Study On The Mechanism And Control Methods Of Surge In Centrifugal Pump Pipeline System

Surges also occur in incompressible liquid systems.When ‘surge' occurs,the system structure vibrates violently,the flow goes up and down,and the system speed and pressure also fluctuate periodically in a wide range,which seriously affects the normal operation of the system.Therefore,it is of great practical significance to study the surge mechanism and control method in pipeline system.In this paper,the phenomenon of ‘surge' in a new type of feed-water system in the secondary loop of nuclear power plant is taken as the research object,and the theoretical derivation,numerical calculation and experimental verification are carried out in order to reveal the mechanism and conditions of surge induction and eventually eliminate the surge phenomenon.Layout of the new feed-water system is firstly clarified.The different functions of the feed-water pump and the no-load running check valve were clearly defined and their flow characteristics were studied respectively.The irregular geometry of the cross section at the main outlet of the valve determines the non-stationarity of the flow characteristics,resulting in the non-continuous relationship between the resistance coefficient and the opening.In addition,the steady-state hydraulic characteristics and transient pressure pulsation of the centrifugal pump are numerically calculated,and the reliability and accuracy of the numerical calculation are verified by experiments.Simulation and calculation results show that the feed-water pump has a ‘hump' hydraulic characteristic.By combining Bernoulli equation of fluid and system driven motion equation,the vibrational differential equation of the piping system is established,with flow rate as a independent variable.A characteristic quantity,characterizing the matching relation between the hydraulic performance of the centrifugal pump and the impedance characteristic of the pipeline system,is less than zero,which is the necessary condition for the stability of the system.By comparing the instability conditions of pump valve system with different structures,flow-rate is a function of time and position.The second order partial differential coupled equations for the stability dynamics of pipeline system are obtained.On this basis,three inducing factors and three external manifestations of surge in centrifugal pump pipeline system are defined.The adaptive aquantity of the system unit is proposed and it is clear that the adaptive unit will increase the degree of freedom of the system.The mechanism of the water-feed system are validated experimentally.The results show that the "surge" frequency is the acoustic frequency of the pipe loop,which is only related to the layout of the pipeline,and does not change with the flow rate or the speed of the feed pump.Flow rate will affect the amplitude and phase of the structure vibration.In the respecitives of fluid,structure and control strategy,the different structure forms of pipeline system which can increase the freedom of the system are clarified.The "hump" of pump's performance curve is one of the inducing factors of the "surge".In order to eliminate the hump characteristic and improve the operation stability of the feed-water pump,the structural design and optimization of volute,impeller and outlet guide were carried out on the premise of ensuring the pump hydraulic performance,and a new feed pump was obtained,Which not only greatly improves the hydraulic performance,but also obviously weakens its "hump" characteristic.In addition,considering the influence of outlet guide vane on pump performance,the influence of guide vane installation position on hydraulic performance and dynamic characteristics of centrifugal pump is studied.It is found that when the volute tongue is located in the middle of two adjacent blades in the outlet guider.Then the new feed pump not only has the optimal hydraulic performance,but also has the minimum pressure fluctuation after these measuresThe no-loading running check vavle is a kind of three-way spring valve with complex structure.It is the most important adaptive unit of the system and also the most direct performing unit of system control strategy.On the basis of studying the regulating characteristics of driving motor,the feed-water pump and the valve,optimizations of the valve structure was carried out in order to realize the linear flow shunting for the loop.In this paper,the optimization scheme of the system control strategy,considering the actual working characteristics of the water supply system,is proposed,such as changing the shape of the throttle orifice,increasing the margin of flow control and adopting the hysteresis function control method and so on.In order to evaluate the important role of the booster pump in inhibiting cavitation and keeping the system running smoothly,the starting process of the feed pump under two different inlet pressures was simulated numerically.The results show that the booster pump can effectively inhibit the initial growth,development,and distribution of cavitation.It can greatly improve the anti-cavitation ability of the feed pump,and play an important role in maintaining the smooth operation of the feed pump in the transient process.Experiments prove that the water-feed system with improved measures of anti-surge and optimized control scheme,not only runs stably without surge,but also greatly improves the structural acoustic characteristics of the system.In a word,surge is a kind of systematic flow instability.It is a kind of self-excited vibration phenomenon formed when the centrifugal pump with the characteristic of "hump" runs in the unstable interval of small flow,and the input energy of the system causes the dispersion of DOFs.It has the characteristics of low-frequency vibration,not changing with the flow rate along with wide fluctuation of flow.Surge occurs mainly in water supply system which has high temperature and high pressure,often accompanied by cavitation and other unsteady flow phenomenon in a pump.