Generally, flow meter selection can be considered from five aspects: Instrument performance, fluid characteristics, installation conditions, environmental conditions, and economic factors. The detailed factors in these five aspects are as follows:
Instrument performance: accuracy, repeatability, linearity, rangeability, flow range, signal output characteristics, response time, pressure loss, etc.
Fluid characteristics: temperature, pressure, density, viscosity, chemical corrosion, abrasiveness, scaling, miscibility, phase change, conductivity, sound velocity, thermal conductivity, specific heat capacity, isentropic index;
Installation conditions: pipeline layout direction, flow direction, straight pipe length upstream and downstream of the sensor, pipe diameter, maintenance space, power supply, grounding, auxiliary equipment (filters, deaerators), installation, etc.
Environmental conditions: ambient temperature, humidity, electromagnetic interference, safety, explosion protection, pipeline vibration, etc.
Economic factors: instrument purchase cost, installation cost, operating cost, calibration cost, maintenance cost, instrument lifespan, spare parts, etc.
The steps for selecting a flow meter are as follows:
Based on the fluid type and five key factors, initially select available meter types (several types should be considered for selection);
Collect data and price information for the initially selected types to prepare for in-depth analysis and comparison;
Use an elimination method to gradually narrow down to 1-2 types, repeatedly comparing and analyzing the five key factors to finally determine the target selection.
Important Notes
Fluid characteristics mainly refer to the pressure, temperature, density, viscosity, and compressibility of the gas. Since the volume of gas changes with temperature and pressure, compensation and correction should be considered.
Meter performance refers to the meter's accuracy, repeatability, linearity, range ratio, pressure loss, initial flow rate, output signal, and response time. When selecting a flow meter, these indicators should be carefully analyzed and compared to select an meter that meets the flow requirements of the metered medium.
Installation conditions refer to the gas flow direction, pipeline direction, upstream and downstream straight pipe lengths, pipe diameter, spatial location, and fittings. These factors all affect the accurate operation, maintenance, and service life of the gas flow meter.
Economic factors refer to purchase costs, installation costs, maintenance costs, calibration costs, and spare parts, which are further affected by the performance, reliability, and lifespan of the gas flow meter.
The accuracy class and functions should be selected based on measurement requirements and application scenarios to ensure cost-effectiveness. For example, in applications such as trade settlement, product handover, and energy metering,accuracy classes such as 1.0, 0.5, or higher are suitable. For process control applications, different accuracy classes are selected based on control requirements. In some cases where only process flow is measured without precise control and metering, a lower accuracy class, such as 1.5, 2.5, or even 4.0, can be chosen. In these cases, an inexpensive insertion electromagnetic flow meter can be selected to measure medium velocity. When measuring general media, the full-scale flow rate of the electromagnetic flow meter can be selected within a relatively wide range of 0.5–12 m/s. The instrument specification (diameter) should not necessarily be the same as the process pipeline. It should be determined based on whether the measured flow rate falls within the specified velocity range. That is, when the pipeline flow velocity is too low to meet the flow meter requirements, or when measurement accuracy cannot be guaranteed at this velocity, the instrument diameter needs to be reduced to increase the flow velocity in the pipe and obtain satisfactory measurement results.
When the flow control instrument system indicates the minimum value, first check the field monitoring instrument. If it is normal, the fault lies in the display instrument. When the field monitoring instrument also indicates the minimum value, check the control valve opening. If the control valve opening is zero, the fault is often between the control valve and the controller. When the field monitoring instrument indicates the minimum value and the control valve opening is normal, the fault is likely caused by insufficient system pressure, system pipeline blockage, pump failure, medium crystallization, improper operation, etc. If the fault is related to the instrument, the causes may include: a blocked positive pressure lead in the orifice plate differential pressure flow meter; a leak in the positive pressure chamber of the differential pressure transmitter; or a stuck gear or clogged filter in the mechanical flow meter.
When the flow control instrument system indicates the maximum value, the monitoring instrument will often also indicate the maximum value. At this point, the regulating valve can be manually opened or closed remotely. If the flow rate decreases, the problem is generally due to process operation. If the flow rate does not decrease, the problem lies with the instrumentation system. Check if the regulating valve of the flow control instrumentation system is functioning correctly; check if the instrumentation pressure measurement system is working properly; check if the instrumentation signal transmission system is functioning properly.
If the flow control instrumentation system reading fluctuates frequently, switch the control to manual mode. If the fluctuation decreases, the problem is with the instrumentation itself or the PID control parameters are inappropriate. If the fluctuation remains frequent, the problem is due to process operation.