The Operational Features of Pumping Stations Equiped with Parallel- Connected Centrifugal Pumps for Land-Reclamation

The required water supply of a pump could be provided by means of different combinations of pressure piping and pumping units at different operational costs. The basic indices describing the operational modes for the individual and group pumping units, including the values of the efficiency, are computed. The adequacy of the calculational results that based on the developed mathematical models is confirmed by the full-scale experimental researches. Based on the presented principles of the load distribution between parallel working pumping units, 5 % of consumed electric power may be saved.


INTRODUCTION
In this study the features of the land-reclamation pumping stations, which deliver water into the irrigating systems have been considered. The study of their operation mode should take in consideration: * The seasonal variation * The temporary decrease of the water supply * The total stoppage for a short period of time under the emergency conditions At such pumping plants, some aggregates operate on one common pressure head pipe lines (PHPL). There is a locking set at each pump of a particular pipe line. The locking set for the small pumping plants is a sliding valve, while disk shutters are applied to the large stations. Such pump stations in Uzbekistan include Khamza, Kiziltepa ,Talimardzhan (HC-7 KMK), Tashkelik, pump stations of Dzhizak and Amu-Zang cascades.
The land-reclamation pumping plants, equipped with long PHPL and several centrifugal pumps are commonly connected in prallel to one PHPL. Their operational modes basically depend on the quantity of pumping units, thier connection schemes and on the water levels in the downstream and upstream of station [1]. At the selection of a pump or group of the pumps, included in the parellel operation, it is necessary to take into account the value and the nature of a change in water folwrate, the duration of the plant operation with one or another folwrate, which in turns minimize the number of switching on and off of the pumping units. The number of startings of the large pumping units with a power of more than 2500 kW is limited to 50-120 times/year [2].

Operation of Pumping Units:
For the economical operation of pumping units, it is recommended to run most of time those units that have higher efficiency. Increasing the operational time for the pumps of higher efficiency and low specific consumption of electrical power from 4500 to 6240 hr/year and decreasing the operational time of the pumping unit with low efficiency and higher consumption of electrical power from 4500 to 2700 hr/year have allowed to reduce the annual power consumption of pump station to 264000 kWh [2]. Discharge control of the land-reclamation pumping plants, equipped with centrifugal pumps operating in parallel on a common pipe line, is accomplished step by step, via starting or turning off of pumping unit. For the organization of the operational mode for different types of pumping units, a time selection criterion is needed for the transition from one pumping unit to another. To achieve that, it is expedient to build for each pumping unit (or group of units) the curves of the specific consumption of electric power for the pumping of 1 m 3 of water. The points of intersections of these curves determine the discharge quantity at which a certain unit is to be run while the other is disconnected [2,3].

The Criterion for the Load Distribution:
The criterion of the correct load distribution between the pumps is the minimum power consumption of the simultaneously operating pumping units. For the majority of the national pumps, the graphs of characteristic N=f(Q) have the form of curves' concave downward, the minimum power consumption is ensured, if power increment for each of the parallel pumping units is equal [ 2 ].
The presented principles of load distribution between the parallel pumping units ensure 5% saving of electrical power. Optimum load distribution between the pumping units, gives the equality efficiencies or specific consumption of the electric power of these units. the rotational speed of these pumps is determined from the condition of the economic operation of pumping units, The station output is given by: Where:  5. From the initial head characteristic curves the supply of each unit is determined on the base of the known full head. 6. Proceeding on the calculated values of discharge and pump head, efficiency and the required power are determined by formula: Where : -density of flowing liquid; g -acceleration due to gravity; N i -shaft power of pump in kW.
The task of determining the optimum distribution of the station productivity for each aggregate is reduced to the selection of such a vector of the pumping units' discharge, for which of the function of the following form is minimum: This task is considered as a parametric optimization [ 3,4 ].

Determination of Q i for all Pumping Units:
From the analysis of HC-7 modes it may be concluded that the closest to the given discharge of the pump station can be ensured by means of different combinations of running units and pressure piping lines, while each of them will have its own power consumption and ,consequently, the different types will have also different running costs. Taking into account the dependency of the geometric height of pumping on the transition mode through the panel of plant water-discharge of individual head losses during the confluence of flows on pipe crosses, the steady-state regime of a group of pumping units, operating in parallel on a common delivery pipe line is described by the system of equations of the following form (for two aggregates):

CONCLUSION
In this study a procedure for the caculation of the pumping plant operation equipped with centrifugal pumps has been developed with the consideration of head lossess during the confluence of flows into the common delivery pipe lines. The comparison between the caculational and experimental results confirms the adequacy of the developed mathematical models and procedure.