Analysis On The Fluid Driven Torque Of Rotary Valve In The Measurement-While-Drilling System

Drilling fluid pressure wave generator is the core equipment of MWD system,the advanced drilling fluid information telemetry system abroad uses a drilling fluid pressure signal generator based on rotary valve or shear valve to carry out phase modulation of pressure carrier to realize the transmission of downhole data,the fast servo and stability control of rotary valve speed is the key to realize the pressure phase shift keying modulation of drilling fluid.In the process of drilling fluid pressure phase shift keying,the speed of the rotor must follow the change of control pulse quickly.Because the load torque of the rotor of the rotary valve has serious nonlinear characteristics,the speed control equation is nonlinear equation.To realize the speed control of the rotary valve,it needs to be linearized.The calculation model of the load torque of the rotating valve is accurately described when the load torque model is used for the linearization of the speed control system,the fluid driving torque generated by the flow of the drilling fluid acting on the rotary valve rotor is an important part of the load torque of the rotary valve.At present,CFD simulation is used to obtain the change law of the rotation angle and the influence factors,but the mechanism of how the structure of the rotary valve and the parameters of the drilling fluid affect the torque of the fluid is still unknown.In order to determine the correctness of the simulation experimental law of the torque,there is a certain question about the authenticity of the fluid driven torque obtained by the simulation.In this paper,the mechanism and influencing factors of fluid driving torque are systematically analyzed and studied.According to the CFD simulation analysis and the rotating valve structure of the flow field of the rotary valve,the paper determines the force form of the rotor blade end face.Through the momentum theorem and the fluid energy analysis of the bifurcated pipe,the mathematical model of the force of the rotor blade end face and the fluid driving torque is established,and the change law of the fluid driving torque with the rotor angle is studied.The effects of sub thickness,rotational speed,clearance between stator and rotor,drilling fluid flow and density on fluid driving torque are analyzed.The results show that the variation law of fluid driving torque obtained by theoretical calculation is basically the same as that of CFD simulation.In the process of turning valve and closing valve,drilling fluid is obstructed or loaded on the rotor movement when the rotor has less clearance.With the further increase of the clearance,the drilling fluid is gradually driving the motion of the rotor;in the process of valve opening,the drilling fluid is hindering the movement of the rotor when the rotor opening is small,along with the opening of the valve.Step by step,the alternation of drilling fluid to drive and obstruct rotor movement is complicated.The theoretical analysis and numerical calculation show that the discharge pressure of the fixed and rotor clearance causes the eddy current and the impact force on the end face of the rotor.The resultant force formed by the impact force and the main valve hole pressure produces the fluid driving torque,and the reduction of the gap thickness will greatly reduce the outlet pressure of the clearance,and then cause a greater effect.The impact force of the eddy current is increased and the torque of the fluid is increased.Because the pressure loss of the valve hole is related to the square of the drilling fluid flow,the torque of the fluid is most affected by the flow of the drilling fluid.As the flow of the drilling fluid is in the turbulent state,the torque of the fluid is almost unaffected by the change of the viscosity of the drilling fluid,and the thickness of the rotor affects the force area of the rotor end face,so it is thicker.The rotor will make the fluid drive torque increase;because of the fluid inertia pressure and rotor speed,the higher speed will produce larger fluid drive torque.The research of this paper theoretically explains the mechanism of drilling fluid on the torque generated by the fluid driven by the rotor and the influence of the drilling fluid parameters and the structure parameters of the rotary valve on the driving torque of the convective body.It has an irreplaceable theoretical guidance for the structure design of the rotary valve and the speed control of the rotating valve.