Abstract on the theme of master's work

Contents

• Introduction
• the purpose of the work, tasks
• an overview of the methods and means of measurement of liquid flow in the pressure pipe of large diameter
• 1.1. Basic concepts and definitions
• 1.2. Classification of liquid flow meters in closed pressure pipelines
• 1.3. Tachometric flow meters
• 1.4. The flowmeters of variable pressure difference
• 1.5. Vortex flowmeters
• 1.6. Non-contact flow meters
• 1.6.1. Ultrasonic flowmeters
• 1.6.2. Electromagnetic flow meters
• 1.7. Flowmeters, operating on the principle of measuring the average velocity in the pipe cross-section
• 1.7.1. Flow meters, pressure devices
• 1.7.2. Flow meters with electromagnetic transducers speed
• Opinion
• List of sources

  Introduction

  Water resources play a vital role in the development of the national economy and the existence of society. That is why one of the key problems of infrastructure development in the Donetsk region is ensuring reliable operation of water supply systems. The problem is significant, as its solutions will help to improve the quality of services provided to consumers, and therefore to preserve the health of the population and increase its welfare.

  Rational use of water resources in turn is carried out by improving the management of technological processes for cleaning, supply and distribution of water. The measures considered necessarily provide for the use of effective modern water metering systems, which should replace existing obsolete flowmeters.

  Purpose, tasks

  The purpose and objectives of research. The purpose of the master's work is to justify the structure of the flowmeter of drinking water in pressure pipelines of large diameter.

  To achieve this goal you need:

  – to develop a mathematical model of an MHD velocity converter with a local magnetic field;

  – to study the dependence of the output signal of an MHD converter with a local magnetic field on the transverse velocity gradient on the basis of a study of the spatial distribution of the magnetic field;

  – on the basis of the results of the performed studies, to determine the optimal ratio of the design parameters of the MHD velocity converter;

  – justify the structure of the measuring device being developed.

  Overview of methods and means for measuring fluid flow in a large-diameter pressure pipeline

  To date, a large number of liquid flow meters have been developed and used, which implement various measurement methods. They differ in their metrological characteristics, scope, and performance. However, all modern flowmeters are subject to unified stringent requirements, which can be divided into two groups:

  – to the first group include individual requirements relating to instruments for measuring flow: high accuracy, reliability; the independence of measurement results from the physical and chemical properties of the medium being measured, as well as the change in the density of matter; speed and a wide range of measurements;

  – the second group includes the requirements that characterize the whole group of flowmeters: the need to measure the consumption of a variety of nomenclature of substances that differ in their physico-chemical properties; the need to measure different flow rates at different pressures and temperatures.

  None of the existing types of flowmeters can fully meet all of these requirements at once. Therefore, the correct choice of the measurement method and the means is a necessary condition for the successful solution of the given measurement problem. When choosing a measuring instrument, one should start from the actual conditions of its operation, the properties of the substance being measured, its parameters and its flow rates, and also with reasonable requirements for measuring accuracy. In addition to the metrological characteristics of the measuring instrument, it is also necessary to take into account its consumer qualities: the degree of complexity of the measuring device, the cost, energy consumption, material consumption, functionality, ease of operation.

  1.1. Basic concepts and definitions

  The flow rate of a liquid is called a measuring device or a set of devices designed to measure the flow of liquid (gas).

  The mass flow meter measures the mass flow of liquid (gas). The volumetric flow meter measures the volumetric flow of liquid (gas) Mass M and volumetric Q costs are related by the relationship: M =? · Q where? - fluid density [2].

  The method of measurement is a method for the experimental determination of the value of a physical quantity, that is, a set of physical phenomena and means of measurement used for measurements.

  The measuring transducer is a means for measuring the flow rate of a liquid (gas), designed to generate a measurement information signal in a form suitable for transmission, further transformation, processing and (or) storage, but not directly perceivable by an observer.

  The information measuring system is a collection of measuring instruments connected by communication channels, designed to receive signals of measurement information in a form convenient for automatic processing, transmission and (or) use in automatic control systems [3].

  1.2. Classification of liquid flow meters in closed pressure pipelines

  The main existing instruments for measuring liquid flow in closed pressure pipelines can be classified according to various characteristics. If the main feature is to accept the presence or absence of a moving element in the hydraulic path of the measuring transducer, the flow measurement means can be divided into two groups [4]:

  – instruments whose primary measuring transducers contain movable elements in the hydraulic path;

  – instruments whose primary measuring transducers do not contain movable elements in the hydraulic path.

  The first group of devices are:

  – flowmeters of constant pressure drop (rotameters, float flowmeters);

  – volumetric flowmeters (piston, disk, ring, screw, with oval gears, lobed);

  – tachometric flowmeters (vane, turbine, ball);

  – flowmeters frontal resistance (with angular displacement, with axial displacement).

  The second group of devices combines flowmeters with flow transformation (violation of the flow field) and without transforming it. Instruments with a flow transformation are:

  – flowmeters of variable pressure drop (diaphragms, nozzles, pipes and venturi nozzles;

  – flowmeters with the generation of pressure pulsations (vortex flowmeters, flowmeters with swirling flow).

  The following devices are referred to instruments without flow transformation:

  – flow meters (with a thermal label, with a radioactive label, with an optically opaque label);

  – ultrasonic flowmeters;

  – calorimetric flowmeters;

  – electromagnetic flowmeters.

  The following are the measurement tools that are most common for measuring liquid flow in closed pressure pipelines.

  1.3. Tachometric flowmeters

  The most common tachometric instruments for measuring fluid flow are turbine flowmeters. A typical design of a turbine flowmeter is shown in the figure. 1.1. It consists of three main elements: the turbine primary converter 1, the secondary converter 2, the reference system (registrar) 3. The turbine converter is an axial blade turbine, supported by impact bearings or bearings 4. The flow of the medium to be measured, leaning on the inclined turbine blades, imparts rotational motion with angular velocity ?, proportional to the flow rate Q [2].

  The secondary converter is an induction coil. In general, turbine blades are made of ferromagnetic material. When the coil magnetic field intersects the turbine blades in the coil, a pulsating current is induced. The frequency of the ripple of the induced current is proportional to the angular velocity of rotation of the turbine, and, consequently, to the flow rate. As secondary converters, inductive coils are also used, in which a periodic change in inductance is produced during rotation of the ferromagnetic turbine, causes a corresponding change in one of the current parameters in the coil.

  

  Figure. 1.1 Turbine Flowmeter Construction

  The electromagnetic sensing element creates a braking effect on the turbine, which at low speeds can affect the angular velocity of the turbine. Therefore, when measuring low flow rates of the substance, other designs of sensitive elements are used, for example, photoelectric [2,5].

  Pulsed current pulses are registered by the counting system. The number of pulses recorded by the system per unit of time characterizes the measured flow of matter. Assuming no turbulence acts on the turbine, the relationship between the angular velocity of the turbine rotation and the flow rate will be determined by the following expression:

  ω=A × Q     (1.1)

  where ω – angular speed of rotation of the turbine;

  A – coefficient determined by the design parameters of the turbine sensor;

  Q – measured flow rate.

  Actually, the moments of the forces of hydraulic friction of the fluid, moments of frictional forces in the supports, and others act on the turbine. The action of these moments will characterize the so-called zone of non-sensibility of the device, that is, the smallest flow Q ₀, which is necessary in order to overcome the moments of resistance and move the turbine from its place or change its constant speed of rotation. Taking this into account, using expression (1.1), we obtain the working equations of turbine meters [2]:

  ω = A × (Q-Q ₀)

  The greatest influence on the readings of turbine flow converters is provided by local hydraulic supports that create a strong one-sided flow compression, as well as supports that cause screw motion.

  The advantages of turbine flowmeters are the simplicity of design, high sensitivity and a large range of measurements, the ability to measure both small and large liquid flow rates with a wide range of physicochemical properties, low inertia, and therefore relatively small dynamic errors in the measurement of averages and instantaneous values ??of expenses. Another important advantage of such flowmeters is the linear dependence of their output signal on the flow velocity in the range specified for the device.

  The shortcomings of turbine flowmeters include: the need for individual calibration and, as a consequence, the need for high-precision calibration equipment; influence of viscosity change of the measured medium and hydrodynamic flow parameters on the instrument readings; the presence of supports that wear out dramatically shortens the life of devices.

  1.4. Flowmeters of variable differential pressure

  The principle of operation of this type of flowmeters, united by a single measurement method, based on the measurement of the pressure drop produced as a result of a local change in the flow velocity of the liquid. To measure the flow rate of a fluid with a differential pressure, three devices are required that are united by the common notion of a variable differential pressure flowmeter [2]:

  – a narrowing device is the main part of such flowmeters and creates a pressure drop in the flow of the medium to be measured due to local variation in the flow velocity;

  – measuring device - a manometer that directly measures the pressure drop;

  – connection device, transfers the pressure drop from the flow to the pressure gauge.

  The narrowing device creates a local narrowing of the stream. The nature and distribution of pressure and velocities along the flow as the substance passes through a portion of the pipeline in which the tapering device is installed is shown in the figure. 1.2

  

  Figure. 1.2. Distribution of pressure and velocity in a normal diaphragm

  The flow velocity in the narrowed section II rises, part of the potential energy of pressure becomes kinetic. As a result, the static pressure in section II becomes less than the static pressure in section I, where the influence of the narrowing device has not yet affected the flow. The difference in these pressures depends on the speed (or flow) of the fluid flowing in the pipeline. In practice, the places for the selection of pressures are most often chosen directly from the inlet and outlet of the flow [6, 7].

  The basic equation for the flow of an incompressible fluid through a narrowing device is:

  

  where? - coefficient of consumption of the narrowing device;      S ₀- the area of the aperture of the narrowing device; g is the acceleration due to gravity; y is the density of the liquid; Δp = (p1-p2) measured differential pressure

  Currently, the most common narrowing devices of three types: a normal diaphragm, a normal nozzle and a venturi (tube).

  A normal aperture is a thin disk with a hole with a sharp rectangular edge on the input side of the stream [2].

  The pressures in a flat diaphragm are selected by means of separate drilling. To averaging the pressure, several holes are drilled, uniformly distributed around the circumference of the pipe in the pressure collection planes. Pipes coming out of the holes are combined by two collectors, from which the pressure is transferred to the pressure gauge.

  The pressures in the chamber diaphragms are taken from the chambers connected to the pipe by annular gaps [2]. The advantage of chamber diaphragms is the selection of actual mean pressures and, in this connection, somewhat less stringent requirements for the length of straight sections of the pipeline before and after the diaphragm; disadvantage – the need for special sealing devices to seal the chambers [2].

  A normal nozzle is made in the form of a nozzle, has a smooth rounding on the input side, and a developed cylindrical part from the exit side. At the outlet of the nozzle is a bore, which protects the original edge from damage. Selection of static pressures is carried out in the same way as in the case of normal diaphragms [4].

  The Venturi nozzle differs from the normal nozzle in that it has an elongated cylindrical part that passes directly to the diffuser [4].

  The venturi consists of the inlet part – confusor, middle part – cylindrical and output parts – diffuser. Before the confusor is installed in a rectilinear section. Such pipes are used on pipelines with a diameter (100 ... 800) mm. Pressure is taken through the annular chambers connected to the pipeline in at least six locations.

  The venturi needs 2 times as much a straight section as the diaphragm, and has a wide (10: 1 and even 20: 1) measuring range. The Venturi tube is used for pulps and contaminated liquids. The truncated venturi is used for clean fluids. When using a straightener, the minimum straight section in front of the venturi becomes equal to two conditional pipe diameters [4].

  The advantages of variable differential pressure flowmeters with narrowing devices are the absence of moving parts, the simplicity of mass production, the verification by the calculation method, the low cost. The disadvantages of this type of devices include: the requirements for the length of straight sections of the pipeline in front of the device and after it, the relatively large measurement error in the range of measured flow rates, large errors in the measurement of pulsating flows, the requirement of constancy of the phase state of the substance, the impossibility of mounting and dismantling without stopping the flow in pipe [4].

  1.5. Vortex Flowmeters

  Vortexes are called flowmeters, whose action is based on the dependence of the frequency of pressure oscillations arising in the flow in the process of vortex formation or jet oscillation, on the flow [7].

  The principle of operation of flowmeters with a body located in the flow, based on the fixation of flow oscillations that arise when flowing around the body [8]. The body, which is in the flow path, changes the direction of the streams flowing around it, and increases their speed due to a corresponding decrease in pressure. In the midsection section of the body, the reverse process of decreasing velocity and increasing pressure begins. Simultaneously, an elevated pressure is created on the anterior part of the body, and a lowered pressure on the posterior part. The boundary layer surrounding the body, having passed its midsection section, detaches from the body and under the influence of the reduced pressure behind the body changes the direction of motion, forming a vortex (Figure 1.3). This occurs both in the upper and lower points of the streamlined body. But since the development of a vortex on one side of the body hinders the same development on the other hand, the formation of vortices on either side takes place in turn. In this case, a streamlined body forms the vortex path of Karman [7].

  Several variants of converting the vortex flow oscillations into an output signal are used. In general, periodic fluctuations in pressure or velocity of jets are used on both sides of the body.

  

  Figure. 1.3 Vortex formation scheme

  The relationship between the flow rate and the frequency of vortex shedding has the form:

  Q = (S × d/Sh)× f

  where S is the area of the smallest cross section of the flow around the flow body;     d is the characteristic size of the body of flow;     Sh is the Strouhal criterion characterizing the dimensionless frequency of the process.

  Cylinders, triangular and tetrahedral prisms are most widely used as flow bodies. The error in measuring the flow throughout the normalized range is + 1.0% [8].

  The merits of vortex flowmeters include the absence of moving parts, the simplicity and reliability of the flow converter, the independence of readings from pressure and temperature, a large measuring range, linearity of the scale, a small error of + (0,5 ... 1,5)%, frequency measuring signal, stability of readings, relatively simple measuring circuit, possibility of obtaining universal calibration. The drawbacks of vortex flowmeters include a significant pressure drop reaching (30 ... 50) kPa, and some limited possibilities for their use: they are unsuitable for low speeds because of the difficulty of measuring a signal having a small frequency and are only made for pipes having diameters (25 ... 300) mm. Their application for large pipes causes difficulties, and for very small diameters there is a stable vortexing [7].

  1.6. Non-contact flowmeters

  Special attention is paid to non-contact flowmeters that provide flow measurement without direct contact of their sensitive elements with the measured medium and without disturbing the field of flow. Such a measurement can be made only by the results of the interaction of the flow field and the auxiliary physical field, the source of which must be located in the flowmeters of the non-contact flowmeter.

  It should be noted that sometimes to non-contact flowmeters include those of their types, the sensitive elements of which have direct contact with the measured medium, although they do not change its nature of the flow. In addition, some types of non-contact flowmeters may contain an additional hydraulic converter that changes the initial nature of the flow of the medium to be measured in order to expand the functionality of the flowmeter. Such flowmeters can be referred to non-contact conditionally and can be called quasi-noncontact [9].

  Methods of non-contact measurements, as well as flow measurements without the use of devices located inside the pipeline, are the most promising. First of all, they include electromagnetic and ultrasonic methods [10].

  1.6.1. Ultrasonic (acoustic) flowmeters

  The basis of the ultrasonic method of flow measurement is the dependence of this or that effect on the passage of ultrasonic oscillations through the flow of a liquid.

  Ultrasonic flowmeters implement various measurement methods. Most often, in practice, two measurement methods are used that differ in the spatial arrangement of the directions of the fluid velocity vector and the propagation vector of the sound wave.

  The first method is based on determining the deviation of the moving medium by an ultrasonic beam directed at right angles to the flow vector. In fact, it measures the decrease in the amount of acoustic energy entering the receiver with increasing flow velocity.

  The disadvantage of this method is low sensitivity, so a variation of this method is often used, which consists in that the beam is guided at a small angle with respect to the diameter of the tube and is taken after repeated reflection from the pipe walls, as shown in the figure. 1.4, a). Thus, an increase in the distance that the beam passes is achieved. The sensitivity of such a design is higher than that of the main method, but it should be noted that the indications are significantly dependent on the degree of corrosion and contamination of the internal surfaces of the tube reflecting vibrations. In addition, the speed of sound in a medium depends on the temperature of the liquid, its kinematic viscosity and the degree of contamination of the liquid. This dependence leads to an increase in the error when the physico-chemical properties of the measured liquid change.

  

  Figure. 1.4 Constructions of transducers of ultrasonic flowmeters

  In order to exclude the effect of the dependence of the speed of sound on various factors, another method is used in which the fluid flow is determined from the difference in the speed of sound propagation in and out of the flow direction. Ultrasonic vibrations are directed at an angle to the direction of flow. On the image. 1.4, b) shows a single-channel, and in the figure. 1.4, c), d) – two-channel designs of flowmeter transducers that realize this method of forming and receiving acoustic oscillations

  If the planes of the emitting and receiving acoustic transducers are located at an angle? to the axis of the tube, then the sound oscillation travels in the liquid to be measured along a path length L at an angle α equal α = 90° - θ. We denote by νL the flow rate averaged over the length L. The projection of the velocity L on the direction νL is equal to L cos&alpha. If the acoustic vibrations are directed towards the flow, then the time τ1 of the distance L passing by them is given by:

  τ₁ = L / (c + ν_L × cosα) (1.1)

  where c is the speed of propagation of acoustic oscillations in a stationary fluid.

  In the opposite direction of acoustic oscillations, the time? t ₂of the same distance is determined by the expression:

  τ₂ = L / (c + ν_L × cosα) (1.2)

  Subtracting (1.2) from (1.1), we obtain:

  

  Given that, we get:

  

  There are several varieties in this method.

  In the time-pulse method, the time difference ΔT is measured; the passage of short pulses in the direction of the flow and against it a path of length L. This difference is related to the average flow velocity ?L by the expression (1.3). In this method, the dependence on the speed of ultrasound in the medium remains, but there are opportunities to compensate for this dependence, for example, by installing an additional pair of resonators.

  In the frequency-pulse method, each pulse (or several pulses) arriving at the receiver excites the generation of a new pulse. By measuring the repetition rate of pulses along and against the flow, the flow is calculated. The advantage of this method is the independence of the characteristic from the speed of sound.

  The phase method involves measuring the phase difference of a signal across a stream and against it. The method is based on the fact that when the flow rate changes, the signal comes to a receiver with different phases. Indeed, if the initial phases of both sound oscillations, which have a period T and a frequency f, are exactly the same, we get:

  
  Substituting the values of (1.3) into (1.4), we obtain an expression defining the relationship between Δφ; and the average flow velocity ? L at a distance L:
  

  where ω = 2πf is the circular frequency of the oscillations.

  There is another kind of ultrasonic flowmeters - Doppler flowmeters. The Doppler method is based on the occurrence of a frequency shift when the sound beam is reflected from a moving particle or the flow is inhomogeneous, for example, from a gas bubble. The disadvantage of the method is the requirement for such inhomogeneities.

  Several basic designs of the primary converter are used to realize the ultrasonic method.

  In a monoblock design, the converter is a piece of pipe with flanges. The piezoelectric transducer is permanently mounted on the pipe. This inverter is calibrated in a calibration installation and delivered fully ready for use.

  In the design with embedded piezo-transducer radiator and receiver are mounted permanently on the existing pipeline.

  In a design with an overhead piezo-transducer and a receiver, they are mounted on an existing pipeline using special clamping devices.

  Of the listed designs, small errors in flow measurement in a monoblock design. The worst errors in a design with overhead piezo-transducer radiators. The advantages of the last two designs include the ability to measure flow in large diameter pipes (>1000 mm) [8].

  1.6.2. Electromagnetic flowmeters

  At present, the electromagnetic flowmeter occupies a firm position among devices for measuring the flow of liquid substances. This type of flowmeter most fully meets the requirements. It has a sufficiently high measurement accuracy, a wide linear dynamic range. In addition, it does not have mechanical parts in contact with the liquid, and therefore it is able to easily meet the sanitary and hygienic requirements.

  The work of the electromagnetic fluid flowmeter is based on the Faraday effect. In a conductor, the lines of force of the field intersect, an EMF is induced that is proportional to the velocity m and independent of the physical properties of the moving medium. In this case, the direction of the EMF arising in the conductor is perpendicular to the direction of motion of the conductor and the direction of the magnetic field [2].

  According to the measured effect, electromagnetic flowmeters can be assigned to three types [11]:

  – Conductive electromagnetic flowmeters are devices whose action is based on measuring the induced electric field (Figure 1.5);

  

  Figure 1.5 Conduction Electromagnetic Speed ??Meter

  – Induction electromagnetic flowmeters are devices whose action is based on measuring the induced secondary magnetic field (Fig. 1.6);

  

  Figure 1.6 Induction Electromagnetic Speed ??Meter

  – ponderomotive electromagnetic flowmeters are devices whose action is based on measuring the interaction force of electric currents induced in a liquid with the magnetic field of the converter (Fig. 1.7).

  

  Figure 1.7 Ponderomotive electromagnetic velocity meter

  Conductive electromagnetic flowmeters, in which the measured value is not the EMF induced by the motion of the liquid, but the potential difference, is removed from the electrodes of the converter [11].

  The liquid flow converter itself is a pipe with a circular section located between the poles of a permanent magnet and an internal diameter equal to the diameter of the pipeline. The converter has two electrodes, by which the induced voltage is removed. Because of the simplicity and reliability of the design, electromagnetic flow transducers have become popular [12].

  In the figure. 1.8 schematically shows the device of an electromagnetic flowmeter. The measuring tube 1 with two metal electrodes 3 placed in it is made of a non-magnetic material with a large electrical resistivity. This guarantees the least distortion of the magnetic flux by the walls of the tube and smaller losses for eddy currents. The magnetic field is created by an electromagnet 2 feeding the power supply 6. The voltage, which is removed from the electrodes, is converted into an amplified electric signal by the electronic amplifier 4, registered by the counting system 5 [2].

  The resulting potential difference E on the electrodes is determined from the electromagnetic induction equation [2]:

  E = –K × B × D × ν_{СР, (1.5)}

  where K – coefficient, depending on the type of magnetic field;     B – magnetic induction in the gap between the poles of the magnet;     D – internal diameter of the pipeline;     ν_{СР,is the average fluid flow rate.}

  For the case of a constant magnetic field, the equation K = 1 is valid. If the magnetic field varies in time t with frequency f, then

  K = sin(2πft) [2].

  

  Figure. 1.8 Structure of the electromagnetic flowmeter

  Electrodes are usually made of stainless steel or another material, for example platinum. They are placed in the holes of the wall of the measuring tube. Inside, the end of the electrode is flush with the inner surface of the wall. The outer end of the electrode is provided by cutting and one or two nuts to tighten the electrode when it is sealed. The measured EMF does not depend on the physical properties of the medium flowing in the pipe, and is determined only by the flow velocity, the magnetic field strength and the distance between the electrodes. EMF, developed in converters of electromagnetic flowmeters, is insignificant, its value rarely exceeds 10 mV [13].

  Expressing in (1.5) the average flow velocity through the volume flow of the medium to be measured, we obtain the measurement equation for electromagnetic flowmeters [2]:

  – for the case of a constant magnetic field

  E = –(4 × B / π×D)×Q;

  – for the case of an alternating magnetic field

  E = –(4 × B ×sin(πft) / π×D)×Q;

  Thus, electromagnetic flowmeters can be made with both a permanent and an alternating magnetic field. These electromagnetic flowmeters have their advantages and disadvantages that determine the areas of their application [2].

  The main disadvantage of electromagnetic flowmeters with a constant magnetic field, which limits their use for measuring the flow of quasi-stationary flows, is the polarization of the measuring electrodes. It is accompanied by a change in the resistance of the sensor, distorts the readings of the device. To reduce the polarization, electrodes with platinum or tantalum coating, as well as carbon and calomel electrodes [6] are used.

  Using an alternating magnetic field can significantly reduce the effect of polarization of the electrodes. However, there are other effects that lead to distortion of the useful signal.

  Firstly, this is a transformer effect, when a transformer EMF is induced in the coil formed by the liquid in the pipeline, electrodes, connecting wires and secondary devices, the source of which is the primary winding of the magnetic field excitation system. Transformer noise can reach (20 ... 30)% of the useful signal. For their compensation, special additional devices are introduced into the measuring circuit of the device.

  Secondly, there is a capacitive effect that arises because of the large potential difference between the magnetic field excitation system and the electrodes and the parasitic capacitance between them (connecting wires, etc.). The means to combat this effect is careful screening.

  Third, there may be an effect of the change in the frequency of the current feeding the excitation system of the magnetic field. Compensate this effect by installing special stabilizing devices, which makes measuring schemes more difficult and reduces the reliability of devices [14].

  There are known methods of feeding a system of excitation of a magnetic field by rectangular current pulses, as well as periodically increasing and decreasing current, allows to get rid of the influence on the measured signal of harmful factors arising in electromagnetic flowmeters with an alternating magnetic field [8,10].

  The electromagnetic flowmeter has a number of advantages that make this type of appliance widely used.

  An important advantage of electromagnetic flowmeters is that the principle of their operation and signal recording is electrical. They can be connected without additional converters to electrical systems designed for measurement and automatic control. For the same reason, electromagnetic flowmeters are used for remote signal recording (although in the case of poor fluid conductivity, the length of the conductors connecting the flowmeter to the measuring device must be limited). Electromagnetic flowmeters, because their work is based on electrical phenomena, low-inertia and allow to study unstable flows even at sufficiently high rates of change of speed [15].

  The universality of the electromagnetic measurement method is also due to their wide functionality, which makes it possible to create an inertia-free measuring instrument with a linear calibration characteristic, the character of which does not depend on the physical and chemical properties of the medium being measured [16] .

  The majority of electromagnetic flowmeters have channels that provide an unobstructed flow of liquid, so such devices do not violate the hydrodynamic flow structure and do not create additional hydraulic resistance [17]. Electromagnetic flowmeters do not have drainage or other openings in which a solid can accumulate, which can lead to additional difficulties associated with cleaning. In circular cross-section flowmeters with a transverse magnetic field, the concentric deposition of the solid phase on the walls does not affect the instrument readings, unless the solid matter and the liquid have the same electrical conductivity [15].

  These advantages have provided a fairly wide application of electromagnetic flowmeters, despite their relative constructive complexity and the need for thorough daily maintenance (zero adjustment, adjustment etc.) [2] .

  Difficulties in creating large-scale electromagnetic flowmeters are largely due to the large consumption of iron and copper. To build a magnetic system of such flow meters, their cost significantly increases, and the dimensions and mass of the primary measuring transducers complicates their installation and makes it impossible without stopping the flow in the pipeline.

  1.7. Flowmeters operating on the principle of measuring the average speed in the cross-section of the pipe

  The problem of measuring the flow of water in large diameter pipelines is an independent measuring task, which is solved by its specific methods. This is due to the fact that the use for these purposes of flowmeters that implement traditional methods, requires enormous costs both for the creation of the instruments themselves and for the creation of means for their individual calibration and verification [2].

  The method of measuring the flow rate by flow velocity at one point is based on the regularities of turbulent flow in pipes, according to which the flow velocity at a certain point of the pipe section is proportional to the average velocity in a given section [1 ]. Flowmeters operating on this principle cause very low head losses, they can be installed and dismantled without interruption of water supply through pipelines [5].

  When determining the costs of these methods, it is necessary to measure the local flow rate at a single point in the cross section of the measuring section of the pipe by the primary converter, and also to measure the area of ??this section. The consumption Q is determined by the formula [1]:

  Q = K × V × S

  where K = V × C × P / V – ratio of the average flow velocity in a given section to the flow velocity at the measurement point; V is the local flow velocity, m / s; S is the cross-sectional area of the pipe, m².

  If the local flow rate is measured at points where it is equal to the average velocity in a given section (at points of average velocity), the coefficient remains constant and equal to unity over the entire range of turbulent flow. The points of medium speed with the developed turbulent flow of the measured medium are located at a distance (0.242 ± 0.013) r from the inner surface of the pipe wall, where r is the internal radius of the pipe in the measuring section [1] .

  According to [1], the following conditions must be met during the measurement:

  – the flow in the pipeline must be formed and turbulent, and the movement – constant;

  – the area of the measuring section must remain constant throughout the measurement;

  – on the walls of the pipe there should be no deposits and build-up in the measured medium and corrosion products.

  Next we consider the main types of instruments designed to measure the average flow velocity of the liquid in the pipeline and which have become most widespread in the flow measurement practice.

  1.7.1. Flowmeters with pressure devices

  Pressure devices create a differential pressure, depending on the dynamic pressure of the flow. They convert the kinetic energy of the flow into a potential flow [18].

  A classic example of a pressure device is a L-shaped tube with a hole facing the flow, which is called a Pitot tube named French scientist, used it to measure the speed of the river. Such a tube perceives the total pressure equal to the sum of the dynamic RD and static PC flow pressures.

  In order to measure the velocity v in the pipeline using such a tube, in addition to the Pitot tube, one must also have a tube for selecting only the static pressure of the PC. Then the pressure gauge, which measures the pressure difference ΔP = РП - РС = РД, will serve to determine the speed by the formula [18]:

  

  where k_T is the tube coefficient;

  k _SZis the compressibility coefficient, for a liquid is equal to one.

  In most cases, the tubes for selecting the total and static pressures are structurally combined (Figure 1.9). Such a device is called the Pitot differential tube. For such tubes manufactured in accordance with the standards ISO 3354-75, ISO 3966-77, GOST 8.361-79 and GOST 8.439-81, the coefficient kT = 1 + 0,0025 [18 ].

  

  The picture. 1.9 Pitot Differential Pipe

  The part of the tube parallel to the axis of the pipeline is called the end, and the part perpendicular to this axis is called the holder. The nose of the tube has a streamlined shape: conic, hemispherical or semi-ellipsoidal. The distance a from the beginning of the tube into the openings of the static pressure collection must be at least (6 ... 8) d, and the distance b from the holder to the same apertures is not less than (8 ... 14) d, where d is the outer diameter of the outer tube . This is necessary for the correct selection of the static pressure pc. Usually the total length of the end is in the range (15 ... 26) d. The diameter d1 of the hole for receiving the total pressure is (0.1 ... 0.4) d, and the diameter d2 of the hole for receiving static pressure is (0.1 ... 0.2) d, but not more than 1.6 mm , and the number of these holes must be at least six. The end with the holder is connected by an arc with radius R (3 ± 0,5) d or flush.

  In order to avoid the negative effect of the tube on the flow in the pipeline, it is desirable that the area s of the projection of the tube together with the holder on the surface perpendicular to the axis of the pipeline be no more than 2% of the cross-sectional area sT of the pipeline. If 2% s / sT <6%, then the result of measuring the difference in pressure will be overestimated. For s / sT>6%, use of pressure tubes is not recommended[18].

  The advantages of pressure devices are low pressure loss, the possibility of measuring in pipes and channels, the cross section of which is not circular, the availability of local velocity measurements in experimental and other works. The disadvantage is very low sensitivity at low speeds [18].

  1.7.2. Flowmeters with electromagnetic velocity converters

  The need for measuring fluid flow in large diameter pipelines led to the development of flowmeters with electromagnetic flow velocity converters. As such converters, magnetohydrodynamic (MHD) converters with a magnetic field localized in the area of ??the measuring electrodes are used. Such converters are simpler and cheaper than conventional electromagnetic flow converters, and with the increase in pipeline diameter, the significance of these advantages increases [18].

  The readings of MHD converters are generally determined not only by local flow conditions and the magnetic field at the measurement point, but also can depend on the velocity gradients V and the induction of the magnetic field B. This is due to the fact that the signal of the MHD converters is the potential difference, which is removed from its electrodes. According to Ohm's law, it is determined by the distribution of the electric current induced in the moving fluid in the magnetic field of the MHD converter:

  

  This current in turn depends on the nature of the velocity distribution and the magnetic field in the entire flow. Therefore, studies are needed to select the design parameters of MHD converters, in which it was possible to reduce this dependence to an acceptable minimum and to obtain a high-precision flowmeter.

  The advantages of MHD speed converters include the fact that they have small dimensions, minimally affect the measured flow, are technologically strong, have low metal consumption, are characterized by low energy consumption, are easily installed by means of standard inlets in continuous pipelines. The disadvantage is the same requirements for the length of the straight sections of the pipeline, and for pressure pipes [1].

  Conclusion

  An overview of the methods and means of measuring the flow of liquid showed a wide variety of devices developed and used today. At the same time, the analysis of each type of flowmeter revealed their characteristic features and disadvantages, which limit the use of these devices to measure the flow rate of liquid in large diameter pipes. Based on the analysis, the following conclusions can be drawn:

  1. The flowmeter of the variable pressure difference with the narrowing devices has a simple design, so that they are widely used in the flow measurement practice. But they are of little use for use in large diameter pipelines due to relatively large errors, large dimensions and weight, high material consumption of the narrowing devices, significant head losses and impossibility of mounting and dismantling without stopping the flow. Flowmeters with pressure devices require large lengths of straight sections before and after the installation of the primary converter.

  2. Traditional tachometric flowmeters are not used on large diameter pipes because of the large mass and large dimensions of the primary converter. Flowmeters with hydrometric turntables installed in the cross-section of the pipe at the point of medium speed, due to the presence in the hydraulic path of a permanently movable element, require constant maintenance. Otherwise wear of bearings and bearings of the moving element leads to the need for frequent re-calibration of the device.

  3. The use of vortex flowmeters with a body located in the flow in large diameter pipes hinders the coincidence of the frequency of free oscillations of the body with the frequency of the vortex shedding and the low efficiency of vortex formation at small values ??of the relative diameter of the body, as well as the unacceptability of its large values ??due to the bulkyness of the primary converter and a decrease in the frequency vortex formation. Similar to flowmeters with narrowing devices, they create significant head losses.

  4. Ultrasonic flowmeters monoblock design, have a relatively high accuracy, require the installation and dismantling of a complete stop flow in the pipe. Flowmeters with overhead radiators do not provide the required accuracy of measurements.

  5. Conventional electromagnetic flowmeters of large dimensions have significant mass, dimensions, material consumption and high cost, for their installation and maintenance, a complete flow stop in the pipe is required.

  6. A flowmeter with MHD velocity converters placed in the cross-section of the pipe at a point of medium speed combines both the advantages of conventional electromagnetic devices - relatively high accuracy, reliability and an output signal represented as an electrical quantity. And the convenience in operation on continuous pipelines, without requiring for installation and maintenance of the flow stop. Thus, flowmeters with an MHD converter most fully meet the requirements and, obviously, very promising.

  When writing this essay, the master's work is not yet complete. Final completion: May 2018.

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