Do the upstream and downstream control valves of a metal tube float flow meter affect the true value of the flow rate?
Thread Content
To measure the nitrogen flow rate, a metal tube float flow meter is used. 1. The nitrogen pressure at the front end of the flow meter is 4 MPa; the diameter is DN15, and the flow rate is measured by allowing the gas to flow directly to the outside at the back end. 2. The flow meter was calibrated at normal pressure and in the presence of nitrogen when it was purchased. 3. In accordance with standard practices, control valves are installed at both the front and back ends of the flow meter. Question: When adjusting the flow rate, it is necessary to manually adjust the control valve at the exit of the flow meter at the back end. Is the flow rate indicated in this case the actual flow rate? I’ve been very confused: when using the control valve at the back of the flow meter, the pressure at the flow meter changes from its calibrated value; therefore, the indicated value does not represent the actual flow rate. So why is it still necessary to adjust the flow in this way? (It’s no longer allowed.) Thank you!
Pressure changes can affect measurement accuracy.
I have seen the flow calculation formulas in the instructions included with the packaging of gas rotary flow meters; it seems that pressure has little effect on the flow rate displayed. Looking forward to a professional answer.
Changes in pressure affect the measurements, for which there are correction formulas
I previously sought out relevant information; the original poster may find it useful to know that a gas rotor flow meter is calibrated based on the volumetric flow rate of a certain gas under standard conditions. For glass rotor flow meters, the scale indicates the flow rate under standard conditions at the time of calibration, while for metal rotor flow meters, the scale provides an analog or digital output signal representing the flow rate under standard conditions at the time of calibration. If the medium under test and the actual operating conditions are identical to those used during calibration, the scale reading of the rotameter represents the actual volumetric flow rate under standard conditions, and no conversion is required. However, since the gas medium or operating conditions during actual measurement differ from those at the time of calibration, the flow rate value represented by the rotor at the same position in the measuring tube is different from that at the time of calibration. To use a gas rotameter correctly, it is necessary to know the flow rate conversion relationship between them.
It is necessary to know at what pressure the metal tube float flow meter is designed, so that pressure compensation can be applied in the system; temperature changes have little impact. Metal tube rotameters are used to measure gases, with an accuracy of around 20%; this has been verified through weighing in actual field conditions, and it is difficult to obtain precise measurements – it is merely a rough indication of whether the flow rate is increasing or decreasing. Detailed parameters can be provided for a further response.
The accuracy of gas float flow meters is rather low; they are flow meters of low precision. The values indicated by them are certainly not the actual flow rate, and they are suitable only for indicating general increases or decreases in flow rate – no high accuracy can be expected from them. Of course, pressure does have an impact, but this can be compensated for using fluid state equations in order to eliminate its effects.