Numerous industrial processes require an exact monitoring of the values for the flow of liquids and gases. The selection of the optimal flow meter depends on the specific application and the process media. Crucial criteria here include process pressure and temperature ranges, viscosity and chemical aggressivity.
The most commonly used method in industrial measurement technology is based on the principle of differential pressure measurement. A narrowing of the cross-section in the pipeline causes a decrease in pressure and a proportional increase in the flow rate.
Thus, the exact determination of the differential pressure before and after the restriction is sufficient to calculate the mass flow or the volume flow. The derivation of the quantities is based on the determination of the flow coefficients with the help of the Bernoulli equation. This method is also known as the differential pressure method.
Fields of application for flow meters
In refineries, flow nozzles are used, for example, to measure the volume flow of steam flows that, under high pressure, escape into the atmosphere. Calorimetric flow switches with an integrated LED display are suitable for monitoring coolant circuits.
Volume flows in ventilation pipes in air conditioning technology are measured using compact air flow meters with electrical output signals. The exact control of the room air supply is used for energy-efficient air conditioning in accordance with applicable EU standards.
In the chemical and petrochemical industries it is mainly robust primary flow elements which are used.
For high accuracies of ≤ ±1.0 % of the actual flow rate, orifice plates with longer measuring paths are used.
Orifice plates
Due to their simple design according to ISO 5167-2 and their easy installation even in existing piping systems, orifices are perfectly suited for flow calculation in numerous applications in the chemical and petrochemical industry as well as in water treatment. A large number of variants can be used for precise flow measurement of gases, liquids and vapors.
Depending on the operating conditions, RF, FF or RTJ sealing surfaces are available. To ensure optimum installation, the bore and geometry of the orifice plates are adapted to the customer's specifications for the respective application. For high accuracies of ≤ ±1.0% of the actual flow rate, plug-in orifices with longer measuring distances are used.
Dynamic pressure probe (pitot tube)
This version of the flow meter is based on the so-called dynamic pressure principle. These rod-shaped flowmeters have several pressure tapping openings upstream and downstream of the flow direction and are installed vertically in the pipeline. This measuring principle allows a more accurate profile for velocity inside the pipe and thus an exact flow measurement.
The flow causes a dynamic pressure on the front side while the pressure value remains constant on the back side. The resulting pressure difference serves as the basis for calculating the mass and volume flow. For control in a remote control room, a differential pressure transmitter can be connected to the dynamic pressure probe to generate a constant signal proportional to the flow rate. Differential pressure gauges or differential pressure switches are used for local indication of the flow rate.
Venturi nozzles (flow nozzles)
With increasing flow through a constriction, turbulence occurs which can lead to inaccuracies. Venturi tubes are flow meters with conical constrictions are used to reduce these disturbances as far as possible. This variant is particularly suitable for flow measurements of steam, chemical substances, gases and other media, even with high process temperatures. The accuracy corresponds to ≤ ±0.1 % of the actual flow rate and the repeatability of the measurement is 0.1 %.
Flow switches
Electronic flow switches offer the operator a local display of the flow values and at the same time an output of an electrical signal (PNP, NPN or 4-20 mA) for transmission to a remote control room. The FSD-3 flow switch is based on the calorimetric measuring principle. The temperature difference that occurs due to the cooling effect of flowing media is measured here. The flow rate can thus be calculated on the basis of the temperature difference. Furthermore, the design of these flow meters allows exact monitoring of the temperature without the need to set up an additional measuring location. Calorimetric flow switches are also known as thermal flow monitors and are ideal for monitoring coolant circuits or for controlling filter units.
Flow monitors do not need any power supply and work on the float measurement principle. A float activates a reed contact as soon as the flow rate exceeds a preset value. Corrosion-resistant materials enable applications in areas such as the chemical industry, machine building, the pharmaceutical industry and in medical engineering.
Air flow sensors
For the monitoring of air flows in ventilation and air-conditioning systems, flow sensors are suitable for volume flow calculation based on the measured differential pressure and the fan constant (K factor) of the connected radial fans. The exact measurement of the smallest differential pressures is crucial for the cost-effective and technical monitoring of room air systems. PID controllers and pitot tubes are also used for this.
Wedge flow meter
Wedge flow meters are suitable for highly viscous and particle-laden process media such as wastewater, sewage sludge, tar sand or cement. The differential pressure is achieved by a wedge-shaped narrowing of the pipeline. This design enables robust, bidirectional measurements in numerous applications of almost all flow types. Process media with extremely low or very high Reynolds numbers of up to several million are no problem for these highly robust primary elements. The simple design allows continuous operation with low maintenance costs.
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