How to Retrofit Tower Internals When Welding Inside the Existing Vessel Is Not Allowed
Retrofit projects often assume that new tower internals can simply be welded to the existing vessel.
In reality, hot work may be prohibited.
This is common when the tower:
- is made from FRP or thermoplastic;
- contains a corrosion-resistant lining;
- operates in a hazardous process area;
- cannot easily obtain a hot-work permit;
- must avoid damage to the existing shell.
The engineering problem then changes.
The question is no longer only:
What internal should be installed?
It becomes:
How can the internal be supported, assembled, and secured without welding to the vessel?
First Determine Why Welding Is Restricted
The reason matters because it determines what alternatives are acceptable.
For an FRP vessel, conventional metal welding to the shell is impossible.
For a lined steel vessel, welding may damage:
- rubber lining;
- plastic lining;
- protective coating.
In hazardous plants, the vessel itself may technically allow welding, but site safety procedures may prohibit hot work during the available shutdown.
Each situation requires a different mechanical strategy.
Reuse Existing Support Structures Where Possible
The simplest retrofit usually uses existing:
- support rings;
- beams;
- brackets;
- mounting points.
Before reuse, inspect:
- corrosion;
- deformation;
- dimensions;
- load capacity.
Existing hardware can become the foundation for a removable internal system.
Bolted Assemblies Are Common
Segmented internals can often be assembled using:
- bolts;
- clamps;
- interlocking frames;
- removable beam systems.
This is especially useful for:
- packing supports;
- demister frames;
- distributors;
- hold-down grids.
The complete assembly can pass through a manway in sections and be connected inside the tower.
Mechanical Restraint Must Still Be Positive
“No welding” does not mean the internal can simply rest loosely inside the vessel.
Gas velocity, liquid flow, vibration, and maintenance activity can move unsecured components.
The design should define how the internal is restrained against:
- vertical movement;
- rotation;
- lateral movement.
FRP Vessels Require Special Load Consideration
FRP should not be treated like steel.
A concentrated mechanical load applied to a small shell area can damage the laminate.
Loads should normally be transferred through suitable:
- molded supports;
- reinforced zones;
- existing structural rings;
- properly designed bearing surfaces.
The vessel manufacturer may need to confirm allowable loads.
Plastic Internals Need Thermal Movement
PP and other thermoplastics expand more than steel.
If a large plastic distributor is rigidly clamped at every point, temperature changes may create stress or deformation.
Non-welded mounting systems should therefore consider:
- thermal expansion;
- sliding allowance;
- controlled clearances.
Removability Has Maintenance Advantages
A no-weld design can actually improve future maintenance.
Bolted or segmented internals can often be removed without:
- cutting;
- grinding;
- damaging the vessel.
This can reduce future shutdown work.
But Too Many Connections Create New Risks
A heavily segmented assembly can increase:
- installation time;
- number of fasteners;
- alignment requirements;
- possibility of loose parts.
The goal is not maximum segmentation.
It is the minimum segmentation necessary for access and installation.
Hydraulic Performance Must Be Preserved
Mechanical adaptation should not create excessive obstruction.
For example, additional support beams may solve the structural problem but reduce gas-flow area.
The engineering team should check:
- free area;
- gas velocity through support openings;
- local liquid drainage.
What Should Be Confirmed Before Fabrication?
For a no-hot-work retrofit, obtain:
- Vessel material.
- Lining material.
- Existing support details.
- Tower internal diameter.
- Manway dimensions.
- Required internal load.
- Process temperature.
- Installation constraints.
- Site hot-work restrictions.
- Vessel-manufacturer limitations where relevant.
Final Engineering Principle
When welding is prohibited, tower internals should be designed as mechanically supported and removable systems from the beginning.
The solution should protect the vessel while still providing adequate structural restraint, hydraulic openness, thermal movement, and maintainability.