Units issues for flow meters
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May I ask everyone: Generally, when process engineers provide requirements for flow meters to the automation specialists, the units of flow rate are typically: for liquids (m³/h), for gases (Nm³/h), and for steam (t/h). 1. If a mass flow meter is selected for measuring liquids or gases, should the measurement range listed in the data sheet be expressed in terms of mass flow rate, i.e., converted from m³/h or Nm³/h to t/h? 2. If measuring steam or gas and a vortex flow meter is chosen, should the measurement range listed in the data sheet have t/h and Nm3/h converted to volumetric flow rate in m3/h?
In principle, Chinese legal units of measurement shall be used. For instruments on the same pipeline, upstream and downstream, unless special metering requirements apply, those with identical functions must use the same units; the main units are as follows. Liquid flow rate: l/h, m3/h, kg/h, g/h, t/h. Gas flow rate: Nm3/h. Steam flow rate: kg/h, t/h. Liquid level: %, m, mm. Pressure: MPa, kPa, Pa, kPa (A), Pa (A). Temperature: ℃. Density: kg/m3. Electrical conductivity: pS/cm. Composition: % (V), % (mol), % (WT). Viscosity: mPas. For gases: ppm, %OEL, %LEL. It is possible to determine whether conversion is needed by consulting with the relevant specialists downstream. The type of instrument selected is based on the specific operating conditions and requirements
1. If you choose a mass flow meter to measure liquids or gases, then you need to convert the flow unit from volumetric flow rate (such as m³/h or Nm³/h) to mass flow rate (such as t/h). A mass flow meter measures the mass flow rate of a fluid; therefore, the measurement range on the data sheet should be expressed in units of mass flow rate. To convert volumetric flow rate to mass flow rate, you need to know the density of the fluid. The conversion formula is: mass flow rate (t/h) = volume flow rate (m³/h) × density (t/m³). 2. If you choose a vortex flow meter to measure steam or gas, you need to convert the flow units from mass flow rate (such as t/h) or standard volume flow rate (such as Nm³/h) to volume flow rate (such as m³/h). Vortex flowmeters measure the volumetric flow rate of the fluid; therefore, the measurement range on the data sheet should be expressed in units of volumetric flow rate. To convert mass flow rate to volume flow rate, you need to know the density of the fluid. The conversion formula is: volumetric flow rate (m³/h) = mass flow rate (t/h) / density (t/m³). For gases, you also need to consider the effects of temperature and pressure in order to convert the standard volumetric flow rate (Nm³/h) to the actual volumetric flow rate (m³/h). .
This post was last edited by mutianly on 2023-9-26 09:13. It’s better this way; it’s more accurate. A mass flow meter is designed to measure mass directly, whereas a vortex flow meter measures volumetric flow rate.
First, you need to know what the medium you’re measuring is. Then, based on that medium, you can select the appropriate type of flow meter. For example, an vortex flow meter can measure gases (non-corrosive) as well as liquids (non-corrosive and free of impurities). However, it measures volume flow rate. If you need to measure steam in terms of tons per hour, then temperature and pressure compensation is required (i.e., you must know the density value). Only then can you calculate the mass flow rate by multiplying the volume flow rate by the density value.
Even without performing any conversions, if you provide the operating conditions to the instrument manufacturer, they can still produce a flow meter suitable for those conditions. Then, you can check what unit the signals they output are in, and you can perform the conversion in the control system backend