Study On The Performance And Mechanism Of Wear Of The Stator And Rotor Of Screw Pump In The Coal-bed Methane(CBM) Wells Draining And Collection

Screw pumps are the commonly-used mechanical equipment in Coal-Bed Methane(CBM)drainage and production.However,the service life of screw pumps needs to be improved constantly due to the harsh and complicated service environment.Wear between stator and rotor is one of the main reasons which cause screw pump to fail.The failure has affected CBM production and become an issue of CBM production.This dissertation mainly focuses on screw pumps,and investigates wear behavior and wear mechanism between stator and rotor,systematically and thoroughly in the areas of motion dynamics analysis,contact pressure analysis,wearing rate calculation,and physical wears.In order to describe actual moving patterns of rotor inside stator,a motion trajectory of the position of the two center points on the stator is used to represent the motion of rotor moving inside stator.The planetary motion of rotor moving inside stator is transformed to a pure roll motion in which a moving rotor rolls on the surface of a fixed stator,assumed no separation between the moving rotor and the fixed stator.This transformation of motion makes it possible to establish a “Roll-Slide” kinematics model for screw pump’s rotor moving along the center line of stator as in the initial motion condition.According to the principle of minimal deformation under external forces in the above “Roll-Slide” model,the finite element method(FEM)is used to analyze the motion trajectory of rotor’s two points which load is also applied onto,resulting in a formula to describe the true motion trace of a screw pump’s rotor moving inside its stator.The relative velocity of contact points between stator and rotor is calculated mathematically and analyzed virtually by FEM.The good agreement between the two results verifies the accuracy and the validity of the “Roll-Slide” kinematics model for screw pumps.A finite element contact equation is established applied contact constraint condition through penalty functions.Then FEM is used to analyze the maximum contact stresses of the stator and rotor.Shear stresses are also analyzed.Due to the effect of pumping and draining fluid characteristics,volume,pressure difference between the two adjacent cavities,rubber’s melding and dwelling,pump speed,and stator’s interference are defined as independent variables so that all these related effects can be used as boundary conditions for FEM analysis.ANSYS Workbench is used to solve the maximum contact stress and contact width.It can be found that the contact width on two sides around two central contact points is unsymmetricalon two sides.This phenomenon is caused by the rubber contact surface undergoing a deform-aggregate-spring back sequence during a full range of rotor “Roll-Slide” motion inside stator.Also,it can be concluded that contact position and interference obviously affects the contact stress between rotor and stator.Therefore,a total of 32 different combination cases with different interferences,different friction coefficients,and different velocities at rotor’s “Roll-Slide” contact locations are analyzed,and 3-D surfaces of stator’s contact stress contour and rotor’s shear stress contour are generated.Accordingly a mathematical formula is established.Furthermore,the contact width of the two sides of a contact point is used to fit a3-D surface and its mathematical formula.Contact stress theory verify aforementioned FEM prediction which is further used to construct the contact stress distribution function.On the other hand,the shear stress on rotor’s point contact and line contact is also analyzed,and found shear stress change pattern for different interference,rotor speed,and time.The tribological system identification method is used and the study of test and analysis on the wear rate of rubber under dynamic change of coal slime’s concentration is performed.A numerical method for stator’s wear rate under different coal slime’s concentration is proposed.Firstly,based on Archard wear equation,a basic numerical model of wear rate calculation equation for the stator and rotor is built.Secondly,since the media flowing through screw pumps is water mixed with coal slimes,it is necessary to analyze the stochastic behavior of coal-slime particles,the motion path of single coal-slime particle,settling rate of particles of low density coal slimes,and ways how motion trajectory acts on the pump.Lastly,the concentration change of worn particles is used as the input,the worn amount of stator is used as the output,and the M-series stochastic signal M based on coal-slime concentrations is used as an input parameter.With the worn volume from physical tests and the sampling time history curves,interrelated function curves and impulse response function curves are generated,which further leads to the modeling transfer functions of wearing rate on stator’s rubber point contact and line contact.Based on the basic numerical model of wear rate,a comprehensive numerical model of stator’s wear rate on point contact and line contact in dynamically-changed coal-slime concentrations is built.Physical tests are used to prove the accuracy of this model,and the error analysis is implemented.After studying wear patterns of wear-failed screw pumps from CBM fields and analyzing contact stress,shear stress and wear rate,wear mechanism between rotor and stator used in CBM drainage and production is revealed.Rotors and stators prototype parts used for CBMfield tests are section-cut,and SEM,EDS,and XRD are used to study the cutting surfaces.The whole screw pump is considered as two study objects,upper body and lower body.For each of study objects,the hardness,macro pattern,and micro pattern of both rotor and stator are studied.Based on physical test data,wear rate calculated,and analysis results of contact stress and shear stress,point contact and line contact’s wear mechanism of both rotor and stator is discussed.Following conclusions can be drawn:(1)The majority wear on stator’s line contact is adhesive wear and abrasive wear,and the wear happens on the straight line section.When contact patterns change from line to point,wear gradually changes to abrasive wear and fatigue wear;(2)Both upper stator and lower stator have similar worn locations,worn amount,and macro wear patterns.However,the lower rotor wears a lot more than the upper rotor does,especially on the lower rotor the welding-dwelling occurred and is easily observed;(3)When the bottom of rotor and stator are the line contact,with the condition of temperature rise,swelling,and a small amount of water drainage caused by friction heat,the surface material of stator rubber will continue to adhere to the surface of steel rotor,then the adhesive wear occur.As part of the surface of the rotor steel chrome layer off,it lead to friction chemical reaction between steel and rubber,with the condition of the collision between the rotor and stator and sliding friction,The Fe oxide of the rotor surface transfer to the stator surface;(4)In the early work of screw pump,the rotor surface coating can improve the fatigue limit,but with the increase of screw pump working time,the coating begin to wear off. It will produce oxidation wear and adhesive wear in the CBM liquid environment.It will produce more serious concentration slip with the increase of wear surface crack under the steel matrix,and it promote the fatigue crack.So fatigue wear is the main form in the late work of screw pump.