Selection and Assembly Matching Requirements for High-Pressure Bolts of Pressure Vessel Flanges
The use of ordinary low-pressure bolts for connecting pressure vessel flanges is strictly prohibited. The core reason lies in the complex operating conditions of the equipment. Ordinary low-pressure bolts are mostly made of A3 steel or low-carbon steel, and their material performance is relatively weak. During equipment operation, frequent fluctuations in temperature and pressure generate alternating stress, which can easily cause ordinary bolts to loosen and preload to decay, ultimately leading to loose flange sealing and medium leakage, resulting in equipment failure and safety hazards.
High-pressure dedicated bolts are made of materials with excellent performance, capable of adaptive deformation in response to changes in operating temperature and pressure, with strong stress stability and resistance to loosening and failure, ensuring the long-term reliability of flange sealing. The core difference between the two isdifferent material mechanical property grades。
I. Bolt usage standards corresponding to pressure ratings
In the assembly specifications for pressure vessel flanges, a pressure rating of 2.5 MPa is considered a low-pressure operating condition, while 6.4 MPa and above are considered medium- and high-pressure operating conditions.All medium-pressure pressure vessel flanges must be connected uniformly using high-pressure double-ended studs; ordinary bolts are prohibitedto meet the equipment's operating requirements for pressure containment, temperature resistance, and fatigue resistance.
II. Material Grade Matching Requirements for High-Pressure Bolts and Nuts
High-pressure flange assembly is subject to strict material grade matching specifications: bolts and their matching nuts must never be paired at the same grade, and thebolt material grade must be one grade higher than the nut. This design prevents galling, seizing, and fatigue fracture between bolts and nuts, improving reusability after disassembly and connection stability.
Typical matching example: when high-pressure bolts are made of 25MoVA material, the matching nuts must be downgraded to 20MoVA material, strictly following the assembly principle of "high-strength bolts paired with nuts one grade lower."
III. Matching requirements for high-pressure flange sealing surfaces and gaskets
For flange connections of medium- and high-pressure pressure vessels, the sealing surface must usea male-female tongue-and-groove sealing structureand ordinary flat sealing forms are strictly prohibited. At the same time, high-strength sealing gaskets must be used in matching. Common types include: steel washers, graphite serrated gaskets, and pure graphite gaskets. By relying on a precise sealing surface structure and suitable gaskets, changes in stress under operating conditions are offset, completely avoiding flange leakage problems under high-pressure and high-temperature operating conditions.
