We continue this series by discussing flowmeter accessories.
Regardless of the design of the variable-area flowmeter, the flow measurement is taken at some equilibrium point where the fluid-flow force is balanced by an opposing force exerted by a “flow element” (such as a float). Either the force of gravity or a spring is used to return the flow element to its resting position when the flow lessens. Gravity-operated flowmeters, Fig. 1 (A-C) must be installed in a vertical position, while vane or spring-operated devices, Fig. 1 (D-F) can be mounted in any position.
Some variable-area flowmeters can be provided with position sensors and transmitters (pneumatic, electronic, digital or fiber optic) for connecting to remote displays or controls. Most flowmeters have only flow-alarm output signals, although some provide a continuous signal that represents the flow rate.
A variable-area flowmeter or rotameter is typically provided with calibration data and a direct-reading scale for air or water (or both). To size a meter for other service, the actual flow must be converted to a standard flow. Instrument manufacturers use different standard flow units. For liquids, the standard flow is the water equivalent in gallons per minute (gal/min) at 70ºF and 10 psi (21ºC, 69 kPa). For gases, it’s the air equivalent in standard cubic feet per minute (scfm) at 70ºF (21ºC) and atmospheric pressure. Tables listing standard water and/or air equivalent values are available from flowmeter manufacturers, which also may provide slide rules, nomographs or computer software for flowmeter sizing.
Features and Advantages
Common features and advantages of all variable-area flowmeters are: mechanical flow measurement with just one moving part, ensuring measurement reliability; application versatility and availability of a variety of construction materials, inlet and outlet sizes and types; easy installation with generally no straight pipe requirements; low pressure drops; linear scales, allowing easy flow-measurement interpretation; and electronic output availability, preserving the mechanical-flow measurement.
Advantages oftapered-tube rotameters include: low instrument cost (when glass or plastic metering tube is used); and they can be used for very-low flow rates.
Advantages of slotted-cylinder flowmeters include: flow measurement accuracy is determined by the precision of the slot manufacturing operation. (a good flow range of 25:1 results); instrument specifications can be changed by field replacement of the slotted tube and float, without having to re-pipe the flowmeter vessel; ability to handle high flows and pressures; and improved immunity to pulsating flows, with no minimum back-pressure.
Limitations common to both tapered-tube and slotted-cylinder variable-area flowmeters include: required vertical mounting and moving parts.
The references for part 3 can be found at the end of part 2.