Process improvement of special high pressure gate

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Improvement of the process of special high-pressure gate valve for oil field

the gate valve for oil production wellhead device (Christmas tree) is the special gate valve with the largest consumption in the oil field. During the manufacturing process of the gate valve, the qualification rate of hydrostatic strength test and sealing performance test of the valve body is too low (30% ~ 70%), which has troubled the production plant for a long time. Based on the production practice of our factory, this paper analyzes the crux of the above problems and puts forward solutions

1. Improvement of blank casting process

our wellhead gate valve adopts sand mold cast steel blank. During the processing of the valve body, there are often casting defects such as porosity and shrinkage at the internal thread matched with the valve seat. Through the macro analysis of the valve body profile, it is found that there are shrinkage and porosity in different degrees in the hot spot area. In order to solve the above problems, we have carried out many improvement tests on the casting process

figure 1

1) improve the gating and riser system. Change the gating and riser system set on the cylindrical surfaces on both sides of the valve body to the horizontal gating system set at the bottom of the valve body as shown in Figure 1

2) change sand mold casting to investment casting

3) change the side pouring method to the top pouring method, and make the middle flange downward. Because investment casting has better permeability, faster and more uniform cooling conditions than sand casting, the structure is more dense. Since the molten steel is poured from the top and directly from the riser, the position of the riser is close to the hot spot area. It creates extremely favorable sequential solidification conditions for castings, and the hot spot area is fed in time. Therefore, the castings produced after the process improvement have a dense structure, eliminate casting defects such as shrinkage porosity and shrinkage cavity, and the product qualification rate reaches more than 99%

2. Measures to improve machining manufacturing accuracy

sealing effect is the key element of valve manufacturing. Sealing has always been a difficult problem in the machining industry. In order to improve the manufacturing accuracy of valves and ensure the sealing effect of valves, the following improvement measures are summarized through theoretical research and production practice

valve body is the most critical machined part in the production of gate valve. Due to the structural requirements, the sealing surface of its valve seat is inclined by 3O ~ 6O with the central symmetrical plane For the valve body of small and medium-sized gate valves, we use horizontal lathe and auxiliary bending tooling (see Figure 2) to complete the processing of the inclined surface of the valve seat. However, it must be ensured that the bending angle (3O ~ 6O) is consistent with the process requirements of the valve body. At the same time, we should strictly ensure that the center height of the machine tool coincides with the center height of the bending tooling. The allowable error is controlled between 0.02mm and 0.04mm after unloading and repeated recalibration. In addition, the accuracy and reliability of one-time pressing and positioning should be considered during the processing of the valve body. As shown in Figure 2, the heavy straight surface close to the main axis of the lathe should be regarded as the reference datum, and the wedge groove on the vertical plane of the bending plate and the sliding of the wedge block should be used to make the flange surface at one end of the valve body positive pressure. The financial funds of local governments are relatively tight, and the two sides should be processed respectively. In addition, during the processing of the valve body, it is necessary to complete the processing of the inner cavities of the valve bodies at both ends by rotating the rotary table for 180o when the cobalt proportion in the waste hand motor pool of the lathe tooling is above 60%. The rotary accuracy is guaranteed by the repeated positioning of two symmetrically distributed conical locating pin holes on the rotary flange. The transposition error is 0.01mm ~ 0.02mm. Fluorine lined butterfly valve

figure 2

in addition, it is necessary to select the appropriate counterweight to match with the bending plate. As we all know, the static balance of the bending tooling on the lathe directly affects the machining accuracy and production efficiency of the lathe, so we have made great efforts in the shape and layout of the counterweight of the bending. First, we have reduced the turning radius of the counterweight as much as possible, and second, we have adopted the structure of the outer circle and the inner side in the layout to improve the dynamic balance performance in the circumferential direction, so as to improve the machine speed and the machining accuracy of the workpiece as much as possible

finally, it is necessary to ensure the coordinated processing of all components in the sealing pair:

that is, the consistency of the slope between the valve body and the ram; Consistency between the bending slope of the valve body and the processed valve body; Consistency between the ram and the inclined disc for turning; Consistency between the inclination of the ram and the grinding slope table

the four kinds of consistency are interrelated, mutually restricted and interdependent. Four kinds of consistency, we are based on the inclination of lathe bending, after many times of screening and grinding out the standard inclination plate, and then use the standard inclination plate to repair the inclination tooling for ram lathe, so as to control the four kinds of consistency, improve the productivity and ensure the assembly accuracy

in addition, the grinding process of the valve seat should also be effectively controlled to ensure that the flatness of the valve seat surface is not less than 100:0.01, and that the surface roughness surface is not affected by scratches, bumps, and uneven grinding sand granularity, so as to achieve the best effect after grinding and ensure the accuracy of the valve seat

through the above series of effective measures and quality control, the qualification rate of wellhead valves produced by our factory has reached more than 98%

Author: Chen Yesheng, Zhang Wei, Wang Shugui, Chen Xiaohua, Daqing Petroleum Administration Bureau

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