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What Is Bolting? | Piping Engineering Glossary
What Is Bolting?
In piping engineering and process engineering, bolting refers to the studs, bolts, nuts, and washers that hold bolted joints together, along with the practices used to select, install, and tighten them. In process piping, bolting most often means the fasteners that clamp flanged joints, where they must keep the gasket compressed against internal pressure, temperature changes, and piping loads.
Bolting Materials and Specifications
Most flanged joints use threaded stud bolts with a nut on each end rather than headed bolts. Common bolting for general carbon steel service is ASTM A193 Grade B7 studs with A194 Grade 2H nuts. Low-temperature service often uses A320 Grade L7, corrosive services may use B8 or B8M stainless grades, and high-temperature services use alloys that resist creep and relaxation. The piping specification defines the bolting for each flange pressure class and service.
Bolting must provide enough clamping force to seat the gasket and keep it sealed, without overstressing the bolts, the flange, or the gasket. Coatings such as zinc or fluoropolymer are often applied to improve corrosion resistance and make later disassembly easier.
Applications
Flanged Joint Assembly
Bolting is tightened by controlled bolt torque or hydraulic bolt tensioning to reach a target bolt stress. Tightening follows a defined cross pattern and number of passes so the gasket compresses evenly.
Turnaround and Maintenance
During outages, bolting is inspected, replaced where corroded or damaged, and reassembled under a flange management program. Records show who tightened each joint and to what value.
High-Temperature and Critical Service
Joints in hot, cyclic, or toxic service may use higher-strength bolting, longer studs, or disc spring washers to maintain load as temperatures change. These details reduce the risk of leaks during startups and shutdowns.
Benefits
Reliable Sealing
Correct bolt load keeps gasket compression within the range needed to seal. It is the single most important factor in preventing leaks at flanged connections.
Safer Maintenance
Standardized materials, lubricants, and procedures make joints easier and safer to open and reassemble. Coated bolting also reduces seizing and the need for hot work to remove nuts.
Fewer Leaks
Good bolting practice reduces flange leakage, fugitive emissions, and startup problems. It also lowers the cost of rework and hot bolting.
Limitations
Torque Is Indirect
Torque only approximates bolt stress because friction varies with lubrication, thread condition, and the nut face. The same torque can produce very different bolt loads from joint to joint.
Relaxation Over Time
Gaskets creep and bolts relax, especially at high temperature, so joints can lose load after startup. Some joints are retightened under controlled conditions once they reach operating temperature.
Corrosion and Cracking
Bolting exposed to moisture, chlorides, or hydrogen can corrode or crack. Material selection and coatings must suit the environment, particularly in marine and sour services.
FAQ
What is the difference between a stud bolt and a regular bolt?
A stud bolt is threaded along its full length or at both ends and uses two nuts, while a regular bolt has a head on one end. Studs are standard on piping flanges because they are easier to tighten evenly and to replace.
What bolting is used for most carbon steel flanges?
ASTM A193 B7 studs with A194 2H heavy hex nuts are the most common choice for general service, though project specifications may require other grades for low-temperature, corrosive, or high-temperature conditions.
Why is lubricant important in bolting?
Lubricant on threads and nut faces reduces friction, so more of the applied torque turns into bolt stretch. Using the specified lubricant is essential for consistent torque results.
How many times should flange bolts be tightened?
Assembly procedures typically use several passes in a cross pattern, increasing torque gradually, followed by a final check pass at full torque until the nuts no longer turn.
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