Reliability of thermal power systems tests. Tests in the discipline "heat supply and heating". Ductile fracture process
A modern energy enterprise (thermal power plant, boiler house, etc.) is a complex technical system consisting of separate units connected by auxiliary technological connections.
An example of such a technical system is the thermal circuit diagram (PTS) of a thermal power plant, which includes a wide range of main and auxiliary equipment (Fig. 5.1): a steam generator (steam boiler), a turbine, a condensing unit, a deaerator, regenerative and network heaters, pumping and draft equipment, and others
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 costs of the working fluid of the installed equipment. PTS is usually depicted as a single-unit and single-line diagram. The same equipment is conventionally shown once in the diagram, technological connections of the same purpose are also shown as a single line.
In contrast to the basic thermal diagram, the functional (full or expanded) diagram of a TPP contains all the main and auxiliary equipment. That is, the complete diagram shows all units and systems (working, reserve and auxiliary), as well as pipelines with fittings and devices that ensure the conversion of thermal energy into electrical energy.
The complete scheme defines the number and dimensions of the main and auxiliary equipment, fittings, bypass lines, starting and emergency systems. They characterize the reliability and level of technical excellence of the TPP and provide for the possibility of its operation in all modes.
According to the functional purpose and impact on the reliability of the operation of the power unit or TPP 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 the 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 electric power and supplied heat (draft fans, 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 affecting the generation of electrical and thermal energy (for example, regenerative heaters).
The reliability of the work of all these groups is interrelated.
The 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.
Structural diagrams allow you to determine such a number or such a 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 block diagram is determined by the nature of the tasks being solved. As elements of the block diagram, it is necessary to choose such equipment or a system that has a certain functional purpose and is considered as an indecomposable whole that has data on reliability.
Quantitative indicators of the reliability of thermal power plants can be obtained by calculating the known characteristics of the reliability 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 TPPs and their block 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 methods of calculation are used.
Therefore, in the future, special attention will be paid specifically to 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 determine to a large extent the reliability indicators of boilers.
On fig. 2.10 shows the distribution of failures of boiler equipment of TPP power units. As can be seen, the greatest damage to boiler equipment occurs due to operating errors. A significant proportion of failures occurs due to design flaws and poor repair quality.
Typical failures due to design flaws on boilers are large thermal scans on the heating surfaces, their accelerated ash wear. In the process of manufacturing boilers, there are violations of 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 normative 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 heat exchange pipes. The low quality of water and steam leads to a sharp increase in deposits, an increase in the temperature of the metal of the pipes 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
The share of failures of the main elements of the boiler plant of the 300 MW power unit
From Table. 2.1 it can be seen that the vast majority of boiler plant 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 failures in general for the heat exchange surface is fairly evenly distributed over the characteristic surfaces (Fig. 2.11). Somewhat more often, screen tubes and pipes of superheaters (KPP1 and KPP2) are damaged.
Screen pipes in operation are exposed to radiant energy, a corrosive environment of fuel combustion products, which, at a low circulation rate and violations of the water regime, leads to their damage and failures in the operation of boilers (Fig. 2.11).
Rice. 2.11.
by elements
A noticeable effect on the damageability of the pipes of the gearbox is exerted by the uneven temperature field leading to thermal distortions along the height of the gas duct, in which the superheater is located.
Superheaters are also damaged because during long periods of time at temperatures above 500 ° C, the metal structure undergoes undesirable changes.
During the operation of solid fuel boilers, the wear of gas ducts by fly ash occurs due to the impact of its particles on the surface. As a result, the oxide film on the bounding surfaces breaks down and erosion develops. Wear is most often uneven. The highest wear intensity takes place in zones of increased 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. The most intensive corrosion occurs during the combustion of high-sulphur fuel oil. Fistulas (Fig. 2.12) occur in contact welded joints due to misalignment of pipes, pinching of the internal section, lack of fusion, 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 fluctuations in the load of the unit, and the quality of installation of the water economizer.
In most cases, when a damage occurs in one pipe, bend or weld, the outflowing jet 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.
Damage to the screens protecting the walls of the combustion chambers (radiation superheater and radiative water economizer) are typical for furnaces.
The view of the damaged pipe of the front screen is shown in fig. 2.13.
Breaks of cyclones, perforated and louvered sheets, fasteners occur in the drums of boilers, which, falling into the holes of the culverts, block them. The speed of movement of the steam-water medium in the screens decreases, the metal of the pipes overheats and collapses.
Welds in the screens are damaged, fistulas are formed.
In supercritical pressure boilers, the pipes of radiative superheaters are damaged due to high-temperature corrosion, which leads to significant wear of the walls on the fire heating side. This occurs at high thermal loads. Thermal distortions are caused by an uneven temperature field along the height of the flue.
Creep and accompanying damage to pipes (microcracks) appear more intensively in bends than in straight pipes. This makes it necessary to periodically change individual elements or entire stages of the superheater.
Failures also occur from uneven expansion of pipes, unequal weight loads - welds are in a complexly stressed state.
Sharp fluctuations in the load of boilers also lead to the occurrence of unacceptable stresses in welds and near-weld zones, causing the formation of cracks, breaks in fasteners and pipes.
Damage to drums and pipelines
Boiler drums and unheated pipe bends are of particular importance in ensuring the reliability of boilers. Although great attention is paid to the reliability of drums in design, manufacture, operation and repairs, they often experience damage, leading to long shutdowns of the boilers.
Rice. 2.14.
These are cracks located in the zone of 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 the intra-drum devices to the casings (Fig. 2.14), as well as defects in the main circumferential and longitudinal seams.
The main reason for the formation of damage is the excess of the material's yield strength by the acting stresses, leading to the appearance of permanent deformation. Increased stresses arise due to the presence of a temperature difference along 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 the inner side of the walls during sudden changes in temperature. These non-stationary modes of the boiler are especially dangerous when it 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 in the main welds - they create the danger of major damage. More often than others, longitudinal and transverse cracks are found in the weld on the inner surface. Lack of penetration, slag inclusions, shells, pores are observed.
The depth of cracks is different, but there are cases when in 1 year it reached 70% of the thickness.
On pipelines, bends are most often damaged. This is where corrosion-fatigue damage occurs. Insufficient compensation of thermal elongations causes increased stresses.
The bends of the feed, drain and steam pipes are brittle, the bends of superheated steam pipelines operating under creep conditions are deformed during destruction.
The article was prepared on the basis of the materials of the collection of reports of the VI International Scientific and Technical Conference "Theoretical Foundations of Heat and Gas Supply and Ventilation" of NRU MGSU.
An analysis of the operation of heat supply systems, carried out by employees of the Research Laboratory "Heat and Power Systems and Installations" (NIL TESU) of UlSTU in a number of Russian cities, showed that due to the high degree of physical and obsolescence of heat networks and the main equipment of heat sources, the reliability of systems is constantly decreasing. This is confirmed by statistical data, for example, the number of damages during hydraulic tests in the thermal 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 heat pipelines occurred during the maintenance of high temperatures and pressures in heating networks, therefore, even in severe frosts, the temperature of the coolant at the outlet of the heat source is not raised above 90-110 ° C, then there are heat sources that are forced to work with systematic undercooling of network water to the standard temperature (“underheating”).
Insufficient costs of heat supply organizations for the renovation and overhaul of heat networks and heat source equipment lead to a significant increase in the number of damages and to an increase in the number of failures of centralized heat supply systems. 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. Under such conditions, designers and builders in a number of cities are refusing to supply heat to new residential areas and envisage the construction of local heat sources there: rooftop, block boilers 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 the organization of heat supply in cities. Despite the fact that decentralized heat supply systems do not have the thermodynamic advantages of heating systems, their economic attractiveness today is higher than centralized from CHP.
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 apply when choosing and designing heating systems in accordance with Federal Law No. 190-FZ “On Heat Supply” and SNiP 41-02-2003 “Heat Networks”. The normative level of reliability is determined by the following three criteria: the probability of failure-free operation, availability (quality) of heat supply and survivability.
The reliability of heat supply systems can be improved either by improving the quality of the elements of which they are composed, or by redundancy. The main distinguishing 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 ways of functional redundancy is the joint operation of various heat sources.
In order to improve the reliability and energy efficiency of heat supply systems, NIL TESU UlSTU created technologies for the operation of combined heat and power systems with centralized main and local peak heat sources, which combine the structural elements of centralized and decentralized heat supply systems.
On fig. Figure 1 shows a block diagram of a combined heat and power system with serial connection of centralized main and local peak heat sources. In such a heat supply system, the CHPP will operate with maximum efficiency at a heat supply coefficient of 1.0, since the entire heat load is provided by heat extraction of turbine steam to network heaters. However, this system provides only a backup of the heat source and an increase in the quality of heat supply due to local regulation of the heat load. The possibilities of 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 are proposed, with parallel inclusion of centralized and local peak heat sources, which, when the pressure or temperature drops below the set level, allow hydraulic isolation of local heat supply systems from the centralized one. Changing the peak heat load in such systems is carried out by local quantitative regulation for each of the subscribers by changing the consumption of network water circulating through autonomous peak heat sources and local subscriber systems. In case of an emergency, the local peak heat source can be used as the base one, and the circulation of network water through it and the local heat supply system is carried out using a circulation pump. The analysis of the reliability of heat supply systems is carried out from the standpoint of their ability to perform the specified functions. The ability of the heating system to perform the specified functions is determined by its states with the corresponding levels of power, performance, etc. In this regard, it is necessary to distinguish between a healthy state, partial failure and complete failure of the system as a whole.
NIL TESU UlSTU created technologies for the operation of combined heat and power systems with centralized main and local peak heat sources
The concept of failure is central to assessing the reliability of a heat supply system. Given the fact that thermal power plants and systems are recoverable objects, failures of elements, assemblies and systems should be divided into operability failures and operation failures. The first category of failures is associated with the transition of an element or system at time t from an operable state to an inoperable (or partially inoperable) state. Functioning failures are due to 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. It is obvious that the failure of the operability of an element or system does not mean a failure of functioning. And, conversely, a failure of functioning can occur even in the case when a failure of operability has not occurred. With this in mind, the choice of indicators of system reliability is made.
Known indicators can be used as individual indicators of the reliability of elements or systems of heat supply as a whole: λ(τ) is the intensity (failure flow parameter) of failures; μ(τ) is the intensity of recovery; P(τ) is the probability of failure-free operation during the period of time τ; F(τ) is the probability of recovery over a period of time τ .
Let us compare the reliability of traditional and combined heat and power systems with the same heat load of 418.7 MW, of which the base load of 203.1 MW is provided by a CHPP with a T-100-130 turbine (network water consumption is 1250 kg/s), and the peak load is 215.6 MW of peak heat sources. CHP 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. 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).
Three water-heating boilers are installed in the consumer's boiler room, one of which is a reserve one.
As can be seen from fig. 1 and 2, any heating system is a complex structure. The calculation of the reliability indicators of such multifunctional systems is a rather time-consuming task. Therefore, to calculate the reliability indicators of such systems, the decomposition method is used, according to which the mathematical model for calculating the system reliability indicators is divided into a number of submodels. This division is carried out according to technological and functional features. In accordance with this, the main heat source (CHP), a system for transporting heat from the CHP to consumers, a decentralized peak heat source and a distribution network system to cover heating loads are allocated in the heating system. This approach allows the calculation of reliability indicators for individual subsystems independently. The calculation of the reliability indicators of the entire heating system is carried out as for a parallel-series structure.
From the point of view of reliability, the heating unit of a CHPP is a complex structure of series-connected elements: a boiler unit, a turbine, and a heating plant. For such a block diagram, the failure of one of the units leads to the failure of the entire installation. Therefore, the availability factor of the heating unit is determined by the formula:
Where k d CHP, k g k, k g t and k rtu are the availability factors of the entire CHPP, boiler unit, turbine and heating plant, respectively.
Stationary values of availability factor k r for the corresponding elements of the circuit are determined depending on the intensity of restorations }
