How to Allow for Thermal Expansion in Metal Tower Internals

Tower internals are installed at one temperature and may operate at another.

A stainless-steel distributor fabricated at workshop temperature can expand when exposed to a hot process.

If a large full-diameter internal is restrained too tightly, thermal expansion may create:

• buckling;
• distortion;
• support loads;
• binding;
• damaged attachments.

Thermal movement therefore matters even when the vessel and internal use similar metals.

All Materials Change Dimension With Temperature

The basic relationship is:

ΔL=αLΔT\Delta L = \alpha L \Delta T

where:

• ΔL = change in length;
• α = coefficient of thermal expansion;
• L = original length;
• ΔT = temperature change.

For small components, dimensional growth may be insignificant.

For multi-meter internals operating across large temperature changes, it can become meaningful.

Diameter Magnifies Movement

A short bracket may expand only slightly.

A distributor several meters across the tower can accumulate more total movement.

Large-diameter towers therefore deserve greater attention to thermal clearance.

Radial Clearance Has More Than One Purpose

Clearance between an internal and the shell may accommodate:

• fabrication tolerance;
• shell out-of-roundness;
• installation;
• thermal movement.

These requirements must be balanced.

Too little clearance may cause binding.

Too much clearance may create process bypass or poor fit.

Expansion Should Not Destroy Levelness

Gravity distributors depend on suitable levelness.

If one side is rigidly anchored while another is free to move unpredictably, thermal expansion may distort the assembly.

Support and attachment philosophy should allow necessary movement while maintaining functional geometry.

Sliding Supports May Be Appropriate

Some designs can use supports that:

• carry vertical load;
• restrain required directions;
• allow controlled thermal movement.

The detailed arrangement depends on the internal and vessel design.

Not every support should automatically be welded rigidly in every direction.

Bolted Connections Can Still Restrain Movement

A bolted assembly is not automatically a sliding joint.

If bolts clamp components tightly, friction can restrict movement.

The designer should distinguish between:

• structural connection;
• locating point;
• sliding interface.

Differential Expansion Matters

If the vessel and internal use different materials, their thermal expansion rates may differ.

Examples include:

• carbon-steel vessel with stainless internal;
• steel vessel with polymer internal;
• lined vessel with metallic equipment.

The relative movement can be more important than expansion of one material alone.

Temperature Gradients Can Cause Distortion

An internal does not always heat uniformly.

A hot feed may locally heat one region of a distributor before the rest reaches operating temperature.

Uneven expansion can create transient:

• warping;
• local stress;
• loss of levelness.

This is especially relevant during startup and shutdown.

Expansion Can Affect Segment Joints

Segmented collectors or distributor panels should not be designed so tightly that thermal movement:

• jams joints;
• opens unwanted gaps;
• transfers excessive force into bolts.

Joint design should consider both assembly and operation.

Hold-Down Devices Need Care

A component may need restraint against:

• uplift;
• vibration;
• process loads.

But restraint should not unnecessarily eliminate required thermal movement.

This balance is particularly important for large-area thin internals.

Check the Governing Temperature Range

Use more than normal operating temperature where appropriate.

Consider:

• installation temperature;
• startup temperature;
• normal operating temperature;
• maximum credible operating case;
• steam-out or cleaning temperature if applicable.

Maintenance conditions can sometimes produce a larger ΔT than normal operation.

Polymer Internals Need Separate Treatment

Plastic components can have much larger thermal expansion than metal.

They may also experience creep.

Metal-design clearances should not simply be copied to PP, PVDF, or PTFE internals.

Thermal Expansion Is an Interface Problem

The analysis should connect:

Internal→ Support→ Vessel Attachment→ Shell

A perfectly calculated internal expansion is not useful if the support ring prevents all movement.

Engineering Takeaway

Thermal expansion review should consider:

Material + Diameter + Temperature Range + Restraint + Sliding Direction + Segment Joints + Vessel Material

The objective is not to make every internal loose.

It is to provide enough controlled movement that operating temperature changes do not distort or overload the equipment.

Summary:Metal tower internals can expand significantly across large diameters and temperature changes. Support arrangements, radial clearances, segmented joints and restraint points should accommodate controlled thermal movement without creating excessive bypass, loss of levelness or structural binding.

URL:https://www.pxdaier.com/tower-internals-thermal-expansion-allowance.html

Progress: F033 / 50

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Final Supplement F034

How to Evaluate Galvanic Corrosion at Dissimilar-Metal Tower Internal Interfaces

A tower may contain more than one metal.

For example:

• carbon-steel vessel;
• SS316L distributor;
• stainless fasteners;
• alloy support clips.

Each material may be individually suitable for its intended service.

When dissimilar metals are electrically connected in the presence of a conductive liquid, however, galvanic corrosion may become a consideration.

The risk depends on much more than simply whether two different metals touch.

What Creates a Galvanic Couple?

Three conditions are generally required:

• different electrochemical behavior;
• electrical contact;
• conductive electrolyte.

Inside a wet chemical tower, these conditions may exist simultaneously.

The Less Noble Material Is Usually at Greater Risk

When a galvanic couple forms, one material acts more anodically.

That material may experience accelerated corrosion compared with its uncoupled condition.

The actual behavior depends strongly on:

• environment;
• temperature;
• surface condition;
• area ratio.

Area Ratio Matters

A small anodic component connected to a large cathodic surface can experience concentrated attack.

For example, a small carbon-steel attachment associated with a large stainless internal may deserve attention.

The opposite area ratio may behave differently.

Therefore, material names alone do not define severity.

Process Chemistry Controls Conductivity

Galvanic effects are usually more important where the liquid is electrically conductive.

Factors include:

• dissolved salts;
• acids;
• alkalis;
• chloride concentration;
• temperature.

A dry gas service is not equivalent to continuously wetted conductive service.

Carbon-Steel Vessel With Stainless Internals Is Common

This combination can be successful in many applications.

The interface should nevertheless be reviewed where:

• wetted conditions exist;
• corrosion consequences are significant;
• small carbon-steel attachments contact large stainless surfaces.

The solution depends on the complete corrosion design.

Isolation Can Be Considered—But Must Be Practical

Electrical isolation may sometimes be achieved using compatible:

• insulating pads;
• sleeves;
• washers;
• coatings.

But isolation introduces other questions:

• mechanical strength;
• chemical resistance;
• temperature;
• long-term integrity.

A poorly selected insulating material can create a new failure mechanism.

Welded Interfaces Cannot Be Electrically Isolated

If a stainless component is directly welded to a carbon-steel attachment, the metals are electrically continuous.

Material transition and weld design then require appropriate engineering.

Do not assume an insulating washer elsewhere removes the welded galvanic connection.

Coatings Need Careful Detail Design

A coating can protect a metal surface.

But a damaged coating near a galvanic junction may expose a small anodic area adjacent to a large cathodic surface.

Therefore coating strategy should be coordinated with the interface.

Fasteners Can Create Local Couples

Small:

• bolts;
• washers;
• clamps

may introduce another alloy into the joint.

Fastener material should therefore not be selected independently of the components it connects.

Crevice Corrosion Can Be Confused With Galvanic Corrosion

A dissimilar-metal joint often also creates:

• narrow gaps;
• stagnant liquid;
• deposits.

Localized attack may therefore involve both:

• galvanic effects;
• crevice chemistry.

Failure analysis should not assume one mechanism without evidence.

Process Cleaning Can Change the Environment

Normal operating fluid may be mild.

Cleaning solution may be:

• more acidic;
• hotter;
• more conductive.

Material-interface review should include credible cleaning conditions where relevant.

Material Compatibility Should Follow the Complete System

The review should consider:

Vessel

• Support Ring
• Beams
• Internal
• Fasteners
• Process Fluid

rather than asking only:

Is SS316L corrosion resistant?

Engineering Takeaway

Galvanic corrosion risk depends on:

Metal Pair + Electrical Contact + Electrolyte + Area Ratio + Temperature + Exposure

A dissimilar-metal interface is not automatically unacceptable.

But it should be deliberately reviewed where tower internals connect different alloys in wet corrosive service.