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Utemuratov Makhset Maratovich, Master of sciences
Scientific Research Institute of Irrigation and Water Problems
Senior Research Fellow E-mail: [email protected]
CORROSION OF PRESSURE PIPELINES AND CONCRETE STRUCTURES OF LARGE PUMPING STATIONS IN UZBEKISTAN
Abstract: for the needs of irrigation and maintenance of land reclamation, according to the Ministry of Agriculture and Water Resources of the Republic of Uzbekistan (MAWR), 8,940 pumping stations are operated, including 24 large ones with a total water consumption of 6.4 billion m3. The most important strategically important facilities for the Republic are the Amu-Zang, Amu-Bukhara and Karshi cascades of pumping stations that provide almost all irrigated farming and the population of the Surkhandarya, Kashkadarya, Bukhara and Navoi regions with water. Along with this, the Amu-Bukhara and Karshi machine channels provide water to the Bukhara oil refinery, Shurtan, Mubarek and other oil-gas chemical complexes, as well as Talimard-jan power plant with a capacity of 3200 thousand kW. The largest of them — the Karshi cascade of pumping stations raises the Amudarya water to a height of 132 m.
Keywords: maintenance and operation, pumping station, pressure pipelines, irrigation canal, machine irrigation, water intake, suspended sediments, transportation, reservoir, cleaning, upstream, downstream, concrete, metal and concrete construction, chemical processes, microorganisms, corrosion, pumping units, substance, pumping units.
At present, 4268.1 thousand hectares of land are irrigated in The riverbed wanders, and in most cases, moving away from the left
the republic, including 1533 thousand hectares (38.3% of the total area) - machine irrigation. The cost of electricity with a machine water lift for irrigation of more than 1.5 million hectares is 7.5-8 billion kW. H per year.
On the balance of the MAWR there are more than 1,580 interfarm pumping stations with a total installed capacity of3,355,000 kW, and the annual volume of water supply is about 50 km3.
Of the total number of pumping stations 76 pcs. - unique (supply more than 100 m3/s) and large, 496 pcs. - medium (feed 1-10 m3/s) and 561 pcs. -small (supply less than 1,0 m 3/s). A significant part of the pumping stations is functioning in conditions when the suspended water contains from 2 to 20 kg/m3 of suspended sediment in the pumped water. In this case, a particularly difficult situation has arisen with engine water supply in the presence of a large number of sediments in the basins of the Amudarya and Zarafshan rivers. In the design of pumping stations in conditions of rising water with a high content of sediments, special settling tanks were provided, but most of them, due to poor operation and design flaws, either did not work at all, or by now do not fulfill their functional purposes. In particular, during the construction of the Karshi cascade of pumping stations, the construction of a special river hydroelectric complex at the Kyzyl-ayak section of the river was planned. Amudarya, and on the right bank of the river a reservoir for reliable supply of water to the pumping units of the first rise of the reservoir. The latter, providing the necessary horizons for the non-cavitation operation of the pumping units of the first rise of the Karshi cascade, was to retain the bulk of the suspended sediments. Nevertheless, the Kyzyl-ayak hydroelectric complex and reservoir on the right bank of the Amudarya River were not built, and the supply channel to the first pumping station remained tied to a temporary, in-situ water intake from the river. The Amudarya River, in its average flow, in particular in the Kyzly-ayak line, flows in an unstable course.
bank, where the water intake is located in the Karshi Machine Canal, rectification work is carried out to create normal water intake conditions. Hydro mechanical method of cleaning the feeding channel contributes to the disturbance, the eroded sandy part of its profile, which causes the flow of water into the machine channel with more than a natural turbidity. All this prevents the normal operation of the entire complex of pumping station facilities. Experience in the operation of irrigation pumping stations, especially in the basins of the river. Amudarya and Zarafshan, shows that because of the large amount of suspended sediments in the water, the functioning of the vestibule, the water intake, is complicated, and in places where there are narrowing or expansion of pipes, turns, junction or distribution nodes, and others. Elements, there are local resistance. Their presence in the pressure pipelines of pumping stations determines the accumulation, and in the future — the formation of serious obstacles to the movement of water. The presence of suspended sediment significantly affects the operation mode of the pumping units, reducing the efficiency, increasing the energy costs of water lifting and the most significant contributing to the danger of corrosion destruction of pressure water conduits due to the deposition of solid materials of mechanical and organic origin in the inner part ofpipes.
The urgency of this research is due, first of all, to a sharp increase in energy prices, the cost of pumps and power equipment, an increase in the specific energy costs for water lifting, a decrease in the reliability of operation of the complex of pumping station facilities, and the growing scarcity of water resources to meet the needs of the Uzbek economy.
Introduction
Experience in the operation of irrigation pumping stations, especially in the basins of the rivers Amudarya and Zarafshan, shows that because of the large amount of suspended sediment in the water, the functioning of the vestibule, the water intake, is complicated,
and in places where there are narrowing or expansion of pipes, turns, junctions or distributions and other elements, its appears local resistance. Their presence in the pressure pipelines of pumping stations determines the accumulation, and in the future — the formation of serious obstacles to the movement of water. The presence of suspended sediment significantly affects the operation mode of the pumping units, reducing the efficiency, increasing the energy costs of water lifting and the most significant contributing to the danger of corrosion destruction of pressure water conduits due to the deposition of solid materials of mechanical and organic origin in the inner part of pipes.
1. Measurements of the thickness of the shell of pressure pipelines
Investigations of the motion of suspended particles have shown that there are no abrasive materials in the solid components of the water flow, as sand material with a diameter of more than 0.01 mm settles in the pumping station's outposts. Therefore, continuous wear of the entire internal surface of the steel pipeline is not observed.
Systematic supervision of the wear of the pipeline sheath is necessary to obtain data that provides:
- checking the static strength of pipelines, as a beam structure;
- checking the stability ofpipelines, which determines the ability of the shell to withstand the external (vacuum) and internal pressure of the pumped water;
- analysis of the wear dynamics of the shell material in time, along the cross-section and along the pipeline route to localize the work to protect the inner surface of the shell from corrosion-abrasive wear.
The measurements are made in the characteristic (beginning, middle and end of each section of the pipeline between adjacent intermediate supports) and fixed sections of the pipeline of the Sher-abad pump station in the Surkhandarya region.
On the pressure pipelines, 52 control sections were assigned, the layout of the sections was agreed with the department of pumping stations "Uzgiprovodkhoz".
The measurements were carried out according to a standard procedure with an ultrasonic thickness gauge of the UT-93 P brand with the following main characteristics:
- measuring range - 0.6 ... 300 mm;
- Discreteness of the digital reading device - 0.1 mm;
- limit of the allowed value of the basic error
In the working range - 0.1 mm.
Analysis of the measurements showed that the thickness of the pipeline shell is distributed relatively evenly along the length and perimeter, where the minimum thickness was 9.4 mm and the maximum thickness was 9.8 mm.
In general, taking into account that the device gives a certain measurement error, it can be concluded that at first the operation of the station for the period from 1966 till 2006. The thickness of the shell of the pipeline has undergone insignificant changes, and according to the measurements the wear averaged from 0.2 to 0.5 mm.
2. Corrosion of metal pressure pipelines
A significant danger to the operability and safety of the pumping station is the considerable wear of the pressure pipelines. In the study of all the above pumping stations, it was found that the most
common type of pipeline damage is the formation of fistulas and tears due to fatigue failure when the wear of the pipeline wall and vibration are combined. In conditions of pumping over river water of increased turbidity, the main type of wear of pressure pipelines is the corrosive wear of the inner surface of the wall of the pressure pipeline. In addition, as studies at the pumping stations Babatag, Amu-Zang-2 have shown, an important role in the wear of pressure pipelines is played by the usual and focal biological corrosion of the metal. Calculations made from the measurements of the residual thickness of the walls of pressure pipelines show that at an average wear rate of 0.08-0.15 mm per year, the wear rate of the wall ofpres-sure pipelines is 45-55%. On the perimeter of the section wear is distributed unevenly, significant wear values fall on the lower part of the pipeline.
Static and hydrodynamic calculation of the stability of the pressure pipeline from the action of internal and external loads shows that at some pumping stations pressure pipelines are operated in conditions close to critical. In view of the constant reduction in the thickness of the wall of the pressure pipeline during its continuous operation, it becomes necessary to check this pipeline for strength against the loads perceived by it, taking into account the residual wall thickness of the pressure pipeline. Loads occurring in the pressure pipeline can be divided into: annular stresses, axial longitudinal stresses and stresses arising from negative pressure. They arise, mainly, from the static head during the operation of the pump unit. Investigations of the sediment transport regime in pressure pipelines have shown that excessive wear of individual sections of the pipeline is caused not only by the mechanical action of the flow of solid particles. The formation of cracks in pressure pipelines, uneven wear of the pipeline led to the need to study, among other things, the causes of corrosion of the pipeline. For this purpose, special investigations were carried out to detect corrosive and stress corrosion damage in pipelines of the pump station.
The object of the research was samples of mud, sand, rust and water, taken from separate sections of the pipeline, suction pipe and settler. As a control, water supplied to the station from the supply channel served. The microbiological analysis was carried out according to the generally accepted classical methods of investigation. The work used nutrient media to identify physiologically active groups of microorganisms involved in the corrosion process:
1. Fish hydrolyzate for saprophytic forms of microorganisms;
2. Glucose-mineral medium for the detection of acid-forming microorganisms;
3. Giltay medium for detection of denitrifying bacteria;
4. The environment of Vinogradsky for nitrifying bacteria;
5. Czapek's environment for mycelial fungi;
6. 9K medium for iron-oxidizing bacteria;
7. Medium for sulfate-reducing bacteria.
The pH of the liquid samples was 6.8-7.2, the humidity of the solid samples was 10-20%, the chloride content was up to 60 mg per 100 g of sand.
The averaged number of different physiological groups of microorganisms isolated from selected samples of samples on the investigated media are presented in Tables 1 and 2. The data of the tables show that in the overwhelming majority of the samples,
microorganisms of all these groups were detected. The most common are acid-forming, denitrifying and nitrifying bacteria. Their number varied from 102 cells/ml to 105/ml in liquid samples, and in solid samples from 103 cells/g to 106 cells/g. It should be noted
Table 1. - The averaged number of physiological groups
that in water samples taken from the suction branch of pumping units, pressure pipeline and settler, the number of microorganisms ranged from 102 to 105 cells/ml, while iron-oxidizing and sulfate-reducing bacteria were not detected.
microorganisms in the waters of the pump station, cl/ml
of
No. of samples Place of selection Iron-Oxide Sulphate Reducing Denitrifying Nitrifying Acid-forming Saprophytes Mineral-Mushrooms
1 2 3 4 5 6 7 8 9
9 Entering water to the pump station - - 2,0 x 10 3 2,5 x 10 2 2,0 x 10 3 5,2 x 10 5 2,0 x 10 1
4 Water from suction nozzle - - 2,5 x 10 5 2,5 x 10 2 5,3 x 10 5 5,2 x 10 5 1 x 10 3
2 Water from the pipeline No.7 - - 6,0 x 10 4 - 3,5 x 10 4 5,6 x 10 5 2 x 10 2
3 Water from the sump - - 5 x 10 3 2,5 x 10 3 3,0 x 10 5 8,4 x 10 6 5 x 10 3
Table 2. - The averaged number of physiological groups of microorganisms in silt, sand, rust and other solid samples
No. of samples Place of selection Iron-Oxide Sulphate Reducing Denitrifying Nitrifying Acid-forming Saprophytes Mineral-Mushrooms
1 2 3 4 5 6 7 8 9
4 Sand from the suction pipe - - 2,5 x 10 4 2,5 x 10 4 4,0 x 10 3 4,8 x 10 6 3,5 x 10 2
7 Sand from the pressure pipe 2,5 x 10 1 0,6 x 10 1 2,5 x 10 6 7,0 x 10 4 9,0 x 10 6 4,0 x 10 3
6 Silt from the compensator of the pipeline no.8 2,5 x 10 1 2,5 x 10 3 2,5 x 10 6 2,5 x 10 6 7,0 x 10 4 9,0 x 10 6 5,0 x 10 3
5 Rust precipitate from the suction nozzle - - 2,5 x 10 4 4,0 x 10 4 4,0 x 10 5 4,8 x 10 6 1,0 x 10 3
8 Rust precipitate from the pipeline no.6 2,5 x 10 1 0,6 x 10 1 2,5 x 106 - 5,4 x 10 5 7,0 x 10 6 2,2 x 10 2
The total number of saprophytes was 105 ... 106 cells/ml, and the acid-forming bacteria - 102 ... 103 cells/ml in water samples and 104 ... 105 cells/g in solid samples, which indicates the presence of microbial corrosion in the investigated objects.
The presented data reflect the integral activity in the studied groups of microorganisms and can, in our opinion, serve as the main criterion for the potential hazard stress of corrosion damage to pressure pipelines of pumping stations.
Thus, preliminary studies indicate the development of corrosive processes, which can lead to the propagation of corrosive foci of destruction of pressure pipelines.
3. Corrosion of metal pressure pipelines The question of the harmful impact on the concrete of hydraulic engineering structures of river, sea, ground, artesian, polluted subsoil, sewer, factory, and waste and swamp waters has not been studied sufficiently. From the above categories, uncontaminated river water has the least harmful effect on concrete structures. Groundwater, artesian and other groundwater, which in most cases contains relatively little harmful chemicals, can also be considered more or less
safe, with the exception of some mineral and thermo mineral springs. Marine and salt lake water, containing a relatively large amount of sulfuric acid and other compounds, are the most dangerous with respect to the destruction of concrete.
The noted adverse conditions are also largely encountered in contaminated sewage containing, as well as swamp waters, a large number of decomposition products of organic substances.
All the phenomena of damage and destruction of concrete flow almost exclusively in the wet state of concrete, and moreover from the direct permanent aggressive action of such liquids and gases that contain in considerable quantities mainly sulfuric, partly chlorine and nitrogen compounds, organic acids and free carbon dioxide.
Table No.3 gives the maximum standards for the content in waters of different categories of those principal substances whose excess content causes a destructive effect on concrete. Grasping or freshly hardened, as well as lean and cast concrete, especially porous, are much easier to cope with the destructive effects of the above chemicals than dense, tamped and little water-permeable concrete.
Table 3. - Permissible content of hazardous chemicals in water
No. Most dangerous Chemi- Content in mg for 1 liter
cals of water
1 Sulfur (SO3) Up to 100
2 Chloride (MgCI2) 20-30
3 Free carbon dioxide (CO2) 10-15
4 Organic matters (Total oxidation) 10-12
5 Nitrogen (N2O5) 50
In fresh water, the fresh exchange of these substances enhances the processes of leaching and destruction of concrete. It should be specially noted that the significant fact that the question of the safety of concrete structures subject to the action of water should not be viewed from the point of view of only the aggressive influence of one or another substance, but in dependence on the aggregate qualitative and quantitative action of all those factors that Can cause weakening and even gradual destruction of concrete. Such processes sometimes cause such a significant decrease in concrete resistance in the critical parts of the structure that a catastrophe with them becomes inevitable. Most of the studies and observations on the state of hydraulic structures show that the strong destruction of concrete foundations and bulls is due to the content of sulfuric acid salts (up to 140 mg per liter) in soil and river water penetrating into porous concrete and forming calcium sulphoaluminate. This cement bacillus in turn tears the inside of the porous concrete, due to its swelling (32 times) in the water.
The conducted inspections of the hydro technical structures of the Sherabad pump station, in particular, the station buildings, showed that the concrete masonry and the foundation of the pumping station building are exposed to chemically aggressive influences, which threaten the strength and stability of the structures. And that the choice of these or other measures can be made only after a detailed study of the chemical processes that took place in the concrete.
Carefully conducted studies of damaged concrete showed that the bulk of the destroyed concrete (78%) consists of calcium carbonate and magnesium; A small amount of gypsum shows that the destruction of concrete was not caused by sulfuric sodium salt. The formation of nests and the erosion of concrete cannot be explained by the leaching of lime and magnesia from it.
This could only happen if the carbon dioxide is aggressively attacked by carbonates and the carbon dioxide lime that is soluble in water is produced. Groundwater analyzes showed a carbon dioxide content of 8.14 mg/l.
The destruction of concrete could not be explained by the impact of carbon dioxide contained in groundwater. The state of the concrete made it necessary to search for the cause of its destruction by calcium sulphoaluminate, the so-called «cement bacillus», which crystallizes into a compound with water (3CaO • Al2O3 • 3CaSO4 + + 3OH2O) and increases its volume by 32 times. This assumption was confirmed by chemical analyzes showing a high content of sulfuric anhydride in the concrete in the range from 1.9 to 5.9% to the cement content, while the maximum allowable content of sulfuric anhydride for Portland cement should not exceed 2.5%. The content
of sulfuric anhydride was greatest at the bottom of the basement and decreased to the top.
Groundwater analyzes showed a significant content of both table salt and magnesium chloride and gypsum, as can be seen from Table No. 4.
Table 4. - Results of water analysis
No. Name of chemical substances Groundwater
Test No. 1 Test No. 2
1 Common salt, mg/l 317 227
2 Sulfuric anhydride 70 102
3 Lime 99 81
4 Magnesium 34 26
The revealed causes of destruction of concrete, as well as the proposed measures to combat the destructive effect on concrete carbon dioxide and sulfur dioxide, showed that the most reliable means in this case is to increase the water tightness and density of concrete, which can be achieved by increasing the amount of cement and the appropriate selection of the particle size distribution Inert materials.
It should be noted that in addition to the phenomenon of corrosion of concrete in many hydraulic structures, corrosion of reinforcement, which is part of reinforced concrete structures, is noted.
In accordance with the basic principles of fracture mechanics, the products have initial defects, which continue to develop during the operation when various loads are applied to them. The experience of operation of large pumping stations shows that different types of corrosive factors act on reinforced concrete structures simultaneously with power loads. Corrosion of reinforcement in concrete occurs due to the penetration of filtration water through the pores and cracks formed in the concrete. The following basic types should be considered among the corrosive factors:
• Aggressive action of groundwater and filtration water;
• Constant exposure to electric current.
All these factors lead to a significant decrease in their strength properties and to the violation of hydraulic structures. At many pumping stations on the back side of the building, the water accumulates at the basement, which comes as a result of leakage of pressure pipelines. So, as a result of the research carried out on the «Sherabad» and «Amu-Zang-2» pumping stations, the above-mentioned processes in the form of corrosion of concrete and reinforcement are clearly manifested. The most typical destruction of hydraulic structures, which occurs during the corrosion of reinforcement in concrete, is a gradual decrease in its working section due to the transition of the outer layers of metal into corrosion products. There are different types of corrosion of metal:
• Uniform throughout the surface for a certain length of the reinforcement.
• Acute local, in the form of ulcerative lesions.
The conducted surveys of hydraulic structures, in particular, at the pumping stations «Amu-Zang-2» and «Sherabad» have shown that the most common defects are the destruction of the protective layer with exposure of the reinforcement, corrosion of the reinforce-
ment and corrosion of concrete, mainly in the form of stains and smudges Lime leaching.
The studies carried out above have mainly revealed the cause of excessive wear and destruction of metal and concrete structures of pumping stations. It should be noted that the process of manifestation of biological corrosion at pumping stations, primarily due to the presence ofwater with suspended sediments, which are the focus of microbiological processes. At present, studies of the temperature regime on the activation of biological organisms that cause corrosion of the metal are conducted.
In science, methods for preventing and methods of protecting pressure water lines from biological corrosion are known. The methods of prevention include the choice of a scheme for constructing pressure pipelines and the creation of favorable hydraulic conditions for sediment transport, as well as coating the metal surface with anticorrosion. To protect the structure from corro-
sion, first of all, it is necessary to determine the type of iron-eating bacteria, and then to choose the means of protecting the surface of structures.
Conclusion
1. As a result of field studies and laboratory experiments in which leading scientists of the Institute of Microbiology of the Academy of Sciences of the Republic of Uzbekistan took part, it was established that the destruction of metal shells of pressure pipes is due to corrosive processes.
2. Corrosion processes mainly take place at the places of sedimentation and adherence of sediments whose particle diameter is less than 0.01 mm. In this case, focal accumulations of solid materials occur in places where the formation of local hydraulic resistances.
3. The presence of sediments, materials of biological origin leads to the development of corrosive processes in other structures, in particular, on reinforced concrete structures of pumping stations.
References:
1. Shterenlicht D. V. Hydraulics. - Moscow: Energia, - 1991. - 351 p.
2. Fayzullaev D. F. "Laminar motions of multiphase liquids in pipelines". - Tashkent. "Fan", - 1966.
3. Khamidov A. A., Khudaykulov S. I. "The theory of jets of multiphase viscous liquids". - Tashkent: Fan, - 2003. - 140 p.
