Design And Investigation Of Dynamic Characteristics Of The Direct Acting Relief Valve In Lubricating System

The steam turbine power generation technology is widely used in coal-fired power generation,natural gas power generation,nuclear power generation,solar energy and biomass power generation,steam turbine is the core equipment of the power plant.The main function of turbine lubricating oil system is to provide the required lubrication oil with enough pressure and flow rate to the turbine,and supporting the turbine shaft and cooling the bearings,it can ensure steam turbine running safely and reliably.At present,the domestic steam turbine industry is in the development of the accelerated period,the turbine technology has been developed from the supercritical unit to the ultra supercritical unit,the power of the turbine can reach 1000 MW,and the shaft system becomes more and more large,the flow rate of lubricating oil supplied to the steam turbine might over ten thousand liters per minute,the change of overflow of the relief valve might to several thousand liters per minute,in such a large lubrication flow and overflow discharge change,it is required to have high setting pressure accuracy and good dynamic performance of the relief valve for the lubricating system.So far,the batch production of the relief valve in domestic are mostly concentrated in low pressure small flow or big pressure high flow,the type of low pressure high flow rate relief valve suitable for the working condition of steam turbine lubricating system is mainly imported from abroad,the price is high,the existing lubricating relief valve in domestic turbine unit was mainly producted through surveying and mapping imitation of the foreign product,its specifications are small,can not meet the needs of the development of steam turbine,and lack of relevant design methods and theoretical guidance,these conditions have seriously hampered the development of low pressure large flow relief valve technology and the improvement of the products quality,the reliable operation of the steam turbine was also affected.So in this dissertation,theoretical analysis,numerical simulation and test are combined to investigate and development the low pressure high flow rate direct acting relief valve applicable to steam turbine lubricating system,the linear analysis,the nonlinear time domain analysis and fluid structure coupling dynamic analysis methods were used collaboratively to investigate the dynamic and static characteristics of the valve systematically from the aspects of the basic structure of the valve,the shape of valve orifice,the damping configuration,the determination of key structure parameters of the valve,the distribution of inner flow field parameters to the dynamic process of the valve.The main results of the dissertation are as follows:1)A general form of linear mathematical model of two kinds of direct acting relief valve which direct action and differential action structure was derived,the relationship between structure parameters and performance parameters of two types of direct acting relief valve was also analyzed and a conditional expression of flow gain met stability and good transient response characteristics was found,which could be used as a design guideline in the determination of key structural parameters of direct acting relief valve.2)A new directing action relief valve structure suitable for low pressure high flow condition in lubrication system was proposed,which characterized by variable flow gain,asymmetric damping,three stages coaxial and damping parallel to the oil inlet.3)A fluid structure coupling numerical simulation method for direct acting relief valve in oil cavitation flow was explored,which could be used to obtain the distributions of internal flow field parameters and the transient response characteristics of valve simultaneously,to capture the impedance characteristics of the throttle or damping element and to extract the fluid structure coupling vibration characteristic parameters of the valve.4)The prototype of the low pressure high flow direct acting relief valve has been developed and tested,the bench test results show that the prototype has good static and dynamic characteristics,which could met the requirement of variable working conditions of steam turbine lubricating oil system.The main contents of this dissertation are as follows:In the first chapter,the background and significance of the research was presented,simultaneously the domestic and overseas present research and development status of the direct acting relief valve including its structure,new development and application of various analytical methods were reviewed.The main research contents of this dissertation were summarized.In the second chapter,the transfer function mathematical models of two kinds of typical structural direct acting relief valve were built using classical control theory,through model analysis,simplification and comparison,a general form of linear mathematical model of direct acting relief valve including direct action and differential action structure was derived,then,the key factors affecting the static and dynamic characteristics of the direct acting relief valve were analyzed,a conditional expression of flow gain met stability and good transient response characteristics was found and a new scheme with decoupling structure and variable flow gain suitable for low pressure and high flow rate conditions was established.In the third chapter,using the lumped parameter modeling method,the nonlinear time-domain models for three different structuring forms of low pressure high flow rate direct acting relief valve was built,the influence of different damping structure and structural parameters on the static and dynamic characteristics of three structural low pressure high flow rate direct acting relief valve was investigated by numerical simulation.An asymmetric damping structure scheme was proposed to improve the dynamic response characteristics of the valve and its mathematical model was also established to study the effects of the asym-metric damping structural parameters on the transient characteristics of the low pressure high flow rate direct acting relief valve.In the fourth chapter,by the analysis,selection and adjustment of wall functions,cavitation models and model's parameters in the FLUENT software and,a numerical simulation method verified by test results available in the literature for calculation the pressure thrust force on poppet valve in cavitating flow was found,and a fluid structure coupling numerical simulation method combined the calculation method mentioned above,the dynamic grid technology and UDF programming technology was explored,which could be used to obtain the distributions of internal flow field parameters and the transient response characteristics of valve simultaneously.In the fifth chapter,the flow characteristics and the flow force characteristics of the low pressure high flow rate direct acting relief valve were analyzed in depth using steady flow field numerical simulation method,the numerical results were provided as a basis for the simplified treatment of the valve discharge coefficient and the hydrodynamic characteristics in the nonlinear time domain analysis of the valve in the third chapter,and the guideline for determining the valve cavity geometry in downstream was also put forward.The effect of different damping structural parameters on the fluid structure coupling characteristics of the relief valve was investigated using the methods obtained in the fourth chapter.In the sixth chapter,the operating principle,test conditions,test equipment and test methods of the test bench of low pressure large flow direct acting relief valve were introduced,and the static and dynamic characteristics of the valve were tested.In the last chapter,some conclusions of this dissertation were summarized and the future research proposals were suggested.