| Nitrogen pump products with independent intellectual property rights and capable of withstanding high pressure, ultra-low temperature, large displacement are being developed in the country to meet the high difficulty of exploitation of unconventional reservoirs and increase production of existing oil and gas wells. A special structure of fluid end is needed to meet the needs of liquid nitrogen, and production of materials of the fluid end must be met with sufficient strength, toughness and fatigue life at cryogenic conditions and long-term effects of alternating stress, so as to meet the construction site requirements, reduce mining costs, improve economic efficiency.By combining theoretical analysis and experimental research methods, and learn from previous studies on liquid nitrogen pump fluid end structure and materials, the thesis mainly work carried out in following areas:(1) The structural characteristics of ordinary fracturing pump fluid end were studied, and the similarities and differences between liquid nitrogen pumping and ordinary fracturing fluid pumping were analyzed. An fluid end meet fluid nitrogen pumping was designed based on the existing hydraulic fracturing pump ends, and related parts of the fluid end were designed to calculate and strength check.(2) Cryogenic material9Ni steel was selected for fluid end manufacturing, two kinds of heat treatment quenching and tempering(QT), two-phase zone heat treatment(QLT) were developed to heat treatment9Ni steel. The metallographic observation of9Ni steel was down and the consisting characteristics of9Ni steel were analyzed after completion of the heat treatment.(3) Single pass compression thermal simulation experiment of9Ni steel was completed, and a constitutive model of9Ni steel was established by using Fruit fly optimization algorithm(FOA) to optimize the least squares support vector machine(LSSVM) parameters, which can provide a reference for thermoplastic processing of9Ni steel.(4) Cryogenic instrumented impact test and tensile test were completed, and the test specimens fracture were observed by scanning electron microscopy(SEM), the mechanism of crack propagation and fracture of9Ni steel at cryogenic conditions were analyzed. The heat treatment process provide sufficient cryogenic toughness and strength was selected to heat treat9Ni steel for fatigue test.(5) The fatigue test of9Ni steel under QLT treatment was completed, and the test data were fitted by using a least squares fit to get Basquin equation, and fatigue life curve under50%,90%,99%reliability were drawn. The material S-N curve was corrected by using of Goodman method to obtain parts S-N curve. And the fatigue test specimens fractures were observed by SEM to analyzed the composition characteristics of the fracture and material fatigue fracture mechanism.(6) Fatigue life prediction of cylinder, valve of the designed fluid end were down by using MSC.Ftigue software based on the fatigue test data, and the fatigue life distribution, and the easy fatigue damage sites of cylinder, valve and plunger were got. Also the influence of suction pressure to the stress and the fatigue life of cylinder, valve were analyzed.The structure study of liquid nitrogen pump fluid end provides the basis for high-presure liquid nitrogen pump fluid end design and manufacturing. And the material study of fluid end provids a simple and feasible method for fatigue life prediction of the main componnents of the fluid end. In conjunction with the actual and results of this study will have a certain guiding significance to on-site production, fabrication and on-site constrution. |
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