Control measures to ensure the sealing performance

2022-08-05
  • Detail

Control measures to ensure the sealing performance of API oil pipe thread connection Abstract: Based on years of experience in oil pipe thread processing and quality control, this paper expounds the control measures taken to ensure the sealing performance of API oil pipe thread connection in the aspects of regrinding and installation of oil pipe thread comber, taper error, pitch error, tooth error and quality inspection of oil pipe thread. This paper mainly analyzes the advantages, disadvantages and tooth profile errors of rough turning and fine turning of oil pipe thread directly with a carding tool or rough turning and fine turning of oil pipe thread with an ordinary thread turning tool, and points out that the correct processing method of oil pipe thread should be rough turning with an ordinary thread turning tool first, and then fine turning with a carding tool. It is suggested that the manufacturing tolerance of the cutting height parameters of the top and bottom of the oil pipe thread comb cutter should be increased from ± 0.02mm to ± 0.01mm, and the cumulative pitch tolerance of the oil pipe thread should be controlled within ± 0.02mm, so as to improve the sealing performance of API oil pipe thread connection

in "on the factors affecting the sealing performance of API oil pipe thread", the author analyzes the influence of taper error, pitch error, tooth error and other factors on the sealing performance of API oil pipe thread connection from the tooth characteristics and sealing mechanism of API oil pipe thread; The causes of taper error, pitch error and profile error in tubing thread machining are briefly described. In this paper, the author will elaborate the control measures to ensure the sealing performance of API oil pipe thread connection according to the experience of oil pipe thread processing and quality control and the processing process of API oil pipe thread, and compare with peers

regrinding and installation of tubing thread comb

the regrinding and installation control of tubing thread comb is described by taking two tooth edge external comb and two tooth four edge flat internal comb as examples

1. Regrinding of the comb knife

when the oil pipe thread comb knife is worn to a certain extent or the comb knife tooth shape is defective, the comb knife must be reground before reuse, otherwise the processed oil pipe thread will be difficult to meet the requirements of connection sealing performance. However, the regrinding of the comb cutter should ensure that the shape and size of the cutting edge of the comb cutter after regrinding are basically unchanged

Figure 1 shows the grinding diagram of oil pipe thread comb

a. external comb cutter B. internal comb cutter

Fig. 1 Schematic diagram of oil pipe thread comb cutter grinding

the formed cylindrical surfaces of oil pipe thread internal and external comb cutters are all back cutting surfaces. The included angle between the rake face and the rear face of the edge external comb is 82 °, and the rear angle of the top edge installed on the special external comb bar is 6 ° (8 ° per inch for 8 teeth external comb); The included angle between the front face and the rear face of the flat body comb is 84 °, and the rear angle of the top edge installed on the special inner comb bar is 6 °. It can be seen from the calculation of the section depth of the prismatic comb cutter that when the front angle of the comb cutter is ground to +6 °, the tooth height of 10 thread will increase by 0.023mm under the condition that the rear angle of the same comb cutter is unchanged; When the rake edge of the comb is ground to +10 °, it will increase by 0.047mm. Therefore, in order to ensure the correct cross-section shape and smooth cutting of the regrinding comb, the following control measures shall be taken for regrinding of the comb:

1) the inner and outer comb of the oil pipe thread must be sharpened on the tool grinder or surface grinder

2) the grinding angle of the outer comb is 8 ° +1 ° (Fig. 1a), and the grinding angle of the inner comb is 2 ° ± 30 '(Fig. 1b)

3) the arc radius of the tooth bottom and the top of the corrected tooth of the sharpened inner and outer comb cutter shall be smooth, the tooth side shall be straight and free of defects

2. Installation of the comb cutter

both the inner and outer comb cutters of the oil pipe thread are installed on the special inner and outer comb cutter rods. The installation quality directly affects the machining error and working reliability of the parts. In this regard, the following measures have been formulated:

1) before installation, the tool bar, the blade positioning groove and the blade should be wiped clean. After the blade is compressed, use a feeler gauge to check whether the contact between the blade and the tool bar positioning groove surface is intact. After processing several parts, use a feeler gauge to recheck to ensure reliable blade positioning

2) when the inner and outer comb tool bars are installed on the tool holder of the machine tool, the dial indicator must be used to correct that the alignment plane of the tool bar is parallel (inner comb tool bar) or vertical (outer comb tool bar) to the rotation axis of the machine tool, and the error is within 0.02mm

3) when processing oil pipe threads with small sizes such as 1.050, 1.315, 1.660 TBG or up TBG, if the spiral rise angle is not considered on the special comb bar, it is allowed to take certain measures on the comb bar to make the comb blade form a positive edge angle, so as to reduce the impact of the spiral rise angle on the thread surface quality. The specific method is as follows: when machining the internal thread, add a gasket at the rear end of the tool bar, and the inner rear end of 100mm is 1.2 ~ 1.6mm higher than the front end, forming a positive edge inclination of 0 ° 41 ′ ~ 0 ° 55 ′; When machining external threads, place a 5mm wide and 120m long gasket with a thickness of 0.3 ~ 0.4mm on the inner side of the tool holder in the width direction of the tool bar to form a positive edge inclination of 0 ° 41 ′ ~ 0 ° 55 ′ (when the tool bar is 30mm wide)

control of taper error of tubing thread

taper error accounts for a small proportion in the accumulated hand tight clearance value during tubing thread connection. Correspondingly, it has little influence on the sealing performance. However, the taper error causes the position of the tight part of the connection to change, which has a great impact on the stability, reliability, connection strength and connection stress of the tubing thread connection

generally, the processing of oil pipe thread cone is divided into two steps, that is, rough turning with cylindrical turning tool and fine turning with oil pipe thread combing tool. In order to reduce the influence of taper error on the sealing performance of oil pipe thread connection, the following measures are formulated in view of the causes of large taper error during the processing of oil pipe thread:

1) during the processing of oil pipe thread parts, the taper angle error shall be controlled within the range of 1 ° 47 ′ 24 ″ (+5 ′, -3 ′), and the taper error shall be checked and corrected frequently. Because the outer cone is measured accurately, the cone can be turned out first, and then the bevel angle can be measured with a sine gauge. The inner cone shall be colored and checked with a cone plug gauge of the same specification, and the contact area shall be greater than 60%

2) the mold base of the profiling lathe shall be overhauled every three months, and the accuracy of the profiling lathe and the CNC lathe shall be overhauled every six months

3) after rough turning of the inner and outer cones of the oil pipe thread, 0.3 ~ 0.5mm shall be reserved at one side to ensure the integrity of the tooth profile when combing the oil pipe thread. In order to ensure sufficient machining allowance for the internal thread of the oil pipe, the diameter of the prefabricated blank hole of the coupling (Table 1) is specified, and the tolerance zone is h14

4) the straightness of the tubing thread cone shall be checked with a ruler through light transmission. When the plain line is in the middle concave state, it shall not be greater than 0.01mm, and when the plain line is in the middle convex state, it shall not be greater than 0.03mm. Table 1 preformed blank hole diameter (straight hole) of coupling type tubing internal thread mm

thread @10, running distance

code 1.050

tbg1.315

tbg1.660

tbg1.900

tbg23/8

tbg27/8

tbg3

tbg4

tbg4

tbg blank hole diameter 21.52836.642 5105 thread

code 1.050

up tbg1.315

up tbg1.660

up tbg1.900

up tbg23/8

up tbg27/8

up tbg3

up tbg4

up tbg4

up TBG blank aperture 2831 586.699111.5

control of pitch error of oil pipe thread

pitch error accounts for the largest proportion of the accumulated hand tight clearance during oil pipe thread connection, up to 50%, so pitch error is the most important factor affecting the sealing performance of API oil pipe thread. The processing of oil pipe threads on the profiling lathe and the CNC lathe is the rotation of the workpiece, while the thread turning tool (or comb tool) translates along the axis of the workpiece according to the required pitch parameters. The pitch accuracy of the workpiece is mainly guaranteed by the motion accuracy of the relevant parts of the lathe. Although the pitch error of tubing threaded parts is very rare to approach or exceed the cumulative pitch tolerance, once it occurs, the impact on the sealing performance of batch parts is fatal. In order to reduce the screw pitch error of the workpiece and ensure the sealing performance of the oil pipe thread connection, the following control measures are taken:

1) the screw pitch error is reduced to no more than 0.02mm for any two threads within the length of l4-g, so that the proportion of the screw pitch error in the cumulative clearance value of the oil pipe thread connection is reduced to 12%, and the maximum cumulative hand tight clearance value of the oil pipe thread can also be reduced from 0.26mm to 0.146mm, Less than 0.158mm (10 threads per inch) clearance that can be eliminated by two turns of machine tightening

2) regularly check the machine tool to eliminate and reduce the spindle rotation error, guide rail guide error and transmission chain transmission error of the machine tool, so as to improve the pitch accuracy during oil pipe thread processing. The specific methods are as follows: ① adjust the spindle and lead screw of the machine tool to eliminate the axial series movement; ② Reasonably adjust the meshing clearance of exchange gear and the runout of main shaft, lead screw and exchange gear journal; ③ Install automatic compensation device for transmission error on general CNC lathe

3) use the internal and external thread pitch meter or other measuring means in the API petroleum pipe thread single measuring instrument to regularly check whether the pitch error is within 0.02mm

control of profile error of oil pipe thread

oil pipe thread is a very special connecting thread. Except that the tooth side is the main working face, its tooth top and tooth bottom are also working faces, which also has a significant impact on the sealing performance of oil pipe thread connection. Although there are only two tooth profile errors (tooth profile height tolerance and tooth profile angle tolerance) specified in the national standard and API Std 5b, and the two tolerances are large on the surface, in fact, the tooth profile error and cross-section roundness error caused by workpiece clamping and processing method during tubing thread processing should be included

theoretically, when the tooth height of the internal thread of the coupling is the maximum and the tooth height of the external thread of the oil pipe is the minimum (increase or decrease the same value according to the flattening height of the tooth top and bottom), the contact of the two hand tight connections is shown in Figure 2

Figure 2 Schematic diagram of HN maximum and HS minimum connection

at this time, the hand tight gap between the top of the internal thread and the bottom of the external thread decreases from 0.076mm to 0.0005mm, while the hand tight gap between the top of the external thread and the bottom of the internal thread increases to 0.1525mm. In this case, if the soft metal coating is not used as the gap filler and the sealing grease with excellent performance is used, the oil pipe thread connection will leak after tightening

in the processing of oil pipe thread, the tooth profile of the workpiece is completely guaranteed by the tooth profile of the oil pipe thread comb. Although the manufacturing tolerance control of the tooth profile angle of the inner and outer comb cutters of the oil pipe thread and the cutting height parameters of the top and bottom of the tooth are relatively strict, the above extreme cases may still occur if the oil pipe thread comb cutter is improperly used or the thread profile of the working hours is not fully combed

1. Tooth profile error during rough turning and fine turning with the comb cutter

in order to ensure that the tooth profile of the workpiece is completely consistent with the tooth profile of the comb cutter, the radial feed mode is adopted in the processing of oil pipe threads, and the thread pitch parameter is taken as the cutting amount. According to the size and specification of the thread, the cutting performance of the material, the rigidity of the part, etc., the reasonable cutting speed and times are selected. In production practice, due to the good rigidity of carbide prismatic comb cutter and the high strength of special comb bar, some operators adopt a higher cutting speed when machining the external thread of oil pipe, and only use 3 to 4 times of cutting to turn the thread. Although this processing method has good surface roughness and high production efficiency, it has the following disadvantages:

1) the cutting load of the tool tip is large, and it is easy to prick and break the edge, which reduces the service life of the comb tool

2) the wear of the comb cutter is accelerated and uneven, the wear of the cutter tip and tooth side is large, and the wear of the tooth bottom is small. After continuous operation for a period of time, the workpiece will form the extreme situation mentioned above, resulting in shallow teeth of the workpiece and affecting the connection sealing performance

3) because the tubing thread parts are generally hollow thin-walled parts, and the back angle of the comb is small, the front angle is basically zero, and this large cutting amount is more than 5kn

Copyright © 2011 JIN SHI