Effect of parallel connection of pumping units on operating costs of pumping station

In Uzbekistan, as well as in many other countries of the world, pumping units operate in parallel into a common pressure pipeline at some pumping stations of the irrigation system. This was done mainly to save on the capital costs of the pipeline. Analysis and hydraulic calculation of the existing large irrigation pumping stations with pumping units operating in parallel showed that at some of them the conditions of parallel operation of pumping units were impaired. As a result, a negative hydraulic process arises, in the form of self-induced vibrations of the water flow in the pipeline, which leads to various damage and deformation of the pressure pipeline and pipeline supports. The calculations showed that with different modes of pumping units operating together, the power consumption at the Amu-Bukhara-1 and Namangan pumping stations, accounting for one unit increases by 0.72 ... 6.53%, and at the Amu-Bukhara-2 and Amu-Zang-2 pumping stations, this index, on the contrary, decreases by 0.32 ... 1.67%. Therefore, a properly selected operating mode of parallel-connected pumping units will lead to a decrease in operating costs, i.e. decrease in the prime cost of raised water.


Introduction
Currently, a large number of pumping stations operate in the irrigation systems of Uzbekistan and other countries of the world. 1668 pumping stations operate under the Ministry of Water Management of the Republic of Uzbekistan, which include more than 5000 pumping units. The total supply of these pumping stations is about 7000 m³/s, and about 2.3 million hectares are irrigated with them, which is more than 53% of all irrigated lands. Each year, about 8 billion kW·h of electricity is consumed for the operation of these pumping stations, which is about 15% of the total electricity generated in the republic per year. The age of all pumping stations is over 30 years [1]. At some pumping stations, pumping units operate in parallel and are connected to a common pressure pipeline through connecting pipelines (in order to save on the capital expenditures, at some pumping stations the pumping units are arranged in a chequered manner) and return pipeline twins [2,3,4,5]. The pressure pipelines of these pumping stations wear out due to the duration of operation on the one hand [5,7,10], and on the other hand due to the inability to provide equal pump heads at their connecting nodes, i.e. on return pipeline twins [5,10].
When designing pumping stations, the following hydraulic conditions were not initially provided [1,2,5,6,7,8,9] such as: -equality of the length of the connecting pipelines, since the pumping units are chequered and in order to achieve their equality, various hydraulic resistances are installed. Moreover, no sections with a uniform motion of flow L 30d are left between them, which led to an increase in the values of the local resistance coefficients, thereby increasing the equivalent length of the connecting sections; -local hydraulic resistance -return pipeline twin, depending on the number of simultaneously operating pumping units, operates as different resistances (rotation by a certain angle, abrupt enlargement, return pipeline twin, etc.), and its value is assumed to be constant (these hydraulic properties of the return twin appear at pumping stations, where pumping units are arranged in a chequered manner, as well as on those where pumping units are located in one row).
As a result, the conditions of parallel operation of pumping units gets impaired, a negative hydraulic process occurs, in the form of self-induced vibration of water flow in the pipeline [1,5,10]. This leads to various damages and deformations of the pressure pipeline and its supports. So, for example, there was a breakdown of the flanges of the K-2-2 pumping station of the Tashkent region and the breakdown of fixed supports (with a rupture of the welded joint of the pipeline) of the Babatag pumping station of the Surkhandarya region. The mentioned negative hydraulic process by origin and decay time differs from the fluid line shock. Also differs from the instability of pumping units due to inconsistency of the characteristics of the pump and pipeline [1, 5, 10-20].

Materials and methods
We studied the hydraulic properties of the return pipeline twins installed on the pressure pipelines of the large pumping stations of Babatag and Amu-Zang-2 of the Surkhandarya region, Amu-Bukhara-1 and Amu-Bukhara-2 of the Bukhara region, as well as the Namangan pumping station of Namangan region. It is determined that the value of the hydraulic resistance of the return pipeline twin [1,5,10] depends on the following indices: -the number of simultaneously operating pumping units in a common pressure pipeline; -proportionality or disproportionality of junction of flows; -measure of the angle of junction of flows; -cross-sectional area ratios of the (common) flows to be merged and merged. The pressure losses on the return pipeline twin are determined by the following formulas [2,6]: for direct flow passage: for lateral flow passage:  gfree-fall acceleration, m/s 2 ; cos αjunction angle of flows.

Results and Discussion
Calculations of the local resistance coefficient values of the return pipeline twin at these pumping stations showed that, depending on the junction angle of flows, it varies from 0.26 to 24.34. At relations of cross sections of the merged and total flows ω п /ω о =0.1  1.0 it makes 24  0.5 and changes when the junction angle changes from 2 to 190%, and at nonproportional junction it changes by 2  65% [1,5,10].
An example of the results of calculating the pressure losses and the local resistance coefficient on the return pipeline twins installed on the pressure pipeline of the Babatag pumping station (Fig. 1, 2) is given in Table 1.   Analysis of table 1 shows that the value of the pressure and the resistance coefficient of the return pipeline twin varies depending on the number of simultaneously operating units. In addition, it turned out that when 3 pumps operate, a vacuum appears at the connecting node. We believe that just this is one of the reasons for the formation of a self-vibrating water motion in a pressure pipeline.
It should also be noted here that the beginning of the self-vibrating water motion in the pressure pipeline appears from the moment the pump unit is connected to an already operating network and will continue until the pressure of the pump units is equalized. We consider that this phenomenon was the cause of the breakdown of the flange connection of the K-2-2 pump station and the breakdown of the support of the Babatag pump station [1].
In addition, the calculations showed that the difference in the sum of losses of the return pipeline twin with the simultaneous operation of the pumping units No.1 and No.2 of the Babatag pumping station was 5.5 cm. All these and previous proofs mean that the pumps operate with a predetermined mode of self-vibrating flow in the pressure pipeline.
By forming up the characteristics of a group of pumps connected to a common pressure pipeline, as well as forming up based on hydraulic calculations of the characteristics of pipelines, we determined the operating modes of the above-mentioned pumping stations. The results of the calculations are given in Table 2.  ----------------- At Amu-Bukhara-1 and Namangan pumping stations, connection of pumping units to a pressure pipeline is performed through a collector. With an increase in water consumption in it, the pressure losses also increase, which leads to an increase in power accounting for one pumping unit. At Amu-Bukhara-2, Amu-Zang-2 and Babatag pumping stations, such connections performed through individual connecting pipelines and connection nodes consisting of a T-joint. Therefore, at these pumping stations, apart from the Babatag pumping station, with an increase in the number of pumping units operating in parallel, the power consumption accounted for one unit decreases. At the Babatag pumping station with parallel operation of 3 units, the power accounted for one unit increases by 0.15%. This is explained by the impairment of symmetry of the poured out flows at the connection node (twin), where the lengths of the connecting pipelines are different, since the pumping units in the building of the pumping station are arranged in a chequered manner. As a result, the