1. How do turbine flow meters work?
A turbine flow meter (also known as a turbine flow sensor) is a device used for measuring the volume flow rate of a liquid or gas. The sensor consists of a turbine rotor within a cylindrical housing. The housing includes a flange or pipe thread on either end for integration of the sensor into the system. As the fluid flows through the cylindrical housing, it causes the rotor to spin at a rotational velocity proportional to the flow rate. A magnetic pick up (explained within the next paragraph) measures the rotor’s rotational velocity and outputs a sine wave signal at a frequency proportional to the flow rate. The hubs, pictured below, house the bearings which support the rotor shaft. An additional and important function of the hubs it to straighten the flow because repeatability is maximized when flow velocity is uniform. The bearings are commonly made of ceramic, due to its low coefficient of friction. The two predominant bearings types used in turbine flow meters are ball bearings and journal bearings. The considerations behind the choice of bearings are discussed within the purchasing tips section.
The rotor’s rotational velocity is measured by a magnetic pickup, which is coil of copper wire wrapped around a permanent magnetic core. As the ferrous rotor blade approaches the magnet, it interacts with the magnetic field lines, causing a shift in the magnetic field. The periodic shifts in the magnetic field induces an AC voltage in the coil at a frequency equal to the number of rotor passes per second. The magnetic pickup is a passive device and therefore does not require a supply voltage. However, the sensor includes built in electronics which amplifies the coil output and may also convert it into a square wave. The built in electronics do require a supply voltage. There are additional methods (some of them very similar to the magnetic pickup) for measuring rotor velocity, these include Hall effect, inductive and capacitive proximity sensors.
2. General characteristics of turbine flow meters
Turbine flow meters measure volume flow rate by detecting the rotational velocity of a turbine. They are accurate flow sensors, well suited for measuring the flow rate of gases and clean liquids of reasonably low viscosity. However, their outputs are effected by fluid density and viscosity and so fluid specific calibration is required for high accuracy measurements. The rotor shaft is supported by either ball bearings or journal bearings, which are most commonly made of ceramic. Turbine flow meters have lifetimes of many years when used for measuring the flow rate of clean liquids with good lubrication properties. However, lifetime is significantly reduced when operating with fluids that contain solid particulates or have poor lubrication properties. Solid particulates are particularly hazardous because they wear the rotor, causing a gradual reduction in accuracy rather than sudden failure. For this reason, it is recommended that a fine filter be placed at the inlet to the flow meter. To obtain accurate flow readings, a minimum length of straight pipe is required at the inlet and outlet to the flow meter, which may complicate system integration in some applications. Most turbine flow meters output a square wave signal whose frequency is proportional to the flow rate.
3. Input and output signals
The magnetic pick up used to measure the turbines rotational speed is a passive device. However, the small voltage generated by the magnetic pickup must be amplified by the sensors built in electronics. The flow meter requires a fixed supply voltage, typically 10-30 Vdc, to power the built in electronics. The sensor outputs a sine wave or square wave signal, whose frequency is proportional to the volume flow rate. The constant of proportionality is known as the k factor. Larger k factors provide higher measurement resolution. Some sensors convert the output of the magnetic pickup to a 4-20 mA signal and may include user programmable upper and lower bounds (known as ‘teach-in’ mode).
4. Applications of turbine flow meters
Turbine flow meters are extremely popular across many branches of industry. They are one of the go to sensors for measuring the flow rate of gases and clean, low viscosity liquids. Applications include metering of flow volume within the process manufacturing and oil & gas industries. They are also commonly used as feedback devices in pump control systems.
5. Typical specification
|Cost||Low to Medium|
|Measurement range||0.01-0.1 L/min to 20-200 m3/min)|
|Fluid temperature||-40 to 85 ° C (up to 400° C available)|
|Max. pressure||10-500 bar|
|Viscosity||0.3-100 Cst (500 Cst possible)|
|Lifetime||Dependent on fluid*|
|Supply voltage||10–30 Vdc (occasionally AC)|
|Output signal||Square/sine wave or 4-20 mA|
|Passive / active||Active**|
|Contact / non-contact||Contact|
* Life depends on the lubrication properties of the fluid and its cleanliness
**The magnetic pickup is passive. However, its output signal requires amplification
6. Purchasing tips
- Material compatibility: Turbine flow meters are normally made from a combination of stainless steel (housing, turbine and shaft) and ceramic (bearings). However, occasionally other materials are used including Nylon and PTFE. For applications with corrosive liquids, it is important to check the chemical compatibility of the sensor materials. ceramic rotors are preferred for corrosive and abrasive liquids.
- Bearings: Turbine flow meters can be purchased with either ceramic ball bearings or journal bearings. Both types of bearings provide high performance, but ball bearings tend to reduce friction and increase accuracy, particularly at lower flow rates. Journal bearings have excellent wear characteristics at high flow rates. However, they may suffer accelerated wear at low flow rates.
- Uni/bi-directional: Most turbine flow sensors can only accurately measure flow in one direction. There are turbine flow meters designed to measure flow rate in both directions. However, not all bi-directional flow meters are able to determine the direction of flow.
- Pressure drop: All turbine flow meters cause a small pressure drop due to the increased flow resistance. The magnitude of the pressure drop at full scale flow is typically 0.1-1 bar depending on the sensor model. The pressure drop can be reduced by purchasing an oversized sensor. This will however result in a decrease in measurement accuracy and resolution.
- Viscosity: Turbine flow meters are intended for relatively low viscosity liquids. Still, the allowable viscosity range can vary significantly between sensors, with some able to operate at viscosity as high as 500 cSt.
- Output signal: The sensors magnetic pickup outputs a sine wave, from which flow rate is determined according to its frequency. The sensor includes built in electronics which amplifies the output and may convert it into a square wave. Some sensors convert the output into a 4-20 mA signal.
- Turndown ratio: Most sensors have a turndown ratio of about 1:10 (ratio between minimum and maximum flow). There are however sensors available with significantly larger turndown ratios. All sensors have a minimum flow rate, below which flow cannot be measured accurately.
- K factor: The K-factor is the number out output pulses (i.e. peaks and troughs of the sine or square wave output) per volume of fluid and often has units of Pulses per liter. The K factor defines the resolution of the sensor.
- Temperature compensation: Turbine flow meters are sensitive to fluid viscosity and must therefore be calibrated at their operating temperature to achieve high accuracy. Flow meters with built in temperature compensation are available for variable temperature applications. These sensors measure the fluid temperature and calculate a correction factor to counteract changes in viscosity.
7. Advantages of turbine flow meters
Turbine flow meters:
- Have a fast dynamic response to changes in flow rate.
- Are of high accuracy.
- Are rugged and suitable for high temperature and pressure applications.
- Models are available covering a very wide range of flow rates (though the turndown ratios of individual models are limited).
8. Disadvantages of turbine flow meters
Turbine flow meters:
- Not suitable for high viscosity liquids.
- Are susceptible to rotor wear due to particulates, causing a reduction in accuracy rather than sudden failure.
- Their output it dependent on the density and viscosity of the fluid and so they require calibration at the precise operating temperature to achieve high accuracy. The effect of viscosity is particularly significant at low flow rates.
- Straight sections of pipe must be installed upstream and downstream of the turbine flow meter to straighten the flow.
- Have a limited turndown ratio and are therefore not able to measure very small flow rates, irrespective of their resolution.
- Most turbine flow meters can only measure flow in one direction.
9. Application tips
- Calibration: Calibration is performed by passing known flow rates through the sensor and measuring the sensor output. As well as correcting for long term drift, calibration is required for each type of fluid because the sensor output is density and viscosity dependent. Advanced flow sensors may include built in temperature compensation.
- Mounting: Turbine flow meters may be mounted horizontally or vertically. If mounted vertically, the flow direction must be upwards. Most turbine flow meters are uni-directional i.e. can only accept flow in one direction.
- Straight pipe sections: Turbine flow meters require a straight section of pipe before and after the flow meter to help straighten the flow, thereby maximizing accuracy. The minimum recommended straight pipes lengths are 15 pipe diameters at the inlet side and 5 pipe diameters at the outlet side. For pipes under 6 inches in diameter, the straight sections should be even longer.
- Filters: It is recommended that a filter is placed at the sensor inlet to remove particulates. The recommended filter size may vary slightly between different manufacturers. Failure to remove particulates will result in accelerated wear of the turbine and bearings, leading to a gradual decline in accuracy. Sensors can be purchased with ceramic turbine rotors, which are significantly more resistant to wear than stainless steel rotors.
- PNP/NPN transistor: The sensor output is governed by a transistor of either PNP or NPN type. A transistor is a switch that either conducts electricity or does not. A PNP type transistor conducts during the logic 0 portion of the square wave and an NPN transistor conducts during the logic 1 portion. When the transistor is not conducting, its output is floating and is likely to pick up background noise. The output must be tied to either to the supply voltage (for PNP type) or to the ground (for NPN type). This is performed using a pullup resistor (PNP) or a pulldown resistor (NPN). Process controllers may have built in pullup or pull down resistor. However, care must be taken to make sure that the sensor is connected to the corresponding port type on the controller. Additional information is available on the Azom and se websites.