Experimental and theoretical studies of pumps of irrigation pumping stations

The article presents the results of a study of the wear of individual pump elements, a decrease in their efficiency and significant losses. The nonidentical operation of the extreme and middle pumps was established. The uneven approach of water to the extreme units created reverse flows of water in the suction pipes, work with vibrations of modes close to cavitation. The modernization of the pumps improved the cavitation characteristics due to the elimination of the cavitation center at the impeller inlet in the area of the driven disk, which increased the reliability and durability of the pump. Diagnostics of irrigation pumps made it possible to substantiate proposals for the modernization of their main units and operating modes. The distribution of pumps according to the general level of vibration is shown, indicating the direction at which the maximum value of vibration displacement was recorded and the results of a comparative measurement of the vibration of the units of the pumping station-1 of the Karshi Main Channel (KMC).Studies of the pumps of large irrigation pumping stations have shown a reduction in vibration and an improvement in basic operating parameters. With the participation of the authors, new complex methods of operation are being developed, which allow increasing its reliability by predicting the technical condition.


Introduction
In the XXI century, the systems of pumping water in the Republic of Uzbekistan amount to more than 2.4 million hectares (50% of irrigated arable land). The area of non-irrigated farmland: non-irrigated arable land, pastures, hayfields is more than 2.3 million hectares. Pumps are widely used in these areas. The Republic of Uzbekistan is the most pumped republic, more than half of the pumping capacity of the entire Central Asian region is concentrated here. The experience of operating pumping stations (PS) is of great importance for developing countries, where climatic and hydrological conditions are similar [1,2].
At the present stage, reliable and efficient operation has become one of the most important requirements for many types of pumps. Therefore, the issues of reducing the intensity of noise and vibration of pumps at the source of their origin have acquired particular urgency [3,4]. A review of works devoted to improving the operating modes of centrifugal pumps showed that the developed mathematical models describe individual processes and most of them are not suitable for developing the most profitable operating modes; the problem of unsteady water movement in the PS has not been fully resolved [5,6].

Methods
The studies of the authors have established the action of axial forces on the working bodies of the pumps. If the coefficients are established empirically for a certain pump operating in a given mode, then the derived equations can be used to determine the maximum permissible suction height or the minimum required head for cavitation conditions. The results of our experiments confirm the correctness of this reasoning.

Results and discussion
To develop effective ways to increase efficiency, reduce noise and vibration, it has become necessary to study the influence on them of energy parameters, design and technological factors and operating conditions of vane pumps. However, many of these factors defied calculation. The task was complicated by the specificity of measuring the vibroacoustic characteristics, for the determination of which it was necessary to create special conditions for testing the pumps excluding the influence of noise and vibration disturbances [7,8].
The limitation of noise and vibration of pumps is dictated by many circumstances, including direct technical requirements, operating conditions, the need to reduce the harmful effects of vibration and noise. Noise and vibration levels are general indicators of pump reliability and quality. Modern pump research is a complex task of significant technical complexity, requiring a large number of experiments [9,10].
The flow path of the pump serves to convert part of the kinetic energy of the flow leaving the wheel into pressure. In this case, inevitable energy losses occur, the minimum of these losses usually determines the position of the maximum efficiency of the pump as a whole.
The phenomena of shock and flow separation, pulsations of pressure and velocities at the exit from the wheel and in flow elements, navigation processes are the main reasons for the occurrence of oscillatory processes in the pump, which are recorded in the form of vibrations and airborne noise. Modern calculation methods pursue the goal of obtaining the maximum energy performance of the pump at the design operating mode [11,12]. However, this design approach does not always provide minimal pump losses.
The above information substantiates the need for research to improve the operating modes of PS with blade units.
The authors carried out research on the PS "Babotag" with horizontal double-entry pumps DVL-1400x900 with a flow rate of 4.44 m 3 /s; head 78.28 m; Efficiency 86.0%; cavitation headroom (NPSHR) 10.2 m.
The modernization of the pump improved its cavitation characteristics by eliminating the center of cavitation at the inlet to the impeller in the area of the driven disk, which increased the reliability and durability of the pump [3,13].
In order to improve the cavitation characteristics, at the suggestion of the authors, the fairing on the shaft side can be made with a cavity in which the guide baffles are located and fixed in the body by means of ribs. The axial force F 1a acts on the wheel as the difference in external pressure on the side discs of the wheel on the right and left sides.
Internal pressure on the sidewalls will be balanced. From the inlet side, the pressure on the side wall will be less, since the pressure in the inlet sp p from the inside is always less than the pressure from the outside. The pressure from the outside is determined by the rotation 2 u of the fluid at a speed in the gap between the wheel wall and the housing at a radius 2 r . Its value is determined by the formula: The pressure in the gap, counted from the outer diameter, drops more slowly than in the wheel, since the fluid in the gap rotates at half the angular velocity. The pressure forces on the lateral surfaces at radii larger than the entry radius sp r are mutually balanced.
Unbalanced pressure forces occur over the area defined by the inlet diameter.
For the established deviations of hydrodynamic quantities from their steady-state values, a mathematical model of the unsteady flow of an incompressible medium has been developed in one of the following forms [6,9].    p or Q . It is more difficult to experimentally determine impulse transient functions than transient functions caused by a step change in the specified values; therefore, instead of relations (4) or (5), we use the related relations: p .
Axial force arises from the change in the amount of movement of water in the axial direction. This force, as reactive, can be calculated from the difference in the momentum in the axial direction at the outlet and inlet to the impeller a c1 .
The axial speed at the exit from the wheel 0 2  a c , therefore the axial force P 2a is found by the formula This force is the reaction force of the inflowing jet. Forces a F 1 and a F 2 are directed in different directions. Total axial force a F ; will be found as the difference of these forces The axial thrust loads the bearings, complicating and heavier the pump design. This effort must be reduced, i.e. relieve the wheel from axial force. The wheel is sealed at the front and rear. On the rear side, at a radius smaller than the radius of the seal, there are holes called unloading holes [14,15]. The pressures on the front and back sides are equalized. This method of axial thrust unloading is used on many pumps during retrofitting at the impeller inlet (Fig. 1). At high pump flow rates, the growth of the hydrodynamic inhomogeneity of the flow and the presence of intense vortices in the outlets and the wheel contributes to the fact that the pressure in the vortex regions decreases, provoking the appearance of cavitation.
In such conditions, cavitation and hydrodynamic inhomogeneity of the flow can lead to increased vibration of the pump body parts. As a normalized vibration parameter, the rootmean-square value (RMS) of vibration displacement in the operating frequency band of 10...1000 Hz is taken during stationary operation of the pump. Additionally, the assessment of the frequency components of the vibration spectrum in the characteristic frequency ranges, causally related to the general dynamic state of the unit [17][18][19][20][21][22][23][24].
The distribution of pumps according to the general vibration level, indicating the direction at which the maximum value of vibration displacement is recorded, is presented in Table 1.  As can be seen from Table 1, the overall vibration level of the extreme pump № 8 exceeds the vibration level of the pump № 5. To explain the features of the modes, the hydraulic flow pattern was determined, that is, the boundaries of the transit flow, eddies and stagnant zones.
When the number of operating pumps changed, it was clarified by measurements how the sizes of the vortex and stagnant zones change, the intensity of funnel formation [2,9]. On various units, usually the extreme and one of the middle ones, the distribution of velocities in the water intake and the inlet section of the suction pipes were measured and compared in order to identify the features of the water supply (Fig. 2). The probability of occurrence of certain deviations from normal operating conditions and the degree of danger of these special modes are different for pumps of different types and are determined by their size, as well as the purpose of the PS, its equipment with control and automation means. For example, an increase in head above the maximum design value poses practically no danger for small centrifugal pumps, but it is highly undesirable for large centrifugal and even more so for axial pumps.
The main difficulties in the operation of pumping and power equipment of large PS are the hydraulic conditions of the flow supply.
The practice of operating the head PS-1 of the largest Karshi cascade, the operation of 4-6 units at relatively low levels is accompanied by vibration of a cavitation nature. Normal operation of the units is achieved by an additional increase in the level of the water horizon in the front chamber (penetration of the impeller).
Inspection of the technical condition of the PS showed that the level in the PS-1 avancamera, which ensures the pump operation without cavitation, should be at least 5.5-6.0 m.
The permissible indicators for the OPV11-260 pumps and the new 300VO-37 / 26C pumps were supposed to be formalized after the end of their trial operation period. Such norms are still absent, and the evaluation criteria set forth in the general technical interstate standards 10816-1-97 and 10816-3-2002 "Vibration. Monitoring the state of the pumping unit based on the results of vibration measurements on non-rotating parts ", where the rootmean-square value of vibration displacement in the operating frequency band of 2 ... 1000 Hz is set as a normalized parameter of the general vibration level during stationary operation of the pump.
From the point of view of vibration strength in units, the most dangerous are oscillations of a periodic nature, which are the result of mechanical, electromagnetic and hydraulic processes with pronounced discrete components. Such dangerous fluctuations are mainly strong diagnostic signals (that is, they stand out well against the background of vibration noise) [25][26][27].
In The reason for the non-uniformity of the flow is the asymmetric flow around the rotating blades during the formation of vortex zones. The inhomogeneity of the flow leads to the appearance of vibration on the impeller chamber at the blade frequency f b and its higher harmonics.
For worn out spherical chambers and impellers, in the absence of technological capabilities for turning, a combination of the above reasons prevails.
At the same time, the non-identical operation of the extreme and middle pumps was also established. The uneven approach of water to the extreme units created reverse flows of water in the suction pipes, work with vibrations of modes close to cavitation ( Table 2).  [19,20].
A new centrifugal pump with changed radii of flow entry into the impeller, blades at the entrance and exit from the impeller was introduced at the Alat pumping station as part of the Amu-Bukhara canal [4,21]. The introduction of new pumps reduces losses in the pump flow path and leads to energy savings.

Conclusions
1. The wear of individual elements leads to a deterioration in the operation of the pumps, a decrease in their efficiency and significant losses. The reasons for the increase in vibration can be the wrong location of the unit in relation to the downstream water level, associated with the peculiarities of their operation, partial mechanical and hydraulic imbalance of the impeller, inhomogeneity of the flow in the impeller. The conducted studies of pumps of large irrigation PS showed a decrease in vibration and an improvement in the main operational parameters during the modernization of the impeller.
2. The modernization of the pump improved its cavitation characteristics by eliminating the center of cavitation at the inlet in the impeller in the area of the driven disk, which increased the reliability and durability of the pump. At the same time, the non-identical operation of the extreme and middle pumps was also established. The uneven approach of water to the extreme units created reverse flows of water in the suction pipes, work with vibrations of modes close to cavitation.
3. Diagnostics of irrigation pumps made it possible to substantiate proposals for the modernization of their main units and operating modes. The distribution of pumps according to the general vibration level is shown, indicating the direction at which the maximum value of vibration displacement was recorded and the results of comparative vibration measurements of PS-1 KMC units. 4. The most rational type of dynamic testing is controlled operation, systematic monitoring of changes in parameters and wear of elements of an operating pump. Under controlled operation, there is no need to accurately measure the absolute values of the parameters; it is necessary to record with high accuracy their time variation and vibration activity. With the participation of the authors, new complex methods of operation are being developed, which allow increasing its reliability by predicting the technical condition.