Precautions for installing metal tube float flowmeters

Thread Content

The metal tube rotameter, also known as the metal tube float flowmeter, is widely used. It is generally employed for measuring medium and low flow rates as well as very low flow rates. The media measured are typically clean liquids and gases, or steam, that do not tend to crystallize or solidify easily and have low viscosity; it is required that the flow rate of these media change slowly. Whether the instrument is installed correctly is fundamental to ensuring its stable operation in the future. The ability to install an instrument properly is an important criterion for evaluating an instrument installation technician. In light of the various factors that can affect installation, this article introduces to readers the key aspects that need to be taken into consideration when installing metal tube flowmeters. I. Installation orientation of the instrument: The vast majority of float flow meters must be installed vertically on a vibration-free pipeline, without any significant tilt, with the fluid flowing upward through the instrument. Figure 6 shows an example of pipeline connection, equipped with a bypass system to allow maintenance without interrupting flow. The angle between the center line of the float flow meter and the vertical line generally does not exceed 5 degrees; for instruments with high precision (grade 1.5 or higher), θ ≤ 20°. If θ=12°, an additional 1% error will be incurred. There is no strict requirement for the length of the straight upstream pipe section for the instrument, but some manufacturers do require a length of (2-5)D; however, this is not really necessary. 　 II. Installation for dirty fluids: A filter should be installed upstream of the instrument. When a metal tube float flow meter with magnetic coupling is used for fluids that may contain ferromagnetic impurities, a magnetic filter should be installed in front of the instrument. It is necessary to keep the float and cone tube clean, especially in instruments with small diameters; the cleanliness of the float significantly affects the measurement values. For example, in the case of a 6mm diameter glass float flow meter, when measuring what appears to be clean water in the laboratory, the flow rate is 2.5 L/h. After operating for 24 hours, the flow rate reading increases by a certain percentage; foreign substances that are not visible to the naked eye adhere to the surface of the float. By removing the float and wiping it with gauze, the original flow rate reading is restored. If necessary, a flushing pipe can be installed as shown in Figure 7 to enable timed flushing. 　 III. Installation of pulsating flow: If there is pulsation in the flow itself, such as the presence of a reciprocating pump or control valve upstream of the location where the instrument is to be installed, or significant load fluctuations downstream, it is necessary to change the measurement location or take corrective measures within the piping system, such as installing a buffer tank ； If the oscillation is caused by issues with the instrument itself, such as too low gas pressure during measurement, valves upstream of the instrument not being fully open, or the control valve not being installed downstream of the instrument, appropriate measures should be taken to address these problems, or an instrument equipped with a damping device should be used instead. 　 IV. Installation with an expanded range: When the required flow rate range is wide, that is, when the range exceeds 10, it is common to use two or more glass tube float flow meters with different flow rate ranges in parallel. One or more of these meters are then connected in series depending on the flow rate being measured. For low flow rates, the reading from the meter with the lower flow rate range is used, while for high flow rates, the reading from the meter with the higher flow rate range is used. The series connection method is simpler to operate as there is no need to frequently open and close valves; however, it results in greater pressure loss. It is also possible to place two floats of different shapes and weights in one gauge; for low flow rates, the reading is taken from the lighter float, while when the float reaches the top, the reading is taken from the heavier float. This allows the measurement range to be expanded to 50–100. 　 V. To remove all liquid, use a corner-type metal float flow meter whose gas inlets and outlets are not aligned in a straight line. When using it with liquids, pay attention to whether there is any air remaining in the extension tube that transmits the float’s movement; this air must be removed completely ； If the liquid contains tiny bubbles, they tend to accumulate inside the casing during flow, so it is even more necessary to vent regularly. This is even more important for small-diameter instruments; otherwise, it has a significant impact on the flow reading. 　 VI. Necessary conversions of flow rates: Unless instruments are custom-made by the manufacturer based on parameters such as density and viscosity of the medium, liquid-related instruments typically have their flow rates calibrated using water, while gas-related instruments are calibrated using air, with the values set at standard engineering conditions. When the fluid density, gas pressure, and temperature under operating conditions do not match those used for calibration, necessary conversions must be made. The conversion formulas and methods are detailed in the instruction manuals of each manufacturer. 　 VII. Calibration and verification of float flowmeters: For the calibration and verification of float flowmeters, standard table methods, volume methods, and weighing methods are commonly used for liquids ； For gases, the bell jar method is commonly used, while the soap film method is used for low flow rates. Some foreign manufacturers have achieved dry-method calibration for their bulk products, that is, by controlling the dimensions of the conical tube and the weight of the float to indirectly determine the flow rate and thereby reduce costs; only high-precision instruments require precise flow calibration. In China as well, some manufacturers strictly control the inner diameter and taper at the starting point of the tapered tube, as well as the size of the float; flow verification is used only to check the quality of the inner surface of the tapered tube. The instruments, cone tubes, and floats produced by such manufacturing plants are designed to be interchangeable, eliminating the need for a complete replacement. 　 Calibrating float flowmeters using the standard gauge method is an efficient approach that is adopted by various manufacturers and in applications. Some manufacturers produce glass tube float flowmeters with a smaller taper over several segments for a specific flow range, thereby extending the scale length of the standard table and improving its accuracy, which allows for high-precision and efficient calibration work. 　 Metal tube float flow meters are suitable for measuring the flow rate of media with small diameters and low flow velocities ； Reliable in operation, requires minimal maintenance, and has a long service life ； Lower requirements are imposed on straight pipe sections ； Wider flow ratio of 10:1 ； Dual-line large LCD display; optional on-site instantaneous/accumulated flow display; single-axis sensitivity indication with backlight ； Non-contact magnetic coupling drive ； All-metal structure, suitable for high temperatures, high pressures, and highly corrosive media ； Suitable for flammable and explosive hazardous environments ； Optional two-wire, battery, and AC power supply options ； Multi-parameter calibration function ； It features data recovery, data backup, and power loss protection. Chengfeng Instruments welcomes inquiries for cooperation: 18362232387