identified, the meter reading is known and the actual flow is known for at least two points on the flow curve. When calibration in place is required, the anticipated accuracy is in the range of ±0.5% of full scale or 1% of indicated flow, whichever is larger.
Repeatability of the readings is usually on the order of 0.5%. Linearity, except at the extremes of the flow range is also expected to be no worse than 0.5% and over full design range of the meter is normally accurate within the ±1% of full rated flow.
To realize the accuracies discussed above, careful attention must be paid to the correct installation specifically with regard to the insertion depth.
The impeller flow sensors are supplied in materials of construction compatible with a broad range of aqueous solutions, of both high and low pH and with deionized water. Flow streams with high concentrations of solids and specifically fibrous solids should be carefully reviewed when the impeller sensor is under consideration.
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Impeller flowmeters are extensively used in the following fields:
Custodial transfer applications are usually unsuitable for the impeller sensor.
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