Process improvement of oilfield special high pressure gate valve

The gate valve for oil production wellhead (tree) is the special gate valve with the largest oilfield usage. During the gate valve manufacturing process, the pass rate of the hydrostatic strength test and sealing performance test of the valve body is too low (30%~70%), and it has long plagued production. plant. Based on the actual production of our factory, this paper analyzes the crux of the above problems and proposes solutions.

1. Improvement of blank casting process

The well gate valve of our factory adopts sand die casting steel blank. During the processing of the valve body, casting defects such as porosity and shrinkage often occur in the internal thread matched with the valve seat. After macroscopic analysis of the valve body profile, it is found to be different. In order to solve the above problems, we have carried out several improvement tests on the casting process.

1) Improve the pouring riser system. a horizontal gate system provided at the bottom of the valve body of the pouring riser system disposed on the cylindrical surface on both sides of the valve body;

2) Changing the sand mold casting to investment casting;

3) The side casting method is the top casting method, and the middle flange is downward. Since investment casting has better gas permeability, faster and more uniform cooling conditions than sand casting, the structure is more compact. Since the molten steel is poured from the top and poured directly from the riser, the position of the riser is close to the hot section. The casting has created an extremely favorable sequential solidification condition, and the hot joint zone is timely replenished. Therefore, the casting structure produced by the process improvement is compact, and the casting defects such as shrinkage and shrinkage are eliminated, and the product qualification rate is over 99%.

2. Measures to improve machine manufacturing precision

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

The valve body is the most critical machining part in the production of gate valve. Due to the structural requirements, the seat sealing surface and the central symmetry plane are inclined by 3o~6o. For the valve body of small and medium-sized gate valve, we use horizontal lathe and auxiliary bending tooling. Finish the processing of the inclined surface of the valve seat. However, it must be ensured that the angle of inclination of the curved plate (3o ~ 6o) is consistent with the process requirements of the valve body. At the same time, it is necessary to strictly ensure that the machine center height and the curved plate tooling center are highly coincident. The allowable error is controlled between 0.02 mm and 0.04 mm. In addition, the valve body needs to consider the accuracy and reliability of one compression and positioning during processing. As shown in Fig. 2, the heavy straight surface of the main axis of the lathe should be regarded as the reference datum. The wedge surface of the valve body should be positively pressed by the sliding of the wedge groove and the wedge block on the vertical surface of the curved plate, and the two sides are respectively pressed. Processing. In addition, during the machining process, the valve body needs to be rotated 180o on the lathe tool to complete the machining of the inner cavity of the valve at both ends. The accuracy of the rotation is ensured by the repeated positioning of the two symmetrically distributed conical pin holes distributed in the slewing flange. The indexing error is from 0.01 mm to 0.02 mm.

In addition, choose the right weight and the curved plate. As we all know, the static balance of the bending tool on the lathe directly affects the machining accuracy and production efficiency of the lathe. Therefore, we have made great efforts in the weight shape and layout of the curved plate. First, we should reduce the radius of gyration of the counterweight as much as possible. The inner circumference of the outer circle is used in the layout to improve the dynamic balance performance in the circumferential direction, so that the machine speed and the machining accuracy of the workpiece are improved as much as possible.

Finally, it is necessary to ensure the coordinated processing of the various components in the sealing pair:

Namely: the consistency of the valve body and the slope of the gate; the consistency of the valve body and the slope of the curved body of the processing valve; the consistency of the ram and the inclined disk for turning; the slope of the ram and the grinding inclination Consistency.

The four kinds of consistency are interrelated, mutually restrictive, and dependent on each other. Four kinds of consistency, we use the slope of the lathe bending plate as the basis, after many times screening and grinding the standard inclination plate, and then using the standard inclination plate to repair the slant lathe with the inclined tooling, control Four consistency, increased productivity and guaranteed assembly accuracy.

In addition, the grinding process of the valve seat should be effectively controlled to ensure that the flatness of the valve seat surface is not less than 100:0.01, to ensure that the surface roughness surface is not affected by factors such as scratches, bumps, uneven grain size of the grinding sand, etc. In order to achieve the best results after grinding, the accuracy of the valve seat is guaranteed.

Through the above series of effective measures and quality control, the qualified rate of wellhead valve products produced by our factory is over 98%.