A device for measuring flowing water. Instruments for measuring liquid flow

Story

The first water meter was invented by Carl Wilhelm Siemens in 1851. The counter had an vane design and, by means of a gear counting mechanism, transmitted the rotation of the impeller to the dial. The beginning of the use of water meters in Germany was recorded in 1858.

Principle of operation

The principle of operation of water meters (mechanical, tachometric) is to count the number of rotations of the impeller located inside the meter and rotating under the pressure of the water flow. The mechanism of the meters responsible for the accuracy of the readings is located in a separate part, which is isolated from the ingress of water into it.

According to the principle of operation, water meters can be divided into tachometric (the basis of the work is a turbine or impeller placed in a fluid flow, which is connected to a counting mechanism), vortex, ultrasonic, electromagnetic (used in industry) - differ from tachometric by the presence of electronic devices and the absence of moving parts. By design, they are divided into separate and compact. By the number of serviced pipelines, water meters are divided into single-channel, two-channel and multi-channel.

Standard cold water meters operate at a temperature of 40 ° C, hot water meters at temperatures up to 90 ° C, the water pressure level in them is 1 MPa. Water meters are used to record the amount of water consumption in apartments and businesses. Accordingly, depending on the power of heating and water supply systems, meters are individual and industrial. Water meters regularly show accurate readings at temperatures up to 60 ° C and relative humidity up to 98%.

Varieties

Single jet

This is a dry-running single-jet water meter, the principle of operation of which is based on measuring the number of revolutions of an impeller rotating under the influence of a single flow of water in the pipeline. The rotation of the impeller is transmitted to the counting mechanism by means of magnetic couplings. The counting mechanism of the dry-running counter is protected from water, which ensures long-term stability of measurements.

Advantages:

• the design of the device provides protection against an external magnetic field (antimagnetic protection of the water meter);
• all devices can be equipped with a pulse output, which provides the possibility of remote reading of readings (the pulse output module is installed inside the water meter housing).

Multi-jet

These meters differ from single-jet meters in that the water flow is divided into several jets before it hits the impeller blade. Due to this, the error of the turbulence of the flow is significantly reduced.

Advantages:

• minimum labor costs of dismantling and installation during periodic verification (only the upper easily removable part of the water meter is subject to verification);
• through additional adapter sleeves, the front panel of the meter is set to the level of the decorative surface (adapter sleeves of various sizes);
• all water meters can be equipped with a pulse output, which allows remote reading of readings (the pulse output module is installed inside the water meter case).

valve

The principle of operation of this dry-running meter is similar to the devices described above: the water flow through a special channel enters the flow chamber and is discharged further into the water supply system. The design of the device provides for the possibility of installing a valve inside the meter, which allows you to turn off the water. According to this function, the counter was called "valve"

Advantages:

• installation does not require complex and expensive work;
• the indicator part of the device can be rotated 360° (in three planes) for easy reading;
• all devices can be equipped with a pulse output, which provides the possibility of remote reading of readings (the pulse output module is installed inside the device case).

Turbine (Woltmann meters)

Mechanical meters for measuring the consumption of cold or hot water starting from a diameter of 50 mm for various types of water supply systems, automatic control systems, regulation and control of technological processes and other areas of activity that require accounting for consumed water. They are installed at the inlets of water supply systems of industrial enterprises, multi-storey buildings and in the water supply system. For the first time these counters were put into production in 1862, using the Woltmann principle.

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Class 42e, 2.) PATENT HA INVENTION

DESCRIPTION OF A DEVICE FOR MEASURING THE AMOUNT OF FLOWING LIQUID, To the patent of S. P. Skrylnikov, filed on March 14

1929 (filing certificate No. 42688).

The proposed device belongs to

r the number of those devices for measuring the amount of flowing liquid, in which spools controlled by an electromagnet are used, with the inclusion of electric meters in the circuit. The device works by successively passing liquid through two chambers - receiving and measuring, with the help of a float device and a spool that rises by the attraction of an electromagnet and descends with a break in the circuit due to its own gravity. The author believes that such a device can reliably take into account the amount of flowing liquid at the smallest flow rates and pressures.

The drawing depicts the device in a vertical section.

Liquid is poured into the receiving chamber 4 through the inlet pipeline 1 through the upper holes 18 in the spool 8 and in the walls of the device. When the cavity of the lower measuring chamber is emptied, the electric current from source 12 flows in this way: through an additional electromagnet 9, contact 15 of the lower float 7, fixed contacts 16, through contact 14 of the upper float 6, through electromagnet 10 and through electric meter 11.

The coil 10 pulls up the spool 8, which in the raised position closes the inflow and outflow of liquid, respectively, through pipes 1 and 2, communicating at the same time through holes 18 and recess 17 the upper receiving chamber 4 of the device with the lower measuring chamber 5; as a result, the last chamber will be filled with liquid that has overflowed into it from chamber 4.

Until the lower chamber is completely filled, the spool remains raised all the time, and only the floating float 6, having opened contacts 14 and 16, will interrupt the current: then the spool 8 will lower, and the float 7, having detached from the electromagnet 9, floats up. At the same time, the flow of liquid through the pipeline 1 will resume and flow out from the outlet pipe 9 below: the cycle of operation is repeated, each time receiving a mark in the electromagnetic counter 11 or in the counter of the number of odes attached to the spool.

To regulate the amount of flowing liquid, a conventional plunger 8 is used; screwing in or screwing out of which, respectively, changes the e 1 bone of the measuring chamber o.

P r e d i e t p a t e. n t a.

1. A device for measuring the amount of flowing liquid, equipped with a spool controlled by an electromagnet included in the electric meter circuit, characterized by the use of two electromagnet 10 located inside the measuring chamber o

"" in, Hydrogr. Uyr. Uzhravl. BA!. S and R;, b:: „l.:::. inar: d, alayan Gl, A and irylistva. at its different heights of floats b, 7, 1 equipped with supporting gontacts 24, 16, of which the lower float 7, made of magnetic material, is in the sphere of attraction located under it and included in the circuit of the same electromagnet 10 additional electromagnet. 9, 2. The form of execution described in and. 1 measuring instrument, different application, in order to change!

Ф chamber capacity o, conventional regulating plunger 8.

3. The form of implementation of the measuring device described in paragraphs 1 and 2. differing in that between the intake. opening of the measuring chamber and under. the leading pipeline 1 includes a receiving chamber 4, which communicates with the latter through a separate bypass channel - a spool in the lower position of the latter.

Water in aquatic breeding establishments is usually passed through either closed pipelines or open channels, and the volumes of water are quite significant. The cost of water supply directly depends on the volume of liquid being moved and the total head of the system created by the pumps. To avoid unnecessary water movements, it is necessary to know exactly how much water is going through pipelines and how much water should go. The various methods for determining water flow are outlined below.

Various flow measuring instruments can be classified according to different criteria. In this book, the following classification of devices for direct measurement is adopted: variable pressure flow meters; constant differential pressure flowmeters; different flow meters; counters for flow measurement in open channels.

Devices for direct flow measurement

Volume counters. For direct measurements of volume, volume meters are used. According to the principle of measurement, they can be divided into two groups: in the meters of the first group, the incoming liquid is measured in separate doses equal in weight; in counters of the second group - in separate doses, equal in volume. The number of doses displaced by the counter for a certain period of time is summed up. Based on the data obtained, the flow rate is determined. So, the amount of liquid passed through the counter, which works according to the volumetric principle, is calculated by the formula

where Q is the amount of liquid passed through the counter in one minute; V is the volume of the measuring chamber of the counter; n is the number of doses displaced by the counter per minute.

If the meter works according to the weight principle, the mass flow is determined by the formula

A volume counter with tilting chambers, working on the principle of counting weight doses (Fig. 10.20), consists of two chambers located one above the other, with the water inlet arranged above the upper chamber. The liquid enters the counter, fills the upper chamber and begins to overflow into the lower one. The filling of the lower chamber continues until the center of gravity shifts so much that the chamber loses its balance and topples over. At the same time, the accumulated water is drained. After complete emptying, the counter takes its original position. During tipping, the upper chamber fills with water and feeds it to the lower one when the latter takes its original position.

The piston counter (Fig. 10.21) refers to volumetric meters with forced displacement of liquid and works as follows. Water enters through the inlet into the chamber located to the right of the piston. The piston begins to move to the left, displacing the fluid that has accumulated in the chamber located to the left of the piston. Not reaching the extreme left position, the piston shifts the valve through which the liquid flows into the chamber lying to the left of the piston, at the same time, the hole connecting this chamber to the outlet of the device closes. Since the water pressure is now acting on the piston on the left side, it moves to the right, forcing water out of the right chamber through the outlet. Not reaching its extreme right position, the piston shifts the spool valve to the right, as a result of which a hole is opened connecting the inlet pipe to the right chamber. For one cycle of operation, the piston displaces a certain volume of liquid from the device. The number of piston movements is summed up by the counting mechanism, and the amount of liquid passed through the device is determined by multiplying the number of cycles by the volume of liquid displaced in one cycle of the piston. In industry, meters are used not with one, but with several reciprocating pistons, which ensures smoother operation. The accuracy of the readings of the device depends on the amount of fluid leakage between the wall of the measuring chamber and the working body. This leakage has a great influence on the meter reading error. If it is eliminated, volumetric counters with cylindrical pistons work with high accuracy, the error can be as low as 0.2-0.3% (Eckman, 1950) . Since meters of this type work on the principle of measuring volume, the density and viscosity of the liquid have practically no effect on the accuracy of measurements. Meters with reciprocating pistons are used in various water supply systems with a mass flow rate from 37 to 3785 l / min. However, their use is limited by special requirements for the liquid, which must not be corrosive and not too viscous (Eckman, 1950).

Counter with disc piston. To measure the amount of fluid flowing through the system, widely used counters with disk | | piston (Fig. 10.22). The widespread use of these meters as water meters is explained by their simple design, compactness, and relatively low cost. In the center of the counter, a ball is installed in a spherical seat, on which a flat disk is fixed. During the operation of the counter, the ball, together with the disk, sways in a spherical seat around a common geometric center, but does not rotate. Under the action of the pressure of the liquid entering the device through the inlet, the disk lowers or rises depending on its position relative to the inlet pipe. When the liquid flows through the counter chamber, the disk plane shifts along the walls and the ball, together with the disk, rotates in its seat. Under the action of the pressure difference in the inlet and outlet nozzles, water flows around the ball with the disk and is directed to the outlet nozzle. During the movement of the ball, the protrusion on its upper part moves along the surface of the cone, the top of which coincides with the center of the ball. Since the inlet and outlet are separated by a partition (not shown in Fig. 10.22), the water must flow through the inlet and around the ball, all the while remaining under the disk. The axis of the disc actuates a counting mechanism that registers the number of movements of the ball with the disc. This number, multiplied by the volume of liquid displaced in one cycle, allows you to determine the volume of liquid that has passed through the device. The meters described can operate at any pressure and temperature of the measured medium. However, the accuracy of the readings can be affected by the density and viscosity of the liquid, since liquid leakage through gaps is possible in instruments of this design. With flow rates from 55 to 1890 l/min, the relative error of the oscillating disc meters! usually does not exceed 1%.

Rotary counter with straight blades. A schematic diagram of a rotary counter with straight blades is shown in fig. 10.23. The main element of the device is a rotor mounted eccentrically in the housing, equipped with blades. When the rotor rotates, the blades under the action of the springs always remain pressed against the inner surface of the housing. Flowing through the meter, the liquid presses on the blades and sets the rotor in rotation, which in turn distills the liquid to the outlet pipe. The number of revolutions of the rotor is fixed and determines the volume of liquid passed through the counter. The density and viscosity of the liquid do not affect the measurement accuracy of rotary counters, since this design is characterized by minimal leakage of the measured substance. The relative measurement error of rotary counters with "straight blades" does not exceed 0.2-0.3%.

Variable pressure flowmeters

Using the Bernoulli equation and the jet continuity equation, it is possible to establish a mathematical relationship between the flow rate of an incompressible fluid and the pressure drop:

The flow coefficient C takes into account the loss of liquid flow in the meter, and the flow coefficient K is the product of C and M:

Venturi nozzles. On fig. 10.25 shows a Herschel Venturi nozzle. A standard Venturi nozzle consists of a tapered inlet L 1 , a middle portion, the so-called neck, L 2 with a minimum cross section and a smoothly expanding outlet L 3 . The profile of the inlet and outlet parts of the nozzle is selected in such a way that the head loss is minimal. Since the fluid flows at maximum speed through the nozzle throat, the static pressure in the constriction will be less than the pressure before the constriction. The selection of pressure values ​​is carried out in the area of ​​the greatest expansion of the inlet part of the nozzle and in the neck. The measured differential pressure is converted to flow rates using Equation (10.13).

Knowing the diameter of the pipeline in which the Venturi nozzle is installed, and the diameter of the nozzle neck, the value of the constant M can be calculated. The flow coefficient C is usually taken from tables or a graph (Fig. 10.26), while the flow coefficient is determined as a function of the Reynolds number. With sufficiently large Reynolds numbers, starting from the value of 2.5·10 5 , the flow rate becomes! permanent. The C values ​​lie on the solid line. The dotted curves limit the range of C values. The flow coefficients are determined for nozzles installed in pipes with a diameter of 5.08 cm or more, and p values ​​in the range of 0.3-0.75 (β is the ratio of the areas of the openings of the mouth of the Venturi nozzle and the pipeline). Unfortunately, there is very little data for low Reynolds numbers and for pipes with a diameter of less than 5.08 cm. However, this is not an obstacle to the widespread use of flow meters with a Venturi nozzle and other variable pressure flow meters, since the theoretical method is extremely rarely used in flow measurement technology. . Usually, in practice, the pressure is measured with a manometer, and the flow rate corresponding to each pressure difference is determined either by the method of direct volume measurement, or by another pre-calibrated measuring device. Thus, points are obtained for plotting the pressure drop versus flow rate. When measuring the flow, it is enough to determine the pressure drop and find the corresponding flow rate from the graph.

Normal nozzles. On fig. 10.27 schematic diagrams of two normal nozzles are presented. Normal nozzles, like Venturi nozzles, operate on the principle of measuring a variable differential pressure. Since the ratio of inlet to outlet diameters is larger for normal nozzles, they cause a greater pressure loss compared to Venturi nozzles due to a significant increase in turbulence. However, normal nozzles have the advantage over venturi nozzles in that they require less space and can be installed between pipeline flanges.

Typically, pressure is taken from normal nozzles at three points. When pressure is tapped using separate holes in the pipeline (see Fig. 10.28), the high pressure tapping point is separated from the nozzle inlet at a distance equal to one pipeline diameter, and the low pressure tapping point is taken above the nozzle outlet at a distance of one pipeline diameter from the inlet nozzle opening for nozzles with a high β value (β>0.25) or one and a half nozzle throat diameters from the nozzle inlet for nozzles with a low β ratio (β
In the normal nozzle shown in Fig. 10.29, pressure tapping holes are drilled in the throat of the nozzle. High pressure is taken at a point separated from the nozzle inlet by a distance equal to one pipeline diameter. A hole for low pressure sampling is drilled in the wall of the nozzle mouth at a distance of 0.15 of the throat diameter from the nozzle outlet. This pressure tapping method makes it possible to control the actual pressure inside the nozzle chamber. Holes drilled in the mouth of the nozzle are useful if the nozzle is in communication with the atmosphere.

On fig. 10.30 shows two ways of taking pressure from the narrowing device at the interface between the nozzle flange and the inner surface of the pipeline. In the diagram at the top of Fig. 10.30 shows an annular chamber communicating with the internal cavity of the pipeline with an annular slot (width not more than 0.02D) or several holes evenly distributed along the circumference of the pipeline. This arrangement of the annular chambers allows the impulse tubes to be drilled directly through the pipeline wall. The second way (see bottom of figure 10.30) is to drill the holes for the impulse tubes at an angle to the pressure tap. Dimensions; holes and the angle of inclination are selected so that the diameter of the inlet part of the hole after finishing does not exceed 0.02 of the inner diameter of the flange.

The method of pressure extraction through holes drilled in the nozzle throat is used relatively rarely, which is explained by: the complexity of laying connecting pipes between the low-pressure chamber and the differential pressure gauge. In addition, such measurements place high demands on the cleanliness of the surface of the pressure taps, since the flow velocity in them reaches its maximum value and the slightest roughness can lead to significant errors in pressure measurements. The method of pressure sampling using impulse tubes installed at an angle is characterized by the largest error compared to the other methods considered. In addition, in this case, the axial length of the hole plays a significant role. The easiest way is to install impulse tubes passed through the holes in the pipeline. It is this method of selection: pressure is most often used in engineering practice.

To determine the flow rate, use the formula (10.14).

The theoretical method for determining the flow rate using formula (10.14) is used very rarely. The calibration of normal nozzles on the test stand is done in the same order as the graduation of Venturi nozzles.

Normal diaphragms. A normal diaphragm is a thin flat disc with a concentric hole. The diameter of the pipeline in which the diaphragm is installed must be greater than the diameter of the diaphragm opening (Fig. 10.32). The fluid flow passed through the pipeline enters the diaphragm, which narrows its cross section. Since the flow velocity in the orifice is greater than in the pipeline, the static pressure in the restricted section will be less than the pressure in the pipeline before the orifice. This differential pressure can be converted into velocity or flow rate values.

Diaphragms are concentric, eccentric and segmented. In concentric diaphragms, the axes of the opening of the diaphragm and the pipeline coincide. In eccentric diaphragms, the axis of the meter is the same as the diameter of the pipeline. - Segmental and eccentric are a segment of a circle of approximately the same diameter as the diameter of the pipeline. Segmental and eccentric diaphragms are used only in special circumstances requiring special conditions (for example, complete drainage of the pipeline), therefore, these diaphragms are not considered below.

There are five different ways to take pressure from normal diaphragms.

1. Impulse pipes are led through the flanges. In this case, the axis of the flange outlet on the high pressure side should be at a distance of 2.54 cm from the front surface of the diaphragm, and the axis of the flange outlet on the low pressure side should be at a distance of 2.54 cm from the opposite surface of the diaphragms (see the lower part Fig. 10.32).

2. Pressure is taken at points separated from the diaphragm by distances equal to one diameter and half the diameter of the pipeline. On the high pressure side, the distance between the axis of the impulse tube and the front surface of the diaphragm should be equal to one pipeline diameter, and on the low pressure side, half the pipeline diameter from the same diaphragm surface. These distances remain constant for all values ​​(see top of Figure 10.32).

3. The impulse tube is brought to the narrowed flow section at the shortest distance from the back surface of the diaphragm. High pressure is taken in the daughter; spaced from the front surface of the diaphragm at a distance equal to 1/2-2 pipeline diameters; usually this distance is taken equal to one-diameter of the pipeline. For low pressure sampling, the impulse tube is inserted into the narrowed flow section at the point of minimum pressure; the nature of the change in the static pressure behind the diaphragm is expressed by the curves shown in fig. 10.33.

4. Impulse tubes are brought to the points of conjugation of the pipeline with the diaphragm. The pressure is taken both before the diaphragm and after it, at the junctions of the inner wall of the pipeline with the diaphragm disk. Options for connecting impulse tubes with this method of measurement are shown in fig. 10.30. For all types of narrowing devices, these options are the same.

5. Impulse pipes installed along the pipeline. In this case, the pressure is measured at those points on both sides of the diaphragm where the flow is steady. In fact, this is how the value of the non-recoverable pressure loss in the diaphragm is determined. Pressures are taken at a distance of 272 pipeline diameters before and 8 pipeline diameters after the front surface of the diaphragm. This method of pressure sampling is used relatively rarely, since the pressure drop measured in this case reflects flow changes to a lesser extent compared to the other listed methods. Hence the large error in the readings during measurements.

The calculated flow formula for concentric orifices is as follows:

The relationship between Q and P 1 -P 2 for an orifice operating under specific conditions can be determined on a test bench using another direct volume measurement device, as described above for the Venturi nozzle. Graph obtained during calibration; the dependence of the pressure drop Р 1 -Р 2 on the flow rate Q is used for practical measurements.

Comparative analysis of Venturi nozzles, normal nozzles and orifices. On fig. 10.35-10.37 shows curves of distribution of static pressure built on the basis of experimental data when normal nozzles, Venturi nozzles and normal diaphragms are installed in the pipeline. The largest pressure drop is noticeable for the diaphragm, the minimum for the Venturi nozzle and the average for the normal nozzle. The greater the pressure drop, the greater the energy loss associated with vortex formation and flow friction against the pipeline walls. Thus, non-recoverable pressure losses in the Venturi nozzle are much less than in nozzles and diaphragms. On fig. 10.38 shows pressure loss curves for normal orifice devices, expressed as a percentage of the pressure drop value, as a function of β, the ratio of the diameters of the nozzle throat or diaphragm opening and the pipeline. As expected, for all types of narrowing devices, the pressure loss is the smaller, the larger β, since as β increases, the speed and turbulence of the flow decrease. The above graphs also show that the pressure loss in the Venturi nozzle is much less than in nozzles or diaphragms, which is; main advantage of the Venturi nozzle.

Venturi nozzles are characterized by high measurement accuracy and do not require frequent calibration like conventional nozzles or diaphragms, as they are more wear resistant, which is especially important when working with liquids containing mechanical impurities. However, venturi nozzles require significantly more installation space and are more expensive. In terms of cost, wear resistance, the nature of the distribution of static pressure and the required length of the straight section of the pipeline, normal nozzles occupy an intermediate position between Venturi nozzles and diaphragms. An important condition for obtaining good results is also the careful installation of normal nozzles in pipelines. Diaphragms are relatively easy to install and do not require a long straight section of pipeline, but they wear out quickly and need frequent calibration. Due to low mechanical strength, they often fail under sudden pressure changes. At the same time, diaphragms are cheaper than all the constriction devices considered, which has led to their widespread use.

Centrifugal flow meters. Curvilinear sections of the pipeline, in which the action of centrifugal forces in the fluid flow is manifested, can also be used for flow measurement. Under the action of centrifugal forces, the flow is squeezed out to the outer wall of the curved section, in connection with this, the pressure on the outer wall of the curved section will be greater than on the inner one. The pressure difference measured at two points in the flow cross section can be converted into velocity values. On fig. 10.39 and 10.40 schematically show flowmeters operating on this principle. One of them is made on the elbow of the pipeline, and the other is a loop-shaped tube. An angle flow meter has become more widespread because it is easier to manufacture, never clogs, and can operate for a long time without recalibration to the required accuracy. The latter is explained by the increased wear resistance of the angular flowmeter. Impulse tubes for pressure sampling are located along the common axis of symmetry of the curved sections of the outer and inner walls of the elbow (see Fig. 10.39).

Pitot tubes. Pitot tubes are also among flowmeters operating on the principle of measuring variable differential pressure. As a rule, they are used for gas flow measurements, but Pitot tubes can also be used for liquid flow measurements. Pitot tube consists of two chambers (Fig. 10.41) - internal and external. The inner chamber with its open end faces the flow of the measured substance; an opening is provided in the outer chamber, the axis of which is perpendicular to the direction of the moving flow. The pressure in the inner chamber of the Pitot tube is the sum of the static and dynamic flow pressures (full head); only static pressure is measured in the outer chamber. The pressure drop measured across the two chambers is actually dynamically driven by the flow pressure and is related to the flow velocity.

Mathematically, the total pressure P t is the sum of the dynamic pressure P d and the static pressure P S:

Mass and weight are related as follows:

After performing simple transformations, we get

The kinetic energy of the flow is equivalent to the dynamic pressure. Therefore, equation (10.16) can be written as follows:

Typically, Pitot tubes are made of small diameter in order to minimize the influence of the inhomogeneity of the medium being measured. Pitot tubes measure the velocity at any point in the flow cross section, and the flow velocity varies across the cross section, so the average flow velocity is determined, which is usually about 0.83 of the maximum velocity (Beckwith and Buck, 1961). The Pitot tube is installed along the axis of the pipeline and the flow velocity is measured in the center of the section. Multiplying this value by 0.83 (correction factor), the average flow velocity is obtained, which is substituted into the continuity equation. The solution of the system of equations gives the flow rate.

Pitot tubes must be installed against the moving flow so that they respond to dynamic pressure. The angle between the axis of the moving flow and the axis of the pitot tube (angle of deflection) must be zero, otherwise significant errors will occur.

Variable differential pressure flow measurement devices have been discussed above for incompressible liquids such as fresh or salt water. All of them can also be used to measure compressible media, such as air, but in this case, a correction factor is introduced into the working flow formula that takes into account the effect of compressibility when air passes through the constriction device. Consideration of compressible fluids was not part of the author's task, so readers interested in this issue can refer to the work published by the American Society of Mechanical Engineers “Flowmeters. Theory and Application” (1959).

Constant Differential Pressure Flowmeters

Rotary flow meter. Among the devices operating on the principle of measuring flow at a constant differential pressure is a rotary flow meter. In this case, the cross section of the flow is variable, and the pressure drop remains constant at all flow rates. According to the method of transmitting readings, the rotameter shown in Fig. 10.42 refers to rotameters with direct readout on a linear scale. The device consists of a vertical, conically expanding transparent tube and a “float” moving freely in it. Since the density of the "float" material is greater than the density of the liquid, the name "float" is arbitrary. The tube of the device must be installed strictly vertically. The flow of the measured substance enters through a narrow inlet section of the tube and passes from the bottom up. Two forces act on the float: its gravity and the lift due to the action of the flow. The float rises until these forces are balanced. Starting from this moment, the float hangs at a certain height. A scale is applied to the surface of the tube, which makes it possible to determine the exact position of the float relative to the beginning of the scale. Since the height of the float is a measure of flow, the scale can be calibrated directly in liters per minute or in other flow units, however, the method of grading the scale in dimensionless units from 0 to 100 is more commonly used, which are converted to actual flow values ​​using calibration curves.

Mathematically, the flow rate of a fluid passing through a rotameter can be expressed as follows (Schoenborn and Colburn, 1939):

Index 1 refers to the liquid, index 2 to the float.

The value of the flow coefficient C should be determined empirically for the specific liquid or gas with which the rotameter will be operated. Calibration of rotameters can be done on a test meter with flow measurement by direct measurement or using another calibrated flow meter, as described above for the Venturi nozzle. Built the calibration curve is the dependence of the height of the position of the float, observed on the scale of the rotameter, on the flow rates within the required measurement limits.Usually for rotameters, this dependence is expressed by a straight line.Next, determine the position of the float on the scale of the instrument and, using the calibration curves, set the appropriate flow rate.

A necessary condition for obtaining reliable measurements is a strictly vertical installation of the rotameter. Rotameters cannot be used to measure the flow rate of liquids with a high content of mechanical impurities, especially large sizes, as well as for opaque liquids. Instruments for measuring the flow of liquids with high temperature and pressure are very expensive. However, rotameters have many advantages over other flowmeters. These include: the convenience of a linear scale covering the entire measuring range of the instrument, and a constant pressure drop across all flow rates. The measurement limits of the device are easy to change, for this it is enough to take another tube or float. Rotameters, in particular, are convenient for measuring the flow rate of corrosive liquids, such as salt water, since the surfaces in contact with the measured substance can be made of any material, such as glass, plastic, etc. The float is made either entirely from -metal, or covered with a plastic shell on top. The use of corrosion-resistant materials increases the cost of the device. During operation, you can monitor the flow.

Immersed Piston Flowmeter

Special flow meters

The method of flow measurement using hot-wire anemometers is quite convenient and provides high measurement accuracy. However, its scope is limited due to the extreme fragility of the heated wire. Wire-wound thermo-anemometers are primarily intended for measuring the flow of gases and only in exceptional cases are used for measuring the flow of liquids.

Turbine meters. The instrument kit includes an impeller or propeller and a counting device that converts the rotational speed of the impeller into impulses (Fig. 10.44). The speed of rotation of the turbine is proportional to the speed of the measured flow, since the blades are mounted on its body at a certain angle to the axis of rotation, and the axis of rotation "of the turbine coincides with the direction of flow. Figure 10.45 shows an industrial sample with tubular flow rectifiers and electromagnetic devices that perceive the rotation of the turbine. This device is suitable for measuring flow rates in large diameter pipelines, in open channels, rivers, as well as for measuring the speed of currents in oceans and lakes.There are many varieties of turbine meters, from cup-type instruments used by meteorologists to determine wind speed, to the example shown Figure 10.45 For flow measurements in open channels, rivers, lakes and oceans, a modification of this sample is used, which is equipped with a plate rigidly attached to the outer surface of the flowmeter parallel to the axis of rotation of the impeller.The purpose of this simple device is to hold the flow meter in a certain position, when the axis of rotation of the impeller is parallel to the flow. Under the action of the flow, the plate constantly rotates, trying to take a position in which its resistance to flow will be the least.

Turbine flowmeters have found wide application in measurements in non-stationary conditions, since, providing sufficient measurement accuracy, they are mechanically durable, easy to operate and do not require complex recording instruments. Another advantage of this device is its low cost. The measurement error of industrial devices does not exceed 0.5% of the upper limit of measurements.

Electromagnetic flowmeters The principle of electromagnetic flowmeters (Fig. 10.46) is that the moving medium, which must have at least a minimum electrical conductivity, is considered as a conductor moving in a magnetic field. The pipeline is installed in a magnetic field in such a way that the direction of flow is perpendicular to the lines of the magnetic field. The EMF induced in a liquid is directed perpendicular to the magnetic field lines and the liquid flow. EMF is removed by two electrodes, which direct the received signal to a device that measures the potential difference.

According to Faraday's law, the value of the induced emf

Since the medium itself is considered as a moving conductor, the EMF induced in the liquid is proportional to the flow velocity.

There are two main modifications of the electromagnetic flowmeter. In one of them, a liquid with low electrical conductivity is passed through a pipeline made of glass, plastic, or other non-conductive material. The electrodes are built into the pipeline walls and are in direct contact with the liquid. Devices of this type produce a weak signal that requires amplification. The second option, unlike the first one, provides for the placement of electrodes on the outer wall of the pipeline, which is made of an electrically conductive material. In this case, the liquid must also have high electrical conductivity (for example, liquid metal) - a condition necessary for the operation of this type of flow meters. In this system, there is no direct contact between the liquid and the electrodes. The use of the device does not require re-equipment of the existing pipeline and does not cause any technical difficulties during installation. Typically, the output signal of such a flow meter is the greater; the higher the electrical conductivity of the measured liquid, and can be transmitted directly to the recording device without pre-amplification.

The main disadvantage of electromagnetic flowmeters of all types is their high cost. However, this disadvantage is compensated by the reliability of the device, in which there are no moving parts. The accuracy of measurements provided by flowmeters of this type is quite high.

Ultrasonic flow meters. These meters use 100 Hz ultrasonic vibrations (Beckwith and Buck, 1961). Piezoelectric or magnetostrictive elements are mounted on the pipeline at intervals of several centimeters, serving one as an ultrasound emitter, the other as a receiver. Ultrasonic waves travel through a liquid at different speeds depending on whether the directions of sound propagation and liquid flow coincide or are opposite. The phase difference of the oscillations coming from the receivers recorded by the sensor is proportional to the fluid velocity. The sensitivity of the circuit can be increased by automatically replacing the functions of a pair of piezoelectric elements with opposite ones. The fast periodic change in the functions of a pair of emitter and receiver (up to 10 times per second) provides the ability to measure the Phase Shift of ultrasonic vibrations directed simultaneously upstream and downstream. The output pulse of the frequency difference of ultrasonic vibrations is doubled compared to the main circuit for the same flow rate.

Flow measurement in open channels

weirs. A barrier placed in the way of the flow of water through which the overflow of water occurs is called a spillway. It can have a cutout of various shapes. On fig. 10.47 shows one of the weirs. Since weirs are used exclusively in open channels, they can only be used to measure the flow of liquids. Most weirs in engineering practice serve to measure the flow of water, and only a few of them, as a rule, in laboratory conditions, are used to measure the flow of other liquids.

The types and designs of weirs are very diverse. Weirs with a sharp edge (i.e. weirs, along the perimeter of the cutout of which a metal sheet with a sharp edge is fixed) according to the shape of the hole in the wall are divided into weirs of rectangular, triangular (V-shaped), round and special sections. Special spillways include trapezoidal and parabolic sections. These profiles ensure that the flow rate is constant or that the flow rate is directly proportional to the head.

On fig. 10.48 shows the main dimensions of the weir. The weir sill (or crest) is the underside of the weir cut. The threshold length L is measured as the distance between the side walls of the slot (see Fig. 10.48). For a rectangular section, the length of the threshold is equal to the width of the weir cut. In a spillway with a triangular cross section, the threshold length approaches zero. Weir static head h is the distance from the crest of the weir to the highest level of the free water surface, measured above the weir (see Fig. 10.48), since the free surface begins to decline even before the weir.

The flow of water passed through the weir is called a flat stream behind the weir. With sufficient flow and a drop between the crest of the spillway and the horizon in the downstream, the space under the jet communicates with the atmosphere. Such a jet is called free or unflooded. The value of head for a free jet is determined by a number of factors, including the sharpness of the edge of the weir, the thickness of the crest, etc. It has been established that this value should be in the range from 1 to 3 cm (ASME, 1959). If the distance between the crest of the sill and the horizon at the downstream of the weir is insufficient, the space under the jet is isolated from the atmosphere and the jet sticks to the wall of the weir. Such a jet is called stuck or flooded.

If the length of the weir is less than the width of the channel Lk (see Fig. 10.48), such a weir is called a weir with lateral compression, and the flow passed through this weir is called a compressed stream. In a compressed flow, the direction of movement of fluid particles of the extreme streamlines flowing to the weir cutout from the side walls of the channel is measured. In this regard, when the liquid flows through the weir, a lateral deformation of the flat jet occurs immediately behind the weir, or “flow compression”. Since the flow compression is reflected in the flow rate, it is taken into account in the calculations by the appropriate correction. It is possible to ensure that the flowing edge streamlines do not create compression of the flow cross section. This is possible provided that the difference between the channel width L c and the threshold length L w is at least 4 times the maximum expected head. Mathematically, this condition can be expressed by the following formula:

The deformation of the cross section of the flow in the vertical plane and some other factors are taken into account by the dimensionless coefficient C, which is introduced into the formula for determining the theoretical value of the flow rate and is the ratio

Thus, the working flow formula for a rectangular spillway takes the form

This measurement method for a rectangular weir has two limitations. First, at too high flow rates, the increase in flow velocity begins to be significantly reflected in the head value, therefore, the head value measured on the spillway must be corrected for the dynamic head v 2 / 2g (v is the flow velocity in the channel), which is added to water pressure. Second, the rectangular weir sill should be at least 15 cm long (ASME, 1959). At smaller values ​​of the threshold length, mixing of the incoming lateral streamlines with each other is observed. At too low flow rates, which make it difficult to freely overflow the liquid in rectangular weirs with a threshold length of 15 cm, it is preferable to use triangular weirs, which in such cases provide better results.

The flow formula used for practical calculations is obtained from equation (10.27) taking into account the coefficient C, which includes the constants (2/3 and √ 2g):

Equation (10.29) is the basic flow formula for a rectangular weir, obtained without taking into account the lateral compression of the jet section (i.e., provided that the length of the threshold is equal to the width of the channel). In engineering practice, to correct this factor, it is assumed that the effective length of the weir threshold is less than the actual length by 0.1h on each side. Thus, for a spillway with bilateral lateral compression, the effective length of the threshold L w is 0.2h less than the actual length. The last condition is entered into the flow formula (10.29), which now in its final form will look like this:

Flow formula for a triangular weir with a right angle at the top

Advantages and disadvantages of weirs. The main advantages of weirs include: high measurement accuracy; simplicity of design and minimum maintenance; mechanical impurities of small sizes can freely pass through the weir without affecting the flow rate; long service life.

Weirs have the following main disadvantages: significant pressure losses in the system; the possibility of clogging with large inclusions, which affects the consumption characteristics and requires cleaning, which is usually done manually; decrease in measurement accuracy when changing the shape of the canal bed to the spillway or with a significant accumulation of alluvial sediments.

Flow Depth Measurement. To determine the flow rate using weirs and flumes, it is necessary to determine the depth of the flow. It is measured at a distance of at least 4h from the front wall of the weir, i.e. before the start of lowering the surface level. Usually, a hook depth gauge is used to measure depth, since this device is highly accurate. The hook of a depth gauge (preferably with a blunt cone) connected to a movable scale is lifted out of the water until its end appears on the surface of the water. A moving scale that moves along a fixed depth indicator shows the depth at the measurement point. At greater depths, you should use a modification of this device, which differs in that the depth indicator, in turn, is equipped with a vernier, which allows you to increase the accuracy of measurements.

There are several other varieties of depth gauges, both with direct readings and working in conjunction with recording devices. The measuring kit includes a level sensor - a conventional float or a device that is sensitive to pressure changes, a scale of indications or a recorder and a clock mechanism (for a recording type device). Level sensors have been described in detail above.

Since the liquid is in continuous motion in channels with weirs or flumes, it is often advisable to use special chambers in which the liquid will be at rest when measuring depth. The calming chamber is a piece of pipe or a box connected by an opening to a moving stream. Inside the calming chamber, the water rises to a level corresponding to the depth of the stream. The small surface area contained within the calming chamber is immobile, allowing high accuracy depth measurements. This method of measurement gives good results if the surface area inside the stilling chamber is about 100 times the area of ​​the opening connecting this chamber to the moving stream (Israelsen and Hansen, 1962).

Weir operation. The width of the channel and the depth of the channel in front of the weir or in the weir box should be sufficient so that the flow velocity at the approach to the weir does not exceed 15 cm/s. The spillway box is installed in such a way that its center line is parallel to the direction of flow. The spillway is installed strictly vertically with a sharp edge towards the overflowing stream. The distance between the lower edge of the weir cutout and the channel bottom should be within 2-3h, and for weirs with bilateral lateral compression, the distance from the side edge of the weir cutout to the side wall of the channel should be at least 2A. To obtain good results, it is necessary that the water depth above the weir crest be at least 5 cm. In weirs of rectangular and trapezoidal sections, the value of h should not exceed one third of the threshold length. Depending on the type of falling jet, different methods are used to determine the flow rate. The water jet behind the weir will appear as a free jet under all flow conditions, unless the weir is designed specifically to produce a flooded jet. The depth gauge scale must be adjusted so that its zero mark coincides with the threshold level. This can be done using a carpenter's level or level. During the operation of weirs, it is necessary to monitor the condition of the canal bed after the weir and maintain its original shape.

Water trays. Parshell flumes. The method of measuring the flow rate with Parshell flumes is based on measuring the amount of water flowing through the narrowed section of the channel, while the static head is partially transformed into dynamic. The Parshell flume reduces the cross section of the flow in the horizontal direction, while at the same time there is a section with a slope in the bottom of the flume (Fig. 10.53). Static head is measured in still chambers A and B. Under conditions of free fluid flow (i.e. when the static head in chamber B is 60% or less of the static head in chamber A), good results can be obtained by measuring the static head only in chamber A. In table. Figure 10.5 shows the flow rates for various static heads in chamber A under the condition of free flow of liquid in the Parshell flume. If the pressure in the lower chamber B is 70% or more, this will distort the measurement in the upper chamber. At the same time, sufficiently high accuracy can be achieved even with flooding values ​​up to 90% if the static head is measured in both chambers L and B and a correction is made to the value obtained in chamber A. The correction values ​​are published in special tables (Israelsen and Hausen, 1962).

Several factors influence flume measurement accuracy, including proper selection and installation, level of maintenance, and static head measurement accuracy. The choice of a water flume involves determining its size depending on the specific conditions of use. When solving this problem, the maximum and minimum flow rates and the maximum allowable static head loss are given, which is a function of the hydraulic slope of the channel and the freeboard height (i.e., the distance from the water level to the upper edge of the channel wall). The flow movement must meet the requirement of free fluid flow.

Example 10.1. Choice of Parshell flume. Select a flume for a flow rate between 0.2 and 1.5 m 3 /s, provided that the maximum head loss is 18 cm and the flow pattern meets the requirement for free flow of liquid. The maximum allowable depth in the channel is 60 cm.

Solution. Since the maximum allowable flow depth in front of the water meter flume is 60 cm, the static head h a measured in this section of the flow cannot exceed 60 cm. According to Table. 10.5 it can be found that with a head of 60 cm or less and a flow rate of 1.5 m / s, a flume with a mouth width of at least 180 cm is required.

It is desirable to maintain the mode of free flow of liquid. For this, it is necessary that the degree of flooding of the lower chamber does not exceed 60% of the flooding of the upper chamber; in other words, the head loss must be at least 40% of the static head ha measured upstream. Due to the hydraulic slope of the channel and the requirements for the water surface, the maximum head loss should not exceed 18 cm. ).

Below are the values ​​of the width of the mouth of the water flume depending on the value of the static head in the upstream for the maximum flow rate (1.5 m 3 /s).

The depth of water measured in the upper chamber for the selected water flume should not exceed 60 cm. Therefore, the height of the sill will be 60 cm - head loss at maximum flow = sill height;

60-16.8 \u003d 43.2 cm from the bottom mark of the bottom of the tray.

It is desirable to have a freeboard upstream of the canal. Sometimes the height of the threshold is reduced for this, but the threshold should not be lowered too much, as this can lead to a violation of the free flow of the liquid.

The industry produces water-measuring trays of Parshell in standard sizes. They are usually made of fiberglass or other similar materials. However, sometimes a Parshell flume needs to be made on site. In table. 10.6 and in fig. 10.54 and 10.55 show all standard sizes of Parshell flumes. They can be made of concrete, brick, wood, metal or other materials. Particular attention in the construction of trays must be paid to the observance of the main dimensions.

Trapezoidal flumes. The schematic diagram of this type of flume is shown in fig. 10.56. The tray is an artificially narrowed part of the channel with a trapezoidal cross section and a flat bottom. As a result of the narrowing of the cross section of the flow, its velocity in this section increases. The head loss in a flume is directly proportional to the velocity of the flowing substance, therefore, the head loss can serve as a measure of flow.

Indications for this type of flume flume do not depend on the state of the water surface on the way to it. This makes it possible to measure flow rates that fluctuate over a wide range with a relatively small head loss. Unlike rectangular water meter flumes, trapezoidal water meter flumes do not require high manufacturing precision. At the same time, the measurement accuracy of trapezoidal flumes is somewhat lower, which is explained by a relatively small pressure drop. The main advantage of this type of gutter flume is that its cross-sectional shape coincides with the main cross-sectional shape of most open channels.

The flow rate of a trapezoidal flume is determined by the formula (Robinson and Chamberlain, 1960)

The coefficient C depends on the type of the flowing liquid, the geometric shape of the flume, the speed and depth of the flow. In this regard, formula (10.33) has limited practical application. Trapezoidal flumes must be individually calibrated for specific application conditions.

Bibliography

• ASME - American Society of Mechanical Engineers (1959). Instruments and Apparatus. Part 5, Measurement of Quality of Materials. Chapter 4, Flow Measurement. Supplement to ASME Power Test Codes.
• Beckwith, T. G. and N. Lewis Buck (1961). mechanical measurements. Addb son-Wesley, Reading, Mass.
• Christiansen, J. E. (1935). Measuring Water for Irrigation. California Agr L cultural Experiment Station Bulletin 588.
• Eckman. Donald P. (1950). Industrial Instrumentation. Wiley, New York.
• Engineering Field Manual for Conservation Practices (1969). U.S. Department of Agriculture, Soil Conservation Service, Washington, D.C.
• Fluid Meters, Their Theory and Application, 5"th ed. (1959) Report of the American Society of Mechanical Engineers Research Committee on Fluid Meters. American Society of Mechanical Engineers, New York.
• Frevert. Richard K., Glenn O. Schwab, Talcott W. Edminster, and Kenneth K. Barnes (1962). Soil and Water Conservation Engineering, 3rd printing. Wiley, New York.
• Fribance, Austin E. (1962). Industrial Instrumentation Fundamentals. McGraw Hill, New York.
• Israelsen. Orson W. and Vaughn E. Hansen (1962). Irrigation Principles and Practices. Wiley, New York.
• King, Horace W., Chester O. Wisler, and James G. Woodburn (1948). hydraulics. Wiley, New York.
• Norton, Harry N. (1969). Handbook of Transducers for Electronic Measuring Systems. Prentice Hall, Englewood Cliffs, N. J.
• Parshall, R. L. (1950). Measuring Water in Irrigation Channels with Parshall Flumes and Small Weirs. U.S. Department of Agriculture, Circular No. 843, Washington, D.C.
• Robinson, A. R. (1959). Trapezoidal Measuring Flumes for Determining Discharges in Steep Ephemeral Streams. Colorado State University Research Foundation, Civil Engineering Section. Fort Collins.
• Robinson, A. R. (1968). Trapezoidal Flumes for Measuring Flow in Irrigation Channels. Agricultural Research Service Publication ARS 41-140, Washington, D.C.
• Robinson, A. R. and A. R. Chamberlain (1960). Trapezoidal Flumes for Open Channel Flow Measurement. Transactions of the American Society of Agricultural Engineers 3(2): 120-124.
• Schoenborn, E. M. and A. P. Colburn (1939). The Flow Mechanism and Performance of the Rotameter. Transactions of the American Institute of Chemical Engineers 35(3): 359.
• Streeter, Victor L. (1962). fluid mechanics. McGraw Hill, New York.
• U.S. Department of Agriculture Farmer's Bulletin 813.

WATER METER

a device for measuring the amount of water supplied or consumed. Water pipes are used for: 1) volumetric, measuring the amount of flowing water by alternately filling a certain volume and recording the number of fillings (Fraget water meter); these V. give the most accurate account, but they are cumbersome; 2) high-speed, built on the principle that the amount of water flowing in the pipe is proportional to the speed of its movement; 3) Venturi water meters and diaphragms, the operation of which is based on the fact that the amount of water flowing is proportional to the pressure difference in the wide and narrowed sections of the device. In railway. water supply, the most common are high-speed Voltmann water meters installed in pumping stations, and "vane" meters - on the distribution network, near water dispensing points. W. Woltman consists of a celluloid pinwheel 1, placed in the body 2, transmission mechanism 3 and counter 4. V. is inserted into straight sections of the water supply. When water moves through the pipeline, the spinner rotates and each revolution corresponds to a certain volume of flowing water. The rotation of the turntable is transmitted to the counting mechanism, which shows the amount of water that has passed through the water meter. The "winged" V. differs from V. Voltman in that instead of a turntable it has a paddle wheel and the movement of water is directed perpendicular to the axis of the wheel.

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