Shanghai Fanggong believes that the pump valve is one of the key components and wearing parts of the drilling pump, and its design directly affects the working performance and service life of the pump. On the one hand, to improve the erosion resistance of the pump valve contact surface, it is necessary to increase the surface hardness of the contact surface, and excessive hardness will weaken the impact resistance. On the other hand, in order to improve the material’s ability to withstand impact loads, it is necessary to ensure that the material has high toughness, and the corresponding hardness will be affected. In addition, although the overall performance of the pump and valve is good, various performances will not work at the same time under different working conditions, and the processing cost of the pump and valve will also increase accordingly. Therefore, studying the failure mechanism of pump valves plays an important guiding role in the design and manufacturing of pump valves.
​Generally speaking, there are two reasons for the failure of drilling pump valves: impact fatigue damage and erosion abrasive wear (hydraulic abrasive wear). However, analysis of the macro and micro morphology of scrapped drilling pump valves in mines shows that impact fatigue damage is the main mechanism of pump valve failure. Therefore, when designing pump valves, it is important to consider the impact fatigue resistance of the pump valve materials and the components made of them. The fatigue strength is determined by the local stress state.
Based on the simplified model of the pump valve in the closing stage and the finite element dynamic model of the pump valve impact process, this paper focuses on the study of the stress form and degree of the stress concentration on the contact surface between the valve disc and the valve seat during the pump valve impact, and through The fatigue life curve of the pump valve performs fatigue check on the maximum stress area to estimate the service life of the pump valve. According to the fatigue life curve, taking the weakest area of the pump valve as the target, by improving the structure of the pump valve to reduce the peak stress, a feasible solution is proposed for efficient use of the pump valve.
​1 Pump valve stress analysis
As the piston reciprocates, the valve disc produces intermittent impact on the valve seat, and the pump valve bears the impact load. The stress on the contact surface goes from the moment of closing to the maximum stress to the moment when the pump valve is opened. With such cyclic impact, it can be determined that the pump valve is subject to pulsating cyclic stress.
In the simplified model of the pump valve closing stage, it is assumed that within a small hysteresis height, the force on the valve disc remains unchanged and it moves downward at a uniform acceleration until it closes. According to this model, the speed and acceleration of the valve disc when the pump valve is closed are obtained.
In the literature, taking the 7# valve widely used in oil fields as an example, the cone angle is selected as 45° (the cone angle is the angle between the cone valve bus line and the axis), the crank angle φ=25° is set when the valve is opened, and the number of strokes is 120 times/min, the pump pressure is 15MPa, and the crank angle φ=25°~180°, a numerical simulation of Adolph’s exact differential equation of the drilling pump valve is performed, and the hysteresis height of the valve disc is obtained to be 0.0056m. Here The speed at is -0.4067m/s2. Using the simplified model, the speed of the valve disc when the pump valve is closed can be found to be -19.3676m/s, and the acceleration is -33476.65m/s2.
Using the velocity and acceleration of the valve disc at the closing moment obtained from the simplified model as motion boundary conditions, ANSYS/LS-DYNA software was used to build a three-dimensional model of the pump valve and simulate the process of the valve disc impacting the valve seat. The overall model of the pump valve is established according to the actual size of the pump valve, the sealing ring is omitted, the material properties and geometric constraints are set according to the actual working conditions of the drilled pump valve, and the 8-node hexahedral element is used for meshing to establish the model.
Apply dynamic theory to analyze and deal with collision and sliding contact interface problems, and obtain the stress distribution diagram when the maximum local stress is generated in the closing stage of the 7# valve disc with a cone angle of 45°.
When the valve disc impacts the valve seat, the maximum local concentrated stress is 0.955×109Pa. It can be seen that the stress concentration area at the lower end of the pump valve cone bears a pulsating cyclic load of 0.955×109Pa, with a period of 0.5s (the impulse of the pump valve times: 120 times/min)
Under the action of pulsating cyclic stress, the stress concentration area at the lower end of the cone surface is more likely to form fatigue cracks, which significantly reduces the fatigue strength of the pump valve. This is completely consistent with the phenomenon of severe plastic deformation in the lower part of the cone surface in the macroscopic morphology of valve seat failure. . It can be seen that the impact fatigue caused by stress concentration during the impact of the pump valve is the main reason for the failure of the pump valve.
This paper uses a three-dimensional geometric solid model to replace the two-dimensional plane model in the literature, and applies various types of dynamic loads to specific loaded parts of the structural model to simulate the real collision process. Using the ANSY/LS-DYNA software finite element explicit nonlinear dynamic analysis solution program, a more accurate stress solution can be calculated, and a more intuitive understanding of the direction of the stress distribution can be obtained.
2 S-N curve of pump and valve materials
40Cr steel is widely used as the manufacturing material for drilling pump valves. 40Cr steel is a low-alloy medium-carbon structural steel. After quenching and tempering, it has the characteristics of good plasticity, high fatigue strength, low notch sensitivity, and excellent low-temperature impact toughness.
Table 1 Mechanical properties of pump valve material 40Cr steel in quenched and tempered state