Research and Discussion on Bolt Fastening of Steel Pipe Tower Flange

The general line project of the Huainan-Shanghai UHV AC transmission demonstration project is the world's first UHV double-circuit transmission line. The iron towers all use steel tube towers. The average height of the tower is 110m and the average weight of the single base is 180t. All are flanged. During the tightening process of the iron tower flange bolts that have been completed in the north-south-Shanghai section, especially in the early stage, the bolts are tight and loose, and the bolts are tightened and tightened, resulting in the tightening rate of the tower bolts failing to meet the acceptance criteria. And the low-efficiency repeated construction also causes huge waste of human resources for the construction unit.

In response to the above situation, Huxi Owners Project Department and Shandong Integrity Supervision Project Department organized the construction project department to carry out special research on the fastening of flange bolts, and invited technical backbones of bolt manufacturers and torque wrench manufacturers to analyze the reasons and formulate Measures, on-site verification, summary evaluation, to solve the problem of flange bolt fastening.

This paper mainly discusses the difficulties and methods of bolting of steel pipe tower flanges.

1 Features and difficulties of flange bolt fastening

The general line of the Huai'nan-Shanghai UHV AC transmission demonstration project is a double-circuit steel tube tower. The main materials of the steel tube tower are all flanged. Analysis shows that the steel pipe tower flange bolt fastening has the following difficulties:

(1) Bolt tightening work: The flange bolts are from M20 to M56. There are 12 kinds of specifications. The average number of single-base iron bolts is 15000. The flange bolts account for more than half, and the tightening workload is large. It has not appeared in previous projects.

(2) High bolt torque value requirements: According to the China Electric Power Engineering Consulting Group Co., Ltd. document (Electrical Regulation Grid [2012] No. 99) of the Electric Power Planning and Design Institute, the final bolt torque determined by the AC Construction Department is 2500 Nm (M56), as opposed to In the past, the transmission line engineering requires a lot of torque values. For example, the Xiangjiaoba-Shanghai ±800kV DC transmission line project spans the steel tube tower. The torque value of the large flange bolt is only 1/3 of the project.

(3) Difficult construction work on the tower: The height of the tower of the project is 110m on average, the height is high, the weight of the electric fixed torque wrench is large, it is inconvenient to carry, and the construction personnel are difficult to construct during the construction of the high place, and the action is limited to the correct It is very difficult to operate the electric torque wrench in posture.

(4) Restriction of fastening tools: The electric fixed torque wrench promoted and applied in this project is an irreplaceable tool for flange bolt fastening. However, due to the first large-scale promotion and use, the manufacturer has insufficient manufacturing experience and the tool performance is not perfect. In addition, the lack of construction personnel's operating experience has become a major factor limiting the bolt fastening efficiency.

2 Flange bolt fastening tools selection

The main fastening tools for the flange bolt fastening of this project are the electric fixed torque wrench, manual torque wrench and torque multiplier as supplementary tools.

2.1 Electric torque wrench

Electric torque wrench parameters and characteristics: (1) Torque range is 100 ~ 3000Nm, the predetermined torque can be set steplessly; (2) torque accuracy is ± 5%, repeatability accuracy is ± 3%; (3) reaction force The arm stuck button is automatically retracted and can be rotated 360°; the working head is also rotated 360°; (4) the working voltage is a conventional 220V digital display, which is more intuitive.

2.2 Manual torque wrench and torque multiplier

2.2.1 Torque multiplier parameters and characteristics: (1) The torque factor of each class gear is generally 5, the conventional ratio is 1:5, 1:15, 1:15.5, etc.; (2) The reaction arm can be 360 ° Rotation; the input multiplier torque accuracy can reach ± ​​4%; (3) the gear is made of alloy steel special processing and heat treatment, high strength.

2.2.2 Manual torque wrench parameters and features: (1) no long handle, light weight, easy to operate, digital display, more intuitive, high precision, can reach ± ​​1%; (2) indication tracking: during the afterburner Tracking shows the added torque value; (3) Peak hold: During the afterburning process, the display value increases with the increase of the reinforcement torque. Once the force is stopped, the peak hold can show the maximum torque value before the force is applied. The user can check the magnitude of the torque value; (4) preset alarm: pre-set the required tightening torque before tightening, during the tightening process, when the tightening torque reaches the predetermined torque value, the preset alarm light point When the light is on, the alarm sounds, prompting to stop the force.

2.3 Characteristics of the use of two fastening tools

As the main tool for the flange bolt fastening of this project, according to the use situation in the construction process, the characteristics and applicable conditions of the electric fixed torque wrench, manual torque wrench and torque multiplier are summarized as follows:

2.3.1 Electric torque wrench: (1) The torque of the electric fixed torque wrench can be adjusted to adapt to the tightening of bolts of various specifications; (2) The electrification construction greatly improves the bolt fastening efficiency, saving time and effort; (3) Using digital display mode, the reading is intuitive and easy to operate; (4) The high-torque electric wrench has a large weight and has great limitations in working at height; (5) the angle of the wrench needs to be kept at 90° during operation. When the station is difficult, the wrench is easy to be damaged; (6) When the power line exceeds 100m, there will be a significant voltage drop, which will affect the torque of the wrench; (7) the generator is required to be higher, the generator power is not satisfied or the frequency is sinusoidal. The difference between the torque value of the electric torque wrench and the electric torque wrench can not be started; (8) The same bolt can not be bolted with the same torque value twice. If the same torque is used for secondary tightening, the electric torque is fixed. When the wrench is repeatedly tightened, the motor should be rotated first and then tightened twice. When the motor is not tightened in place, the motor will be tightened with the same torque.

2.3.2 Manual torque wrench and torque multiplier: (1) The manual torque wrench is very light, and the output torque is large with the torque multiplier. It avoids the use of the extended handle and is easy to operate. (2) It is a supplement to the electric torque wrench. It is suitable for places where high-altitude operation and electric torque wrench are not easy to operate; (3) The price is moderate and not easy to damage; the fastening efficiency is relatively low, which is time-consuming and labor-intensive.

3 Field test of flange bolt fastening

3.1 Test content

The largest flange bolts for this project are M56 and 8.8, and the torque value is designed to be 2500Nm. Such a large torque is used for the first time in the transmission line. In the past, there was no precedent for application. For this reason, we chose the flange bolt of M56 for the fastening test. The test was carried out on the construction site of the 20-segment Jiangsu power transmission and the test equipment was electric. Torque wrench and torque test wrench. Two fastening tests were carried out on the two main material flanges for a total of 56 M56 flange bolts. The bolts were symmetrically numbered before the test. The center of the tower and its four bolts in the 90° direction, the remaining bolts are fastened in symmetrical order (see Figure 1 for the tightening sequence and numbering), and each flange bolt is tightened three times. Three days after the completion of the test, the supervision project department re-examined the 56 bolts tested.

In the first test, a single set of symmetrical tightening was performed with a set of electric fixed torque wrenches to ensure that the torque value was up to standard and tightened in three times. The initial tightening torque is 1500 Nm, the second tightening torque is 2200 Nm, and the third tightening is tightened according to the design value of 2500 Nm:

The first step: first tighten the flange bolts, adjust the torque of the electric torque wrench to 1500Nm, and tighten the bolts according to the cross symmetry method.

The second step: with the torque of 2200Nm, the first tightening of the bolts is tightened for the second time. The first bolts are symmetrical and tightly tightened. After the tightening is completed, the torque detecting wrench is used for testing. All the 8 bolts reach the torque of 2200Nm. value.

Step 3: Continue to use 2200 Nm of torque and tighten the remaining untightened bolts by the cross symmetry method. After completion, it is detected by the test wrench. The bolts that have been tightened all reach the torque value, but 5 bolts of the 8 bolts tightened in the second step are loose.

The fourth step: the electric fixed torque wrench is adjusted to the torque of 2500Nm, and the bolt is tightened for the third time. The first cross is symmetrically tightened. After the tightening is completed, the torque detecting wrench is used for testing. The 8 bolts all reach the torque value of 2500Nm. .

Step 5: Continue to tighten the untightened bolts by the cross symmetry method with a torque of 2500 Nm. After completion, the torque detection wrench is used for testing. The bolts that are tightened all reach the torque value, but the four bolts that are tightened in the fourth step are loose.

In the second test, two sets of electric fixed torque wrenches were used for two-way simultaneous symmetrical operation. In order to ensure the torque value reached the standard, the tightening was still carried out three times. The initial tightening torque was 1500 Nm, and the second tightening torque was 2200 Nm. The secondary design is 2500 Nm and the steps are the same as the first test, but only the 2 bolts are loose in the third and fifth steps.

After 3 days, the re-inspection was carried out. 56 bolts were checked with a torque detecting wrench. It was found that 56 bolts could not reach the design torque and the torque loss was large.

3.2 Data Analysis

3.2.1 For a single symmetrical fastening flange with a set of electric torque wrenches, although the tightening sequence is correct, the first tightening bolt is affected because the flanges are not closed in parallel during the tightening process. And loose.

3.2.2 Two sets of electric fixed torque wrenches are used for two-way simultaneous and symmetrical tightening. The effect is obviously improved compared with the use of one set. However, due to the limitations of construction equipment and aerial work, it is impossible to configure more on one flange. Electric fixed torque wrench.

3.2.3 Due to the galvanization of gaskets and bolts, there will be a loss of bolts after tightening for a period of time.

3.2.4 As the tower body rises, the weight of the tower body increases continuously, and the pre-tightening torque decreases for the bolt torque of the bottom pre-tightening.

4 Methods and measures for tightening flange bolts

After the test was completed, the representatives of the owner, supervisor, construction unit, fasteners and fastening equipment manufacturers analyzed and discussed the test results, and summarized the effective fastening methods and measures to prevent flange bolt loosening as follows:

4.1 Flange bolt fastening method

(1) Strictly tighten the bolts one by one in the order of the “Ten” method, and apply two sets of electric (manual) torque wrenches for two-way simultaneous symmetrical operation; (2) Tightly tighten according to the specified torque to ensure the use (3) Keep the surface of the fastener clean and prevent the dirt from increasing the torque coefficient such as dirt; properly use the electric wrench to keep the sleeve and the flange plane perpendicular when tightening; (4 For bolts that cannot be tightened with an electric wrench, the internal thread of the nut and the lower support surface of the nut should be fully coated with butter, and then tightened with a manual wrench; (5) When the bolt is turned, it needs to be fastened with a wrench when tightening. Special buckle; (6) When the construction is on site, the torque value can be increased by 5% when the torque value is set on the electric torque wrench; (7) The three-step fastening method can effectively reduce the pre-tightening due to the influence of bolt plating and other factors. The phenomenon of force loss: the first step, the first time the bolt is tightened 3 times, the first and second tightening is 50%~80% of the design torque, and the third tightening is designed with the torque tightening. ; In the second step, after the tower is set up for several days (the straight tower is 7 days, the corner tower is 15 days), it is re-tightened; the third step is followed by a second re-tightening one month later.

4.2 Organization measures for flange bolt fastening

Increase investment, high-torque electric torque wrench is the first in the power tool industry. It is also the first time to be widely used in UHV engineering. The performance of the wrench is not perfect and easy to damage. The construction unit should plan ahead to ensure electric setting. The number of torque wrenches and the input of the construction personnel can meet the requirements.

Increase the training intensity, organize the operator of the electric torque wrench to conduct unified training and on-site operation guidance, master the operation essentials, and avoid damage to the wrench due to improper operation.

To build a communication platform, the construction unit will promptly feedback the problems encountered in the construction to the manufacturer, and the manufacturer should solve the problem in the shortest time to ensure the continuous construction on site.

The above method is used for fastening, which greatly improves the fastening rate of the steel pipe tower flange bolts, but at the same time, many new problems have appeared in practical applications. For example, some flange bolts are assembled with two sets of electric torque wrenches at the same time. It is difficult to operate the construction workers and the efficiency is low. There is no effective solution to this problem. It needs to be further explored and solved in the subsequent construction. To this end, we should give play to the coordination functions of the owners and the supervision units, strengthen the links between the construction units, learn from each other, learn from each other's strengths, and jointly explore ways to solve the problem of flange bolt fastening, and provide technical support for the subsequent maintenance of the tower to ensure the project. The successful implementation of the target, at the same time, also accumulated experience for the subsequent UHV projects, laying a solid technical foundation.

references

[1] 岑阿毛. Construction technology of transmission and distribution lines [M]. Kunming: Yunnan Science and Technology Press, 2004.

[2] Zhu Yanqing. Transmission line construction (primary and intermediate) [M]. Beijing: China Electric Power Press, 1998.

[3] Wang Hengchang. Transmission line construction (advanced workers) [M]. Beijing: China Electric Power Press, 1998.

[4] State Grid Corporation. 1000kV overhead transmission line tower construction workers

[5] Art Guide (Q/GDW155-2006) [S]. Beijing: China Electric Power Press, 2008.

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