Operation Of Step Pumping Station System Based On Open Channel Transient Flow And Its Applications

Pumping stations play a key role in irrigation,drainage,water diversion,urban water supply and sewage,which protecting the safety of people's lives and property,promoting economic development,improving people's lives and constructing ecological environment.A step pumping station system is a set of pumping station steps connected with channels in series;each step pumping station consists of a number of pump units working in parallel mode.The role of these pump units is to pump water from a low area to a high position,and transfer water by a channel to the next step pumping station.The flow conditions in the real-life pumping station-channel systems usually vary with time,and thus,the flows are unsteady.The analysis of unsteady flow is more complex than that of steady flow because unsteady flow conditions may vary with respect to both space and time.The large step pumping station system usually transfers water with open channels.Whenever the pump units start up or stop,the steady state conditions of flow in the channel are disturbed,transients in the open channels are produced and the flow parameters are changing in a long time;in addition,the depth of transient flow and the pumping station's operation duty influence each other.Therefore,the situation is highly complicated.In this work,the behavior of unsteady flow in a prismatic trapezoidal channel between two step pumping stations and the pumping station's operation duties are studied.Mathematical models are constructed to simulate one-dimensional,unsteady,gradually-varied open channel flow in the system channel.Proposed models are applied on a realistic step pumping station system-Huai'an and Huaiyin step pumping station system in Jiangsu Province,China.The main works and achievements are as follows:(1)In order to solve transient flow in the channels of the step pumping station system,several solving methods of the Saint-Venant equations are analyzed and compared.The mathematical model is constructed which involves hydraulic loss,water loss due to seepage and evaporation and boundary conditions of the pumping stations.The numerical solving method based on Lax diffusive scheme of the mathematical model is proposed.(2)A mathematical model and numerical solving method are applied on Huai'an and Huaiyin pumping station system.After starting pump units in the pumping stations,unsteady flows in the open channel are immediately generated.The accuracy of the model is tested by varying the scheme grid step lengths.In order to guarantee the stability and the computational accuracy at all times,the optimal distance step obtained is ?x = 1 m,while the optimal time step is ?t = 0.01 s.The water surface elevation,the flow rate and the lateral outflow per unit length along the channel in different times are calculated and discussed,as well as several influential factors at various stages along the channel.The computed results show that the water surface profile in the open channel varies continuously over time until it becomes almost constant.Thus,the pumping station operation duties change over time.It also observed that different initial water levels tend towards the same results in terms of water surface profile and flow rate.The time required to reach the almost steady state after the transient flow produced in the open channel also depends on the initial water level value,where,the initial water level values closer to the last value requires less time to converge.The accuracy of the results is validated by comparing the computed results with measured data.(3)When starting two step pumping stations at the same time,two waves are propagated in the channel,which are high positive wave and low negative wave.The wave propagation mechanism is analyzed.The water surface elevation and the flow rate along the channel change violently in the first 2?4 hours after starting the pump units,the measurements at the cross section of the channel of the pumping station flow rate that are performed too soon after starting the pump units will cause substantial error.Over time,the flow rate change becomes increasingly small until a situation is reached when upstream pumping station's discharge satisfies the summation of downstream pumping station's discharge and the lateral outflow along the channel.(4)The transient flow in an open channel between two step pumping stations and the pumping station's operation duties after pump units starting up at the same time under different pumping station operation modes(number and blade angle of operating pump units)are studied.The water surface elevation,the flow rate and the lateral outflow per unit length at various stages along the channel are calculated and discussed.The results show that when the pumping station discharge changes by changing the number of running pump units and pump blade angle,the water surface profile increases,or decreases until it converges to steady state,the time required to reach steady situation depends on pumping station discharge.Higher pumping station discharges lead to short transient period.(5)The transient flow in an open channel between two step pumping stations for different channel lengths and the pumping station's operation duties are studied.The water surface elevation,the flow rate and the lateral outflow per unit length at various stages along the channel are calculated and discussed.The results indicate that the water surface profile increases,or decreases until it converges to steady state,the time required to reach steady situation depends on the channel length,where,shorter channel lengths lead to shorter transient period.(6)The transient flow in an open channel between two step pumping stations under abnormal situations due to breakdown and the pumping station's operation duties are studied.A mathematical model is constructed and applied on the step pumping station system under abnormal situations.The water surface profile,the flow rate and the lateral outflow,at various stages along the channel under abnormal situations,including step 1 pumping station or step 2 pumping station or one pump unit cannot start-up,one of pumping stations or pump units suddenly breaks down after normal stable operation,were calculated and discussed.The results indicate that the situation that the water surface profile increases,or decreases,converges to a new steady state,will influence the pumping station operating duty,flow rate in the open channel and safety of the pump units and dam.So,the management,maintenance and rush-repair of pumping stations should be scheduled according to the transient flow performances.Specially for the abnormal situations whose allowed rush-repair time cannot meet the required rush-repair time,the part of or all pump units of the normal operation pumping station must be stopped or the blade angle of the pumps should be adjusted to the smallest in order there to be enough time to repair the pumping station.In this research,the unsteady flow behavior in a prismatic trapezoidal open channel between two step pumping stations and the pumping station's operation duties for different initial water values,different operation modes of the pumping stations,different channel length values,several abnormal situations expected to occur in the real-life for the large step pumping station system and its applications were studied.The water surface profile,the flow rate and the lateral outflow along the channel were computed and discussed.The achievements have great significance for design,operation and management of large step pumping stations.