Using Rotameters for Flow Measurement

Introduction to Flow Measurement with Rotameters

The main application of the rotameter flow meter is to measure the flowrate of liquids and gases. It is composed of a plastic, metal or glass tube and float. The float response to flowrate changes is linear, and a 10-to-1 flow range or turndown is standard.

The OMEGA™ laboratory rotameters offer a higher flexibility due to the use of correlation equations. The rotameter is simple to install and maintain, and has gained popularity because it has a linear scale, a relatively long measurement range, and low pressure drop.

Working Principle of Rotameters

How to Choose a Rotameter

The following section presents information on how to choose a rotameter and answers questions relevant to how to make the choice, including questions about the flow rates for the flow meter, temperature, size of the pipe, reading of the rotameter, accuracy, requirements, and process pressure.

The rotameter is the most popular variable area flow meter because of its low cost, simplicity, low pressure drop, relatively wide rangeability and linear output.

Furthermore, it operates simply: in order to pass through the tapered tube, the fluid flow raises the float. The higher the level of flow, the higher the float is lifted. In liquid service, the float rises due to a combination of the buoyancy of the liquid and the velocity head of the fluid.

By contrast, with gases, buoyancy is disregarded, and the float responds mostly to the velocity head.

The metering tube in a rotameter is installed vertically (Figure 2-15) and the small end is at the bottom. The fluid that is going to be measured enters at the bottom of the tube, passes upward around the float, and exits the top. If there is no flow, then the float stays at the bottom. When fluid enters, the metering float begins to rise.

The fluid flow rate and the annular area between the float and the tube wall, and the moving of the float up and down are proportional. As the float rises, the size of the annular opening increases.

This is counter proportional to the differential pressure across the float. When the upward force exercised by the flowing fluid equals the weight of the float, a stable position is reached in the float.

There is a correspondence between every float position and a particular flowrate for a particular fluid's density and viscosity. Therefore, you have to size the rotameter for every single application. The flow rate can be determined by matching the float position to a calibrated scale on the outside of the rotameter, when the sizing of the rotameter has been done in a correct way. Many rotameters come with a built-in valve for adjusting flow manually.

A number of shapes of float are available for various applications. An early design of the rotameter was created with slots, which caused the float to spin in order to achieve stabilizing and centering. The term rotameter was coined because of that rotating float.

Rotameters usually have calibration data and a direct reading scale for air or water (or both). In order for the rotameter to be sized for other service, the actual flow has to be first converted to a standard flow.

In terms of liquids, the standard flow is the water equivalent in gpm; for gases, the standard flow is the air flow equivalent in standard cubic feet per minute (scfm). Tables listing standard water equivalent gpm and/or air scfm values are provided by rotameter manufacturers. Rules, nomographs, or computer software for rotameter sizing are also frequently provided by manufacturers.

Choose the Right Type of Rotameter for Your Application

Glass Tube Rotameters

The basic rotameter is the glass tube indicating-type rotameter, which is a basic rotameter, whose tube is formed of borosilicate glass for accuracy, and the float is precisely machined from metal, glass or plastic.

Stainless steel is typically what the float is made up of, so that it can provide resistance to corrosion. The float also consists of a sharp metering edge and a tube-and-float combination.

A scale is installed alongside the tube and it is there where the reading is observed. The metering edge also has end fittings and connection of a number of materials and styles.

The tube-and-float combination is another important part, because it is the part that actually provides the measurement. Furthermore, similar glass tube and stainless steel float combinations are generally available, regardless of the type of case or end fittings the application can demand, so as best to meet customer requirements.

To achieve direct reading of air or water, the scale of the rotameter could be calibrated, or it may include a scale to read a percent of range or an arbitrary scale to be used with conversion equations or charts. Generally, safety-shielded glass tube rotameters are used widely in the industry to measure both liquids and gases.

They provide flow capacities to about 60 GPM, and are manufactured with end fittings of metal or plastic to meet the chemical characteristics of the fluid being metered. There are, however, several fluids for which the meters are not suitable.

Those include the fluids which attack glass metering tubes, such as water over 90 °C (194 °F), with its high pH which softens glass; wet steam, which has the same effect; caustic soda, which dissolves glass; and hydrofluoric acid, which etches glass.

The main limitations of the rotameters for general purposes include the pressure and temperature limits of the glass metering tube. Small tubes of 6 mm (1/4") are suitable for working pressures up to 500 psig, but the operating pressure for a large 51 mm (2") tube may be as low as 100 psig.

The feasible temperature limit for glass rotameters is 204 °C (400 °F), although operation at such high temperatures reduces the operating pressure of the meter to a large extent. Overall, the relationship between the operating temperature and pressure is linear.

Metal Tube Flow Meters

Metal tubes have a different application from the glass tubes. The metal tubes are used for higher pressure and temperature in the cases where temperatures are above the practical range in which glass tube can be used.

The metal tubes are usually manufactured in aluminum, brass or stainless steel.

The piston position is decided based on the magnetic or mechanical follower that can be read from the outside of the metal metering tube.

Similar to glass tube rotameters, the spring-and-piston combination determines the flowrate, and the fittings and materials of construction must be chosen in order to satisfy the demands of the applications.

The metal meters are used in situations where the high operating pressure or temperature, the water hammer or other forces would be harmful for the glass metering tubes.

Spring and piston flow meters can be used for most fluids, including corrosive liquids and gases. They are particularly applicable to steam applications, where glass tubes are unacceptable.

Heavy Duty/Industrial Pressure Transducers

Heavy Duty/Industrial Pressure transducers have a more rough design than other transducers. They are meant to accommodate heavy industrial environments and frequently feature a scalable 4-20 mA transmitter.

This transducer ensures much greater immunity to electrical noise. Such noise is common for industrial environments, but this type of transducer can mitigate the issue.  

Design Variations

A variety of materials is available for floats, packing, O-rings, and end fittings. Rotameter tubes for such safe applications as air or water can be made of glass, making the glass tubes the most common tubes, formed with precision of safety shielded borosilicate glass. As mentioned above, for more unsafe conditions where breakage is possible, metal tubes are provided.

Glass, plastic, metal, or stainless steel are usually what floats are made from in order to ensure corrosion resistance. Other float materials include carboloy, sapphire and tantalum. End fittings are made from metal or plastic. Some fluids damage the glass metering tube, such as wet steam or high-pH water over 194 °F (which can soften glass); caustic soda (which dissolves glass); and hydrofluoric acid (which etches glass).

Floats have a sharp edge at the point where the reading should be observed on the tube-mounted scale. The reading is observed on that tube-mounted scale, which the floats have at their sharp edge.

To ensure accuracy, the glass-tube rotameter has to be installed at eye level. The scale can be calibrated for direct reading of air or water, or can read percentage of range. Generally, glass tube rotameters can measure flows up to about 60 gpm water and 200 scfh air.

Figure 2-15 shows how a reading is taken from the scale of a correlation rotameter. To get the real flow in engineering units, the reading is compared to a correlation table for a particular gas or liquid. Correlation charts are prepared and available for nitrogen, oxygen, hydrogen, helium, argon and carbon dioxide.

The correlation meter is not as convenient as a direct reading device is, but it is more accurate. This is because a direct-reading device is accurate for only one specific gas or liquid at a particular temperature and pressure. A correlation flow meter can be used with a great number of fluids and gases under various conditions. Different floats in the same tube can measure different flow rates.

When working with pressures up to 500 psig, the more appropriate tube is a small glass tube. However, the maximum operating pressure of a large (2 inch diameter) tube may be as low as 100 psig. The practical temperature limit is about 400° F, but such high-temperature operation significantly reduces the operating pressure of the tube.

Normally, the relation between operating temperature and pressure is a linear one. Glass-tube rotameters are more frequently used in situations where a single fluid is being exhausted via several different channels (Figure 2-17).

Another appropriate application of the glass-tube rotameters is when several streams of gases or liquids are being metered at the same time or mixed in a manifold. Multiple tube flow meters allow up to six rotameters to be mounted in the same frame.

Operating a rotameter in a vacuum is also possible. For rotameters that have a valve, it is placed at the outlet at the top of the meter. A dual-ball rotameter is used for measurements at a wider range.

This instrument has two ball floats: a light ball (typically black) for indicating low flows and a heavy ball (usually white) for indicating high flows. The black ball is read until it goes off scale, and then the white ball is read. An instrument like that can have a black measuring range from 235-2,350 ml/min and a white to 5,000 ml/min.

As discussed, metal tubes are used when the temperature is beyond the practical for the glass tube. Those tubes can be made of stainless steel, and the position of the float is detected by magnetic followers with readouts outside the metering tube.

Furthermore, metal-tube rotameters can be used for hot and strong alkalis, fluorine, hydrofluoric acid, hot water, steam, slurries, sour gas, additives, and molten metals. They can also be applied in cases where high operating pressures, water hammer, or other forces could damage glass tubes.

Metal-tube rotameters are available in diameter sizes from K in to 4 in, can operate at pressures up to 750 psig, temperatures to 540 °C (1,000 °F), and can measure flows up to 4,000 gpm of water or 1,300 scfm of air.

Metal-tube rotameters are readily available as flow transmitters for integration with remote analog or digital controls. The way transmitters detect float position is magnetic coupling.

The indication is usually external in the form of rotatable magnetic helix that moves the pointer. The transmitter can be intrinsically safe, microprocessor-based, and can be provided with alarms and a pulse output for totalization.

Plastic-tube rotameters are rather low cost rotameters that are perfect for applications involving corrosive fluids or deionized water. The tube itself can be made from PFA, polysulfone, or polyamide. The wetted parts can be made from stainless steel, PVDF, or PFA, PTFE, PCTFE, with FKM or Kalrez^® O-rings.

Accuracy of a Rotameter

Accuracy of 0.50% AR over a 4:1 range can be attained by rotameters when calibrated properly, while the inaccuracy of industrial rotameters is typically 1-2% FS over a 10:1 range. Purge and bypass rotameter errors are in the 5% range.

Flow rates can also be set manually through the rotameter by adjusting the valve opening while observing the scale to establish the required process flow rate. Rotameters can be repeatable to within 0.25% of the actual flow rate, only when the operating conditions do not change.

Most rotameters are not influenced by viscosity variations. The most sensitive are very small rotameters with ball floats, while larger rotameters are less sensitive to viscosity effects. The limitations of each design are published by the manufacturer (Figure 2-18). Viscosity limit is influenced by the float shape. If the viscosity limit is exceeded, the indicated flow must be corrected for viscosity.

A rotameter can be designed with two floats (one sensitive to density, the other to velocity) so as to approximate the mass flow rate that results from its sensitivity to changes.

The more closely the float density matches the fluid density, the greater the effect of a fluid density change will be on the float position. Mass-flow rotameters work best with low viscosity fluids, such as raw sugar juice, gasoline, jet fuel and light hydrocarbons.

Upstream piping configuration does not affect the rotameter accuracy. The meter also can be installed directly after a pipe elbow without adverse effect on metering accuracy. Rotameters are inherently self-cleaning because, as the fluid flows between the tube wall and the float, it produces a scouring action that tends to prevent the buildup of foreign matter.

Nevertheless, it is always better if rotameters are used on clean fluids which do not coat the float or the tube. Liquids with fibrous materials, abrasives, and large particles should also be avoided.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.