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Views: 0 Author: Site Editor Publish Time: 2025-02-20 Origin: Site
Due to differences in measurement principles or sensor designs, each type of flow meter is best suited for specific operating conditions and has unique installation requirements. To achieve optimal measurement performance in practical applications, selecting the appropriate flow meter type is essential, while also ensuring that the installation location meets the measurement requirements of the chosen instrument.
The selection of a flow meter requires comprehensive consideration of multiple factors, including:
Ensuring Proper Selection – Various process parameters must be considered, such as the fluid state under minimum, normal, and maximum design conditions, as well as flow rate, pressure, temperature, density, and viscosity. Additionally, changes in operating modes and abnormal operating conditions, such as startup, commissioning, and emergency scenarios, should also be taken into account.
Special Process Requirements – It is important to determine whether there are special conditions, such as multiphase flow, specific fluid compositions, corrosion, erosion, or the presence of solid particles in the medium.
Pipeline Compatibility – Flow elements installed within pipelines are part of the process piping system, meaning they must comply with material and grade requirements. Additionally, the pressure loss of the flow meter should not exceed the maximum allowable pressure loss of the process pipeline.
Measurement Accuracy Requirements – Generally, the accuracy and stability of a flow meter are proportional to its cost. Not all flow measurements in a process system require high precision. Choosing an appropriate instrument based on accuracy requirements is an effective way to optimize cost-performance.
Installation Requirements – Different types of flow meters have varying installation requirements, such as straight pipe length, pipeline vibration conditions, and fluid flow direction.
Electromagnetic flow meters, vortex flow meters, and mass flow meters are commonly used flow measurement instruments in petrochemical plants.
Electromagnetic flow meters operate based on the principle of electromagnetic induction. They are suitable for measuring the flow of most conductive liquids but are not applicable to gases and steam. They can be used to measure dirty, greasy media, as well as untreated wastewater and liquids containing solid particles.
Consideration of Fluid Conductivity: The minimum required conductivity should be referenced according to the manufacturer’s recommendations, generally ranging from 0.5 to 20 μS/cm.
Selection of Lining Material: Common options include PTFE, PFA, ceramic, and rubber:
PTFE: The most commonly used lining material. Since it cannot adhere to the inner wall of the housing, it is not suitable for vacuum conditions. Additionally, some liquids may permeate PTFE, leading to short circuits and corrosion of flow meter components.
PFA: Has lower permeability than PTFE but is prone to deformation under temperature and pressure fluctuations.
Ceramic: Suitable for various applications but is sensitive to temperature fluctuations and pipeline stress.
Rubber: Chloroprene rubber or natural rubber is the most economical lining material, mainly used for water measurement.
Selection of Electrode Material: In addition to corrosion resistance against the measured medium, it is essential to avoid surface effects on the electrodes.
Positioning: Electromagnetic flow meters can be installed at any position in the pipeline that meets the required straight pipe section length, but the flow tube must remain fully filled with liquid.
Electrode Orientation: The electrode axis should be horizontal to prevent contamination and avoid measurement signal loss due to bubble accumulation.
Grounding: Follow the manufacturer’s guidelines for grounding the flow meter (grounding ring).
Avoiding Large Conductive Surfaces: Installation should be away from large conductive surfaces such as metal supports.
Protecting the Inlet Lining: Coarse or abrasive liquids may damage the inlet lining of the flow meter.
Handling PTFE Linings: When tightening the process connection bolts, follow the manufacturer’s specified torque requirements.
Avoiding Incorrect Gasket Materials: To prevent damage to the liner, metal spiral-wound gaskets should not be used. Graphite gaskets should also be avoided as they are conductive and may interfere with measurements.
Dealing with Gas-Liquid Mixtures: Electromagnetic flow meters are insensitive to uniformly distributed low-content gas bubbles. However, when a fluid contains a significant amount of gas, the flow meter reading may be falsely high. Gas bubbles accumulating near the electrodes can cause measurement issues. If gas is present in the liquid, the flow meter should be installed in a vertical pipeline. The minimum required flow velocity is 0.1 m/s, and for applications with low accuracy requirements, the maximum flow velocity should not exceed 12 m/s.
Magmeter Flow Meter
Electromagnetic flow meters
Vortex flow meters offer a wide turndown ratio and can measure the flow of various process media, including liquids, gases, and steam. However, because Kármán vortices require a certain velocity to form, vortex flow meters have a relatively high zero cut-off point. The minimum measurable flow is constrained by factors such as Reynolds number, fluid velocity, and signal-to-noise ratio (frequency interference).
Vortex flow meters are sensitive to fluid oscillations and mechanical vibrations. If vibration frequencies fall within the meter’s measurement range, systematic and random errors may occur.
Reynolds Number Consideration – The Reynolds number should be at least 20,000, with values above 40,000 preferred under both normal and abnormal conditions. The minimum flow rate should ensure reliable measurement per the manufacturer’s specifications. The selected flow meter should have a lower minimum measurable flow than the actual operating minimum to ensure accuracy. Additionally, the selection should balance turndown ratio and permanent pressure loss.
Cavitation Prevention – When measuring liquids, cavitation must be prevented, as it can cause signal loss and damage to both the flow meter and downstream piping.
Unsuitability for Two-Phase Flow – Vortex flow meters should not be used for wet gases, wet steam, or two-phase flow measurements.
Medium Compatibility – These meters are not suitable for particularly viscous, waxy, or erosive fluids.
Pipe Diameter Selection – To ensure operational efficiency at minimum flow rates (such as during startup), the flow meter's diameter is typically one or two sizes smaller than the pipeline diameter.
For example, if the pipeline is DN100, the selected meter should be DN80 or DN50.
If both sizes can accommodate the expected maximum and minimum flow rates, the smaller size should be chosen.
Straight Pipe Requirements – Typically, 15D upstream and 5D downstream of straight pipe are required, but manufacturer guidelines take precedence in special cases. The upstream pipeline or flange transition should be smooth and aligned with the pipe, without burrs. Vortex-derived meters may require shorter straight pipe lengths.
Pipe Orientation Considerations –
Vortex flow meters can be installed on horizontal or vertical pipes.
On horizontal pipes, the meter should be placed in a level position to reduce debris accumulation.
When measuring gas flow in horizontal pipes, avoid installing the meter at the lowest point, as condensate could affect accuracy.
When installed on vertical pipes for liquid measurement, the fluid should flow from bottom to top.
Full Pipe Condition – The pipe should be completely filled with fluid during normal operation for accurate measurement.
Drainage for Condensing Fluids – For gas measurements where condensation is possible, drainage facilities should be installed to prevent liquid buildup during startup.
Installation Timing – The vortex flow meter should be installed only after the pipeline has been flushed and purged.
Vortex Steam Flow Meters
Vortex Flow Measurement
Coriolis mass flow meters measure mass flow directly and can be used for both liquid and gas measurements but are not suitable for steam. Since viscosity variations do not affect their measurements, these meters are ideal for slurries and emulsions, provided the medium does not clog the measuring tube. Within the instrument’s allowable density range, density changes do not affect measurement accuracy.
Coriolis meters support both unidirectional and bidirectional flow and have no moving parts, making them highly durable. However, they are sensitive to oscillatory flow and mechanical vibrations—if external vibrations match the natural frequency of the meter’s measuring tube, random errors may occur.
Flow Capacity Consideration – Coriolis meters have size and capacity limitations. If one meter is insufficient, multiple meters can be installed in parallel.
Application Areas – These meters are widely used for trade measurements, chemical injection, and oil blending. Trade measurement applications require certification from regulatory authorities.
Material Selection for Measuring Tubes – The choice of material depends on the process fluid composition:
304 & 316 Stainless Steel – Common but not suitable for halogen-containing media, as even low concentrations can cause stress corrosion due to pipe vibrations.
Hastelloy (C-22, C-276) – Recommended for halogen-containing fluids.
Titanium Alloy – Used for special applications requiring high corrosion resistance.
Pressure Drop Considerations – The pressure drop across the meter is a key selection criterion. Under no circumstances should cavitation occur, as it may severely damage the meter.
Gas Flow Measurement – Coriolis meters can measure gas flow, but manufacturer consultation is necessary to ensure stability. Low gas density can lead to inaccurate readings.
Unsuitability for Two-Phase Flow – Gas containing small amounts of liquid or liquid containing small amounts of gas will cause measurement instability.
Environmental Protection – In humid or saline conditions, use hermetically sealed sensors to prevent corrosion.
Installation Position – These meters can be installed in any orientation, but the preferred method is one that allows automatic drainage of the measuring tube.
No Straight Pipe Requirement – Unlike other flow meters, Coriolis meters do not require straight pipe runs for accurate measurement.
Vibration and Stress Elimination –
Proper support structures should be used to eliminate vibration.
The meter should be installed on a pipeline free from external stress to ensure stable readings.
For trade measurements, bellows or expansion joints can be used to reduce stress effects.
Pipeline Installation Requirements:
The flow meter must not rely on process connections for support.
A bypass line and appropriate valves should be installed.
Pressure gauges and pulsation dampeners should not be used in pulsating flow conditions.
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