Devices of this type (Figure below) involve the installation of an item that reduces the passage of fluid in the pipe section. There are three types, all detailed in codes of practice standards and ratified (ISO 5167-1 / Al, AGA, ASME / LH Spink)
- Board of holes
- Nozzles
- Tubes / nozzle Venturi
These designs cover a broad spectrum of performance requirements and adapt easily to different process conditions. The differences between them are counted in terms of pressure drop, straight sections of input and output, overhead costs and installation and maintenance.
Figure 22: Measurement principle of step-type flowmeters
reduced (orifice plate as an example).
The figure above shows the effects it has on a step reduced the flow of the pipeline, in this case with an orifice plate creates a narrow passage (d). According to Bernoulli's equation, the fluid velocity increases in the pipeline v1 to v2 in general narrow passage. Consequently, the dynamic pressure (NFIDCs) increases and static pressure (Pestat) decreases in the ratio for the different speeds of the fluid.
The pressure drop caused in this mode is commonly called "pressure differential" (Ap = PDIN - Pest). This pressure differential according to the main flow velocity is a direct measure of the flow through your Beriah. Two capillary tubes communicate differential pressure transmitter, where the measurement is processed and converted to the corresponding output signals.
When combined with Bernoulli's equation and apply other relevant empirical correction coefficients, we obtain a single formula to calculate the mass flow rate (Qm) and the volume flow (Qv)
Symbols (of the equation of flow and above):
The above equation must be slightly modified if you want to measure gases or compressible fluids. Unlike liquids, when trying to pass a gas through a narrow passage, there is a change in density and a change in pressure. Since the mass flow must remain constant, the velocity in the narrow passage to be increased v2, then v2 is a function of the density and cross section. This increase of speed in the narrow passage is at the expense of potential energy (pressure) and internal energy (temperature) of gas. Therefore, a gas passing at high speed by a small diaphragm undergoes a change of both pressure and tempera ¬ ture. ISO 5167 standards or AGA 3 explain in detail this phenomenon.
The description of the factors for the calculation of the above expressions can be found in the relevant implementation rules. These coefficients depend on various factors such as, for example, the ratio of the diameters β (beta), type the particular characteristics of the parent, the Reynolds number, etc.
Loss:
The fluid returns to its original speed downstream of the narrow passage. The excess dynamic pressure is converted almost completely into static pressure. The residual pressure drop (see Figure below) depends on the ratio between the diameters β (beta) and, consequently, the geometry of the narrowing. This pressure drop (Δω), however, is considerably lower con ¬ differential pressure Dp.
Figure 23: Loss of residual pressure (Δω) with parent-hole.
a = plate orifice / nozzle normal, b = nozzle, C = Venturi tube / nozzle Venturi, Pitot tube d =
Field operating flow measurement:
In general terms we can say that the differential pressure gauges to measure flow rates of "zero in on." Uncertainty tends to be higher at the lower end of the field measured by the square root that appears in the expression of flow calculation. The increase in the uncertainty of the differential pressure transmitter is quadratic. The ratio between the values of maximum flow rate (Qmax) and the flow rnínimo (qmin) is called a turndown or "turndown". Minimum throughput is often perceived cau ¬ dal lower than can be measured with a specific accuracy. Typical values turndown (turndown) in most applications are between 3:1 and 6:1.
The turndown can be expanded significantly if it is connected in parallel one second differential pressure transmitter with a smaller turndown. For example, a transmitter configuration as shown in Figure below is enough to extend the turndown (turndown) to a value of 36:1.
24: rangeability measurement through primary element for reduced bore (orifice plate). In a 6:1 turndown is passed to a 36:1
