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Rotameters are safe, reasonably accurate and widely used

Principle of operation, application and specification of a common flow-metering device

July 01, 2014
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By Jim Dillon

Brooks Variable Area FlowmeterThe variable-area flowmeter, often called a rotameter, is a cost-effective solution for liquid and gas flow measurement. Notably, external power isn’t required for operation. The meter delivers a fail-safe flow rate under any circumstance.

The variable-area flowmeter is called a rotameter because the float spins, or rotates, as the flow passes it. The spinning float is a clear indication that fluid is flowing.

Variable-area technology was invented in Germany more than 100 years ago. Today, the variable area meter is capable of measuring flow rates with process pressure approaching 20,000 psi/1350 bar and process fluid temperatures of 750 F/420 C. Even though the technology is more than 100 years old, it is still respected and widely used.  

Principle of operation

There are five primary elements to a rotameter:

1.     Metering tube – a tapered chamber that contains the fluid and float

2.     Float – a spherical or symmetrically shaped object moving up or down based on flow rate

3.     Scale – marked with increments of the flow units used to read the flow rate

4.     Process connections (end fittings) – the flanged or threaded assembly used to connect  process fluid lines

5.     Body or housing – the frame or structure containing the metering tube, float, scale and customer connections

Rotameters measure volumetric flow of liquids and gases. Operation is based on the variable-area principle, where flow raises a float in a tapered tube, increasing the area for passage of the fluid flow. The float will reach a stable position when the force exerted by the flowing fluid plus the buoyancy equals the gravitational force.

A change in flow rate upsets this force balance and the float moves up or down until it again reaches a position where the forces are in balance. Pressure drop across the float is low and remains essentially constant as flow rate changes.

Float response to flow rate changes is linear and usually a 10-to-1 flow range is standard. Variable area meters can be installed directly after pipe fittings or valves without adverse effects on the meter accuracy. The meters are also inherently self-cleaning since the flow of gas or liquid between the tube wall and the float provides a scouring action that discourages foreign matter build-up.

One reason rotameters are widely used is their repeatability, defined as the closeness of the measurement points taken under the same process conditions. Accuracy of a variable area meter is calculated using the full-scale accuracy method as opposed to the percent-of-rate method.

How are rotameters used?

Rotameters are used in diverse applications, from simple to sophisticated. Some typical applications include the following.

Process analyzers measure a target analyte in a process stream. Sampling is the single most critical issue for process analysis. When the sampling system becomes plugged, it can cause big problems and costly, unscheduled maintenance. A variable area flowmeter — with 4-20 mA output — continuously monitors analyzer sample flow — not the case with a flow alarm or flow switch. When flow starts to drop — indicating the onset of plugging — users can schedule maintenance to clear the problem before the analyzer is starved of the sample. Rotameters without 4-20 mA output are often used for sample flow in analyzer applications.

Liquid natural gas takes up to 600 times less space than in its gaseous equivalent, making it safer and more cost-efficient to transport. While LNG production, storage and loading are insulated, small amounts of LNG will re-gasify and need to be flared to prevent system over-pressure. While not a liquid, the vented vapor is still extremely cold, requiring an innovative, low-temperature option variable-area meter, such as the Brooks Instrument 3809G, to measure the flow. Low-temperature VA meters can be the perfect technology for flare applications; the meters are simple, extremely repeatable and durable.

Large rotating equipment requires reliable flow monitoring on a number of fluid supplies — like lubrication fluids, coolants and dry gas seal gasses — to ensure efficient and safe operation. Metal-tube variable-area meters are commonly used to monitor lube oil and coolant flows. Use of a 4-20 mA transmitting variable-area meter means flow can be continuously monitored. Glass-tube or plastic-body variable area meters are frequently used to ensure proper flow to dry gas seals.

On offshore platforms, extreme conditions can prevail. Common applications involve injecting proprietary fluids into high-pressure extraction fluid to prevent corrosion or extraction-fluid freezing, or to add lubrication. In all cases, the goal is local monitoring of extraction, which improves the overall process yield. Since extraction normally happens under very high pressures, flowmeter choices are limited. Variable-area meters can be an excellent choice because they are simple, proven and reliable and available for operating pressures to 20,000 psi/1350 bar.

Used for monitoring process, instrument impulse-line, purge-gas, flushing- or cooling-media, make-up, reactor-gas and liquid- feed flows; additional rotameter applications include the following:

Chemical injection/dosing – controlling flow rate of fluids to be mixed (added) to the primary fluid.

Boiler control – measuring steam flow to a boiler or of gases that heat the boiler.

Purge applications – purge gas or liquids keep process lines clear, by creating a positive pressure or shielding gas in a contained welding application; creating a positive pressure where electrical components installed in an hazardous gas area could otherwise cause ignitions.   

Tank blanketing – measuring inert gas used as a “blanket” over liquid in a tank.

Simple flow measurement – alarm or 4-20 mA output allows flow conditions to be monitored and controlled remotely.

Specifying a rotameter

Most rotameter applications are mechanical-indicator only so no power is needed to measure flow, making rotameters installable in hazardous areas. They can be installed in multiple locations over one process line without significant pressure loss. Manual control valves are available from very small meter sizes to 2-inch line size meters. Options are available for different types of metal tubes.

Variable area flowmeters are unique in that they act as an early warning system when air quality changes. Flow rate is generally impacted if process conditions change. Thus, when specifying meter applications, it’s important to know the normal and maximum operating temperature and pressure; normal, maximum and minimum flow rate; gas reference conditions or fluid density and viscosity; and other parameters.

When the measured fluid is a gas, or some liquids such as water or oils, it is necessary to know the normal operating temperature. Process lines have design temperature and pressure values which often are significantly different from the operating conditions of the meter. Knowing the maximum temperature is necessary because the operating temperature is normally much lower than the maximum temperature.

Knowing an application’s required flow rate is necessary when specifying a rotameter. The goal is to select a meter where the normal operating flow is in 60% to 80% of the meter’s range, because Variable area meters are more accurate in the upper part of its range. Additionally, the meter must also handle the minimum and maximum flows. A final important data point is units. If the unit is too small for the meter selected, the flow number can be either miniscule or too large.

Other aspects

Reference condition and flow type as aspects of flow rate apply only to gases and are used to correct the flow rate to a standard. The two key reference conditions are “standard” in the United States and “normal,” which is common in the rest of the world. The U.S. standard reference condition is 70 degrees Fahrenheit and 1 atmosphere; the normal reference condition is 0 degrees Celsius and 1 atmosphere.

Physical characteristics of gases and liquids must be considered when specifying a rotameter. Fluid density and viscosity are important because these two values allow engineers to select the right meter size. Performance data is usually collected on different VA meters so users know which ones fit the supplied process conditions, including density and viscosity.

Today’s flowmeter project approvals and certifications can be complex, and involving different regional and national standards. In the past, approvals were based on end-user location, but now they are based on destination country. It is important that users select meters with appropriate global certifications.

As stated, rotameter accuracy is computed using a full-scale accuracy method. Rotameters are more accurate in the flow-range upper end, but more variable-area meters are used for repeatability of flow measurement. This means, given the same process conditions, the float will repeat and be at the same scale reading.

While some other measurement technologies are more accurate, VA meters are still widely used, due in large part to their versatility.

Trends and ends

While indicator-only rotameters are still the norm, an increasing number of them are specified with options such as flow alarms and analog output transmitters. Metal tube, instead of glass tube rotameters are today increasingly used, due to their durability.

The latest “armored” technology solves challenges end-users faced from extreme conditions. Today’s armored VA flowmeters allow for accuracy across a wide range of flow rates and operating temperatures. Explosion-proof technology makes armored rotameters suitable for even the most stringent hazardous area classifications worldwide.

Ease of installation and retrofit, ruggedness and reliability, as well as ease of customization for specific requirements are important. It’s also important that a rotameter meet relevant safety certifications and has options for IIC/Class 1 Division 1 fire-hazard areas. These devices should be constructed with rugged stainless steel components and exterior housing for durability. Even in the most demanding applications — measuring gases, liquids or steam — rotameters can deliver 2% full-scale accuracy.

Rotameters are inexpensive flow-metering devices for a variety of industrial processing applications. They provide reliable, accurate and repeatable gas and liquid flow measurement and control, with many design options available so users can specify the best solution for the application at hand.

Jim Dillon is global variable area product manager, Brooks Instrument.

The Brooks Rotameter Company began in 1946 by introducing an innovative rotameter design that not only used bolts to fasten the side plates but also dowel pins. In 1958, Brooks introduced the Sho-Rate, which is an industry standard for low-flow purge meters. With over 60 years of experience, Brooks Instrument today provides Coriolis mass flow, thermal mass flow and variable area flowmeters, as well as vacuum capacitance manometers; pressure controllers, gauges and sensors; and level measurement technologies for critical processes.

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Photos and Captions:

 VA-MT3809G-XP-FL-2-5222

Today’s armored variable area flowmeters, such as the Brooks MT3809, allow for accuracy across a wide range of flow rates and operating temperatures.

Process Analyzer Sample Flow-2.eps

A variable area flowmeter can continuously monitor sample flow to the analyzer, unlike a flow alarm or flow switch.

000000437989

Offshore platforms require reliable flowmeter products that operate under extreme conditions such as high pressure and difficult environmental surroundings.

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