Reliability of thermal power systems tests. Methods for calculating the reliability of thermal power equipment of thermal power plants

A modern energy enterprise (thermal power plant, boiler house, etc.) is a complex technical system consisting of individual installations united by auxiliary technological connections.

An example of such a technical system is a basic thermal diagram (PTS) of a thermal power plant, which includes a wide list of main and auxiliary equipment (Fig. 5.1): steam generator (steam boiler), turbine, condensing unit, deaerator, regenerative and network heaters, pumping and draft equipment, and etc.

The basic thermal diagram of the station is developed in accordance with the used thermodynamic cycle of the power plant and serves to select and optimize the main parameters and flow rates of the working fluid of the installed equipment. The PTS is usually depicted as a single-unit and single-line diagram. Identical equipment is shown on the diagram conditionally once; technological connections of the same purpose are also shown as one line.

In contrast to the basic thermal diagram, the functional (full or expanded) diagram of a thermal power plant contains all the main and auxiliary equipment. That is, the full diagram shows all units and systems (working, backup and auxiliary), as well as pipelines with fittings and devices that ensure the conversion of thermal energy into electrical energy.

The complete diagram determines the number and sizes of main and auxiliary equipment, fittings, bypass lines, starting and emergency systems. They characterize the reliability and level of technical excellence of the thermal power plant and provide for the possibility of its operation in all modes.

Based on their functional purpose and impact on the reliability of operation of a power unit or thermal power plant as a whole, all elements and systems of the functional diagram can be divided into three groups.

The first group includes elements and systems, the failure of which leads to a complete shutdown of the power unit (boiler, turbine, main steam pipelines with their fittings, condenser, etc.).

Rice. 5.1. Functional and structural diagrams of a steam turbine power unit: 1 - boiler; 2 - turbine; 3 - electric generator; 4 - condensate pumps; 5 - deaerator; 6 - feed pumps

The second group includes elements and systems, the failure of which leads to a partial failure of the power unit, i.e., a proportional decrease in electrical power and released heat (draft machines, feed and condensate pumps, boilers in double-block circuits, etc.).

The third group includes elements whose failure leads to a decrease in the efficiency of a power unit or power plant without compromising the production of electrical and thermal energy (for example, regenerative heaters).

The reliability of the work of all these groups turns out to be interconnected.

Calculation of quantitative indicators of the reliability of complex technical systems, such as thermal power plants, requires the preparation of structural (logical) diagrams, which, unlike functional ones, reflect not physical, but logical connections.

Block diagrams make it possible to determine the number or combination of failed circuit elements that lead to the failure of the entire system.

As an example in Fig. 5.1 shows the principal thermal and structural diagrams of a steam turbine power unit.

The degree of detail of the structural diagram is determined by the nature of the problems being solved. As elements of a structural diagram, it is necessary to select equipment or a system that has a specific functional purpose and is considered as an indecomposable whole that has reliability data.

Quantitative indicators of the reliability of thermal power plants can be obtained by calculating using known reliability characteristics of elements and functional-structural diagrams or by processing statistical data on their operation.

Accordingly, all methods for calculating the reliability of thermal power equipment of thermal power plants and their structural diagrams can be divided into three groups:

• analytical methods;
• statistical methods;
• physical methods.

From the introductory part it is already clear that the main object of consideration in this section is a thermal power plant as a complex technical system. To calculate the reliability indicators of such vehicles, taking into account the actual conditions of their operation, structural calculation methods are used.

That's why Special attention In the future, we will focus specifically on analytical methods of calculation.

The operation of power boilers is accompanied by complex physical and chemical processes in the steam-water path, in the gas-air path, in the metal from which the elements of power equipment are made.

The processes of combustion, heat transfer, corrosion, the formation of deposits on heating surfaces, changes in the properties and characteristics of the metal largely determine the reliability indicators of boilers.

In Fig. Figure 2.10 shows the distribution of failures of boiler equipment of power units of thermal power plants. As you can see, the greatest damage to boiler equipment occurs due to operating errors. A significant proportion of failures occur due to design flaws and poor quality of repairs.

Typical failures due to design flaws in boilers are large thermal gaps on heating surfaces and accelerated ash wear. During the manufacturing process of boilers, violations occur in the process of bending, casting, heat treatment of parts made of heat-resistant steels, and welding.

During operation, it is possible that the actual characteristics of coals do not correspond to the standard ones, which leads to a deviation from the specified values ​​of the volumes of combustion products and the temperature at the outlet of the furnace. The consequence of this is a disruption in the operation of the convective part of the boiler and an increase in ash wear of the heat exchange pipes. Poor quality of water and steam leads to a sharp increase in deposits, an increase in the temperature of the pipe metal and to their burnout.

Rice. 2.10.

The failure rate of the main elements of boiler units is not the same. For example, the classification of damage to boiler equipment of 300 MW power units is as follows (Table 2.1).

Table 2.1

Failure rate of the main elements of the boiler installation of a 300 MW power unit

From the table 2.1 shows that the overwhelming majority of boiler installation failures are associated with malfunctions in the operation of heating surfaces.

Reliability, durability and other indicators of the reliability of the heating surface itself depend on the nature and intensity of combustion processes, heat transfer, corrosion, deposits, and changes in the properties of metals. Moreover, the frequency of malfunctions in general for the heat exchange surface is fairly evenly distributed over the characteristic surfaces (Fig. 2.11). Screen pipes and superheater pipes (KPP1 and KPP2) are somewhat more often damaged.

During operation, screen pipes are exposed to radiant energy and the corrosive environment of fuel combustion products, which, at low circulation speeds and disturbances in the water regime, leads to their damage and failure of the boilers (Fig. 2.11).

Rice. 2.11.

by element

The uneven temperature field along the height of the gas duct in which the superheater is located, which leads to thermal distortions, has a noticeable effect on the damageability of gearbox pipes.

Superheaters are also damaged because during long-term operation at temperatures above 500 °C, the metal structure undergoes undesirable changes.

When boilers operate on solid fuel, wear of flues due to fly ash occurs due to impacts of its particles on the surface. As a result, the oxide film on the limiting surfaces is destroyed and erosion develops. Wear is most often uneven. The greatest wear rate occurs in areas of high speeds and in flows with the highest ash concentration.

In order to reduce ash wear, the speed of flue gases in chimneys is limited to 7... 10 m/s. On the other hand, at speeds below 3 m/s, ash drifts occur, causing an increase in resistance and a deterioration in heat transfer.

The strength of welds is affected by temperature changes and corrosion processes. Corrosion occurs most intensely when burning high-sulfur fuel oils. Fistulas (Fig. 2.12) occur in contact welded joints due to misalignment of pipes, constriction of the internal section, lack of fusion, and cracks.

Rice. 2.12.

with a defective seam

The operating time from the start of operation or overhaul to the formation of a fistula depends on the nature and size of the defect and operating conditions, water quality, cyclicity and amplitude of unit load fluctuations, and the quality of installation of the water economizer.

In most cases, when damage occurs in one pipe, bend or weld, the flowing stream of water also destroys adjacent pipes. By the time the boiler is turned off and cooled down, several adjacent pipes are damaged.

Rice. 2.13.

Typical for furnaces are damage to the screens protecting the walls of the combustion chambers (radiation steam superheater and radiation water economizer).

A view of the damaged front screen pipe is shown in Fig. 2.13.

In the boiler drums, breakages of cyclones, perforated and louvered sheets, and fasteners occur, which, falling into the openings of the drain pipes, block them. The speed of movement of the steam-water medium in the screens decreases, the metal of the pipes overheats and collapses.

The welds in the screens are damaged and fistulas are formed.

In supercritical pressure boilers, radiant superheater tubes are damaged due to high-temperature corrosion, leading to significant wear of the walls on the fire heating side. This occurs under high thermal loads. Thermal distortions are caused by an uneven temperature field along the height of the flue.

Creep and accompanying pipe damage (microcracks) appear more intensely in bends than in straight pipes. This forces periodic replacement of individual elements or entire stages of the superheater.

Failures also occur from uneven expansion of pipes and unequal weight loads - welds are in a complexly stressed state.

Sharp fluctuations in boiler loads also lead to the occurrence of unacceptable stresses in welds and heat-affected zones, causing the formation of cracks, breaks of fasteners and pipes.

Damage to drums and pipelines

Of particular importance in ensuring the reliability of boilers are boiler drums and bends of unheated pipes. Although great attention is paid to the reliability of drums during design, manufacture, operation and repairs, damage often occurs in them, leading to long shutdowns of boilers.

Rice. 2.14.

These are cracks located in the area of ​​the pipe holes, in the metal of the cylindrical part of the drum, on the inner surface of the bottoms, in the heat-affected zone of welding of intra-drum devices to the housings (Fig. 2.14), as well as defects in the main annular and longitudinal seams.

The main reason for the formation of damage is the excess of the acting stresses to the yield strength of the material, leading to the appearance of residual deformation. Increased stresses arise due to the presence of a temperature difference across the wall thickness along the perimeter and along the length of the drum.

Of particular importance in this case are cyclic heat changes on the surface layers of the metal on inside walls at sudden shifts temperature. These non-stationary modes of the boiler are especially dangerous during its starts and stops.

The development of cracks is facilitated by the action of corrosive boiler water on the metal. It enhances corrosion-fatigue processes in the metal of drums.

The most dangerous defects are in the main welds - they create the risk of major destruction. More often than others, longitudinal and transverse cracks occur in the deposited weld on the inner surface. Lack of penetration, slag inclusions, cavities, and pores are observed.

The depth of cracks varies, but there are cases when in 1 year it reached 70% of the thickness.

Bends on pipelines are most often damaged. This is where corrosion fatigue damage occurs. Insufficient compensation for thermal expansion causes increased stress.

Bends of supply, water-draining and steam-discharge pipes fail brittlely; bends of superheated steam pipelines operating under creep conditions are deformed upon destruction.

Components of reliability properties.

Reliability is a complex property, which, depending on the purpose of the object and the conditions of its use, consists of combinations of the following properties:

Ø reliability;

Ø durability;

Ø maintainability;

Ø Storability.

Reliability– this is the property of an object to continuously maintain its functionality for a given time.

Durability– this is the property of an object to maintain operability until a limit state occurs with an established system of maintenance and repairs.

Limit state of an object- this is a condition in which its further use is unacceptable due to safety conditions or is not economically feasible, or restoration of its working condition is technically impossible or economically inexpedient. The limiting state of an object can occur, firstly, in an operational installation with an unacceptable decrease in its safety or economic efficiency indicators; secondly, for an installation that is inoperable as a result of such a failure, after which restoration of the facility’s operability is technically impossible or economically unjustified.

Maintainability– this is a property of an object, which consists in adapting, firstly, to preventing and detecting the causes of failures by monitoring the serviceability of constituent elements and systems and, secondly, to maintaining and restoring an operational state by carrying out maintenance and repairs of equipment. To ensure the maintainability of the facility, it is necessary to have effective diagnostics condition of the facility and carrying out high-quality maintenance and repairs.

Storability– this is the property of an object to maintain the values ​​of reliability, durability and maintainability during or after storage and transportation.

Limit state of equipment.

Limit state of an object- this is a condition in which its further use is unacceptable due to safety conditions or is not economically feasible, or restoration of its working condition is technically impossible or economically inexpedient. The limiting state of an object can occur, firstly, in an operational installation with an unacceptable decrease in its safety or economic efficiency indicators; secondly, for an installation that is inoperable as a result of such a failure, after which restoration of the facility’s operability is technically impossible or economically unjustified. IN new edition limit state - the state of an object in which its further operation is unacceptable or impractical for reasons of danger, economic or environmental.

Maintainability of equipment.

Maintainability– this is a property of an object, which consists in adapting, firstly, to preventing and detecting the causes of failures by monitoring the serviceability of constituent elements and systems and, secondly, to maintaining and restoring an operational state by carrying out maintenance and repairs of equipment. To ensure the maintainability of an object, it is necessary to have effective diagnostics of the condition of the object and carry out high-quality maintenance and repairs. In the new edition, maintainability is the ability of an object, under given conditions of use and maintenance, to maintain or restore a state in which it can perform the required function.

Concept of failure of power equipment.

By definition, operability is the state of an object to perform a given function, maintaining the values ​​of specified parameters within the limits established by regulatory and technical documentation. In relation to power plants, their operability is defined as a state in which they can bear electrical and thermal loads with the corresponding parameters within the limits specified in the operational documents.

Failure is called loss of performance, i.e. transition to a state in which the value of at least one parameter characterizing the ability to perform specified functions does not meet the requirements established by regulatory and technical documentation. For power plants failures are associated with a decrease in available power or parameters of electrical and thermal energy.

Reliability characteristics of restored objects.

Maintainability characteristics.

1. The law of object restoration

2. Recovery intensity

3. Average recovery time

4. The law of object durability

5. Average resource and average service life of equipment

6. Assigned resource and service life of equipment

Model of fracture of a body with cracks.

See 28.

The process of ductile fracture.

Ductile fracture occurs after significant plastic deformation. The process of changing the structure of the metal is shown schematically in Fig. The initial structure of the metal, which can be observed under a microscope with 1000x magnification (View 1), is a network of grains of approximately the same size. The field of grains is uniform, there are no visible inclusions of impurities, in particular carbon compounds - carbides. In some cases, it is possible to use lower quality metal, which contains a number of small inclusions that stand out against the background of the grains.

The nucleation and development of discontinuities begins at grain boundaries. The first cracks always originate from the outer surface of the part. The nature of the distribution of metal microdamages depends on the tensile stress. At high stresses, microdamages are localized near the fracture surface; at low stresses, they are distributed evenly along the length of the sample.

At the initial stage, individual pores appear (type 2), with increasing plastic deformation the number of pores increases, individual pores are combined into chains (type 3). Subsequently, chains of pores grow to microcracks, which cover large areas of the material (type 4). During the deformation process, several parallel cracks appear (type 5), which develop inside the cross section until further damage is concentrated on one main crack. This crack is where the part fails.

The concept of reliability of thermal power equipment.

Characteristic distinctive feature power stations from manufacturing enterprises other industries is the requirement to ensure a continuous balance of “electricity generation – electricity consumption”. This condition must be met regardless of the time of day, days of the week, seasonal fluctuations in demand for manufactured products, instability in the quality of fuel supplied to the power plant, etc.

Since generating electricity for future use and storing it is impossible, an unforeseen failure in the operation of power plant equipment, in addition to the costs of restoring this equipment, can lead to significant damage to electricity consumers, cause catastrophic situations in industries with continuous operation, create emergency situations in transport, in connection , significantly complicate the work of public utilities. Therefore, the main task of power plants and energy systems is to ensure uninterrupted power supply to consumers. This task can only be solved if it is in good condition and reliable operation equipment.

GOST R 53480-2009 defines reliability as a property of availability and the properties of non-failure operation and maintainability that influence it, and maintenance support.

Readiness is the ability of an object to perform the required function under given conditions, assuming that the necessary external resources secured.

For a power plant, the concept of reliability can be formulated more specifically. The reliability of thermal power plants is the property of maintaining over time the ability to generate electrical and thermal energy of certain parameters according to the required load schedule with a given system of equipment maintenance and repairs.

The article was prepared on the basis of materials from the collection of reports of the VI International Scientific and Technical Conference “Theoretical Foundations of Heat and Gas Supply and Ventilation” of the National Research University MGSU.

An analysis of the operation of heat supply systems, carried out by employees of the research laboratory “Heat Power Systems and Installations” (NIL TESU) of Ulyanovsk State Technical University in a number of Russian cities, showed that due to the high degree of physical and moral wear and tear of heating networks and the main equipment of heat sources, the reliability of the systems is constantly decreasing. This is confirmed by statistical data, for example, the number of damages during hydraulic tests in the heating networks of the city of Ulyanovsk has increased 3.5 times over eight years. In some cities (St. Petersburg, Samara, etc.), major failures of main heating pipelines occurred during maintenance in heating networks high temperatures and pressures, so even in very coldy the temperature of the coolant at the outlet of the heat source is not raised above 90-110 °C, that is, heat sources are forced to work with systematic underheating of the network water to the standard temperature (“underheating”).

Insufficient costs of heat supply organizations for renovation and major repairs of heating networks and heat source equipment lead to a significant increase in the number of damages and an increase in the number of failures centralized systems heat supply. Meanwhile, urban heat supply systems are life support systems, and their failure leads to changes in the microclimate of buildings that are unacceptable for humans. In such conditions, designers and builders in a number of cities refuse to provide heating in new residential areas and provide for the construction of local heat sources there: roof-mounted, block boiler houses or individual boilers for apartment heating.

At the same time, Federal Law No. 190-FZ “On Heat Supply” provides for the priority use of district heating, that is, the combined generation of electrical and thermal energy for organizing heat supply in cities. Despite the fact that decentralized heat supply systems do not have the thermodynamic advantages of district heating systems, their economic attractiveness today is higher than centralized ones from thermal power plants.

At the same time, ensuring a given level of reliability and energy efficiency of heat supply to consumers is one of the main requirements that are presented when selecting and designing heating systems according to federal law No. 190-FZ “On Heat Supply” and SNiP 41-02-2003 “Heat Networks”. The standard level of reliability is determined by the following three criteria: probability of failure-free operation, availability (quality) of heat supply and survivability.

The reliability of heat supply systems can be increased either by improving the quality of the elements from which they are composed, or through redundancy. The main distinctive feature of a non-redundant system is that the failure of any of its elements leads to the failure of the entire system, while in a redundant system the probability of such a phenomenon is significantly reduced. In heat supply systems, one of the methods of functional redundancy is collaboration various heat sources.

In order to increase the reliability and energy efficiency of heat supply systems, the Research Laboratory of TESU UlSTU has created technologies for operating combined heating systems with centralized main and local peak heat sources, which combine the structural elements of centralized and decentralized heat supply systems.

In Fig. Figure 1 shows a block diagram of a combined heating system with the sequential inclusion of centralized main and local peak heat sources. In such a heat supply system, the thermal power plant will operate with maximum efficiency with a heating coefficient of 1.0, since the entire heat load is provided by the heating extraction of steam turbines to network heaters. However, this system only provides redundancy of the heat source and improves the quality of heat supply due to local regulation of the heat load. The possibilities for increasing the reliability and energy efficiency of the heating system in this solution are not fully used.

To eliminate the shortcomings of the previous system and further improve combined heat supply technologies, combined heating systems have been proposed, with parallel inclusion of centralized and local peak heat sources, which, when the pressure or temperature drops below a set level, make it possible to hydraulically isolate local heat supply systems from the centralized one. The change in peak heat load in such systems is carried out by local quantitative regulation for each subscriber due to changes in the flow rate of network water circulating through autonomous peak heat sources and local systems of subscribers. At emergency situation The local peak heat source can be used as a base heat source, and the circulation of network water through it and the local heating system is carried out using a circulation pump. The reliability of heat supply systems is analyzed from the standpoint of their ability to perform specified functions. The ability of a heating system to perform specified functions is determined by its states with corresponding levels of power, productivity, etc. In this regard, it is necessary to distinguish between an operational state, a partial failure and a complete failure of the system as a whole.

Technologies for the operation of combined heating systems with centralized main and local peak heat sources have been created at the Research Laboratory of Tesu of Ulyanovsk State Technical University.

The concept of failure is the main one when assessing the reliability of a heat supply system. Considering the fact that thermal power plants and systems are recoverable objects, failures of elements, assemblies and systems should be divided into failures of operability and failures of operation. The first category of failures is associated with the transition of an element or system at time t from an operational state to an inoperable state (or partially inoperable state). Operational failures are associated with the fact that the system at a given time t does not provide (or partially does not provide) the level of heat supply specified by the consumer. Obviously, failure of an element or system does not mean failure of operation. And, conversely, a failure of operation can occur even in the case where a failure of performance has not occurred. Taking this into account, the selection of system reliability indicators is made.

Well-known indicators can be used as single indicators of the reliability of elements or heat supply systems as a whole: λ(τ) - intensity (failure flow parameter) of failures; μ(τ)—recovery intensity; P(τ) is the probability of failure-free operation during a period of time τ; F(τ) is the probability of recovery over a period of time τ.

Let's compare the reliability of traditional and combined heating systems with the same thermal load of 418.7 MW, of which a base load of 203.1 MW is provided at a thermal power plant with a T-100-130 turbine (network water consumption 1250 kg/s), and a peak load of 215.6 MW peak heat sources. The thermal power plant and the consumer are connected by a two-pipe heating network with a length of 10 km. In a traditional district heating system, the entire heat load is provided by the CHP plant. In one combined system, the peak heat source is installed in series with the centralized one (Fig. 1), in the other - in parallel (Fig. 2).

The consumer's boiler room has three hot water boilers, one of which is a backup one.

As can be seen from Fig. 1 and 2, any heating system is a complex structure. Calculating the reliability indicators of such multifunctional systems is a rather labor-intensive task. Therefore, to calculate the reliability indicators of such systems, the decomposition method is used, according to which the mathematical model for calculating system reliability indicators is divided into a number of submodels. This division is carried out according to technological and functional criteria. In accordance with this, the heating system has a main heat source (CHP), a heat transport system from the CHPP to consumers, a decentralized peak heat source and a distribution network system to cover heating loads. This approach makes it possible to calculate reliability indicators for individual subsystems independently. Calculation of reliability indicators of the entire heating system is carried out as for a parallel-series structure.

From a reliability point of view, the heating unit of a thermal power plant is a complex structure of series-connected elements: a boiler unit, a turbine, a heating unit. For such a structural diagram, the failure of one of the units leads to the failure of the entire installation. Therefore, the availability factor of the heating unit will be determined by the formula:

Where k g CHP, k g k, k g t i k g tu are the availability factors of the entire thermal power plant, boiler unit, turbine and heating plant, respectively.

Stationary values ​​of availability factor k r for the corresponding circuit elements are determined depending on the intensity of restorations }