The most widely used instruments for measuring the flow of substances flowing through pipelines can be divided into the following groups:
1. Variable differential pressure flow meters.
2. Constant differential pressure flow meters.
3. Electromagnetic flow meters.
4. Counters.
5. Others.
Variable differential pressure flow meters.
Variable differential pressure flow meters are based on the flow rate dependence of the differential pressure created by a device installed in the pipeline, or by the latter element itself.
The flow meter includes: a flow transducer that creates a pressure difference; a differential pressure gauge that measures this difference and connecting (pulse) tubes between the converter and the differential pressure gauge. If it is necessary to transmit the flow meter readings over a considerable distance, a secondary transducer is added to the indicated three elements, which converts the movement of the movable element of the differential pressure meter into an electrical and pneumatic signal, which is transmitted via a communication line to the secondary measuring device. If the primary differential pressure gauge (or secondary measuring device) has an integrator, then such a device measures not only the flow rate, but also the amount of the substance passed.
Depending on the principle of operation of the flow converter, these flow meters are divided into six independent groups:
1. Flow meters with restriction devices.
2. Flow meters with hydraulic resistance.
3. Centrifugal flow meters.
4. Flow meters with a pressure device.
5. Flow meters with pressure amplifier.
6. Impact-jet flowmeters.
Let's take a closer look at flow meters with a restriction device, since they are most widely used as the main industrial devices for measuring the flow of liquid, gas and steam, including at our enterprise. They are based on the dependence on the flow rate of the pressure drop created by the restriction device, as a result of which part of the potential energy of the flow is converted into kinetic energy.
There are many types of constriction devices. So in Fig. 1, a and b standard diaphragms are shown, in Fig. 1, c – standard nozzle, in Fig. 1, d, e, f - diaphragms for measuring contaminated substances - segmental, eccentric and annular. In the next seven positions of Fig. Figure 1 shows restriction devices used at low Reynolds numbers (for substances with high viscosity); so, in Fig. 1, g, h, and diaphragms are shown - double, with an inlet cone, with a double cone, and in Fig. 1, j, l, m, n - nozzles-semicircle, quarter circle, combined and cylindrical. In Fig. 1, o shows a diaphragm with a variable opening area, which automatically compensates for the influence of changes in pressure and temperature of the substance. In Fig. 1, n, p, s, t shows the flow meter pipes - Venturi pipe, Venturi nozzle, Dall pipe and Venturi nozzle with double constriction. They are characterized by very little pressure loss.
Picture 1.
The pressure difference before and after the restriction device is measured with a differential pressure gauge. As an example, consider the operating principle of the 13DD11 and Sapphire-22DD devices.
Figure 2.
The operating principle of differential pressure converters 13DD11 is based on pneumatic power compensation. The device diagram is shown in Fig. 2. Pressure is supplied to the positive 2 and negative 6 cavities of the converter, formed by flanges 1, 7 and membranes 3.5. The measured pressure difference acts on the membranes welded to the base 4. The internal cavity between the membranes is filled with silicone liquid. Under the influence of membrane pressure, lever 8 is rotated at a small angle relative to the support - the elastic membrane of output 9. The valve 11 moves relative to the nozzle 12, fed by compressed air. In this case, the signal in the nozzle line controls the pressure in the amplifier 13 and in the negative feedback bellows 14. The latter creates a moment on the lever 8, compensating the moment arising from the pressure difference. The signal entering the bellows 14, proportional to the measured pressure difference, is simultaneously sent to the output line of the converter. The zero corrector spring 10 allows you to set the initial value of the output signal to 0.02 MPa. The transducer is adjusted to a given measurement limit by moving the bellows 14 along the lever 8. Measuring pneumatic transducers of other modifications are designed in a similar way.
Figure 3.
Differential pressure transducers Sapphire-22DD (Fig. 3) has two chambers: positive 7 and negative 13, to which pressure is supplied. The measured pressure difference acts on the membranes 6, welded along the perimeter to the base 9. The flanges are sealed with gaskets 8. The internal cavity 4, limited by the membranes and the strain gauge 3, is filled with silicone liquid. Under the influence of the pressure difference, the membrane moves the rod 11, which transmits force through the rod 12 to the lever of the strain gauge 3. This causes a deflection of the membrane of the strain gauge 3 and a corresponding electrical signal transmitted to the electronic device 1 through the sealed terminal 2.
Constant differential pressure flow meters.
The principle of their operation is based on the perception of the dynamic pressure of the controlled medium, depending on the flow rate, by a sensitive element (for example, a float) placed in the flow. As a result of the flow, the sensing element moves, and the amount of movement serves as a measure of flow.
Devices operating on this principle are rotameters (Fig. 4).
Figure 4.
The flow of the controlled substance enters the tube from the bottom up and carries the float along with it, moving it upward to a height H. At the same time, the gap between it and the wall of the conical tube increases, as a result the speed of the liquid (gas) decreases and the pressure above the float increases.
The float is acted upon by a force from bottom to top:
G1=P1·S ⇒ Р1=G1/S
and from top to bottom
G2=P2·S+q ⇒ P2=G2/S-q/S,
where P1, P2 – pressure of the substance on the float from below and above;
S—float area;
q is the weight of the float.
When the float is in equilibrium G1=G2, therefore:
P1 - P2=q/S,
since q/S=const, it means:
P1 - P2=const,
Therefore, such devices are called constant differential pressure flow meters.
In this case, the volume flow can be calculated using the formula:
where Fc is the cross-sectional area of the conical tube at height h, m2; F-area of the upper end surface of the float, m2; p-density of the measured medium, kg m3; c – coefficient depending on the size and design of the float.
Rotameters with a glass tube are used only for visual flow readings and do not have devices for transmitting a signal over a distance.
The rotameter should not be installed in pipelines subject to strong vibrations.
The length of the straight section of the pipeline before the rotameter must be at least 10 Du, and after the rotameter at least 5 Du.
Figure 5.
Fluoroplastic pneumatic rotameter type RPF
Rotameters of the RPF type are designed to measure the volumetric flow rate of smoothly varying homogeneous flows of clean and lightly contaminated aggressive liquids with dispersed non-magnetic inclusions of foreign particles neutral to fluoroplastic and converting the flow rate into a unified pneumatic signal.
The RPF consists of a rotametric and pneumatic part (pneumatic head).
The body of the rotametric part 1 (Fig. 5) is a straight-flow pipe with rings 6 welded at the ends.
Inside the housing there are: a float 2 that moves under the action of the measured flow, rigidly connected to twin magnets 7, a measuring cone 4, guides 3, 12.
The body of the rotametric part is lined with fluoroplastic-4, and the guides 3, 12, float 2, and measuring cone 4 are made of fluoroplastic-4.
The pneumatic head is designed to provide local readings and is a round housing 20, which houses: a servo drive 16, a pneumatic relay 13, pressure gauges 18, an arrow 9, a moving mechanism 10, a scale of local readings, and inlet and outlet fittings.
The servo drive 16 is a metal cup 15 in which the bellows assembly 17 is located. The bellows 17 separates the internal cavity of the servo drive from the external environment and, together with the spring 24, serves as an elastic element.
The lower end of the bellows is soldered to the movable bottom, to which the rod 14 is rigidly connected. At the opposite end of the rod 14, a nozzle 25 and a mechanical relay 8 are fixed.
During operation, the mechanical relay ensures that the nozzle is closed by the flap when the flow rate increases and the nozzle opens when the flow rate decreases.
The mechanical relay (Fig. 6) consists of a bracket 1 fixed to the block 3, a shutter 2 installed together with a tracking magnet 5 on cores in bracket 4. Bracket 4 is attached with screws to block 3. Adjustment of the position of the mechanical relay relative to the nozzle is carried out by moving the mechanical relay along the axis of the servo rod.
Figure 6.
The movement mechanism 10 is pivotally connected to the mechanical relay 8 by a rod 11 and converts the vertical movement of the rod 14 into a rotational movement of the arrow 9.
All parts of the air head are protected from environmental influences (dust, splashes) and mechanical damage by a cover.
The principle of operation of the rotameter is based on the perception by the float moving in the measuring cone 4 of the dynamic pressure passing from the bottom to the top of the measured flow (Fig. 6).
When the float is raised, the passage gap between the measuring surface of the cone and the edge of the float increases, and the pressure drop across the float decreases.
When the pressure drop becomes equal to the weight of the float per unit cross-sectional area, equilibrium occurs. In this case, each flow rate of the measured liquid at a certain density and kinematic viscosity corresponds to a strictly defined position of the float.
In principle, the magnetopneumatic converter uses the property of perception by a tracking magnet 6, mechanical movement of twin magnets 7, rigidly connected to the float, and the conversion of this movement into an output pneumatic signal (Fig. 7).
Moving the float upward causes a change in the position of the tracking magnet 6 and the damper 5 rigidly connected to it. In this case, the gap between the nozzle and the damper decreases, the command pressure increases, increasing the pressure at the output of the pneumatic relay 4 (Fig. 7).
The power-amplified signal enters the internal cavity of the glass 15 (Fig. 5). Under the influence of this signal, the elastic element (bellows 17-spring 24) of the servo drive 16 is compressed, the rod 14, rigidly connected to the lower end of the bellows 17, the nozzle 25, the mechanical relay 8, mounted on the rod 14, moves upward.
The movement of the rod 14 occurs until the tracking magnet 5 with the damper takes its original position relative to the twin magnets 7.
Figure 7.
When the float moves downward, the position of the tracking magnet 5 and the valve associated with it changes, while the gap between the valve and the nozzle 25 increases, thereby reducing the command pressure and the pressure at the output of the pneumatic relay. Excess air from the cavity of the cup 15 (Fig. 4) is vented into the atmosphere through the pneumatic relay valve. Since the pressure in the glass 15 has decreased, the rod 14, under the action of the elastic element (bellows-spring) in place with the mechanical relay 8, moves down (towards the movement of the float) until the tracking magnet 5 with the damper takes its original position relative to the twin magnets.
The pneumatic relay is designed to amplify the output pneumatic signal in terms of power.
The operating principle of the VIR flow meter is based on the rotametric measurement method, that is, the measure of flow in it is the vertical movement of the float under the influence of the liquid flow flowing around it. The movement of the float is converted into an electrical signal.
Figure 8.
A schematic electrical diagram of the VIR with a connection diagram to the converter (KSD) is shown in Fig. 8.
VIR is a rotametric pair (measuring cone, float-core), responding to changes in the flow of the measured liquid, through a differential transformer T1, which converts the movement of the float-core into alternating current voltage. The converter (KSD) is designed to power the primary winding of the sensor transformer T1 and convert the alternating current voltage induced in the secondary winding of the sensor differential transformer T1 into readings on the instrument scale corresponding to the flowing fluid flow.
The change in voltage on the secondary winding of differential transformer T2, caused by the movement of the float core in the sensor, is amplified and transmitted to the reversible motor.
The moving core of the differential transformer T2 is a negative feedback element that compensates for the change in voltage at the input of transformer T2. The core moves through the cam when the reversible RD motor rotates. At the same time, the rotation of the reversible motor is transmitted to the instrument pointer.
The rotameter sensor (Fig. 9) consists of a housing 1, a rotametric tube 2, a differential transformer coil 3, a float core 4 and a terminal box 5.
The housing is a cylinder with covers 9, inside which a rotametric pipe passes, and a terminal box with a cover 6 is welded to its side surface, which is secured with six bolts. The housing contains a differential transformer coil filled with compound 10 (VIXINT K-18).
The rotametric pipe is a stainless steel pipe, at the ends of which flanges 7 are welded, which are used to mount the sensor to the production line. Inside the rotametric tube there is a fluoroplastic tube 8 with an internal measuring cone.
Figure 9.
The differential transformer coil is wound directly onto the rotametric tube, the ends of the coil windings are connected to the feed-through terminals of the terminal box.
The float-core consists of a specially designed float made of fluoroplastic-4 and a core made of electrical steel located inside the float.
The float core differential transformer coil constitutes the sensor differential transformer, the primary winding of which is supplied by the transducer and the voltage induced in the secondary winding is supplied to the transducer.
Electromagnetic flow meters.
Electromagnetic flow meters are based on the interaction of a moving electrically conductive liquid with a magnetic field, subject to the law of electromagnetic induction.
The main applications are electromagnetic flow meters that measure the emf induced in a liquid when it crosses a magnetic field. To do this (Fig. 10), two electrodes 3 and 5 are inserted into section 2 of a pipeline made of non-magnetic material, coated on the inside with non-conductive insulation and placed between poles 1 and 4 of a magnet or electromagnet in a direction perpendicular to both the direction of movement of the liquid and to the direction of the magnetic field lines. The potential difference E at electrodes 3 and 5 is determined by the equation:
where – B – magnetic induction; D – distance between the ends of the electrodes, equal to the internal diameter of the pipeline; v and Q0 are the average speed and volumetric flow rate of the liquid.
Figure 10.
Thus, the measured potential difference E is directly proportional to the volume flow Q0. To take into account the edge effects caused by the inhomogeneity of the magnetic field and the shunting effect of the pipe, the equation is multiplied by correction factors km and ki, which are usually very close to unity.
Advantages of electromagnetic flowmeters: independence of readings from the viscosity and density of the substance being measured, the possibility of use in pipes of any diameter, no pressure loss, linearity of the scale, the need for shorter lengths of straight pipe sections, high speed, the ability to measure aggressive, abrasive and viscous liquids. But electromagnetic flow meters are not applicable for measuring the flow of gas and steam, as well as dielectric liquids, such as alcohols and petroleum products. They are suitable for measuring the flow rate of liquids with a specific electrical conductivity of at least 10-3 S/m.
Counters.
According to the principle of operation, all liquid and gas meters are divided into high-speed and volumetric.
Speed meters are designed in such a way that the liquid flowing through the chamber of the device rotates a turntable or impeller, the angular velocity of which is proportional to the flow speed, and, consequently, to the flow rate.
Volume meters. The liquid (or gas) entering the device is measured in separate, equal volume doses, which are then summed up.
High-speed counter with screw spinner.
A high-speed meter with a screw spinner is used to measure large volumes of water.
Figure 11.
Fluid flow 4 fig. 11, entering the device, is leveled by the stream straightener 3 and falls on the blades of the turntable 2, which is made in the form of a multi-threaded propeller with a large blade pitch. The rotation of the turntable is transmitted through a worm pair and transmission mechanism 4 to a counting device. To adjust the device, one of the radial blades of the flow straightener is made rotary, so that by changing the flow speed, you can speed up or slow down the speed of the turntable.
High-speed counter with vertical impeller.
This meter is used to measure relatively small water flows and is available for nominal flows from 1 to 6.3 m3/h with calibers from 15 to 40 mm.
Figure 12.
Depending on the distribution of the water flow entering the impeller, there are two modifications of the meters - single-jet and multi-jet.
Figure 12 shows the design of a single-jet counter. The liquid is supplied to the impeller tangentially to a circle described by the average radius of the blades.
The advantage of multi-jet meters is the relatively small load on the support and the impeller axis, but the disadvantage is that the design is more complex than single-jet meters and the possibility of clogging the jet supply holes. Turntables and impellers of counters are made of celluloid, plastics and hard rubber.
The meter is installed on a linear section of the pipeline, and at a distance of 8-10 D in front of it (D-diameter of the pipeline) there should be no devices that distort the flow (elbows, tees, valves, etc.). In cases where some flow distortion is still expected, additional flow straighteners are installed in front of the meters.
Meters with a horizontal impeller can be installed on horizontal, inclined and vertical pipelines, while meters with a vertical impeller can only be installed on horizontal pipelines.
Liquid volume meter with oval gears.
The operation of this meter is based on the displacement of certain volumes of liquid from the measuring chamber of the device by oval gears that are geared and rotating under the influence of the pressure difference at the inlet and outlet pipes of the device.
Figure 13.
The diagram of such a counter is shown in Fig. 13. In the first initial position (Fig. 13, a) the surface r of gear 2 is under the pressure of the incoming liquid, and the equal surface r is under the pressure of the outgoing liquid. Less input. This pressure difference creates a torque that rotates gear 2 clockwise. In this case, the liquid from cavity 1 and the cavity located under gear 3 is forced out into the outlet pipe. The torque of gear 3 is zero, since the surfaces a1g1 and g1b1 are equal and are under the same input pressure. Therefore, the gear is 2-driven, the gear is 3-driven.
In the intermediate position (Fig. 13, b), gear 2 rotates in the same direction, but its torque will be less than in position a, due to the counteracting moment created by the pressure on the surface dg (d-point of contact of the gears). Surface a1b1 of gear 3 is under incoming pressure, and surface b1 b1 is under outgoing pressure. The gear experiences a counterclockwise torque. In this position, both gears are driven.
In the second initial position (Fig. 13, c), gear 3 is under the influence of the greatest torque and is the driving one, while the torque of gear 2 is zero, it is the driven one.
However, the total torque of both gears for any of the positions remains constant.
During a full rotation of the gears (one cycle of the counter), cavities 1 and 4 are filled twice and emptied twice. The volume of four doses of liquid displaced from these cavities constitutes the measuring volume of the counter.
The greater the fluid flow through the meter, the faster the gears rotate. Displacing measured volumes. The transmission from the oval gears to the counting mechanism is carried out through a magnetic coupling, which works as follows. The driving magnet is fixed at the end of the oval gear 3, and the driven one is on the axis, connecting the coupling with the gearbox 5. The chamber where the oval gears are located is separated from the gearbox 5 and the counting mechanism 6 by a non-magnetic partition. Rotating, the drive shaft strengthens the driven one.
G. I. Sychev
Head of Flowmeters department
Spirax-Sarco Engineering LLC
Properties of water vapor
Flow measurement problems
Ultrasonic flow meters
Vortex flowmeters
Other types of flow meters
The accuracy of steam flow measurement depends on a number of factors. One of them is the degree of dryness. This indicator is often neglected when selecting metering and measuring instruments, and completely in vain. The fact is that saturated wet steam is essentially a two-phase medium, and this causes a number of problems in measuring its mass flow and thermal energy. Today we will figure out how to solve these problems.
Properties of water vapor
To begin with, let’s define the terminology and find out what the features of wet steam are.
Saturated steam is water vapor that is in thermodynamic equilibrium with water, the pressure and temperature of which are interconnected and located on the saturation curve (Fig. 1), which determines the boiling point of water at a given pressure.
Superheated steam is water vapor heated to a temperature above the boiling point of water at a given pressure, obtained, for example, from saturated steam by additional heating.
Dry saturated steam (Fig. 1) is a colorless transparent gas that is homogeneous, i.e. homogeneous environment. To some extent, this is an abstraction, since it is difficult to obtain: in nature, it is found only in geothermal sources, and the saturated steam produced by steam boilers is not dry - typical dryness values for modern boilers are 0.95-0.97. Most often, the degree of dryness is even lower. In addition, dry saturated steam is metastable: when heat enters from the outside, it easily becomes overheated, and when heat is released, it becomes moist saturated.
Figure 1. Water vapor saturation line
Wet saturated steam (Fig. 2) is a mechanical mixture of dry saturated steam with a suspended fine liquid that is in thermodynamic and kinetic equilibrium with the steam. Fluctuations in the density of the gas phase and the presence of foreign particles, including those carrying electrical charges - ions, lead to the emergence of condensation centers that are homogeneous in nature. As the humidity of saturated steam increases, for example, due to heat losses or increased pressure, tiny droplets of water become centers of condensation and gradually grow in size, and saturated steam becomes heterogeneous, i.e. two-phase medium (steam-condensate mixture) in the form of fog. Saturated steam, which represents the gas phase of the steam-condensate mixture, transfers part of its kinetic and thermal energy to the liquid phase when moving. The gas phase of the flow carries droplets of the liquid phase in its volume, but the speed of the liquid phase of the flow is significantly lower than the speed of its vapor phase. Wet saturated steam can form an interface, for example under the influence of gravity. The structure of a two-phase flow during steam condensation in horizontal and vertical pipelines changes depending on the ratio of the shares of gas and liquid phases (Fig. 3).
Figure 2. PV diagram of water vapor
Figure 3. Structure of two-phase flow in a horizontal pipeline
The nature of the flow of the liquid phase depends on the ratio of friction and gravity forces, and in a horizontally located pipeline (Fig. 4) at a high steam velocity, the flow of condensate can remain film-like, as in a vertical pipe; at a medium speed it can take on a spiral shape (Fig. 5) , and at low film flow is observed only on the upper inner surface of the pipeline, and on the lower surface a continuous flow, a “stream”, is formed.
Thus, in the general case, the flow of a steam-condensate mixture when moving consists of three components: dry saturated steam, liquid in the form of drops in the core of the flow, and liquid in the form of a film or jet on the walls of the pipeline. Each of these phases has its own speed and temperature, and when the steam-condensate mixture moves, relative phase slip occurs. Mathematical models of two-phase flow in a steam pipeline of wet saturated steam are presented in the works.
Figure 4. Structure of two-phase flow in a vertical pipeline
Figure 5. Spiral movement of condensate.
Flow measurement problems
Measuring the mass flow and thermal energy of wet saturated steam poses the following challenges:
1. The gas and liquid phases of wet saturated steam move at different speeds and occupy a variable equivalent cross-sectional area of the pipeline;
2. The density of saturated steam increases as its humidity increases, and the dependence of the density of wet steam on pressure at different degrees of dryness is ambiguous;
3. The specific enthalpy of saturated steam decreases as its humidity increases.
4. Determining the degree of dryness of wet saturated steam in a flow is difficult.
At the same time, increasing the degree of dryness of wet saturated steam is possible in two well-known ways: “crushing” the steam (reducing the pressure and, accordingly, the temperature of the wet steam) using a pressure reducing valve and separating the liquid phase using a steam separator and a condensate trap. Modern steam separators provide almost 100% drying of wet steam.
Measuring the flow rate of two-phase media is an extremely complex task that has not yet gone beyond research laboratories. This is especially true for steam-water mixtures.
Most steam flow meters are high-speed, i.e. measure the steam flow rate. These include variable pressure differential flowmeters based on orifice devices, vortex, ultrasonic, tachometer, correlation, and jet flowmeters. Coriolis and thermal flow meters stand apart, directly measuring the mass of the flowing medium.
Let's look at how different types of flow meters cope with their task when dealing with wet steam.
Variable differential pressure flowmeters
Variable differential pressure flowmeters based on orifices (diaphragms, nozzles, Venturi tubes and other local hydraulic resistances) are still the main means of measuring steam flow. However, in accordance with subsection 6.2 of GOST R 8.586.1-2005 “Measurement of flow and quantity of liquids and gases using the differential pressure method”: According to the conditions for using standard restrictive devices, the controlled “medium must be single-phase and homogeneous in physical properties”:
If there is a two-phase medium of steam and water in the pipeline, measurement of coolant flow by variable pressure differential devices with standardized accuracy is not ensured. In this case, “one could talk about the measured flow rate of the vapor phase (saturated steam) of the wet steam flow at an unknown value of the degree of dryness.”
Thus, the use of such flow meters to measure wet steam flow will lead to unreliable readings.
An assessment of the resulting methodological error (up to 12% at a pressure of up to 1 MPa and a dryness degree of 0.8) when measuring wet steam with variable pressure differential flow meters based on orifice devices was carried out in the work.
Ultrasonic flow meters
Ultrasonic flowmeters, successfully used in measuring the flow of liquids and gases, have not yet found wide application in measuring steam flow, despite the fact that certain types of them are commercially produced or have been announced by the manufacturer. The problem is that ultrasonic flowmeters that implement the Doppler measurement principle, based on the frequency shift of the ultrasonic beam, are not suitable for measuring superheated and dry saturated steam due to the lack of inhomogeneities in the flow necessary to reflect the beam, and when measuring the flow rate of wet steam, it is very readings are underestimated due to differences in the velocities of the gas and liquid phases. Ultrasonic flowmeters of the time-pulse type, on the contrary, are not applicable for wet steam due to the reflection, scattering and refraction of the ultrasonic beam on water droplets.
Vortex flowmeters
Vortex flowmeters from different manufacturers behave differently when measuring wet steam. This is determined both by the design of the primary flow transducer, the principle of vortex detection, the electronic circuit, and by the features of the software. The influence of condensate on the operation of the sensitive element is fundamental. In some designs, “serious problems arise when measuring saturated steam flow when both gas and liquid phases exist in the pipeline. Water concentrates along the pipe walls and interferes with the normal functioning of pressure sensors installed flush with the pipe wall." In other designs, condensation may flood the sensor and block flow measurement altogether. But for some flow meters this has virtually no effect on the readings.
In addition, a two-phase flow, running into a bluff body, forms a whole spectrum of vortex frequencies associated with both the velocity of the gas phase and the velocities of the liquid phase (droplet form of the flow core and the film or jet near-wall region) of moist saturated vapor. In this case, the amplitude of the vortex signal of the liquid phase can be very significant and, if the electronic circuit does not involve digital filtering of the signal using spectral analysis and a special algorithm for identifying the “true” signal associated with the gas phase of the flow, which is typical for simplified models of flow meters, then severe underestimation of consumption readings. The best models of vortex flowmeters have DSP (digital signal processing) and SSP (spectral signal processing based on fast Fourier transform) systems, which can not only increase the signal-to-noise ratio, highlight the “true” vortex signal, but also eliminate the influence of pipeline vibrations and electrical interference
Despite the fact that vortex flowmeters are designed to measure the flow of a single-phase medium, the work shows that they can be used to measure the flow of two-phase media, including steam with water droplets, with some degradation of metrological characteristics.
Wet saturated steam with a dryness degree of over 0.9, according to experimental studies by EMCO and Spirax Sarco, can be considered homogeneous due to the “reserve” in accuracy of the PhD and VLM flowmeters (±0.8-1.0%), readings of mass flow and thermal power will be within the errors normalized in .
With a dryness degree of 0.7-0.9, the relative error in measuring the mass flow of these flow meters can reach ten percent or more.
Other studies, for example, give a more optimistic result - the error in measuring the mass flow rate of wet steam using Venturi nozzles on a special installation for calibrating steam flow meters is within ±3.0% for saturated steam with a dryness degree of over 0.84.
To avoid condensate blocking the sensing element of a vortex flowmeter, such as the sensing wing, some manufacturers recommend that the sensor be oriented so that the sensing element axis is parallel to the steam/condensate interface.
Other types of flow meters
Variable differential/variable area flow meters, flow meters with a spring-loaded damper and variable area target flow meters do not allow the measurement of a two-phase medium due to possible erosive wear of the flow part during the movement of condensate.
In principle, only Coriolis-type mass flowmeters could measure two-phase media, but research shows that the measurement errors of Coriolis flowmeters largely depend on the ratio of phase fractions, and “attempts to develop a universal flowmeter for multiphase media are likely to lead to a dead end.” At the same time, Coriolis flow meters are being intensively developed, and perhaps success will be achieved soon, but so far there are no such industrial measuring instruments on the market.
To be continued.
Literature:
1. Rainer Hohenhaus. How useful are steam measurements in the wet steam area? // METRA Energie-Messtechnik GmbH, November, 2002.
2. Good Practice Guide Reducing energy consumption costs by steam metering. // Ref. GPG018, Queen's Printer and Controller of HMSO, 2005
3. Kovalenko A.V. Mathematical model of two-phase flow of wet steam in steam pipelines.
4. Tong L. Heat transfer during boiling and two-phase flow. - M.: Mir, 1969.
5. Heat transfer in two-phase flow. Ed. D. Butterworth and G. Hewitt.// M.: Energy, 1980.
6. Lomshakov A.S. Steam boiler testing. St. Petersburg, 1913.
7. Jesse L. Yoder. Using meters to measure steam flow // Plant Engineering, - April 1998.
8. GOST R 8.586.1-2005. Measuring the flow and quantity of liquids and gases using the differential pressure method.
9. Koval N.I., Sharoukhova V.P. On the problems of measuring saturated steam.// UTSMS, Ulyanovsk
10. Kuznetsov Yu.N., Pevzner V.N., Tolkachev V.N. Measurement of saturated steam using constriction devices // Thermal power engineering. - 1080.- No. 6.
11. Robinshtein Yu.V. On commercial metering of steam in steam heat supply systems. // Materials of the 12th scientific and practical conference: Improving measurements of liquid, gas and steam flow, - St. Petersburg: Borey-Art, 2002.
12. Abarinov, E. G., K.S. Sarelo. Methodological errors in measuring the energy of wet steam using heat meters for dry saturated steam // Measuring technology. - 2002. - No. 3.
13. Bobrovnik V.M. Non-contact flow meters "Dnepr-7" for metering liquids, steam and oil gas. //Commercial accounting of energy resources. Materials of the 16th international scientific and practical conference, St. Petersburg: Borey-Art, 2002.
14. DigitalFlow™ XGS868 Steam Flow Transmitter. N4271 Panametrics, Inc., 4/02.
15. Bogush M.V. Development of vortex flow metering in Russia.
16. Engineering Data Book III, Chapter 12, Two Phase Flow Patterns, Wolverine Tube, Inc. 2007
17. P-683 “Rules for accounting for thermal energy and coolant”, M.:, MPEI, 1995.
18. A. Amini and I. Owen. The use of critical flow venturi nozzles with saturated wet steam. //Flow Meas. lnstrum., Vol. 6, No. 1, 1995
19. Kravchenko V.N., Rikken M. Flow measurements using Coriolis flow meters in the case of two-phase flow. // Commercial accounting of energy carriers. XXIV International Scientific and Practical Conference, St. Petersburg: Borey-Art, 2006.
20. Richard Thorn. Flow Measurement. CRC Press LLC, 1999
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Steam consumption metering. Adventures of instrumentation engineers or vortex flowmeters as a real alternative to restriction devices
Edition: Energy Analysis and Energy Efficiency No. 6. Year: 2006
15.10.2006Currently, issues of energy resource accounting are rightly receiving increased attention. This is determined by the fact that, on the one hand, without reliable information about the resources consumed, it is impossible to competently carry out energy saving measures, which, in the context of constantly rising energy prices, is vital for both individual enterprises and each industry and the country’s economy as a whole. . On the other hand, in the context of a multiple increase in the number of metering devices, the problem of the cost of their maintenance, or rather maintaining them in working order, comes to the fore.
Due to the specific nature of this environment, measuring steam flow is isolated from the scope of gas metering problems. This is determined primarily by high temperatures and pressure in steam pipelines, as well as the presence in them, including as a result of increased wear of pipelines under these extreme conditions, of various mechanical inclusions (corrosion products, scale, etc.), as well as condensate. Therefore, with all the variety of methods for measuring flow, there are really only two alternatives to solve the problem of steam metering:
- flow meters based on the method of variable pressure drop on a restriction device (SU);
- vortex flowmeters (VR).
- Should you choose a flow meter based only on cost, dynamic range (DR), accuracy, and calibration interval (CTI)?
- Do the technical characteristics of Russian-made flow meters really correspond to the best foreign analogues?
The average metrologist has the following characteristics in his mind of the flow measurement methods under consideration:
Accordingly, the conclusion is very simple: if you have the means, then it is better to purchase a vortex flowmeter, since it is more accurate and is calibrated less frequently; if funding is limited, then only the “good old” diaphragm remains.
The article could have been completed with this conclusion, if not for the key points outlined in the preamble. Therefore, we suggest forgetting the images and numbers of the measurement methods being studied and starting the selection of a steam flow meter from scratch.
To begin with, let’s remember what flowmeters on the control system and vortex flowmeters are.
The first consists of a certain restriction device installed in the pipeline. Typically, a so-called diaphragm is used as a constriction device: a disk whose internal diameter is smaller than the internal diameter of the pipeline. Due to local narrowing, the diaphragm creates a pressure difference, the value of which is measured by a differential pressure sensor. The absolute steam pressure in the pipeline and the steam temperature are measured simultaneously. If the diaphragm flow coefficient is known, this information is sufficient to calculate the gas or steam flow rate and, accordingly, determine the amount of product consumed during the reporting period.
The vortex principle of flow measurement is based on the von Kármán effect, which consists in the fact that when a flow of liquid or gas flows around a poorly bluffed body, regular vortex formation occurs, i.e. alternate formation and shedding of vortices on both sides of the specified body, and the repetition frequency of the vortices is proportional to the flow speed. This vortex formation is accompanied by regular periodic pulsations of pressure and flow velocity in the wake behind the bluff body. Accordingly, by measuring the frequency of these pulsations, it is possible to determine the speed or flow rate of gas or steam under operating conditions. In order to determine the amount of steam passed through, it is necessary, as in the case of SU, to additionally measure the pressure and temperature of the steam.
In the article we will consider the characteristics of two subtypes of vortex flowmeters (VR), which have become widespread in Russia, which differ in the method of detecting vortices:
- Pressure or speed pulsations are recorded by sensors located on the surface of the flow part.
- Pressure pulsations affect the sensitive element (wing, tube, piezomicrophone, etc.) behind the bluff body, which transmits them to a sensor hidden deep in the device.
So, let's return to the task at hand - we need to install a steam metering unit.
Most likely, the steam flow rate will vary depending on the time of year, production volumes and other factors, so it is necessary to ensure that the flow meter's measurement range is sufficient.
The standard ratio of the maximum and minimum flow rates measured using a control system is 1:3, but can reach 1:10 (if you use multi-range “intelligent”, but also very expensive, differential pressure sensors). This is already good, but the cost of the node in this case will also be set to the maximum of its “dynamic range”.
A wide dynamic range is an undoubted advantage of vortex flowmeters. This figure varies from 1:20 to 1:40. But not everything is smooth here either. After all, the conversion coefficient of a vortex flowmeter (i.e., the ratio of the frequency of vortex formation to the instantaneous flow rate of the measured medium through the measuring section of the device) is stable in a very limited range of flow rates determined by the Reynolds number Re (hydrodynamic similarity criterion). To achieve maximum accuracy, it is necessary to introduce individual correction factors to ensure measurement accuracy over the entire range. Using an array of coefficients requires good processing power of the processor, so modern intelligent vortex flowmeters must have the latest generation processors. Unfortunately, not all domestic devices use digital signal processing with correction of the Karman dependence, so the measurement error in such devices increases with increasing dynamic range.
Interestingly, the use of digital spectral signal processing has made it possible to overcome another annoying shortcoming of VR in the past. The fact is that the measurement principle involves detecting flow pulsations. In this case, external vibrations could superimpose on the useful signal and even completely block it. Interference led to a decrease in measurement accuracy and the possibility of an output signal appearing in the absence of flow in the pipeline, the so-called “self-propelled” phenomenon.
Modern smart VRs analyze the signal spectrum, cutting out noise and amplifying useful harmonics, thereby guaranteeing measurement accuracy. At the same time, vibration resistance indicators increased by an order of magnitude on average.
Features of steam metering that should be taken into account when choosing a measuring instrument include high temperature of the medium, possible clogging of the pipeline near the flow meter, the possibility of deposits appearing on the internal surfaces of the flow meter, as well as the likelihood of periodic occurrence of water hammer and thermal shock. Let's consider the influence of these factors.
The steam temperature can vary from 100 0C to 600 0C. In this case, flow meters on CS can be used throughout the entire designated range. However, the measurement accuracy of flow meters on the control system will deteriorate with increasing temperature, which is associated with changes in the internal diameter of the pipeline and the diameter of the diaphragm, as well as the additional temperature error of the pressure sensor. The influence of changes in geometric dimensions is especially critical when measuring on pipelines with a diameter of less than 300 mm, and the additional temperature error of a pressure sensor (for example, Metran-100) is 0.9% per 100? C.
The operating temperature range of the VR can be 150, 200, 350, 450 0C, depending on the model and manufacturer. Moreover, the last two values correspond to the characteristics of imported devices. We hope that readers have a clear understanding of the difference between the concept of “the device works and shows something” and “the device works in accordance with the stated characteristics.” Very often, VR manufacturers are silent about the additional temperature error associated with changes in the geometric dimensions of the elements of the flow part. Foreign flow meters automatically correct flow readings based on temperature, sometimes reaching 0.2% for every 100 0C. Domestic smart VR also performs temperature correction. Therefore, do not forget to check with the manufacturer about the availability of such error correction when choosing a flow meter.
Clogging of the pipeline and the appearance of deposits on the main elements of the flow converter over time can nullify your efforts in selecting and installing a metering unit. The reason is simple: the design of the flow meter on the control system assumes the formation of deposits on the bottom of the pipeline near the front wall of the diaphragm. As the clogging increases, its influence on the error control system increases, which sometimes reaches tens of percent. The adhesion of a substance to the surface of the diaphragm, as well as the wear of its edges, helps to transform the metering unit into a sensor for the presence of flow in the pipeline. To prevent this from happening, it is necessary to periodically (every two months) clean the flow meter on the control unit.
What about VR? Contaminants have a significantly less impact on the process of vortex formation than on the pressure drop across the control unit; moreover, there are simply no cavities and pockets where deposits can accumulate in the control unit, so the stability of the readings of the latter is much higher. In addition, it has been experimentally proven that vortex formation leads to self-cleaning not only of the bluff body itself, but also of a section of the pipeline at a distance of approximately 1 nominal diameter of the pipeline (DN) before and 2-4 DN after the bluff body. The use of special shapes and sizes of bluff bodies made it possible to further reduce the influence of these changes in the geometric dimensions of the VR flow part.
Today, manufacturers use specially shaped bluff bodies. They are designed in such a way that their change affects the measurement accuracy significantly less than that of control systems and VRs with rectangular or, especially, cylindrical bluff bodies. However, it should be remembered that rags, wrenches and other types of “mechanical impurities” can sometimes be “transported” in our pipelines along with steam. Therefore, if a filter (at least a large mesh) is not installed before the metering station, then you should pay attention to VR with removable blimp body. Such a device can be cleaned without dismantling and subsequent verification.
An important indicator of the reliability of a steam metering unit is its resistance to hydraulic shocks, which often arise as a result of malfunctions of heat sources and the “personal initiative” of operating personnel. In order for the reader to have respect for this phenomenon, we note that water hammer and usually the subsequent increase in pressure lead to the rupture of heating batteries and are often the main reason for the failure of sensors.
Flow meters on control systems are not afraid of water hammer, but the VRs are divided into two camps. In VR based on pressure pulsations, the sensitive elements are located under a thin membrane and therefore are not protected from water hammer. Manufacturers, as a rule, honestly warn about this, reminding, however, that the warranty for the device in this case is not valid. In VR based on bending stresses the sensing element is separated from the medium being measured, so he knows nothing about water hammer.
When steam is supplied through a cooled pipeline, a sharp increase in temperature occurs, and the sensitive elements of the sensor become very hot on the inside and cooled on the outside. This increase in temperature is called thermal shock and, accordingly, it also dangerous only for VR pressure pulsations, the sensitive elements of which are in close proximity to the measured medium.
Now let's imagine the pipeline on which we will install the metering unit. If the metering unit is installed on the street or in an unheated room, then the control system will require increased attention: the impulse lines connecting the pressure sensor to the pipeline may freeze, so they will need to be heated and purged.
Vortex flowmeters are easy to install and do not require maintenance. We only recommend that you make sure that the device corresponds to climatic version C3 from (-40 to +70) 0C and make sure that the computer is kept warm.
Speaking of computers. The volumetric steam flow rate itself, the values of which are given by the flow meter, is of no practical value. You need to know either the mass of the steam or the thermal energy it transfers. For these purposes, heat calculators are used that calculate the required parameters based on data from flow, pressure and temperature sensors. The necessary and mandatory functions of the computer include maintaining an archive of measured parameters, as well as monitoring and recording emergency situations.
You can connect the flow meter to the computer using a 4-20 mA current signal, which is available, perhaps, in all flow meters, both SU and vortex.
The advantages of vortex flowmeters include additional output frequency signal. Its advantages are higher accuracy. Please note that manufacturers indicate the relative error for the frequency signal, and the reduced error for the current output. The given error means that the accuracy of the values will deteriorate proportionally as you move away from the maximum flow rate. For example, if for a flow meter with a DD of 1:10 a reduced error of say 1.0% is indicated, this means that at the maximum flow rate the relative error will actually be 1.0%, and at the minimum it will correspond to 10%. The conclusion is simple: a frequency signal is preferable. Moreover, all modern computers have a frequency input signal of 0-1000 Hz or 0-10000 Hz.
Foreign manufacturers consider the digital output signal as an additional option, since consumers have long appreciated the benefits of digital communications. In Russia, the situation is currently the opposite: a digital signal is offered as a free bonus, but is actually used in rare cases. This is often facilitated by Russian manufacturers of secondary equipment, considering support for digital input signals unnecessary. In addition, the passage of a digital signal requires higher quality communication lines, which are currently not available everywhere. Nevertheless, the presence of a digital channel in a flow meter can be very useful when automating technological processes or simply when displaying instrument readings on a PC. Let us note an important point: choose devices with standardized, internationally recognized digital protocols HART, Foundation Field Bus, ProfiBus, Modbus. Otherwise, closed standards, understandable only to the device manufacturer, will be of little use.
Let us return, however, to the pipeline and the installation location of the steam metering unit. Most flow measuring instruments must be installed on straight sections of pipeline with a length of 1 to 100 nominal diameters (DN). The longest straight sections from 30 to 100 DN are required for flow meters with control system. Failure to comply with these requirements leads to a distortion of the uniformity of the medium flow and, as a consequence, a decrease in the measurement accuracy.
Compared to control systems, VRs have less stringent requirements for the lengths of straight sections. The corresponding recommendations are 30 DN, with possible reduction to 10 DN depending on the pipeline configuration. In most cases, a reduction to 10Du without deteriorating accuracy is possible only after introducing additional correction factors that take into account the characteristics of the installation location.
Note that some Russian VR manufacturers report “victory over the laws of hydrodynamics” and indicate requirements for straight sections from 3 to 5Dn, which is 2 and even 3 times better than those of foreign models. Let us leave the underestimation of the requirements for the lengths of straight sections to the conscience of these manufacturers. And we recommend that consumers do not engage in self-deception and install VR on pipelines with straight sections of at least 10Du in length, and SU - at least 30Du.
And now we invite readers to strain their imagination and imagine not one, but three identical pipelines with steam and three engineers Shaibov, Fishkin and Vikhrev, each of whom we will entrust to install and maintain a metering unit on one of the pipelines.
The engineers decided to take different paths to solve the problem of steam metering and, accordingly, chose a meter based on the SU, an imported steam metering unit based on the VR, and a domestic steam metering unit based on the VR. At the same time, Shaibov was primarily guided by the cost of the metering unit. Fishkin decided to fork out the cash, believing that “the miser pays twice,” and purchased an imported vortex flowmeter. Vikhrev studied the issue thoroughly and, according to the principle “if there is no difference, why pay more?”, settled on a domestic vortex flowmeter for bending stresses. Let's watch our characters.
Trouble awaited our heroes already at the first stage, when purchasing flow meters.
When making calculations, Shaybov did not suspect that the cost of the pressure sensor would increase by a third due to the fact that the unit would be located in an unheated room, and the impulse lines with valve blocks turned out to be not as cheap as expected. As a result, the cost of the metering unit on the control system was equal to the solution based on the domestic VR.
Fishkin was a little upset when, after 5 weeks of waiting to receive the equipment, he learned that he would have to wait a couple more weeks due to delays at customs.
Vikhrev’s problems at this stage can only be attributed to the difficulty in choosing from a large assortment of computers. (However, we would like not to touch upon the problem of choosing a computer in this article, so we will trust Vikhrev’s choice and will not even ask him what kind of computer he purchased).
Finally, all the engineers received the equipment, all that remains is to install it and the first stage has been completed. Vikhrev managed it the fastest, because the technological insert and a set of mounting parts were supplied along with the flow meter. Shaibov had to spend significantly more time to comply with all the mandatory requirements for installing the diaphragm: ensuring that the diameters of the pipeline and the diaphragm housings match, the alignment of the control system and the pipeline, and connecting the control system chambers with the differential pressure sensor using impulse lines. Shaibov also had to come to terms with the fact that the accuracy of the metering unit will be lower than stated due to unaccounted factors: pipeline roughness and discrepancy between the actual internal diameter of the pipeline and the calculated data.
Installation of the metering unit based on imported equipment went smoothly, thanks to well-illustrated operating instructions. However, a local dealer threw a “fly in the ointment” by refusing to supply a set of mounting parts for the flow meter and shifting its production to Fishkin. Fishkin’s joy over the successful installation of the unit was also short-lived, since programming the devices turned out to be difficult due to the lack of a Russian-language menu and obvious translation errors in the accompanying documentation. A call to the local supplier showed that they did not have a specialist to configure the equipment, so all questions were redirected to the head office of the company's representative office in Russia. And Fishkin waited a long time for answers to his questions. However, Fishkin is already used to waiting...
So, the equipment is installed and connected, the node is commissioned. However, time passed and Shaybov began to suspect that the SU’s testimony was untrue. After opening, cleaning the diaphragm and the adjacent section of the pipeline from blockages and purging the impulse lines, the readings began to correspond to those expected, however, the conclusion was disappointing: the unit needs to be cleaned once every two months.
Fishkin and Vikhrev watched their colleague’s fuss with some gloating, thinking that they would only remember about their BP units in three years, when the time came for their verification. However, the issued decree of the local Center for Migration Dispelled expectations: the region introduced an order to verify all flow meters and heat energy meters every year, regardless of the requirements of federal regulations.
Shaibov’s finest hour had come: the entire verification of the metering unit resulted in the next removal of the diaphragm (over a year of friendship with the control system, the engineer learned to quickly remove the diaphragm, since he carried out this procedure regularly) and measurement of its geometry in the presence of a representative of the Central Monitoring Center, as well as verification of pressure and temperature sensors .
The imported Fishkin flow meter can be verified in two ways: by flushing the device on a water stand or using a no-spill method. The second option turned out to be more preferable. The verification procedure turned out to be quite simple: measuring the geometry of the bluff body and checking the electronic unit. True, Fishkin had to additionally purchase a special, expensive verification kit, which could have been dispensed with if the device had used standard, rather than unique, proprietary connectors.
Vikhrev was ready for the verification procedure and even waited for it, since even at the purchase stage he made a choice in favor of VR bending stresses, which, due to their versatility, can be verified not only on an air, but also on a water verification stand, which is available in any regional center . A pleasant surprise for Vikhrev was the presence of an officially approved spill-free verification method similar to Fishkin’s flow meter.
Finally, we invite you to imagine that the engineers’ flow meters have failed. We only feel sorry for Shaibov: after all, he no longer leaves the control system, being an integral part of the accounting unit. Let the breakdowns of the Fishkin and Vikhrev flowmeters be of the same nature, let’s, for example, imagine that the frequency output of both devices failed due to the fault of a worker who mixed up the polarity of the contacts.
So, having complained about the workers, Fishkin and Vikhrev began to study the operating manuals for the flow meter. Using the built-in self-diagnosis function, Fishkin was convinced that only the frequency output had failed. Having called the service center (SC), he found out that replacing electronics is a five-minute procedure, thanks to the modular design of the device. However, the service center refused to provide repair documentation and a replacement module, explaining such secrecy by the manufacturer’s company policy. Fishkin had to send the device to the service center, where, as it later turned out, just such a module was not currently in stock, so it was ordered abroad. Here's a five-minute procedure for you. However, wait, Fishkin, wait. You're used to it.
Vikhrev also called the SC and, even knowing Fishkin’s misadventures, was ready to send the device there. But at the SC he was pleasantly surprised. Vikhrev was informed that his device could be repaired in the field and was sent repair documentation, offering a choice of either replacing the module yourself, or removing the device and sending it to the nearest service center. Seeing that to replace the electronics, you just need to unscrew a couple of bolts, and there is no need to dismantle the entire flow meter, much less stop the steam supply in the pipeline, Vikhrev decided to carry out the repairs himself. A couple of days later, the manufacturer sent Vikhrev a replacement electronic module, which he received in the morning; and by lunchtime the faulty module was replaced and the device started working again.
- you should choose VR, because The control system requires constant maintenance. Otherwise, the measurement error of the control unit will significantly exceed the stated values;
- all accompanying documents must be in Russian;
- the flow meter must have an officially approved spill-free verification method and be universal to ensure the possibility of its verification on a water stand;
- the sensitive element of the flow meter must be reliably protected from hydraulic and thermal shocks;
- The design of the flow meter must be modular, with the ability to quickly and conveniently replace each module in the field;
- repair documentation must be provided by the manufacturer at the request of consumers;
- The manufacturer's regional service center must provide the ability to quickly repair a failed flow meter, including directly at the site of operation.
To the recommendations of our fictional characters, we will add from ourselves that when choosing a flow meter, you should make a decision not only on the basis of the numbers highlighted in advertising brochures, but also on other important technical and operational characteristics.
Enjoy Your Bath!