Flange connectors play a crucial role in industrial piping systems, and their sealing performance and operational stability directly depend on the precise control of bolt preload. Insufficient preload prevents adequate gasket compression, leading to media leakage; excessive preload can cause flange deformation, gasket failure, or bolt breakage, thus threatening system safety. Therefore, scientifically setting and strictly adhering to bolt preload requirements is a core element in ensuring the reliable operation of flange connectors.
Setting the bolt preload requires comprehensive consideration of the flange structure, gasket characteristics, and operating conditions. The flange stiffness, sealing surface type, and the number and distribution of bolts all affect the preload transmission efficiency. For example, necked flanges, due to their enhanced bending resistance from the neck structure, can withstand higher preload without warping; while slip-on flanges, with lower stiffness, require increased bolt count or anti-loosening designs to distribute the preload. Regarding gaskets, spiral wound gaskets require higher preload to achieve an initial seal, while rubber gaskets require controlled preload to avoid over-compression and permanent deformation. Operating conditions are equally critical. High-temperature environments can induce gasket creep and bolt stress relaxation, necessitating an appropriately increased initial preload. Conversely, low-temperature or vacuum conditions require prevention of leaks caused by gasket cold flow or contraction.
The application of preload must follow a step-by-step increasing principle to ensure uniform gasket compression. A common method is the "cross-tightening method," which involves tightening bolts in symmetrical stages: the first stage uses a lower torque for initial positioning, allowing the gasket to initially fit against the flange sealing surface; the second stage gradually increases the torque to 60%-70% of the target value, eliminating most gaps; the third stage achieves final tightening at the target torque, with repeated checks to compensate for elastic deformation. This method avoids flange tilting or gasket misalignment caused by unilateral overtightening, and is particularly suitable for large-diameter or high-pressure flange connections.
Precise control of bolt preload depends on tool selection and operating procedures. Torque wrenches are commonly used tools, and their accuracy needs regular calibration, with an error range controlled within ±5%. For critical operating conditions, hydraulic tensioners or ultrasonic measurement technology can be used to directly measure the preload by controlling bolt elongation, achieving an accuracy of ±2%. During operation, environmental factors must be considered. Low temperatures can reduce lubricant viscosity, increasing the coefficient of friction and causing the actual preload to exceed the set value; while high temperatures may cause lubricant failure, necessitating the use of high-temperature resistant lubricants or a lubrication-free design.
The condition of the gasket directly affects the required preload. Before installation, check the gasket surface for flatness, ensuring it is free of scratches, wrinkles, or embedded foreign objects; otherwise, replace the gasket. For spiral wound gaskets, confirm the compatibility of the inner and outer rings with the flange sealing surface to avoid localized stress concentration due to dimensional deviations. Gasket thickness tolerance must be strictly controlled; excessive thickness reduces preload effectiveness, while insufficient compression may lead to leakage. Furthermore, the gasket material must be compatible with the medium; for example, for chlorine-containing media, Hastelloy or titanium alloy gaskets should be used to prevent corrosion-induced seal failure.
Flange connector maintenance requires periodic re-inspection of bolt preload. After system operation, bolt preload may gradually loosen due to vibration, temperature changes, or pressure fluctuations. It is recommended to retighten the bolts within 24 hours of initial operation, followed by periodic inspections based on operating conditions. For high-temperature or high-pressure systems, nuts with locking devices or disc spring washers can be used to compensate for preload loss. Before restarting after a long period of inactivity, the preload must be recalibrated to ensure that the sealing performance returns to design requirements.
The bolt preload requirement of the flange connector is a core guarantee of system sealing. From flange selection and gasket matching in the design phase, to tool selection and operating procedures in the installation phase, and maintenance management in the operation phase, every step must be guided by scientific principles and achieve precise application of preload through meticulous control. Only in this way can the flange connector be ensured to operate stably for a long time under complex operating conditions, providing solid support for the safety and efficiency of industrial systems.
