How to Specify Bolts and Fasteners for Removable Tower Internals
Bolts used inside a packed tower are often treated as minor hardware. In practice, an unsuitable fastener can seize during installation, loosen during operation, corrode before the surrounding internal or prevent removal during the next shutdown.
A complete tower-internals specification must define more than bolt diameter and nominal material.
Start with the Fastener’s Function
Fasteners inside a column may perform different duties:
- Carry structural load
- Clamp gasketed panel joints
- Attach removable beams
- Secure packing retainers
- Hold leveling devices
- Position nonstructural components
- Prevent components from lifting during an upset
The required strength, preload and locking method depend on the function. A locator bolt should not automatically be treated like a loaded structural connection, while a structural beam connection should not be specified using only convenient workshop hardware.
“Same Material as the Internal” Is Incomplete
Calling for “SS316 bolts for SS316L internals” does not fully define the fastener.
The specification should identify:
- Applicable material standard
- Alloy group or grade
- Strength condition
- Bolt or stud dimensions
- Nut specification
- Washer material
- Thread form and pitch
- Surface condition or coating
- Required product marking
- Material certification
- Positive material identification requirements, when applicable
The selected alloy must be evaluated against the process fluid, cleaning chemicals, shutdown moisture and operating temperature. Galvanic compatibility, chloride exposure, crevice conditions and temperature-dependent strength may all matter.
Fastener standards used for pressure-boundary bolting are not automatically required for internal, non-pressure-boundary connections. The correct standard should follow the actual mechanical design and project specification.
Stainless Fasteners Can Gall
Austenitic stainless-steel threads can seize during tightening or removal. Adhesive wear damages the mating thread surfaces until the nut and bolt lock together.
Risk increases with:
- Dirty or damaged threads
- High installation speed
- Excessive preload
- Poor thread fit
- Repeated tightening
- Similar stainless mating surfaces
- Installation without an approved lubricant
Controls may include a qualified material pairing, clean rolled threads, slower tightening and a process-compatible anti-seize compound.
Lubrication changes the relationship between applied torque and bolt tension. Therefore, a torque value is incomplete unless the specified thread condition and lubricant are also stated.
For services with strict contamination limits, the lubricant must be approved for contact with the process. A product acceptable in a refinery tower may be unacceptable in oxygen, semiconductor-chemical, food or pharmaceutical service.
Prevent Loose Parts Inside the Tower
A loose nut can fall into the packing, obstruct a distributor outlet, damage rotating downstream equipment or become impossible to retrieve after closure.
The design should define an appropriate retention method, such as:
- Locking nuts qualified for the temperature
- Double-nut arrangements
- Tab washers
- Locking plates
- Lockwire
- Captive fasteners
- Positively retained pins
Polymer-insert locknuts should not be selected without verifying their chemical and temperature limits. A generic spring washer should not be assumed to provide adequate retention for every cyclic or vibrating service.
Fasteners should also remain accessible to the installer. A theoretically removable joint has little value if the nut is hidden behind a beam or cannot be reached with the required tool.
Specify Assembly and Inspection Requirements
The installation procedure should state:
- Whether connections are snug-tight or preloaded
- Required torque or tension method
- Lubricated or dry thread condition
- Tightening sequence for gasketed joints
- Permitted fastener reuse
- Method for confirming locking-device engagement
- Final hardware count
- Inspection marking requirements
For thin distributor panels, excessive tightening can deform the plate, open an adjacent gasket gap or change the level of an outlet zone. Tightness and hydraulic flatness must therefore be checked together.
Plan for the Next Shutdown
Fasteners should be selected for both installation and eventual removal. A connection that survives the process but must be cut apart during every turnaround increases downtime and risks damaging the internal.
The procurement package should include spare quantities for critical sizes and special alloys. Replacement hardware must remain traceable so that maintenance teams do not substitute unidentified bolts during a shutdown.