Analysis of the technology of ball valve with large diameter   Abstract: in order to ensure the sphere machining accuracy and surface quality conform to the requirements of the drawings, valve to obtain excellent sealing performance and low operating torque, prolong the service life of the valve, this paper introduces the use of the center hole as a sphere turning and grinding the same positioning datum, by making a reasonable processing technology, ensure that all the form tolerance and position precision machined part drawings design requirements.   Reliable sealing performance and low operating torque are important indicators to measure the quality of ball valve, and the machining quality of ball valve not only directly affects the sealing performance and operating torque, but also directly affects the service life of the valve. Large diameter ball valve structure with fixed ball, floating seat, by the spring and medium pressure to push the seat, the seat sealing surface and the ball spherical tight fit, produce sealing pressure, prevent media leakage. The smaller the roundness error of the sphere, the smaller the surface roughness, and the smaller the necessary specific pressure to ensure the sealing, so that the valve sealing performance is good, low operating torque.  
  1. Influence of ball precision on sealing performance

The influence of sphere precision on sealing performance is mainly considered from the following aspects:

(1) The roundness error of the spherical sealing surface directly affects the coincidence between the spherical surface and the sealing surface of the seat.

If the fit is high, it will increase the fluid flow

The motion resistance between the sealing surface and the sealing performance are improved.

The surface roughness of the sphere is high, so the sealing must be increased.

Therefore, the spherical roundness error is reduced

Poor, reduce the surface roughness is helpful to reduce the seal required specific pressure, improve the seal performance.

(2) The axial error between the upper and lower journal of the sphere and the sphere, and the distance deviation between the upper and lower positioning plane and the horizontal axis of the sphere will all lead to the deviation of the axis of the sphere and the axis of the seat, resulting in the dislocation of the sphere and the seat sealing belt, resulting in the decrease of the specific pressure of the local seal and the leakage of the seal.

(3) The symmetry and perpendicularity of the groove face of the valve stem will affect the reasonable coordination between the stem and the ball. In serious cases, the ball will get stuck. On the one hand, the ball cannot adjust itself, causing seat seal leakage, and on the other hand, increasing stem torque or stem packing leakage.

Thus it can be seen that the processing quality of the ball directly affects the sealing performance, operating performance and service life of the valve, while the processing technology of the ball directly affects the processing quality of the ball, and ultimately affects the overall performance of the valve.

 
  1. Analysis of ball machining process

Generally speaking, the machining precision and the machining efficiency are considered in the machining of spheres. In order to ensure the assembly, sealing and operation performance of the valve, the 42 “-150lb ball machining drawing shown in Figure 1 is taken as an example to illustrate the process scheme analysis.

(1) Rough machining of each part of the sphere

The rough machining of each part of the sphere is realized on the NC vertical lathe.

(2) The coaxiality between the upper and lower journal of the sphere and the sphere

To meet the coaxiality design requirements of the spherical journal and the spherical surface, the machining method of integrated grinding and one-time clamping must be adopted to achieve, so as to ensure the coaxiality ≤0.05 mm and control the roundness of the sphere ≤0.However, there is no such processing equipment in China at present, so the turning and grinding need to be completed in two equipment, which requires the same positioning datum for journal turning and spherical grinding to ensure the required accuracy. At the same time of machining both ends of the journal, the center hole of the center of the center is machined at the end as the positioning reference for the surface finishing on the lathe or grinding machine, so as to ensure the coaxiality between the upper and lower journal of the sphere and the sphere. If the center hole is only used as the support point, the design of the center hole should meet the requirement of the maximum weight bearing of the workpiece.

(3) Upper journal stem hole working face symmetry, perpendicularity

To meet the design requirements of symmetry and perpendicularity of the opposite side of the valve stem hole, it is generally processed on the CNC milling and boring equipment. After the shaft head outer diameter is processed, the use of coordinates positioning, the valve stem hole is processed, and the machining accuracy can be guaranteed by the equipment itself.

(4) Spherical roundness and full runout For spherical parts with spherical roundness and full runout requirements, they must be processed on the surface grinding machine in order to meet the requirements of drawing design. In order to ensure the grinding accuracy and reduce the grinding allowance, it is required that the same positioning datum should be used for the lathe and grinder, and the spindle rotary accuracy of the lathe and grinder should be higher than that of the machined parts.

(5) Distance between the two planes at the root of the journal and the spherical axis

Generally in the milling and boring machining center, according to the machined sphere can find out the spherical center axis, as a reference to the shaft neck root end face processing in place.

  1. The process route of the sphere

(1) Rough machining of the spherical surface and journal, and the rough finishing of the channel are processed on the CNC vertical lathe.

(2) The outer circle, plane, stem hole and positioning center hole of the shaft neck shall be jammed and machined in place once in the horizontal boring and milling center.

(3) The turning finishing of the sphere is put on the spherical lathe for processing.

(4) The surface grinding finishing is placed on the surface grinding machine for processing.

 
  1. 42 “-150 LB ball processing technology

A105 + ENP(75 m) is the processing material of 42 “-150 LB sphere. The processing technology and procedure are shown in Table 2.

Table 2 processing technology of 42 “-150 LB ball

No.ProcessProcess contentEquipment
1MaterialAfter rough machining, the forgings are normalized and tempered
2According to incoming inspection specifications, qualified inspection after warehousing
3  Rough turnPositioning and aligning clamping with the channel hole and the end plane, leaving a margin of 1mm on each side of the vehicle end face, leaving a margin of 3mm ~ 5mm in the diameter of the spherical surface of the vehicle to the size of the drawing  CNC vertical lathe
4Rough turnWorkpiece turn around, same procedure 3, car end face, channel and spherical, leaving finishing allowanceCNC vertical lathe
5  Rough turnPositioning with special tooling, pressing plate and calibration, one end pivot, outer circle and each end face reserve finishing allowance of 3 ~ 5 mm  vertical lathe
6    Rough turn  The workpiece should be turned around, with the pivot outer circle and lower plane as the reference, compressed and corrected with the pressing plate, the other end pivot, outer circle and each end face reserve finishing allowance of 3 ~ 5 mm    vertical lathe
7        Fine boring      The channel hole is placed perpendicular to the end plane, find the positive pressure, to the sphere as a reference, boring the end face, outer circle, lower end face, milling the stem groove, drilling the center hole, reserve coating (75 m) margin to the drawing size, to ensure the coaxiality of the axis and the sphere and the stem groove symmetry, verticality requirements.Drill process threaded hole.Horizontal milling and boring CNC Machining center
8    Fine boring  Table rotation 180 degrees, u-turn, system alignment position, the other end of the boring pivot end face, outer circle, positioning end face, drilling center hole, reserve coating (75 m) margin to the drawing size, to ensure the axial degree of the upper and lower journal requirements horizontal milling and boring    CNC Machining center
9  Fine lathe  Positioning the center hole of the upper and lower shaft, aligning with the center of the spherical surface, finishing the surface of the spherical surface, with a single-side grinding allowance of 0.3 ~ 0.5mm  Ball lathe
10    Fine grinding    The center hole of the upper and lower axes is positioned, and the center of the sphere is used as a reference to find alignment. The surface of the sphere is refined to the size of the drawing, and the thickness of the surface coating is reserved to ensure that the roundness, full run-out and coaxiality of the sphere and the upper and lower axes meet the requirements of the drawingGrinding Machine    
11    lathe    Positioning of special tooling, clamping with four claws, pressing and correcting with platen, refining the round corners of ball mouth to drawing requirements; Turn around, finish the other end of the ball mouth round corner to the drawing requirements  CNC vertical lathe
12     Based on the horizontal table, the roundness, full runout and the coaxiality error of the sphere and the axis are respectively detectedThree-dimensional measuring instrument
13  Plating  The surface of the sphere is coated with ENP to ensure the thickness of the coating (75 m), and heat treatment to ensure the hardness of the coating HV650 ~ 800, after heat treatment to polish the sphere   
14According to the requirements of drawing size, check qualified and put into storage
 

In addition, heat treatment must be carried out after forging to refine the structure, improve mechanical properties, maintain dimensional stability and increase the anti-deformation ability of the sphere. At the same time, the coating quality of the sphere is also very important, strictly according to the process operation, to ensure that the coating surface corrosion resistance, wear resistance and do not fall off.

5 Conclusion

After grinding with imported mirror grinding machine, the main technical parameters can be controlled in: roundness error ≤0.02 mm, roughness Ra0.1 to 0.2 microns.

The processing equipment is equipped with an electronic probe for real-time precision detection. The maximum machining specification is 64 “. The surface quality and dimensional accuracy of the processed ball are greatly improved, and the precision of the processed large diameter ball meets the requirements of the drawing and the processing technology is reasonable.

In a word, the processing quality of the ball directly affects the sealing performance and operating torque of the valve. Reasonable processing technology, the use of advanced processing center and high-precision mirror grinding equipment, is a crucial factor to improve the precision of the ball and ensure the overall performance of the valve.

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