Which flow measurement techniques are available?
Numerous industrial processes require an exact monitoring of the values for the flow of liquids and gases. The selection of the optimal flow measurement technology depends on the specific application and the type of process media. Crucial criteria here include process pressure and temperature ranges of the medium, viscosity and chemical aggressivity. WIKA offers an extensive portfolio of primary flow elements: among them, orifice plates, meter runs, flow nozzles / Venturi nozzles, Venturi tubes, pitot tubes and restriction orifices.
The method most widely used 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. These methods are known as differential pressure methods.
When are orifice plates used?
Due to the simple design in accordance with ISO 5167-2 and their ease of installation, even in existing piping systems, orifice plates are ideal for flow measurement in numerous applications in the chemical and petrochemical industries and also in water treatment. A number of variants can be used for the exact flow measurement of gases, liquids and vapours.
Depending on the operating conditions, RF, FF oder RTJ sealing faces are available. To ensure optimal installation, the bore and geometry of the orifice plates are adapted to the respective application in accordance with customer specifications. For high accuracies of ≤ ±1.0 % of the actual flow volume, orifice plates with longer meter runs are used.
When is a restriction orifice used?
Restriction orifices are used when the pressure should be reduced or the flow rate restricted. The restriction orifice can, depending on the customer requirement, be used as a single-step restriction orifice or as a multi-step restriction orifice.
How do pitot tubes (pitot probes) work?
This variant of flow measurement technology is based on the dynamic pressure principle. These rod-shaped flow measuring instruments have several pressure tapping openings facing towards and away from the direction of flow and are installed vertically in the pipeline. This measurement procedure allows an accurate profile for velocity in the interior of pipes, and thus an exact flow measurement.
The flow creates a dynamic pressure on the front, while the pressure value remains constant on the back. The resulting pressure difference serves as the basis for calculating the mass and volume flow. For monitoring in a remote control room, a differential pressure transmitter can be connected to the pitot tube to generate a constant signal proportional to the flow. Differential pressure gauges or differential pressure switches are used for the local display of the flow.
When are Venturi nozzles (flow nozzles) used?
With increasing flow through a constriction, turbulence occurs which can lead to inaccuracies. Venturi tubes with conical constrictions are used to reduce these disturbances as far as possible. The Venturi nozzle (flow nozzle) consists of a converging section with a rounded profile and cylindrical neck and, at higher temperature and flow rates, provides greater strength and dimensional stability than an orifice. This variant of flow meters is particularly suitable for steam, chemical substances, gases and other media, even with high process temperatures. The accuracy corresponds to ≤ ±0.1 % of the actual flow volume and the repeatability of the measurement is 0.1%.
Process media with extremely low or very high Reynolds numbers of up to several million are no problem for these highly robust flow meters. The simple design allows for an enduring operation with low maintenance costs.
What are the advantages of Venturi tubes?
Venturi tubes offer features that set them apart from other primary flow elements. A WIKA Venturi tube is made from bar stock or welded sheet. Compared to other differential pressure flow meters, a Venturi tube offers higher pressure recovery and is used when upstream and downstream pipe requirements are low. Due to their reliability, ease of handling and high pressure recovery, Venturi tubes are often used in systems with a low pressure gradient. Since a large proportion of the output pressure is maintained, the cost of pumping is kept to a minimum.
What are wedge flow meters suitable for?
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.
