How to Retrofit Tower Internals When Field Welding Is Not Allowed

Many operating plants restrict or prohibit welding inside existing process towers during retrofit.

Reasons may include:

• hazardous-area controls;
• confined-space requirements;
• vessel metallurgy;
• internal linings;
• shutdown schedule;
• post-weld inspection requirements.

This means replacement internals must sometimes be designed for mechanical installation only.

That constraint should be identified before fabrication.

Why “Just Weld It on Site” Is a Bad Retrofit Assumption

Field welding can require:

• hot-work permits;
• gas testing;
• fire watch;
• weld procedures;
• qualified welders;
• inspection;
• surface repair.

If the plant prohibits these activities, an internal designed around welded installation may become unusable.

Use Existing Supports Where Possible

Existing rings, beams, clips, and brackets can sometimes provide the mounting structure.

Replacement internals may then use:

• bolts;
• clamps;
• locking plates;
• mechanical clips.

The exact system depends on load and service.

Design Bolted Segments

Large internals commonly need segmentation for manway access anyway.

Bolted construction can therefore serve two purposes:

• sectional installation;
• no-weld assembly.

Typical candidates include:

• support grids;
• distributors;
• bed limiters;
• mist eliminator supports.

Prevent Loose Hardware

Bolted equipment inside a process tower must be designed so that hardware cannot easily loosen and fall into the bed.

Consider:

• locking arrangements;
• suitable fasteners;
• vibration;
• corrosion.

The connection should be maintainable but secure.

Check Material Compatibility

Fasteners are part of the wetted internal system.

They should be compatible with:

• process fluid;
• temperature;
• the materials being joined.

Using corrosion-resistant internals with unsuitable bolts defeats the purpose of the upgrade.

Consider Plastic Internals

Plastic tower internals may use different joining methods from metal components.

Possible approaches can include:

• bolted assemblies;
• welded fabrication performed outside the vessel;
• modular sections installed mechanically.

Field restrictions should therefore be communicated to the manufacturer early.

Maintain Structural Integrity

Avoiding welding does not reduce the required load capacity.

Mechanical joints must still withstand:

• packing weight;
• liquid load;
• dynamic gas forces;
• maintenance loads where applicable.

Connection design should be treated as part of structural engineering.

Check Installation Access

Bolts and clamps need working space.

A connection that is theoretically possible may be extremely difficult to assemble inside a narrow tower.

Consider:

• tool clearance;
• hand access;
• worker position;
• sequence.

Simple assembly is especially valuable during short shutdowns.

Avoid Excessive Small Components

Highly segmented systems may eliminate welding but create hundreds of bolts or clips.

This increases:

• labor;
• installation errors;
• loose-part risk.

The design should balance manageable section size with practical assembly time.

Plan Removal as Well as Installation

Mechanical installation has one major advantage:

components may be easier to remove during future shutdowns.

Where possible, design the retrofit so that major components can be dismantled without cutting.

State the Restriction in the RFQ

The purchaser should clearly state:

No field welding permitted inside tower.

This allows the manufacturer to design appropriate segmentation and mechanical connections from the beginning.

Discovering the restriction after fabrication can be expensive.