Rotameter is a device that can measure the flow of liquid and gas. Generally, a rotameter is a tube made of plastic, glass or metal, combined with a float, which responds linearly to the flow of fluid in the tube.
Due to the use of related equations, OMEGA™ laboratory rotameters are more versatile. The advantages of rotameters include: long measuring range, low pressure drop, easy installation and maintenance, and linear scale.
For the above advantages, the rotameter is the most widely used variable area flowmeter. It consists of a tapered tube; when fluid passes through the tube, it raises the float. A larger volumetric flow will put more pressure on the float, thereby raising it higher. In liquid, the speed of the flowing liquid is combined with the buoyancy to increase the float; for gas, the buoyancy is negligible, and the height of the float is mainly set by the speed of the gas and the resulting pressure.
Usually, the pipe is installed vertically. When there is no flow, the float stops at the bottom, but as soon as the fluid flows up from the bottom of the tube, the float starts to rise. Ideally, the height that the float passes through is proportional to the fluid velocity and the annular area between the float and the pipe wall. As the float rises, the size of the annular opening increases, which reduces the pressure difference across the float.
When the upward force exerted by the fluid flow balances the weight of the float, the system reaches equilibrium, the float reaches a fixed position, and the float is suspended by the fluid flow. You can then read the density and viscosity of the specific fluid’s flow rate. Of course, the size and composition of the rotameter will depend on the application. If everything is calibrated and sized correctly, the flow rate can be read directly from the scale based on the position of the float. Some rotameters allow you to manually adjust the flow rate using valves. In early designs, the free float rotated with changes in gas and fluid pressure. Because they rotate, these devices are called rotameters.
Rotameters usually provide calibration data and direct reading scales for common fluids (air and water). Determining the size of a rotameter used with other fluids requires conversion to one of these standard formats; for liquids, the water equivalent is gpm; for gases, the air flow is equivalent to standard cubic feet per minute (scfm). Manufacturers usually provide calibration tables for these standard flow values ​​and use them in conjunction with slide rules, nomograms, or computer software used to determine the size of the rotameter.
The basic rotameter is a glass tube indicator type. The tube is made of borosilicate glass, and the float can be made of metal (usually corrosion-resistant stainless steel), glass or plastic. Buoys usually have sharp or measurable edges, which will point to specific readings on the scale. Rotameters are equipped with end fittings or connectors according to the application. Regardless of the type of housing or terminal fittings, a similar glass tube and stainless steel float combination can usually be used. Since the tube float assembly actually performs the measurement, this is the most important part of standardization.
Scales can be set to provide direct readings of air or water-or they can indicate a calibrated scale, or flow in air/water units, to be converted to the flow of the relevant fluid via a look-up table.
The relative rotameter scale can be compared with the correlation table of gases such as nitrogen, oxygen, hydrogen, helium, argon and carbon dioxide. This will prove to be more accurate, although it is inconvenient to read directly from the scale. The scale is only designed for a fluid at a very specific temperature and pressure, such as air or water. After the conversion is completed, the relevant flowmeter can provide you with the flow values ​​of various fluids under various conditions. Using multiple floats can measure different flow rates at the same time. Generally, installing a glass tube rotameter at the height of the line of sight can make readings easier.
In industry, the safety shield gas flowmeter is the standard for measuring water or air flow under normal conditions. They can measure flow rates up to 60 GPM. Depending on the chemical properties of the measuring fluid, plastic or metal end caps can be used.
There are some examples of fluids where glass tubes cannot be used. Water above 90°C (194°F), its high pH softens the glass; wet steam has the same effect. Caustic soda dissolves glass; and hydrofluoric acid etched glass: For these applications, different pipes must be sought.
Glass metering tubes have pressure and temperature limitations, which are often the factors that limit the performance of glass tube rotameters. Small 6 mm (1/4 inch) tubes can work at pressures up to 500 psig. The larger 51 mm (2 inch) pipe can only work at a pressure of 100 psig. Glass rotameters are no longer practical at temperatures around 204°C (400°F), but since temperature and pressure usually scale with each other, this means that rotameters may actually be unusable at lower temperatures. High temperature will reduce the maximum working pressure of the glass tube.
In the case of measuring multiple gas or liquid streams at the same time or mixing together in a manifold, glass tube rotameters can be used; they are also suitable for the case where a single fluid flows out through several different channels, in this case, Multi-tube flow meters allow you to install six rotameters in a single rack device.
Metal tubes are usually made of aluminum, brass or stainless steel and can be used for higher temperatures and pressures. Since they are not transparent, mechanical or magnetic followers located on the outside of the tube can be used to determine the floating position. Here, the combination of spring and piston determines the flow rate. Choose end fittings and other materials according to the application to avoid corrosion or damage. Generally, they can be used to corrode glass tubes in situations where sudden water hammer is very important, or in situations where higher temperature or pressure (such as steam-related pressure or pressure) will damage the glass rotameter The corrosive liquid.
Examples of ideal metal tube rotameter fluids include strong alkali, hot alkali, fluorine, hydrofluoric acid, hot water, steam, slurry, acid gas, additives and molten metal. They can operate at pressures up to 750 psig and temperatures up to 540°C (1,000°F), and can measure the flow of water up to 4,000 gpm or air up to 1,300 scfm.
The metal tube rotameter can be used as a flow transmitter with analog or digital control. They can detect the floating position through magnetic coupling. Then, this moves the pointer in a magnetic spiral to display the floating position externally. Transmitters usually use microprocessors to provide alarm and pulse output to measure and transmit fluid flow.
Heavy-duty/industrial pressure sensors have elastic coatings and can operate under heavy industrial conditions. Usually use the expandable 4-20 mA transmitter: it has greater resistance to electrical noise, which may be a problem in heavy industrial sites.
As mentioned earlier, there are many possibilities for selecting materials and designs for floats, fillers, O-rings and end fittings. Glass tubes are the most common, but metal tubes can be used under conditions where the glass will break.
In addition to glass, plastic, metal or stainless steel, the float can also be made of carbon steel, sapphire and tantalum. The float has a sharp edge at the point where the reading should be observed with a tube scale.
Rotameters can be used in vacuum. A valve placed at the outlet of the meter can allow this to happen. If the expected flow range is large, a double ball rotor flowmeter can be used. Usually there is a black ball to measure a small flow, and a large white ball to measure a larger flow. Read the black ball until it exceeds the scale, and then use the white ball to read. Examples of measurement ranges include black balls with a speed range of 235-2,350 ml/min, and white balls with a maximum range of 5,000 ml/min.
The use of plastic tube rotators can replace hot water, steam and corrosive liquids at low cost. They can be made of PFA, polysulfone or polyamide. To avoid corrosion, wetted parts can be made of stainless steel with FKM or Kalrez® O-rings, PVDF or PFA, PTFE, PCTFE.
In the range of 4:1, the laboratory rotameter can be calibrated to an accuracy of 0.50% AR. The accuracy of industrial rotameters is slightly worse; usually FS in the range of 10:1 is 1-2%. For purge and bypass applications, the error is about 5%.
You can manually set the flow rate, adjust the valve opening, and observe the scale at the same time to calibrate the process flow rate; when calibrating for a specific process under the same operating conditions, the rotameter can provide repeatable measurements, and the measurement result is within 0.25% of the actual flow rate .
Although the viscosity depends on the design, when the rotor viscosity changes small, the rotameter often does not change too much: the very small rotameter that uses the spherical measurement is the most sensitive, while the larger rotameter is not sensitive. If the rotameter exceeds its viscosity limit, the viscosity reading needs to be corrected; usually, the viscosity limit is determined by the material and the shape of the float, and the limit will be provided by the rotameter manufacturer.
Rotameters do depend on the density of the fluid. If it is easy to change, you can use two floats, one depends on the volume and the other is used to correct the density. Generally, if the density of the float matches the density of the fluid, the density changes due to buoyancy will be more important, resulting in more changes in the float position. Mass flow rotameters are most suitable for low viscosity fluids such as raw sugar juice, gasoline, jet fuel and light hydrocarbons.
The upstream pipe configuration should not affect the flow accuracy; do not install the flowmeter after the elbow is inserted into the pipe. Another advantage is-because the fluid always passes through the rotameter, it should be kept clean and free of debris; however, clean fluid should be used for this purpose, without the possibility of particles or coating the pipe wall, which will cause the rotameter Becomes inaccurate and eventually becomes unusable.
This information has been obtained, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
OMEGA Engineering Ltd. (August 29, 2018). Introduction to rotameter measurement. AZoM. Retrieved from https://www.azom.com/article.aspx?ArticleID=15410 on December 6, 2020.
OMEGA Engineering Ltd. “Introduction to Flow Rate of Rotameter”. AZoM. December 6, 2020. .
OMEGA Engineering Ltd. “Introduction to Flow Rate of Rotameter”. AZoM. https://www.azom.com/article.aspx?ArticleID=15410. (Accessed on December 6, 2020).
OMEGA Engineering Ltd., 2018. Introduction to rotameter measurement. AZoM, viewed on December 6, 2020, https://www.azom.com/article.aspx? ArticleID = 15410.
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