The flow measurement is perhaps the highest percentage axis in terms of variables measuring industrial concerns. No other variable is the importance of this, since no flow measurements would be impossible balance of materials, quality control and even the operation of continuous processes.
There are many methods to measure fluxes, most of which is essential knowledge of some basic characteristics of the fluids for proper selection of the best method to use. These characteristics include viscosity, density, specific gravity, compressibility, temperature and pressure, which we will not detail here.
There are basically two ways to measure the flow: the flow rate and total flow. The flow is the amount of fluid passing a given point at any given time. The total flow amount of fluid through a given point during a specific time period. Let's look at some of the methods used to measure flow.
1 DIFFERENTIAL PRESSURE MEASUREMENT
Use enabled devices that create a pressure differential due to the passage of a fluid through a restriction. The reason for doing this is that the flow is proportional to the square root of the pressure difference between two points, before and after the restriction. One element is the plate - hole or perforated plate. There, the fluid undergoes a decrease in pressure, which is minimal at the point called "vena contracta". Although, the pressure tends to recover, there is finally a loss of pressure.
One-hole plate is placed in a pipe clamped between two flanges. The shape and location of the hole are the hallmark of three types of this device: the plate concentric, eccentric and segmental, the selection of some of these depends on the characteristics of the fluid to be measured. There are three types of pressure taps on both sides of the parent: footage of flanges, pipe outlets and sockets vena contracta. Similarly, here the fluid characteristics influence the choice of one of these.
Typically using a differential pressure transmitter pressure making and sending a signal representing the flow. At this signal however you must remove the square root to obtain a linear response with respect to the flow. Formerly used special instruments for this purpose. Today, this is a software feature on digital instruments.
The perforated plate is finally a simple item, cheap, although somewhat imprecise, as other differential pressure devices. Although functionally is subject to erosion and damage, it is easy to replace.
Figure 12 Measuring plate - orifice and differential pressure transmitter
Another restriction of pipe for flow measurement is the Venturi tube, which is specially designed for the length of the "pipe". It is shaped like two funnels joined at their smaller openings and is used for large pipes, is more accurate than the plate-hole, but is considerably more expensive and more difficult to install.
Figure: Principle Venturi
An average between-orifice plate and venturi nozzle is the flow, which resembles half of a venturi through which fluid enters, this device is as accurate as the Venturi tube, but not as expensive or difficult to install . The pressure used for the venturi, are located at points of maximum and minimum pipe diameter. In the case of the nozzle, are located as recommended by the manufacturer.
Another primary element for measuring flow by differential pressure method is a pilot tube, which in its simplest form, consists of a tube with a small hole at the point of measurement (impacts).
When the fluid enters the tube, its velocity is zero and the pressure is maximum. The other pressure to obtain the differential measurement is taken from a point near the pipe wall. Actually, e! Pilot tube measured fluid speed and they do not necessarily flow and the fluid must be enclosed in a pipe. It could for example be used to measure the flow of water from a river or air flow i be suspended from an airplane.
2 VARIABLE AREA METERS
They differ from previous ones in that in those there is a change in pressure, while the area remains constant. Here, however, what remains constant is the pressure differential due to the large variability of the area. One of these is the rotameter which consists of a vertical tapered tube enclosing a float, which will, depending on the flow, takes a position in the tube that increases or decreases the size of the area and thus maintains a constant pressure. A graduated scale inside the tube is calibrated in units of pressure and get a direct reading of it.
Rotameters can be made from glass tubes, metal and plastic. The latter two are used when the fluid is very corrosive or my obscure to allow placement of an internal scale. In such cases, using a magnetic follower connected to a magnet placed on the internal float and thus mechanically transmit the variation of flow indicator.
3 MAGNETIC METERS
Using Faraday's law of induction, which states that when a current passes through a conductor and a magnetic field exists in the transverse direction to it, creates an electrical potential proportional to the current.
When used for measuring flow, a tube is placed electrically isolated with a pair of electrodes mounted on both sides of the tube and flush with the fluid. Electrical coils placed around the tube so as to generate a magnetic field in a plane perpendicular to both the body axis of the output voltage is proportional to the average velocity of the fluid, it does not matter if it is laminar or turbulent. It is also independent of viscosity, density, temperature and pressure.
While it is required that the fluid has a certain minimum conductivity, the output signal varies with the increase in conductivity, which is an advantage. In applications where it is necessary to measure mass flow, this can be achieved by measuring the density of the fluid and multiplying the two signals.
Magnetic Meter
4 A TURBINE METER
An instrument of this type consists of a turbine wheel precision bearings mounted on a pipe portion, and an electromagnetic coil placed on the wall of the pipe, causing the rotation of the turbine at a rate that varies directly with flow process fluid. The interruption of the magnetic field, with each step of each turbine blade produces an electrical pulse. The frequency of these pulses determines the speed of the fluid.
5 VORTEX METER
The form of measurement is similar to the turbine. However, here a fixed at the entrance to the pipe like a propeller generates a rotary motion to the fluid. Another device, then in charge of restoring the original flow to the fluid. The swing of it at the point of measurement is proportional to flow. These oscillations produce temperature variations in a sensor placed in the area variations are then converted into voltage pulses which are amplified, filtered and transformed into square waves to be later entered into an electronic counter.
Figure 13 Vortex Meter
There are other flow meters such as the impact plate, measuring flow, adding that the fluid force developed on a "white" is a disk plate, and this force is proportional to the square root of flow, ultrasound , employing a transmitter and a receiver (sometimes installed on the same vessel) to measure the frequency deviation in the transmitter signal, due to the fluid velocity.
In the case of flow measurement in open channels, we can mention the dam, the nozzle open and basically dumps where the fluid level is measured, which varies, as it passes through these devices. A pit adjacent to the channel still has a level sensor (usually a float), whose vertical position varies depending on the flow.
6 TOTAL FLOW METERS
Within this type of devices are so-called positive displacement meters, which, separating the current volumetric flow increases individual and have those increases.
The meters are manufactured so that each instrument volume is known accurately and the sum of these increments gives a very aproximada1 the total volume that passes through the meter. Most positive displacement meters are mechanical and used mainly to measure total amounts of fluid to be transferred and is often associated with other devices to get action display, record or control. Among the most commonly used include the oscillating disc, oscillating piston, cycloidal, oval, birrotor, etc.
The mass flow meters in all types and flow computers, today represent a sample of the technological advances in measuring this variable. The Coriolis meter is an example of the former. Here the fluid flowing through a vibrating tube causes a deflection of the tube provides! mass flow. These meters have high accuracy.
