While much attention is given to selecting the proper flange and gasket, the importance of choosing the correct fasteners is often overlooked.
To achieve and maintain a leak-free connection, all elements of a flange assembly must be compatible. Bolts, studs, and nuts—collectively known as fasteners—are essential for this goal. This section discusses bolts, studs, screws, nuts, washers, bolting procedures, and their importance about flanges.
Flange bolts are indispensable components in industrial systems, providing the crucial clamping force needed to secure flanges and establish leak-proof joints. These fasteners play a crucial role in maintaining system integrity, especially in high-pressure and high-temperature environments, where even minor failures can have serious consequences. Flange bolts serve as the backbone of structural and fluid systems, ensuring the safe and efficient operation of pipelines, pressure vessels, and other essential equipment.
What is a Flange Bolt?
A flange bolt is a fastener specifically used to connect two flanges. It has a cylindrical body, threaded on one end and with a head on the other; the shank of the bolt is non-tapered. Bolts are made from a variety of materials, with steel being the most common in engineering applications. The counterpart to a bolt is the nut, which is the internally threaded fastener. Other common fasteners include screws (set screws) and studs.
What is a Flange Nut?
A nut is the internally threaded part of a threaded assembly. A hex nut has internal threads along its central axis and is shaped like a hexagon, with a bearing face and a non-bearing face. The non-bearing face has a 30-degree chamfer and faces away from the flange edge. The bearing face is unchamfered and faces the flange edge, and it should be isolated from the flange using a washer. Some nuts are pre-equipped with washers, making additional washers unnecessary. Nut materials are defined by applicable standards, including ASTM.
Flange Washers
Washers are disc-shaped components with a central hole, placed between fasteners and the components being fastened. Washers fall into three major categories: plain washers, spring washers, and lock washers. Each category has several variations, but generally, they serve one or more of the following functions:
Distribute the load of the fastener – Prevents the edges of bolt heads from digging into the surface of the part being fastened. This is the most common use of washers.
Electrically insulate the fastener from the component – Prevents galvanic corrosion. Washers made from materials such as PTFE, graphite, or elastomers can significantly reduce the risk of failure based on specific applications.
Create residual tension in the fastener – The tension helps prevent the fastener from loosening over time, especially in machinery subject to vibration.
Washers can be used in combination, for example, by placing a flat washer between a spring washer and the component being fastened. A single washer can also serve multiple purposes—such as a flat washer distributing load and electrically insulating at the same time. Washers used for electrical insulation are often made of polymer-based materials due to their insulating properties.
Among the three main categories, flat washers are by far the most common type.
Bolt Material Specifications
ASME B16.5 specifies approved materials for flange bolts to ensure they can withstand harsh industrial environments. Common materials include:
Alloy Steel – Known for high strength and the ability to withstand high pressure.
Stainless Steel – Offers excellent corrosion resistance, making it ideal for chemical and marine applications.
High-Strength Alloys – Materials such as ASTM A193 Grade B7 or A320 Grade L7 are designed for extreme temperature and pressure conditions.
Bolting Procedures
A flange is a flat, disc-shaped component with evenly spaced pre-drilled holes.
The number of required bolts is always a multiple of 4—for example, 4, 8, 12, 16, etc. As flange size and pressure class increase, both the number and diameter of required bolts also increase.
Tightening flange bolts creates a compressive force that holds the flanges together, usually with a gasket in between to form a leak-proof seal. This design ensures safe containment of gases, liquids, or other substances under varying pressure and temperature conditions.
Cross-pattern tightening of flange bolts ensures even stress distribution across the sealing surface. Uneven stress on the sealing face may cause flange misalignment or gasket extrusion—either of which can increase the risk of sealing failure.
Flange Bolt Size Chart (ASME B16.5)
ASME B16.5 is a globally recognized standard that provides comprehensive guidelines for the design, sizing, and pressure ratings of pipe flanges and flange fittings. One of its key aspects is the detailed specification of bolt sizes, materials, and configurations for various flange ratings.
By following ASME B16.5, industries can ensure safe, compatible, and well-sealed flange connections under pressure. The standard eliminates ambiguity and ensures seamless integration of components—flanges, bolts, and washers.
Compliance with ASME B16.5 is not just a technical requirement but a critical safety measure. Errors in bolt selection or flange compatibility can result in catastrophic failure, leakage, or even environmental hazards, especially in high-risk industries such as oil & gas, chemical processing, and power generation.
Key Parameters in the Bolt Size Chart
The flange bolt size chart provides vital information to help engineers and technicians choose the correct bolts for a specific application. Key parameters include:
- Nominal Pipe Size (NPS) – Standard dimension identifying pipe diameter.
- Flange Rating – Pressure rating (e.g., Class 150, 300, 600) that determines flange strength.
- Bolt Diameter – Diameter of bolts required to secure the flange.
- Number of Bolts – Total number of bolts required to complete the connection.
- Bolt Circle Diameter (BCD) – The distance between the centers of bolt holes on the flange.
- Bolt Length – For stud bolts, this includes threaded length and any required protrusion for the nut.
Understanding these parameters ensures accurate bolt selection and compatibility with flanges, gaskets, and nuts.
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