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What is the order of flange bolt tightening when installing flanged valves?

一、Introduction

In industrial piping systems, the installation of flanged valves is a critical task. And the tightening sequence of flange bolts is crucial to ensure the quality and reliability of valve installation. The correct fastening sequence can make the flange connection uniform force, ensure sealing performance, prevent leakage, and extend the service life of the valve and piping system.

二、the basic principles of flange bolt fastening

（1）Symmetrical fastening:

This is the core principle of flange bolt fastening. Tightening the bolts from the diagonal position of the flange enables the flange to be uniformly stressed during the tightening process and avoids deformation of the flange or damage to the sealing surface due to excessive localized stress. By symmetrically tightening the bolts step by step, the flanges can be gradually fitted to ensure the flatness and uniformity of the sealing surface.

（2）Step-by-step tightening:

The flange bolts should not be tightened to the final torque at one time, but the torque should be increased gradually in several steps. This will enable the flange and sealing gasket to gradually adapt to the force state during the fastening process, avoiding damage caused by instantaneous application of excessive force. At the same time, step-by-step tightening also helps to more accurately control the degree of tightening of each bolt to ensure that all bolts are uniformly stressed.

三、the detailed sequence and steps of flange bolt fastening

（1）Preliminary positioning

Install the valve accurately on the corresponding pipeline so that the flanges at both ends of the valve are aligned with the flanges on the pipeline. Ensure that the gap between the flanges is even, the general gap should meet the design requirements, usually around 2 - 5 mm. If the gap is too large or too small, it may affect the tightening effect and sealing performance of the bolts.

Insert all the bolts but do not tighten the nuts. Allow the bolts to pass freely through the flange holes, making sure that the bolts fit well into the flange holes without any sticking or deflection. At this point, the bolts should be in a slack state so that they can be freely adjusted in position during subsequent tightening.

（2）The first symmetrical tightening

From the flange diagonal position to select a pair of bolts, use a wrench to gently tighten the nut. Tighten just enough to initially secure the flange without applying excessive torque. The purpose of this step is to allow the flanges to begin to gradually fit together by applying initial tension in two opposite directions. Next, select another pair of diagonal bolts to perform the same operation. With the initial tightening of these two pairs of diagonal bolts, the flanges are subjected to an even force in all four directions and are able to initially adjust the gap between the flanges to be more even. At this point, the flanges are not yet fully affixed and there is still some room for movement.

（3）Second symmetrical tightening

After completing the first symmetrical tightening, further tighten each pair of bolts again in diagonal order. This time the tightening should be slightly stronger than the first time, but still do not reach the final torque. The bolts can be tightened to about 30% - 50% of the final torque. Through this tightening, the gap between the flanges is further reduced and the sealing gasket begins to come under pressure and gradually performs its sealing function. During the tightening process, pay attention to the fit of the flanges. If the flange is found to have local unevenness or uneven gap, it should be adjusted in time. The tightening degree of individual bolts can be adjusted appropriately to make the flange fit as evenly as possible.

（4）step by step cross fastening

After completing two symmetrical fastening, then in accordance with the clockwise or counterclockwise direction, in turn on the adjacent bolts for fastening. The strength of each fastening should be uniform, and in accordance with the principle of gradually increasing the torque. For example, first tighten all bolts to about 70% - 80% of the final torque. Use a torque wrench on each bolt to ensure that the torque applied is accurate and consistent. As you progressively cross-tighten, keep checking the flatness of the flanges and the fit of the sealing surfaces. A plug gauge can be used to check the flange gap to ensure that the gap is within the allowable range. If it is found that the gap does not meet the requirements, the tightness of the bolts should be adjusted in time. At the same time, pay attention to observe whether the sealing gasket has deformation, displacement and other phenomena, if there is any abnormality, it should be dealt with in time.

（5）Final tightening

After completing the previous steps, tighten all bolts to the specified final torque, again in a clockwise or counterclockwise direction. At this time, make sure that the torque of each bolt reaches the exact value, and check each bolt carefully to prevent leakage or insufficient torque. After the final tightening is complete, use a tool to recheck the tightness of each bolt to ensure that all bolts are tightened in place. You can use a knockout wrench to gently tap on the bolts to determine if they are tightened securely by the sound. If the sound is crisp, the bolt is well tightened; if the sound is muffled, it may indicate that the bolt has loosened and needs further tightening.

四、Special circumstances and considerations

(1) Large or high-pressure flanges

For large or high-pressure flanges, the tightening of bolts is more stringent due to the higher pressure they are subjected to. In the fastening process, it may be necessary to use greater torque and more specialized tools. At the same time, in order to ensure the tightening effect, it may be necessary to increase the number of tightening rounds. For example, for some large high-pressure flanges, it may be necessary to carry out 8 - 10 rounds of fastening, and the torque increase of each round should be reasonably controlled according to the specific situation.

When fastening the bolts of large flanges, it is also necessary to pay attention to the deformation of the flange. Due to the large size of the large flange, in the fastening process is prone to local deformation. Therefore, the deformation of the flange should be closely watched during the fastening process, and tools such as a percentage meter can be used for monitoring. If the flange deformation is found to exceed the permissible range, the tightening sequence and strength should be adjusted in time, and the flange should be corrected if necessary.

(2) Flanges and bolts of different materials

When flanges and bolts are made of different materials, the difference in thermal expansion coefficients of the materials should be considered. In high or low temperature environments, the degree of thermal expansion of different materials is different, which may lead to changes in the tightening force of the bolt. Therefore, in this case, the tightening torque of the bolt needs to be adjusted appropriately according to the actual working temperature. For example, when the flange is made of carbon steel and the bolt is made of stainless steel, in a high temperature environment, the coefficient of thermal expansion of the stainless steel bolt is larger than that of the carbon steel flange, and the bolt will elongate, and the fastening force will decrease. At this time, it may be necessary to appropriately increase the preload of the bolt during installation, or carry out regular tightening checks and adjustments according to temperature changes during operation.

(3) Site environmental factors

If the environmental conditions of the installation site are harsh, such as humidity, dust, corrosive gases, etc., it will have an impact on the tightening and service life of the bolt. In this case, in addition to the necessary protective measures for the bolt, such as applying anticorrosive paint, installation of protective sleeve, etc., it is also necessary to pay special attention to preventing impurities from entering the threads and sealing surfaces in the fastening process. For humid environments, to prevent bolts from rusting, you can choose lubricants with anti-rust properties. For environments with corrosive gases, corrosion-resistant bolts and flange materials should be selected, and effective anti-corrosion measures should be taken. At the same time, after tightening the bolts, the flange connection parts should be sealed and protected in time to prevent the intrusion of external impurities and corrosive substances.