In the installation and maintenance of equipment and pipelines, the tightening of flange bolts and nuts may seem like a basic operation, but it is actually a key link in ensuring equipment sealing performance and avoiding leakage accidents.
Loosening too much can easily cause medium leakage, while tightening too tightly may lead to the bolt breakage and gasket loss, especially for important equipment that is prone to high temperature, high pressure, flammability, and explosion. Even a slight deviation in operation can cause safety hazards. So how should flange bolts and nuts be tightened properly? Which tool should be used in different scenarios? How to do a good job of thermal tightening control during the heating process? Today's informative article will provide a detailed overview of the complete set of standard operating requirements for flange bolts and nuts tightening, from preliminary inspection to tool use, and then to hot tightening specifications, all explained and thoroughly in one go.
Technical requirements for fastening during the installation process of equipment and pipeline flanges
(1) Inspection of flange fasteners
Gasket inspection: During installation, it is necessary to use brand new, clean, and dry gaskets and carefully check for any damage, defects, or other issues. It is strictly prohibited to reuse old gaskets. Before installation, it is necessary to check the gasket specifications and pressure rating to ensure that they are completely consistent with the flange identification information.
Flange inspection: Before installation, check the sealing surface of the flange for scratches, dents, sediment adhesion, corrosion, burrs, and other damages. If the indentation or scratch depth of the water line running radially through the sealing surface of the flange exceeds 0.2mm, and the coverage area exceeds half of the width of the gasket sealing surface, the flange must be replaced or the sealing surface must be reprocessed and repaired.
The supporting surface of the nut on the back of the flange should be kept flat and parallel. The flange alignment inspection must be strictly carried out in accordance with the relevant requirements for pipeline installation in section 6.2 of the "SH3501-2011 Construction and Acceptance Specification for Highly Toxic and Combustible Medium Pipeline Engineering in Petrochemical Industry".
The inspection of bolts and nuts: Strictly follow the equipment and pipeline design requirements, check the bolts and nuts model specifications to ensure correct selection. The threads and contact surfaces must be free of impurities such as dirt, rust, heavy skin, scratches, burrs, and debris that may affect the tightening torque.
It is strictly prohibited to repair bolts through welding or machining methods. After the flange is tightened, at least 2 complete threads should be exposed on the outside of the nut. Before use, bolts and nuts must be lubricated to reduce the coefficient of friction and improve their resistance to slipping and corrosion.
Before applying lubricating oil, the threads of the bolts, nuts, and contact surfaces need to be degreased and dried. Lubricating oil should be uniformly and standardly applied to bolt threads, nut threads, nut bearing surfaces, washers, and flange nut support surfaces. ​High-temperature anti-seize agents can be selected according to the working conditions.
Welding or machining methods are not allowed to repair bolts and nuts. After the flange installation is tightened, at least two threads are exposed outside the nut. Before use, bolts and nuts must be lubricated to ensure a low coefficient of friction and improve their resistance to slipping and corrosion. The thread of the screw, nut, and contact surface must be degreased and dried before applying lubricating oil.
(2) Methods of flange fastening
Torque free plum blossom wrench or impact wrench: suitable for fastening flanges of conventional equipment and ordinary pipelines. Tools should be selected reasonably according to bolt specifications and flange pressure levels. The fastening requirements are as follows.
1) The maintenance unit shall develop a tightening plan, symmetrically tighten the flanges, and number them in the tightening sequence.
2) Use four bolts as washers at positions for positioning, ensuring that the center of the wrapped washer is within the edge of the flange.
3) Tighten the positioning bolt by hand, then insert other stud bolts and tighten them by hand to balance the load, ensuring that at least 2 threads are exposed on each end of the nut.
4) Based on the on-site equipment and flanges, calculate the number of tightening cycles (at least 3 times) and the impact load (force) for each tightening cycle. Tighten the impact load (force) in ascending order from small to large (such as 50%, 80%, and 100% increase), and do not load too quickly or excessively to prevent gasket sealing failure.
5) The sequence of tightening each time with a torque free ring wrench or impact wrench.
Tighten the two radially opposite bolts and nuts to the predetermined hammering load (force) of the bolts and nuts. Tighten another pair of bolts that are approximately 90 degrees apart along the circumference from the previous two bolts and nuts. Continue tightening until all other bolts and nuts are tightened to the predetermined hammering load.
6) Finally, tighten all bolts and nuts in a clockwise or counterclockwise direction with 100% hammering load (force).
