The variable area flow meter is an industrial flow meter used to measure the flowrate of liquids and gases. The variable area meter consists of a tube and float. The float response to flowrate changes is linear, and a 10-to-1 flow range or turndown is standard.
In the case of OMEGA™ laboratory variable area flow meters, far greater flexibility is possible through the use of correlation equations. The variable area flow meter is popular because it has a linear scale, a relatively long measurement range, and low pressure drop. It is simple to install and maintain.
Learn more about variable area meters
Principle of Operation
The meter's operation is based on the variable area principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flowrate. With liquids, the float is raised by a combination of the buoyancy of the liquid and the velocity head of the fluid. With gases, buoyancy is negligible, and the float responds to the velocity head alone. The float moves up or down in the tube in proportion to the fluid flowrate and the annular area between the float and the tube wall. The float reaches a stable position in the tube when the upward force exerted by the flowing fluid equals the downward gravitational force exerted by the weight of the float. A change in flowrate upsets this balance of forces. The float then moves up or down, changing the annular area until it again reaches a position where the forces are in equilibrium. To satisfy the force equation, the variable area flow meter float assumes a distinct position for every constant flowrate. However, it is important to note that because the float position is gravity dependent, variable area meters must be vertically oriented and mounted.
How to select a variable area flow meter ?
What is the minimum and maximum flow rate for the flow meter?
What is the minimum and maximum process temperature?
What is the size of the pipe?
Would you like a direct reading meter or is a look up table acceptable?
What accuracy do you need?
Do you require a valve to regulate the flow?
Will there be back pressure?
What is the maximum process pressure?
Choose the right variable area meter for your application
Glass Tube variable area flow meter
The basic variable area flow meter is the glass tube indicating-type. The tube is precision formed of borosilicate glass, and the float is precisely machined from metal, glass or plastic. The metal float is usually made of stainless steel to provide corrosion resistance. The float has a sharp metering edge where the reading is observed by means of a scale mounted alongside the tube. End fittings and connections of various materials and styles are available. The important elements are the tube and float, often called the tube-and-float combination, because it is this portion of the meter which provides the measurement. In fact, similar glass tube and stainless steel float combinations are generally available, regardless of the type of case or end fittings the application can demand, as to best meet customer requirements. The scale of the meter can be calibrated for direct reading of air or water, or it may have a scale to read a percent of range or an arbitrary scale to be used with conversion equations or charts. Safety-shielded glass tube variable area flow meter are in general use throughout industry for measuring 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. The only fluids for which these meters are not suited are those 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 primary limitations of general purpose meter are the pressure and temperature limits of the glass metering tube. Small, 6 mm (1/4") tubes 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 practical temperature limit for glass variable area flow meter is 204°C (400°F), although operation at such high temperatures substantially reduces the operating pressure of the meter. In general, there is a linear relationship between the operating temperature and pressure.
Metal Tube Flow meters
For higher pressures and temperatures beyond the practical range of glass tubes, metal tubes are used. These are usually manufactured in aluminum, brass or stainless steel. The position of the piston is determined by magnetic or mechanical followers that can be read from the outside of the metal metering tube. Similar to glass tube variable area meter , the spring-and-piston combination determines the flowrate, and the fittings and materials of construction must be chosen so as to satisfy the demands of the applications. These meters are used for services where high operating pressure or temperature, water hammer, or other forces would damage glass metering tubes. Spring and piston flow meters can be used for most fluids, including corrosive liquids and gases. They are particularly well suited for steam applications, where glass tubes are unacceptable.
Plastic Tube variable area flow meter
Plastic tubes are also used in some variable area flow meter designs due to their lower cost and high impact strength. They are typically constructed of polycarbonate, with either metal or plastic end fittings. With plastic end fittings, care must be taken in installation, not to distort the threads. Meters with all plastic construction are available for applications where metal wetted parts cannot be tolerated, such as with deionized water or corrosives.
Purge Flow Regulators
Figure 2-16: Purge Flowmeter Design
If a needle valve is placed at the inlet or outlet of a variable area flow meter, and a d/p regulator controls the pressure difference across this combination, the result is a purge-flow regulator. Such instrumentation packages are used as self-contained purge flow meters. These are among the least expensive and most widely used flow meters. Their main application is to control small gas or liquid purge streams. They are used to protect instruments from contacting hot and corrosive fluids, to protect pressure taps from plugging, to protect the cleanliness of optical devices, and to protect electrical devices from igniting upon contact with combustibles.
Purge meters are quite useful in adding nitrogen gas to the vapor spaces of tanks and other equipment. Purging with nitrogen gas reduces the possibility of developing a flammable mixture because it displaces flammable gases. The purge-flow regulator is reliable, intrinsically safe, and inexpensive.
As shown in the figure, purge meters can operate in the constant flow mode, where P2 - P0 is held constant at about 60 to 80 in H2O differential. In bubbler and purge applications, the inlet pressure (P1) is held constant and the outlet pressure (P0) is variable.The figure describes a configuration where the outlet pressure (P0) is held constant and the inlet pressure (P1) is variable.
They can handle extremely small flow rates from 0.01 cc/min for liquids and from 0.5 cc/min for gases. The most common size is a glass tube variable area flow meter with -in (6 mm) connections, a range of 0.05-0.5 gpm (0.2-2.0 lpm) on water or 0.2-2.0 scfm (0.3-3.0 cmph) in air service. Typical accuracy is ±5% FS over a 10:1 range, and the most common pressure rating is 150 psig (1 MPa).
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