Ph.D., A. V. Kovalenko
The superheated steam meters used determine: pressure, temperature, and, one"Consumption parameter". As already noted, this information is insufficient to determine the heat and mass of wet steam.
In order to ensure the possibility of controlling the heat and mass of wet steam for such meters, it is planned to use calculators with the possibility of entering a correction for the "degree of dryness" parameter. However, such a solution to the problem of controlling the parameters of wet steam, based on the prior art, should be recognized as insufficiently effective.
In the steam lines of superheated steam, the signal of the "flow parameter" of these meters corresponds to the mass flow rate of the controlled flow. Superheated steam flow rate can be represented by the following mathematical expression:
, (1 .1)
where: - consumption of superheated steam;
Superheated steam density;
Superheated steam speed in the steam line;
Controlled flow section.
The density of the superheated steam is a known function of the pressure and temperature of the steam in the controlled steam line.
To determine the flow rate of superheated steam () can be used any acceptable measuring "flow parameter", for example, a measuring diaphragm.
Thus, the flow rate of superheated steam is determined by the measured signals of the “flow rate”, temperature and pressure. This design model is ideal for determining the parameters of superheated steam.
However, superheated steam, in the process of using, or losing, its thermal energy, inevitably becomes wet steam.
The wet steam flow rate can be represented by the following mathematical expression:
, (1.2)
where: - wet steam consumption;
Vapor phase consumption of wet steam (saturated steam phase);
Wet steam liquid phase consumption;
The speed of movement of the liquid phase of the flow.
Saturated steam with saturated vapor temperature; - wet steam; - water with a temperature of saturated vapors.
The densities of the wet steam phases are known functions of the steam pressure in the controlled steam line. Other parameters of wet steam, for example, such as:,,,,, superheated steam meters cannot be determined. In this situation, it makes no sense to correct the “flow rate” signal by the measured value of the degree of dryness, for the reason that this signal does not physically correspond to the flow rate or its phases. Such a signal of a “consumption parameter needs not to be corrected, but ... to be adjusted.
The indicated problem of controlling the heat and mass of wet steam can be shown in detail using specific examples.
Example of a flow measurement system... Steam flow measurement system using pressure pipes of a special design according to the patent for invention No. 2243508 (RU). In this system (device) for determining the flow rate, the static pressure and the pressure difference () are measured between two pressure pipes in a controlled steam flow at the outlet of the reactor, the inlet of one pressure pipe is directed against the flow, and the other is directed downstream.
It is known from published sources that the test results of this system in steam pipelines of nuclear power plants and thermal power plants show the advantage of using pressure pipes over other meters of steam parameters. In particular, their advantage over measuring diaphragms is shown, in reliability and simplicity of design, simplicity and ease of installation, in the practical absence of pressure losses.
In the steam line of a reactor, for example, VVER-1000 power units, wet steam flows with a dryness degree not exceeding 0.98. In this regard, the pressure drop () measured by the two pressure pipes of the device is formed by both phases of the controlled flow. The dependence of this pressure drop across the pressure pipes on the flow parameters can be represented by the following mathematical expression:
(1.3)
where: is the signal coefficient of the two measuring tubes;
True volumetric steam content of wet steam flow;
The speed of movement of the vapor phase of the flow;
The speed of movement of the liquid phase of the flow;
Vapor phase density;
Density of the liquid phase.
The above equation (1.3) containsthreeunknown flow parameters (,,) and coefficient ( ) the signal of the measuring tubes of the device. This system does not receive any other information for solving the problem. In this regard, the problem of determining the flow rate of wet steam cannot be solved without the use of additional information or the introduction of limiting conditions.
The considered device, in order to determine the flow rate of the controlled flow of wet steam, must somehow determine, or, somewhere take values, , and.
This device is used in the control system of the coolant level in the reactors of nuclear power plants. The device's information processing system uses a single-phase flow model. This follows from the text and formulas in its description. Thus, the real presence of a liquid phase, in a controlled flow, is ignored by this device. The main calculation formula of the device under the patent for invention No. 2243508 (RU) can be represented as follows:
(1.4)
That is, equation (1.3) is used with a fixed value (equal to one) of the true volumetric steam content (). Equation (1.4) directly shows how this distorts the calculated value of the velocity parameter of the vapor phase of the flow. The left side of the formula is the measured parameter formed by two phases of the flow moving at different speeds (continuous vapor and, in its volume, dispersed liquid). The right side of the formula is the product of the vapor phase density (a function of static pressure) and the square of the vapor phase velocity of the flow.
Another example... The device according to patent No. 2444726 (RU) contains a steam line with selective (selective) to the properties and parameters of the vapor phase, a meter of "flow parameter" (for example, a Pitot tube, the inlet window of which is directed downstream), a static pressure meter, and a dryness meter.
- On signal static pressure () determine the necessary "tabular" flow parameters, for example: density and specific heat content of its phases:
Vapor phase density;
Density of the liquid phase;
Enthalpy of the vapor phase;
Enthalpy of the liquid phase.
FROM ignored dynamic rarefaction meter (if the coefficient is previously defined or taken somewhere) allows you to determine the velocity of the vapor phase of the flow:
,(2.1)
where: - signal from the dynamic rarefaction meter;
Signal coefficient of the dynamic rarefaction meter;
Vapor phase density;
The vapor phase velocity of the wet steam flow.
- On signal dryness meter determine the ratio of the flow rate of the vapor phase (saturated steam phase) to the total flow rate of the controlled flow:
, (2.2)
The solution of the system of two equations (2.1) and (2.2) with three unknown parameters:,,, and the fourth unknown coefficient is possible only with the involvement of additional information.
The phase slip parameter () can become such additional information for solving the problem. The ratio of the "local" value (true volumetric steam content) to the "consumption" value (consumption volumetric steam content) in the technique referred to as phase slip parameter ( 
). The phase slip parameter () is a weak function of pressure and can be determined by the empirical formula ().
Thus, to solve the problem, a third equation is obtained:
, (2.3)
If we somehow determine or somewhere take the coefficients (,,), the system of three equations (2.1), (2.2), (2.3) with three unknown flow parameters (,,) according to the signals of the device meters (according to patent No. 2444726) allows us to solve the task of controlling the heat and mass of the flow of wet steam. The shown solution looks very cumbersome, but under some implementation conditions, the noted drawback is negligible. It should also be taken into account that the steam parameters determined by this device lag behind the current moment for the time delay of the determined parameter of the degree of dryness (about 30-40 sec).
In the presented work on specific examples shown, that:
- Notable superheated steam meters do not provide the possibility of creating a system for controlling heat and mass of wet and saturated steam.
It should be recognized the futility of units for controlling heat and mass of wet steam using superheated steam meters. By themselves, they do not control the heat and mass of the wet steam flow, and when supplemented with means for controlling the degree of dryness, at best, they form a cumbersome control system that does not provide the required accuracy with a significant delay in the determined steam parameters.
Pay attention to state of the art available for solving control problems heat and mass of wet steam:.
The proposed technical solutions are the core (version) of the system for monitoring the current parameters of wet steam, which provides the ability to standardize the accuracy according to the reference signals of dryness meters. The accuracy of the control of the true volumetric steam content and the velocities of the flow phases is directly normalized. A detailed description of this variant of the system for controlling the heat and mass of the flow of wet steam will be presented later in a separate work.
Literature:
1. Kovalenko A. V. The issue of creating a wet steam control system for accounting tasks
and technological goals. Article on the RosTeplo portal. Published 06.02.2012
2. A.G. Ageev, R.V. Vasilieva, Yu.S. Gorbunov, B.M. Korolkov. Tests of the steam flow measurement system in steam pipelines of steam generators of power unit No. 3 of Balakovo NPP in dynamic modes. / Magazine "New in the Russian electric power industry", No. 11, 2007 /
3. Ageev A.G. and other RF patent for invention No. 2243508. Device for measuring steam flow in a steam pipeline. Bulletin of Inventions, 27.12.2004 / Patent holder of the ERIC/
4. Kovalenko A.V. RF patent for invention No. 2444726 (RU). Device for controlling heat power, mass flow rate, enthalpy and dryness of wet steam flow. Bulletin of inventions No. 7, 2012
5. Tong L. Boiling heat transfer and two-phase flow. M .: Mir, 1969.-344 p.
6. Kovalenko A.V. RF patent for invention No. 2380694 (RU), MKP G 01N 25/60. Method for controlling the degree of dryness of wet steam / A.V. Kovalenko // Bulletin of inventions. 2010. No. 3. No. 2008119269. Priority 15.05.2008
7. Kovalenko A. V. RF Patent for invention No. 2459198 (RU), Device for controlling the degree of dryness, enthalpy, heat and mass flow rates of wet steam. Bulletin of inventions No. 23, 2012
8. Kovalenko A.V. Application for invention No. 2011129977 (RU). Device for determining the degree of dryness of a wet steam stream. Priority from 19.07.2011. Decision to grant a patent for an invention dated 09.07.2012.
9. Kovalenko A.V. Application for invention No. 2011120638 (RU). A method for controlling the true volumetric steam content and the velocities of the phases of the flow of wet steam in the steam line of the steam generator. Priority from 20.05.2011. Decision to grant a patent for an invention dated 12.10.2012.
10. Kovalenko A.V. Application for invention No. 2011121705 (RU). A method for controlling the true volumetric steam content and the velocities of the phases of the flow of wet steam in the steam line on the flow. Priority from 27.05.2011. Decision on granting a patent for an invention dated 12.10.2012.
G.I.Sychev
Head of Flow Meters
Spirax-Sarko Engineering LLC
Water vapor properties
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 its dryness. Often this indicator is neglected when selecting metering and measuring devices, and it is 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 rate and thermal energy. How to solve these problems, we will figure it out today.
Water vapor properties
To begin with, let's define the terminology and find out what are the features of wet steam.
Saturated steam is water vapor in thermodynamic equilibrium with water, the pressure and temperature of which are related to each other and are located on the saturation curve (Fig. 1), which determines the boiling point of water at a given pressure.
Superheated steam - steam 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, it is homogeneous, i.e. homogeneous environment. To some extent, this is an abstraction, since it is difficult to obtain it: in nature it is found only in geothermal sources, and saturated steam produced by steam boilers is not dry - typical values of the degree of dryness 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 is supplied from the outside, it easily becomes overheated, and when heat is released, it becomes moist saturated.
Figure 1. Line of saturation of water vapor
Wet saturated steam (Fig. 2) is a mechanical mixture of dry saturated steam with a suspended finely dispersed liquid in thermodynamic and kinetic equilibrium with steam. Fluctuations in the density of the gas phase, the presence of foreign particles, including those carrying electric charges - ions, lead to the appearance of centers of condensation, which is of a homogeneous nature. As the moisture content of saturated steam increases, for example, due to heat losses or an increase in pressure, the smallest water droplets become centers of condensation and gradually grow in size, while saturated steam becomes heterogeneous, i.e. a two-phase medium (steam-condensate mixture) in the form of fog. Saturated steam, which is the gas phase of the steam-condensate mixture, transfers part of its kinetic and thermal energy to the liquid phase when it moves. The gas phase of the flow carries droplets of the liquid phase in its volume, but the velocity of the liquid phase of the flow is significantly lower than the velocity 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 condensation of steam in horizontal and vertical pipelines changes depending on the ratio of the proportions of the gas and liquid phases (Fig. 3).
Figure 2. PV diagram of steam
Figure 3. Structure of a 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 vapor velocity, the flow of condensate can remain film-like, as in a vertical pipe, and at an average one it can acquire a spiral shape (Fig. 5) , and at low film flow is observed only on the upper inner surface of the pipeline, and at the bottom, a continuous flow, "stream", is formed.
Thus, in the general case, the flow of a vapor-condensate mixture during movement consists of three components: dry saturated vapor, 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 during the movement of the steam-condensate mixture, a relative slip of the phases occurs. Mathematical models of a two-phase flow in a wet saturated steam pipeline are presented in the works.
Figure 4. Structure of two-phase flow in a vertical pipeline
Figure 5. Spiral motion of condensate.
Flow measurement problems
The measurement of the mass flow rate and heat energy of wet saturated steam is associated with the following problems:
1. 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 with the growth of its moisture content, and the dependence of the density of wet steam on pressure at different degrees of dryness is ambiguous;
3. Specific enthalpy of saturated steam decreases with increasing moisture content.
4. Determination of the degree of dryness of wet saturated steam in the flow is difficult.
At the same time, an increase in the degree of dryness of wet saturated steam is possible in two known ways: by "crushing" the steam (by 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 drain. Modern steam separators provide almost 100% dehumidification of wet steam.
Measuring the flow rate of two-phase media is an extremely difficult task that has not yet gone beyond research laboratories. This is especially true for the steam-water mixture.
Most steam flow meters are high-speed, i.e. measure the steam flow rate. These include flowmeters of variable differential pressure based on orifice devices, vortex, ultrasonic, tachometric, correlation, jet flow meters. Coriolis and thermal flow meters stand apart, which directly measure the mass of the flowing medium.
Let's take a look at how different types of flow meters do their job when dealing with wet steam.
Differential pressure flow meters
Differential pressure flow meters based on orifices (diaphragms, nozzles, Venturi pipes 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 the flow rate and amount of liquids and gases by the differential pressure method": According to the conditions for using standard orifice devices, the controlled "medium must be single-phase and homogeneous in physical properties":
In the presence of a two-phase medium of steam and water in the pipeline, the measurement of the flow rate of the coolant by devices of variable differential pressure with normalized accuracy is not ensured. In this case, "one could speak of the measured flow rate of the vapor phase (saturated vapor) of the wet vapor flow at an unknown value of the degree of dryness."
Thus, the use of such flowmeters to measure the flow of wet steam will lead to inaccurate readings.
An assessment of the arising methodological error (up to 12% at a pressure of up to 1 MPa and a degree of dryness of 0.8) when measuring wet steam with variable pressure drop flow meters based on orifice devices was carried out in the work.
Ultrasonic flow meters
Ultrasonic flow meters, which are successfully used for measuring the flow rate of liquids and gases, have not yet found wide application in measuring the flow rate of steam, despite the fact that some of their types are mass-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 for reflecting the beam, and when measuring the flow of wet steam, underestimate the readings due to the difference in the velocities of the gas and liquid phases. On the other hand, pulse-time ultrasonic flowmeters are inapplicable 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. Of fundamental importance is the effect of condensate on the operation of the sensitive element. In some designs, “serious problems arise when measuring the flow rate of saturated steam when both gas and liquid phases exist in the pipeline. Water concentrates along the pipe wall and interferes with the proper functioning of the flush-mounted pressure transducers. " In other designs, condensate can flood the sensor and block flow measurement altogether. But for some flow meters, this practically does not affect the readings.
In addition, a two-phase flow, incident on a body of flow, forms a whole spectrum of vortex frequencies associated with both the velocity of the gas phase and the velocities of the liquid phase (the droplet form of the flow core and the film or jet near-wall region) of wet saturated vapor. In this case, the amplitude of the vortex signal of the liquid phase can be quite significant and, if the electronic circuit does not imply digital filtering of the signal using spectral analysis and a special algorithm for extracting the "true" signal associated with the gas phase of the flow, which is typical for simplified models of flow meters, then strong underestimation of the flow rate readings. The best models of vortex flowmeters have DSP (Digital Signal Processing) and SSP (Fast Fourier Transform Spectral Signal Processing) systems, which not only improve the signal-to-noise ratio, isolate the "true" vortex signal, but also eliminate the influence of pipeline and electrical vibrations. interference.
Despite the fact that vortex flowmeters are designed to measure the flow rate of a single-phase medium, it is shown in the work that they can be used to measure the flow rate of two-phase media, including steam with water droplets, with some degradation of metrological characteristics.
Wet saturated steam with a dryness degree above 0.9 according to the experimental studies of EMCO and Spirax Sarco can be considered homogeneous due to the "margin" in accuracy of PhD and VLM flow meters (± 0.8-1.0%), readings of mass flow and heat 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 rate of these flowmeters 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 by Venturi nozzles on a special installation for calibrating steam flow meters is within ± 3.0% for saturated steam with a dryness degree greater than 0.84.
To avoid blocking of the sensing element of a vortex flowmeter, for example, a sensitive wing by condensate, some manufacturers recommend orienting the flow sensor so that the axis of the sensing element is parallel to the vapor / condensate interface.
Other types of flow meters
Variable Differential / Variable Area Flowmeters, Spring Loaded Flow Meters, and Variable Area Targets do not allow measurement of a two-phase medium due to possible erosive wear of the flow path during condensate movement.
In principle, only Coriolis-type mass flowmeters could measure a two-phase medium, but research shows that measurement errors of Coriolis meters are largely dependent on the phase ratios, and "attempts to develop a universal flowmeter for multiphase media are likely to lead to a dead end." At the same time, Coriolis flowmeters are developing intensively, 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 a two-phase flow of wet steam in steam pipelines.
4. Tong L. Heat transfer during boiling and two-phase flow.- Moscow: Mir, 1969.
5. Heat transfer in two-phase flow. Ed. D. Butterworth and G. Huitt. // M .: Energiya, 1980.
6. Lomshakov A.S. Testing of steam boilers. SPb, 1913.
7. Jesse L. Yoder. Using meters to measure steam flow // Plant Engineering, - April 1998.
8.GOST R 8.586.1-2005. Measurement of 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. // UTSSMS, Ulyanovsk
10. Kuznetsov Yu.N., Pevzner V.N., Tolkachev V.N. Measurement of saturated steam by constriction devices // Thermal Engineering. - 1080.- №6.
11. Robinstein Yu.V. On the commercial metering of steam in steam heat supply systems. // Materials of the 12th Scientific and Practical Conference: Improving the Measurements of Liquid, Gas and Steam Consumption, - SPb .: Borey-Art, 2002.
12. Abarinov, E. G., K.S. Sarelo. Methodical errors in measuring the energy of wet steam by heat meters for dry saturated steam // Izmeritelnaya tekhnika. - 2002. - No. 3.
13. Bobrovnik V.M. Non-contact flow meters "Dnepr-7" for accounting of liquids, steam and oil gas. // Commercial accounting of energy carriers. Materials of the 16th International Scientific and Practical Conference, - SPb .: 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 of heat energy and heat carrier", 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 VN, Rikken M. Flow measurements using Coriolis flow meters in the case of a two-phase flow. // Commercial accounting of energy carriers. XXIV International Scientific and Practical Conference, - SPb .: Borey-Art, 2006.
20. Richard Thorn. Flow Measurement. CRC Press LLC, 1999
- Gas valves (solenoid valves, safety shut-off valves, safety relief valves, shut-off valves and valve blocks)
- Cabinet units with one reduction line and bypass
- Cabinet stations with main and reserve reduction lines
- Manometers, vacuum gauges, pressure gauges showing and signaling
- Pressure gauges, traction gauges and draft gauges showing and signaling
- Associated equipment (diaphragm seals, pulsation dampers, positioners, etc.)
- Temperature meters, meter-regulators and temperature controllers
- Controllers for temperature control in heating systems
- Temperature control devices, multichannel meters and regulators
- Related equipment for level measuring and control devices
- Control valves, mixing valves, shut-off and control valves and water pressure regulators
- Related equipment (tightness detectors, KOFs, thermal covers, etc.)
- Light type industrial gas infrared emitters
- Industrial gas infrared emitters of a dark type
- Air curtains, gas-air heaters, heat generators
- Ceiling, wall (wall) infrared panels and infrared heating tape systems
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Steam consumption metering. The adventures of instrumentation engineers or vortex meters as a real alternative to orifices
Edition: Energy Analysis and Energy Efficiency No. 6. Year: 2006
15.10.2006Currently, increased attention is justly paid to the issues of energy resources accounting. This is determined by the fact that, on the one hand, without the availability of reliable information about the resources consumed, it is impossible to competently carry out energy saving measures, which, in the context of a constant increase in energy prices, is vital both for individual enterprises and each of the industries and the country's economy as a whole. ... On the other hand, in the conditions of a manifold increase in the number of metering devices, the problem of the cost of their maintenance comes to the fore, or rather, maintenance in working order.
Measurement of steam consumption due to the specifics of this medium is distinguished from the field of gas metering tasks. This is primarily determined by high temperatures and pressures in steam pipelines, as well as the presence in them, including as a result of increased pipe wear in these extreme conditions, of various mechanical impurities (corrosion products, scale, etc.), as well as condensate. Therefore, with all the variety of flow measurement methods, there are really only two alternatives for solving the problem of steam metering:
- flowmeters based on the method of variable differential pressure across the restriction device (DC);
- vortex flowmeters (VR).
- Should you choose a flowmeter based only on cost, dynamic range (DD), accuracy, and MTI?
- Do the technical characteristics of Russian-made flow meters really correspond to the best foreign counterparts?
In the head of an average metrologist, the following characteristics of the considered flow measurement methods have developed:
Accordingly, the conclusion is very simple: if there are funds, then it is better to purchase a vortex flowmeter, since it is more accurate and less frequent; if funding is limited, only the "good old" diaphragm remains.
This conclusion could have been the end of the article, if not for the key points outlined in the preamble. Therefore, we propose to forget the images and figures for the studied measurement methods and start the selection of a steam flow meter from scratch.
To begin with, let us recall what are CS flowmeters and vortex flowmeters.
The first consists of a kind of restriction device installed in the pipeline. Usually, a so-called diaphragm is used as a restricting device: a disc whose inner diameter is less than the inner diameter of the pipeline. Due to the local constriction, the diaphragm creates a differential pressure, the magnitude of which is measured by a differential pressure sensor. The absolute pressure of steam in the pipeline and the temperature of the steam are measured simultaneously. If the flow rate of the diaphragm is known, this information is sufficient to calculate the flow rate of gas or steam and, accordingly, determine the amount of product consumed during the reporting period.
The vortex principle of flow measurement is based on the von Karman effect, which means that when a liquid or gas flows around a bluff body, regular vortex formation occurs, i.e. alternate formation and separation of vortices on both sides of the specified body, and the repetition rate of the vortices is proportional to the flow velocity. This vortex formation is accompanied by regular periodic pulsations of pressure and flow velocity in the wake behind the flow body. Accordingly, by measuring the frequency of these pulsations, the velocity or flow rate of the gas or steam under operating conditions can be determined. In order to determine the amount of the passed steam, it is necessary, as in the case of the SU, to additionally measure the pressure and temperature of the steam.
In this article, we will consider the characteristics of two subtypes of vortex flowmeters (VR), which have become widespread in Russia, which differ in the way they detect vortices:
- Pressure or velocity pulsations are recorded by sensors located on the surface of the flow path.
- Pressure pulsations act on the sensing element (wing, tube, piezomicrophone, etc.) behind the body of the streamline, which transmits them to a sensor hidden in the depth of the device.
So, back to the task at hand - we need to install a steam metering unit.
Most likely, the value of steam consumption will change depending on the season, production volumes and other factors, therefore, it is necessary to provide a margin of the measuring range of the flow meter.
The standard ratio of the maximum and minimum values of the flow rate measured by the CS is 1: 3, but can reach 1:10 (if you use multi-range "smart", but also very expensive differential pressure sensors). Not bad already, but the cost of the node in this case will also be set to the maximum of its "dynamic range".
Wide dynamic range is the undoubted advantage of vortex flowmeters. This figure ranges from 1:20 to 1:40. But not everything is smooth here either. Indeed, the conversion factor of a vortex flow meter (i.e., the ratio of the vortex frequency 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 enter individual correction factors that ensure the accuracy of measurements over the entire range. The use of the array of coefficients requires good processing power of the processor, therefore, the latest generation processors must be installed in modern intelligent vortex flowmeters. Unfortunately, not all domestic devices use digital signal processing with correction of the Karman dependence, therefore, 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 measuring principle assumes the detection of flow pulsations. At the same time, external vibrations could be superimposed on the useful signal and even completely block it. Interference led to a decrease in the measurement accuracy and the possibility of an output signal in the absence of flow in the pipeline, the so-called "self-propelled" phenomenon.
Modern intelligent VR analyzes the signal spectrum, eliminating noise and amplifying useful harmonics to ensure accurate measurements. At the same time, the indicators of vibration resistance increased by an order of magnitude on average.
The 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 on the inner surfaces of the flow meter, as well as the likelihood of periodic water hammer and thermal shock. Let's consider the influence of these factors.
The steam temperature can vary from 100 ° C to 600 ° C. At the same time, flowmeters at CS can be used in the entire designated range. However, the measurement accuracy of flow meters at the control system will deteriorate with an increase in temperature, which is associated with a change in the internal diameter of the pipeline and the diameter of the orifice, as well as an 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 the pressure sensor (for example, "Metran-100") is 0.9% per 100 ° C.
The temperature range of BP operation can correspond to 150, 200, 350, 450 0С, depending on the model and manufacturer. Moreover, the last two values correspond to the characteristics of imported devices. We hope that the readers are well aware of the difference between the concept of "the device works and shows something" and "the device works in accordance with the declared characteristics." Very often, BP manufacturers are silent about the additional temperature error associated with a change in the geometric dimensions of the flow path elements. In foreign flow meters, the flow rate readings are automatically corrected by temperature, sometimes reaching 0.2% for every 100 ° C. In domestic intelligent VR, temperature correction is also performed. Therefore, do not forget to check with the manufacturer about the presence of such an error correction when choosing a flow meter.
Clogging of the pipeline and the appearance of deposits on the main elements of the flow transducer over time can nullify your efforts in the selection and installation of a metering unit. The reason is simple: the design of the flow meter on the CS 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 control error increases, which sometimes reaches tens of percent. The adhesion of substance to the surface of the diaphragm, as well as the wear of its edges, contributes to the transformation of the metering unit into a sensor for the presence of a flow in the pipeline. To prevent this from happening, it is necessary to periodically (every two months) clean the flow meter at the control system.
And what about BP? The process of vortex formation is significantly less influenced by contamination than the pressure drop across the BC; moreover, there are simply no cavities and pockets where deposits can accumulate in the BP, so the stability of the latter's readings is much higher. In addition, it has been experimentally proven that vortex formation leads to self-cleaning not only of the flow body itself, but also of the pipeline section at a distance of approximately 1 nominal diameter of the pipeline (DN) before and 2-4 DN after the flow body. The use of special shapes and sizes of the flow bodies made it possible to further reduce the influence of these changes in the geometric dimensions of the flow path of the BP.
Manufacturers today use special shaped streamline bodies. They are designed in such a way that their change affects the measurement accuracy much less than that of a control system and a VR with rectangular or, moreover, cylindrical flow bodies. However, it should be remembered that in our pipelines, together with steam, rags, wrenches and other types of "mechanical impurities" can sometimes be "transported". Therefore, if a filter is not installed before the metering station (at least a coarse mesh), then you should pay attention to VR with removable wrapper... 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 failures in the operation of heat sources and "personal initiative" of the maintenance personnel. In order for the reader to have respect for this phenomenon, we note that water hammer and usually the pressure build-up that follows them lead to rupture of heating batteries and are often the main reason for failure of sensors.
Flow meters at the control system are not afraid of water hammering, and BP was divided into two camps. In BP 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 invalid. In BP based on bending stresses the sensing element is separated from the measured medium, therefore, knows nothing about water hammering.
When steam is supplied through the cooled pipeline, a sharp increase in temperature occurs, while the sensitive elements of the sensor turn out to be highly heated from the inside and cooled from the outside. This increase in temperature is called thermal shock and, accordingly, it also dangerous only for BP pulsations of pressure whose sensitive elements are in close proximity to the measured medium.
Now let's imagine a pipeline on which we will mount 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 can freeze, so they will need to be heated and blown out.
Vortex flowmeters are not whimsical to the place of installation and do not require maintenance. We only recommend that you make sure that the device complies with the climatic version C3 from (-40 to +70) 0С and make sure that the calculator is warm.
By the way about calculators. By itself, the volumetric flow rate of steam, the values of which are given by the flow meter, is of no practical value. It is required to know either the mass of the steam or the heat energy that it carries. For these purposes, heat meters are used that calculate the required parameters based on data from flow, pressure and temperature sensors. The necessary and obligatory functions of the calculator include maintaining an archive of measured parameters, as well as monitoring and recording emergency situations.
It is possible to connect the flow meter to the calculator using a 4-20 mA current signal, which is available, perhaps, for all flow meters, both for control units and for vortex ones.
The advantages of vortex flowmeters include additional output frequency signal... Its advantages are higher accuracy. Note that manufacturers indicate a relative error for the frequency signal and a reduced error for the current output. The quoted error means that the accuracy of the values will deteriorate proportionally with distance from the maximum flow rate. For example, if for a flow meter with a DD of 1:10 the reduced error is, say, 1.0%, then this means that at the maximum flow rate the relative error will indeed be 1.0%, and at the minimum it will already 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.
Overseas manufacturers consider digital output as an additional option, as consumers have long appreciated the benefits of digital communications. In Russia, the opposite situation is still developing: a digital signal is offered as a free bonus, but in reality it is used in rare cases. This is often facilitated by Russian manufacturers of secondary equipment, considering the support of digital input signals unnecessary. In addition, for the passage of a digital signal, higher-quality communication lines are required, 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. An important point to note: choose devices with standardized world-recognized digital protocols HART, Foundation Field Bus, ProfiBus, Modbus. Otherwise, there will be little use from closed standards, which are understandable only to the manufacturer of the device.
Let us return, however, to the pipeline and the place of installation of the steam metering unit. Most flow measuring instruments should be installed on straight pipeline sections with a length of 1 to 100 nominal diameters (DN). The longest straight sections from 30 to 100 DN are required for flowmeters with CS. Failure to comply with these requirements leads to a distortion of the uniformity of the flow of the medium and, as a consequence, a decrease in the measurement accuracy.
In comparison with SU, BP impose less stringent requirements on the lengths of straight sections. The corresponding recommendations are 30 DN, with a possible reduction to 10 DN, depending on the configuration of the pipeline. In most cases, a reduction to 10 DN without deterioration of accuracy is possible only after the introduction of additional correction factors that take into account the specifics of the installation site.
Note that some Russian BP manufacturers report about "victory over the laws of hydrodynamics" and indicate the requirements for straight sections from 3 to 5Du, which is 2 or even 3 times better than foreign samples. Let's leave the underestimation of the requirements for the lengths of straight sections on the conscience of these manufacturers. And we recommend that consumers not engage in self-deception and install BP on pipelines with straight sections of at least 10Du, and CS - 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 go different ways of solving the problem of steam metering and chose, respectively, a meter based on CS, an imported steam metering unit based on BP, and a domestic steam metering unit based on BP. At the same time, Shaibov was primarily guided by the cost of the metering unit. Fishkin decided to fork out, believing that "a miser pays twice," and bought 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 bending stress flowmeter. Let's watch our characters.
Trouble awaited our heroes already at the first stage, when buying flow meters.
During the calculations, Shaibov 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 became 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 include, perhaps, difficulty in choosing from a large assortment of calculators. (However, we would like not to touch upon the problem of choosing a calculator in this article, so we will trust the choice of Vikhrev and will not even ask him which computer he purchased).
Finally, all the engineers received the equipment, it remains to install it and the first stage is completed. Vortex managed the fastest, because the technological insert and the set of mounting parts were supplied together with the flow meter. Shaibov had to spend much more time in order to comply with all the mandatory requirements for the diaphragm installation: to ensure that the diameters of the pipeline and diaphragm bodies match, the CS and pipeline alignment, connect the CS chamber with the pressure drop sensor with impulse lines. Shaibov also had to come to terms with the fact that the accuracy of the metering unit will be lower than declared due to unaccounted factors: roughness of the pipeline and the discrepancy between the actual internal diameter of the pipeline and the calculated data.
The installation of the metering unit based on imported equipment went smoothly thanks to the well-illustrated operating instructions. However, the local dealer threw a fly in the ointment, refusing to supply a set of mounting parts for the flow meter and shifting its manufacture to Fishkin. Fishkin's joy over the successful installation of the node was also short-lived, since the programming of 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 in setting up the equipment, so all questions were forwarded 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 unit is handed over. However, time passed and Shaibov had a suspicion that the testimony of the SU did not correspond to reality. 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 the expected, however, the conclusion was disappointing: cleaning of the assembly is required every two months.
Fishkin and Vikhrev watched their colleague's bustle with some gloating, thinking that they would remember their nodes at BP only three years later, when the time came for their verification. However, the issued resolution of the local UCM dispelled expectations: the region introduced an order on the verification of all flow meters-heat energy meters every year, regardless of the prescription of federal decrees.
Shaibov's finest hour came: the entire verification of the metering unit resulted in another diaphragm removal (during a year of friendship with the SU, the engineer learned to quickly remove the diaphragm, since he carried out this procedure regularly) and measured its geometry in the presence of a representative of the CSM, as well as in the verification of pressure and temperature sensors ...
An imported Fishkin flowmeter can be verified in two ways: by pouring the device on a water stand or using a non-spill method. The second option turned out to be more preferable. The verification procedure turned out to be quite simple: measurement of the geometry of the streamline body and verification of the electronic unit. True, Fishkin had to additionally purchase a special expensive verification kit, which could have been dispensed with if the device used standard rather than unique proprietary connectors.
Vikhrev was ready for the verification procedure and even waited for it, since at the purchase stage he made a choice in favor of BP bending stresses, which, due to their versatility, can be verified not only on air, but also on a water calibration stand, which is available in any regional center ... A pleasant surprise for Vikhrev was the presence of an officially approved method of non-spill verification, similar to the Fishkin flow meter.
Finally, we suggest you imagine that the engineers have out of order flow meters. We only have pity on Shaibov: after all, he already does not depart from the SU, being an integral part of the metering unit. Let the breakdowns of the Fishkin and Vikhrev flow meters be of the same nature, let us, for example, imagine that both devices have a frequency output out of order due to the fault of a worker who has mixed up the polarity of the contacts.
So, complaining about the workers, Fishkin and Vikhrev began to study the operating manuals for the flow meter. Using the built-in self-diagnostics function, Fishkin made sure that only the frequency output was out of order. Having called the service center (SC), he learned that replacing the electronics is a five-minute procedure, thanks to the modular design of the device. However, the SC refused to provide repair documentation and a replaceable module, explaining such secrecy by the policy of the manufacturer's company. Fishkin had to send the device to the SC, where, as it turned out later, just such a module was not in stock at the moment, so it was ordered abroad. So much for your five-minute procedure. However, wait, Fishkin, wait. You are used to it.
Vikhrev also called the SC and even, knowing Fishkin's misadventures, was ready to send the device there. But in 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 on his own, or removing the device and sending it to the nearest SC. Seeing that replacing the electronics requires only unscrewing a couple of bolts, without having to dismantle the entire flowmeter and even more so stop the steam supply in the pipeline, Vikhrev decided to carry out the repair himself. A couple of days later, a replacement electronic module was sent to Vikhrev from the manufacturing plant, which he received in the morning; and by lunchtime the faulty module was replaced and the device started working again.
- you should choose BP, because SU requires constant maintenance. Otherwise, the error in measuring the control system will significantly exceed the declared values;
- all accompanying documents must be in Russian;
- the flow meter must have an officially approved non-spill calibration method and be universal to ensure the possibility of its verification on a water bench;
- the sensitive element of the flow meter must be reliably protected from hydro and thermal shock;
- the design of the flow meter should be modular, with the possibility of quick and easy replacement in the field of each of the modules;
- repair documentation must be provided by the manufacturer at the request of consumers;
- the manufacturer's regional SC should provide the ability to quickly repair a failed flow meter, including directly at the site of operation.
We add to the recommendations of our fictional characters that when choosing a flow meter, a decision should be made not only on the basis of the figures highlighted in advertising brochures, but also on other important technical and operational characteristics.
Enjoy Your Bath!
1. Measuring the flow rate of water vapor
The calculation of the orifice device for measuring the flow rate (Q 0) of water vapor is carried out according to the following procedure
Determine the missing data for the calculation
The absolute pressure of the measured medium in front of the orifice is determined as the sum of the barometric and gauge pressures
where - barometric pressure (P b = 1 kgf / cm 2 = 9.8066 * 10 4 Pa);
Overpressure().
Density of the measured medium under operating conditions (and t = 340 0 С).
Appendix 3
We determine the value of D corresponding to the operating temperature t = 340 0 С of the substance in the pipeline according to the formula:
where is the inner diameter of the pipeline in front of the restriction device at a temperature of t = 20 0 С (D = 200 mm);
Average coefficient of linear thermal expansion of the material of the restricting device (pipeline) in the range from 20 to t ° С, 1 / deg
t is the temperature of the measured medium in front of the orifice (t = 340 0 С).
Dynamic viscosity of the measured medium under operating conditions
|
Temperature, 0 С |
|||||||
|
Dynamic viscosity, 10 -5 Pa * s |
We accept.
We take the adiabatic exponent equal to k = 1.38.
Accept the constriction device Nozzle, guided by the following considerations
a) at the same values of the modulus and pressure drop, the nozzle allows you to measure a higher flow rate than the diaphragm, and at D? 300 mm also provides a higher measurement accuracy in comparison with the diaphragm (especially with small modules);
b) at the same values of the modulus and flow rate, the pressure loss in the nozzle is much less than in the diaphragm;
c) the accuracy of measuring the flow rate of gases and steam when using a nozzle is higher than when using a diaphragm;
d) change or contamination of the inlet profile of the orifice during operation affects the flow rate of the diaphragm to a much greater extent than the flow rate of the nozzle.
1.3. The upper measurement limit of the differential pressure gauge Q P (Q OP, Q NI, Q MP) is selected according to the specified maximum measured flow rate Q max = 0.8 m 3 / s = 2880 m 3 / h so that the standard value of Q P is the closest greater in relation to to the value of Q m ax. Thus, we take Q P = 3200 m 3 / h.
1.4. We accept the narrowing device module for the following reasons:
When using nozzles and Venturi nozzles, the inaccuracy of the correction for the Reynolds number DQ has the least effect on the flow coefficient, when 0.5? m? 0.65.
Thus, we take m = 0.5.
1.5. By the value of m I calculate:
Consumption coefficient a And according to the formula:
a H = 0.9100 + 0.6258m - 1.4m 2 + 1.6667m 3, with m = 0.5 a H = 1.0812;
The value of the consumption coefficient b according to the formula:
a = a I * k 2,
where k 2 is a correction factor for pipe roughness (k 2 = 1.005).
steam pressure analog switch
a = .0812 * 1.005 = 1.0866.
1.6. Determine the limiting nominal differential pressure of the differential pressure gauge DRn. Let the permissible pressure loss in the orifice be given at the highest measured flow rate Qmax.
Determine the permissible pressure loss P PD at a flow rate equal to the selected upper limit of the differential pressure gauge Q P = 3200 m 3 / h.
The limiting differential pressure of the differential pressure gauge ДРн is selected from a number of standard numbers. Therefore, DRn = 250 kPa.
1.7. Determine the Reynolds number at a flow rate equal to Q CP = 2520 m 3 / h.
Because calculated Reynolds number> for a given module m = 0.5, then we continue the calculation further.
1.8. We determine the largest pressure drop in the restricting device for ring, bellows and membrane differential pressure gauges according to the formula:
1.9. We determine the correction factor by the formula:
1.10. Calculating the ratio
1.11. We determine the correction factor by the formula:
1.12. We calculate (with four significant digits) the desired value d 20 of the diameter of the orifice of the orifice at 20 ° C:
1.13. For float differential pressure gauges filled with mercury, above which there is a gas with a density of 14 kg / m 3, or oil, above which there is a gas with a density of 0.9 kg / m3, as well as for ring, bell, bellows and membrane differential pressure gauges, we determine the volumetric flow corresponding to the highest differential pressure
Influence of switching circuits of power unit heaters on the thermal efficiency of heating
The first step in calculating the PFS is to determine the states of steam in the turbine stages. To do this, build the process of steam in the turbine in the h, S-diagram. We use the technique ...
Modernization of the power supply system of the cement plant
The heat balance is carried out: In accordance with VNTP 06-86, we select the steam parameters: T = 187.9 0C P = 1.2MPa Where the heat capacity of fuel oil in kcal / (kg * 0C) is calculated according to the formula cT = 0.415 + 0.0006 * t, t is the temperature fuel, 0С. The average fuel oil temperature is taken in winter - -20, summer - 20 ...
Project of a 450 MW condensing power plant in Nazarovo
The coefficient of underdevelopment of the heating extraction power is: For the first extraction: (4) where is the enthalpy at the outlet of the turbine, kJ / kg; - enthalpy of steam at the inlet to the superheater, kJ / kg; - enthalpy of steam at the exit from the superheater, kJ / kg ...
CHP project with a capacity of 500 MW
The coefficient of underutilization of the power of heating extractions: for the first extraction: (30) for the second extraction: (31) The steam consumption for network heaters is determined from the heat balance equation: (32) (33) Taking the regeneration coefficient Kр = 1 ...
CHP project with the development of invariant ACS
Steam consumption for the turbine is determined by the formula:. Then: kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s, kg / s. Turbine power: = 80 MW - power ...
GRES design
The coefficient of underutilization of the power of the heating selection to the lower network heater: (2.21) where iotb7 is the enthalpy of steam in the selection to the lower network heater from Table 2.2, kJ / kg; iк is the enthalpy of steam in the condenser from table 2.2 ...
This course project uses the variable differential pressure method to measure steam flow. This method is based on the fact that the flow of steam flowing in the pipeline ...
Design of steam flow and temperature control systems
To measure the temperature of the steam, we use a thermoelectric thermometer - thermocouple XK (chromel drops). A thermocouple is two conductors (thermoelectrodes) made of different metals, soldered at one point ...
Designing a thermal diagram of a CHP plant for an industrial enterprise and a residential area
Measurement of the flow rate and mass of substances (liquid, gaseous, bulk, solid, vapors, etc.) is widely used both in inventory and reporting operations, and in the control, regulation and management of technological processes ...
Development of a variable differential pressure flowmeter with a Venturi tube
It is required to calculate the reduced temperature of the superheated water vapor tpr and the reduced pressure ppr to determine the coefficient of dynamic viscosity h. According to the reference book: where t is the temperature of water vapor,? C; t = 500? C ....
Calculation of the basic thermal diagram and technical and economic indicators of a power plant (power unit with a PT-135 / 165-130 / 15 turbine)
power unit steam turbine deaerator Determination of the preliminary steam consumption for the turbine. The coefficient of underutilization of the power of industrial selection:; where Hi = i0-ik, hpr = i0-i3 are the used heat drops in the steam flow. Hi = 3471.4-2063.26 = 1408.14 kJ / kg. hpr = 3471 ...
Calculation of the working circuit of a nuclear power plant
The amount of steam taken for the technological needs of two-circuit NPPs (steam consumption for auxiliary needs of the SN) is determined by the NPP capacity, the features of the operating principle of the NPP and NPP taken into account as a whole ...
Calculation of the thermal scheme of the turbine K-800-240
The calculation of the basic thermal diagram by the method of successive approximations is based on a preliminary estimate of the steam flow rate for the turbine using the regime diagram or by approximate formulas ...
Calculation of the low pressure cylinder (LPC) of the turbine K-300-240-1
The thermal diagram of the installation is adopted according to the prototype. The number of extractions, the steam pressure in the extractions and the steam flow rate in each extraction are selected according to the tables presented in the appendix ...
The state of the steam is determined by its pressure, temperature and specific gravity. The pressure of the vapor contained in the vessel is the force with which it presses on a unit surface of the vessel wall. It is measured in technical atmospheres (abbreviated at); One technical atmosphere is equal to a pressure of 1 kilogram per square centimeter (kg / cm2),
The value of the steam pressure, which the boiler walls, is determined by the pressure gauge. If, for example, the one installed on a steam boiler shows a pressure of 5 atm, then this means that every square centimeter of the boiler wall surface experiences a pressure of 5 kg from the inside.
If gases or vapors are pumped out of a hermetically sealed vessel, then the pressure in it will be less than the external one. The difference between these pressures is called rarefaction (vacuum). For example, if the external pressure is 1 atm, and in the vessel it is 0.3 atm, then the vacuum in it will be equal to 1-0.3 = 0.7 atm. Sometimes rarefaction is measured not by fractions of the atmosphere, but by the height of the liquid column, usually mercury. It is calculated that the pressure of 1 technical atmosphere, that is, 1 kilogram per 1 square centimeter, creates a column of mercury 736 mm high. If the vacuum is measured by the height of the column pTyfra, then in our example it is obviously equal to: 0.7X736 = 515.2 mm.
The vacuum is determined by vacuum gauges, which show it in fractions of the atmosphere, or by the height of the mercury column in millimeters.
Temperature is the degree of heating of bodies (steam, YODA, iron, stone, etc.). It is determined by a thermometer. As you know, zero degrees on the Celsius scale corresponds to the temperature of ice melting, and 100 degrees to the boiling point of water at normal atmospheric pressure. Degrees in Celsius are indicated by ° C. For example, a temperature of 30 degrees Celsius is indicated like this: 30 ° C.
The specific gravity of steam is the weight of one cubic meter (m3) of it. If it is known, for example, that 5 m3 of steam has a weight of 12.2 kg, then the specific gravity of this steam is 12.2: 5 = 2.44 kg per cubic meter (kg / m3). Consequently, the specific gravity of steam is equal to its total weight (in kg) divided by its total volume (in m3).
The specific volume of steam is the volume of one kilogram of steam, i.e. the specific volume of steam is equal to its total volume (in m3) divided by its total weight (in kg).
The higher the pressure under which the water is, the higher its boiling point (saturation), therefore, each pressure has its own boiling point. So, if a pressure gauge installed on a steam boiler shows a pressure, for example, 5 atm, then the boiling point of water (and steam temperature) in this boiler is 158 ° C. If the pressure is raised so that the pressure gauge shows 10 atm, then the temperature of the steam also rises and will be equal to 183 ° C.
Let us now consider how steam is obtained.
Let us assume that there is ioda in the glass cylinder under the piston. The piston fits snugly against the cylinder walls, but at the same time can move freely in it (1, /). Let us also assume that a thermometer is inserted into the piston to measure the temperature of water and steam in the cylinder.
We will heat the cylinder and at the same time observe what happens to the water inside it. First, we will notice that the temperature of the water rises, and its volume increases slightly, and the piston in the cylinder begins to slowly move upward. Finally, the water temperature rises so much that the water boils (1, //). Bubbles of steam, with force flying out of the water, will carry away its particles in the form of splashes, as a result of which the space above the boiling water will be filled with a mixture of steam and water particles. Such a mixture is called wet saturated steam or simply wet steam (I, III).
Continuing to boil, we will notice that there is less water in the cylinder, and more and more wet steam. Since the volume of steam is much greater than the volume of water; from which it turned out, then as the water turns into steam, the internal volume of the cylinder will increase significantly, and the piston will quickly go up.
Finally, the moment will come when the last particle of water in the cylinder turns into steam. Such steam is called dry saturated (1, / K), or simply dry. The temperature of steam and water during boiling (saturation temperature) remains constant and equal to the temperature at which the water began to boil.
If the heating of the cylinder is continued, the temperature of the steam will rise and at the same time its volume will increase. Such steam is called superheated (1, V).
If the heating of the cylinder is stopped, then the steam will begin to give off heat to the environment, while its temperature will decrease. When it reaches the saturation temperature, the steam will turn back to dry saturated. Then it will gradually turn into a liquid, therefore, the steam will become wet. This process takes place at a constant temperature equal to the temperature! kipedia. When; last part | the steam will turn into water, the boiling of the water will stop. Then there will be a further decrease in temperature to the ambient temperature.
From the above, the following conclusions can be drawn.
First, the steam can be wet, dry, and superheated. Dry steam is very unstable and even with the slightest "heating *" or cooling it becomes superheated or humid, so that in practice the steam is only humid or superheated.
In the second, observing the boiling of water in it through the walls of a glass cylinder, one can notice that at the beginning of boiling, when there is still a lot of water in the cylinder, the vapor has a dense milky-white color. As the water boils away, when it becomes less and less in the vapor, the density of this color decreases; the vapor becomes more transparent. Finally, when the last particle of water turns into vapor, it will become transparent. Consequently, water vapor itself is transparent, and the water particles that it contains give it a white color. There can be different amounts of water particles in wet steam. Therefore, in order to have a complete understanding of wet steam, you need to know not only its pressure, but also the degree of dryness. This value shows; what amount of dry steam in fractions of a kilogram is contained in one kilogram of wet steam. For example, if one kilogram of wet steam consists of 0.8 kg of dry steam and 0.2 kg of water, then the degree of dryness of such steam is 0.8. The dryness of wet steam produced in steam boilers is 0.96-0.97.
Thirdly, in the experiment performed, the load on the piston did not change, which means that the pressure of the superheated steam (just like that of blissful dry steam) remained unchanged during the experiment, but its temperature increased as it warmed up. Consequently, at the same pressure, the temperature of the Superheated steam can be different. Therefore, to characterize such a vapor, not only its pressure is indicated, but also its temperature.
So, to characterize wet steam, you need to know its pressure and degree of dryness, and to characterize superheated steam, its pressure and temperature.
In fact, superheated steam began to form only after there was no water left in the cylinder, therefore, when there was. water, only wet steam can be obtained. YU
Therefore, steam in steam boilers can only be wet. If it is necessary to obtain superheated steam, the wet steam is removed from the boiler into special devices - steam heaters, thus separating it from water. In superheaters, steam is additionally heated, after which it already becomes superheated.
Although to obtain superheated steam, a superheater device is required, which complicates the boiler installation, but due to the advantages that superheated steam has over wet steam; it is used more often in ship installations. The main of these advantages are as follows.
1. When superheated steam is cooled, its condensation does not occur. This property of superheated steam is very important. No matter how good the pipes are insulated, through which the steam flows from the boiler to the machine and the steam cylinder of this machine, they still conduct heat, and therefore the steam, in contact with their walls, is cooled. If the steam is overheated, then cooling is associated only with a decrease in its temperature and specific volume. If the steam is wet, it condenses, that is, part of the steam turns into water. The formation of water in the steam line and especially in the cylinder of the steam engine is harmful and can lead to a major accident.
2. Superheated steam gives off heat worse than wet, therefore, in contact with cold walls of pipelines, cylinders, etc., it cools less than wet. In general, when working with superheated steam, a savings in fuel consumption of 10-15% is obtained.