Rotameters

Fig. 22. Common flow meteis (a) orifice, (b) venturi, (c) target, (d) rotameter, and (e) weir. See text.

Instrument Society of America 400 Stanwix Street Pittsburgh, Pa. 15222 Standards l ibrary for Measurement and Control, 12th ed., 1994. Instmmentation standards and recommended practices abstracted from those of 19 societies, the U.S. Government, the Canadian Standards Association, and the British Standards Institute. Covers control instmments, including rotameters, aimunciators, transducers, thermocouples, flow meters, and pneumatic systems (see... 

Fig. 3. Solvent-processing equipment using partial condenser. Level a on the water overflow line to the receiver should be about 3 cm below level b on the solvent-return line. Dimension b—c must be great enough to overcome pressure drop in the vapor piping, condenser, solvent piping, and rotameter. In a 4 m (1000-gaI) ketde, dimension b—c would be at least 1.25 m. The volume of the piping described by the dimension c—d—e should contain twice the volume of dimension b—c, thus providing an adequate Hquid seal against normal ketde operating pressures.

Rotameter A rotameter consists of a vertical tube with a tapered bore in which a float changes position with the flow rate through the tube. For a given flow rate the float remains stationary since the vertical forces of differential pressure, gravity, viscosity, and buoyancy are balanced. The float position is the output of the meter and can be made essentially linear with flow rate by makiug the tube areavaiy hn-early with the vertical distance. 

Rotameters The rotameter, an example of which is shown in Fig. 10-21, has become one of the most popular flowmeters in the chemical-process industries. It consists essentially of a plummet, or float, which is free to move up or down in a vertical, slightly tapered tube having its small end down. The fluid enters the lower end of the tube and causes the float to rise until the annular area between the float and the wall of the tube is such that the pressure drop across this constriction is just sufficient to support the float. Typically, the tapered tube is of glass and carries etched upon it a nearly linear scale on which the position of the float may be visually noted as an indication of the flow. 

Rotameters require no straight runs of pipe before or after the point of installation. Pressure losses are substantially constant over the whole flow range. In experimental work, for greatest precision, a rotameter should be cahbrated with the flmd which is to be metered. However, most modern rotameters are precision-made so that their performance closely corresponds to a master cahbration plot for the type in question. Such a plot is supphed with the meter upon purchase. 

According to Head [Tran.s. Am. Soc. Mech. Eng., 76, 851-862 (1954)], flow rate through a rotameter can be obtained from... 

The ratio of flow rates for two different fluids A aud B at the same rotameter reading is given by... 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

Be sure material being measured is being delivered to rotameter. 

For the tandem arrangement gas seal, a primary seal vent must be pro vided to vent the leakage across the process side seal. This vent ma> lie to flare or other suitable gas disposal point. The back pressure under nor mal conditions should be kept to a low value. A small amount of back pressure is recommended to keep a positive differential across the see ondaiy seal. Leakage measurement may be provided in the vent line to provide health monitoring of the primary seal. Unfortunately, the rotameter, which would be the obvious choice, should not be used because of its lack o reliability. If an orifice or needle valve is used to set the back pressure to the seal vent, pressure upstream of the restriction can be measured for a relative flow measurement. This type of reading does provide trend data that may be used to judge the seal s performance. 

Blockages or faulty pumps. Pumps should be checked periodically as instructed by the manufacturer. They can be calibrated using rotameters or bubble flowmeters. Unless pumps possess a limiting orifice they should be calibrated with the air indicator tube in position. 

Before removing the pump at the end of the sample period, check the flow rate to ensure that the rotameter bail is still at the calibrated mark (if there is a pump rotameter). If the ball is no longer at the mark, record the pump rotameter reading. 

Calibrate personal sampling pumps before and after each day of sampling, using either the electronic bubble meter method or the precision rotameter method (that has been calibrated against a bubble meter). 

Turn on the pump and adjust the pump rotameter, if available, to the appropriate flow rate setting. 

The precision rotameter is a secondary calibration device. If it is to be used in place of a primary device such as a bubble meter, care must be taken to ensure that any introduced error will be minimal and noted. The precision rotameter may be used for calibrating the personal sampling pump in lieu of a bubble meter provided it is (a) Calibrated with an electronic bubble meter or a bubble meter, (b) Disassembled, cleaned as necessary, and recalibrated. It should be used with care to avoid dirt and dust contamination which may affect the flow, (c) Not used at substantially different temperature and/or pressure from those conditions present when the rotameter was calibrated against the primary source, (d) Used such that pressure drop across it is minimal. If altitude or temperature at the sampling site are substantially different from the calibration site, it is necessary to calibrate the precision rotameter at the sampling site where the same conditions are present. 

If possible, eliminate inherently weak equipment like sight glasses, hoses, rotameters, bellows, expansion joints, and most plastic equipment. The spare parts consumption from the shop and warehouse will indicate what is wearing out. 

Hliiull. In v.tive B 5th ijifcni-.O pr< SM re ViilvL-. fail [jpai C)peration Excessive chlorine How to Tower Water Basin - high chlorine level to cooling water -potential for excessive corrosion in cooling water system Rotameter Relief valve an pressure check valve outlet in Nnilk-... 

In another incident the nitrogen flow was too small. The range of the rotameter in the nitrogen line was 0-60 L/min (0-2 fC/min) although 150 L/min (5 ft /min) was needed to keep the oxygen content at a safe level. 

Pitot ube or Pitot Venturi Tube Turbine or Propeller Type Element Rotameter... 

Every 5-8 ft (1.5-2.5 m) along the standpipe use rotameters to regulate aeration flow... 

In practice, deliberate changes in flow are necessary to proportion quantities of flow into various systems, and to determine flow rate by various measuring devices by restrictions, e.g. Venturi meters and rotameters. 

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