Flow Measurement: Rotameter and Flow Computer Solutions
Gas and liquid flow measurement: variable area flow meters (rotameters), sample gas flow meters and natural gas flow computers.
Why is flow measured?
Flow is the amount of fluid passing through a line per unit of time, and it is monitored in almost every process and measurement system. In sample preparation and analysis systems, sample gas or liquid flowing at the correct rate is essential for the analyzer to respond accurately and quickly; low flow causes lag, while high flow degrades filter and cooler performance. On large natural gas and process lines, calculating mass and volume flow is the basis of custody transfer, energy calculation and efficiency monitoring.
Where is flow measured?
- Analyzer sample lines: Flow monitoring at the cooler, filter, pump and analyzer inlet.
- Auxiliary lines: On calibration gas, purge and cooling water lines.
- Natural gas and process pipelines: In differential pressure-based measurements.
Methods for measuring flow
In variable area (rotameter type) flow meters, a float rises in a conical tube according to the flow rate, and the flow is read directly from the scale; flow can be adjusted with an integrated control valve, and low-flow alarm options are available. Different ranges are offered for liquid and gas; this is a simple, locally readable solution, ideal for low-flow sample lines. Flow computers calculate the mass and volume flow of gas and other fluids from the signals they receive from differential pressure, static pressure and temperature transmitters, record totals and transmit them through communication.
Choosing a flow meter
- Fluid type, flow range and temperature/pressure
- Seal and body material (EPDM, Viton, stainless steel) and connection type
- Need for a low-flow alarm and remote monitoring
- Number of inputs and approval (such as CSA) on the flow computer
Flow measurement with Tetra
Tetra supplies Yokogawa RAKD41 and Kytölä rotameters, M&C FM series gas flow meters and Thermo Scientific AutoPILOT PRO and AutoFlex flow computers; its product range also includes the Mega System A1-Aircal digital flow calibrator. Installation, commissioning and service are provided by Tetra; separate solutions are also offered for flue gas flow.
Frequently asked questions
What units is flow expressed in?
For liquids, m³/h or L/min is generally used, and for gases m³/h, Nm³/h or L/min; mass flow is given in kg/h. For gas flow, it must be stated whether the value is expressed at actual operating conditions or at normal or standard conditions, because volume changes as pressure and temperature change.
What happens if a rotameter is used with a different gas or pressure?
A rotameter's scale is calibrated for a specific fluid, density and pressure. A value read with a different gas or at a different operating pressure deviates from the true flow; in that case, a correction factor accounting for the density difference should be applied, or the instrument should be selected for the new conditions. Rotameters must also be mounted vertically and in the direction specified by the manufacturer.
Why should flow be kept constant in an analyzer sample line?
A change in sample flow directly affects the analyzer's response time and, on some analyzers, the measurement result. Low flow increases the risk of lag and clogging, and high flow degrades filter and cooler performance. Flow meters with a control valve and low-flow alarms are therefore common on sample lines.
Which standards does a flow computer use in natural gas calculations?
Flow computers apply standards such as AGA 3 for orifice measurement and AGA 8 for gas compressibility calculation. Corrected volume, mass and energy totals are calculated using differential pressure, static pressure and temperature inputs. Which standard and gas composition data to use is determined by the commercial contract and relevant legislation.
Products measuring this parameter
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