How to Prevent Thread Galling in Stainless-Steel Fasteners During Tower Internals Assembly

Stainless-steel bolts and nuts are widely used for liquid distributors, packing support grids, hold-down grids, collectors, mist eliminator frames and other removable tower internals.

They are corrosion resistant, easy to fabricate into custom assemblies and convenient when internals must be divided into sections for manway installation.

But stainless fasteners have one practical installation problem that is often discovered only when assembly begins:

the nut suddenly locks onto the bolt and can no longer be tightened or removed.

This failure is commonly known as thread galling.

Once severe galling occurs, additional torque usually does not solve the problem. Instead, the threads may tear, seize completely or force the installer to cut the fastener out.

Inside a packed tower, where access is limited and installation time is valuable, a few seized bolts can create unnecessary delays.

Understanding why galling occurs and controlling it before assembly is therefore an important detail in tower-internals installation.

What Is Thread Galling?

Thread galling is a form of adhesive wear.

When two metal thread surfaces slide against each other under load, microscopic high points on the surfaces come into contact. Under unfavorable conditions, local adhesion develops between the mating surfaces.

As tightening continues, these small bonded areas tear apart.

Material transfers from one thread surface to the other, friction rises rapidly, and the damaged surfaces begin to lock together.

With stainless steel, this process can progress very quickly.

A fastener may rotate normally for several turns and then become extremely difficult to move within a short additional rotation.

If tightening continues, the bolt and nut may seize permanently.

This is different from corrosion seizure. Galling can occur during the original installation of completely new, clean stainless fasteners.

Why Stainless Steel Is Particularly Susceptible

Stainless steel maintains corrosion resistance through its passive surface film, but its sliding behavior is different from that of many conventional fastener materials.

When two similar stainless surfaces are forced together under pressure, friction and local adhesion can become significant.

The risk increases when the threads experience high contact pressure combined with rapid sliding.

This is why galling is frequently associated with stainless-on-stainless combinations such as:

SS304 bolt + SS304 nutorSS316L bolt + SS316L nut

The problem does not mean the stainless grade is defective.

It is a tribological problem created by the interaction of material, surface condition, loading and installation method.

High-Speed Tightening Can Trigger Galling

One of the easiest ways to increase galling risk is to tighten stainless fasteners too quickly.

High rotational speed produces rapid sliding between loaded thread surfaces.

Frictional heating can develop locally, while the installer has little time to recognize that resistance is increasing.

Impact tools are particularly risky when used without an appropriate installation procedure.

The tool may continue applying energy even after the thread surfaces begin to damage each other.

For critical stainless tower-internal joints, controlled tightening is generally safer than simply using maximum tool speed.

If resistance suddenly increases before the joint is properly seated, installation should stop and the thread condition should be checked.

Forcing the nut farther can convert an early-stage problem into complete seizure.

Dirty or Damaged Threads Increase the Risk

New fasteners should not automatically be assumed to have perfect threads.

During manufacturing, transportation or site handling, threads can collect:

• metallic particles;
• grinding dust;
• dirt;
• damaged thread crests;
• burrs;
• deformation from impact.

Any condition that increases friction also increases galling risk.

Before assembly, bolts and nuts should therefore be visually checked and should engage smoothly by hand for the initial turns.

A nut that cannot run freely onto a bolt before loading should not be forced into the final assembly.

The cause should first be identified.

Lubrication Can Reduce Galling—but It Changes the Joint

A suitable thread lubricant or anti-seize compound can reduce friction between mating stainless surfaces and therefore reduce galling risk.

However, lubricant selection cannot be treated casually in chemical-process equipment.

The product must be compatible with the project requirements and eventual service.

Possible restrictions may involve cleanliness, process contamination, temperature, chemical compatibility or special plant requirements.

There is another important engineering issue:

lubricated threads produce different bolt tension from dry threads at the same applied torque.

A large portion of tightening torque is consumed by thread and bearing friction. When lubrication reduces that friction, more of the applied torque can become bolt preload.

Therefore a dry-thread torque value should not automatically be used after adding anti-seize.

Where controlled bolt preload matters, the tightening procedure should correspond to the actual lubrication condition.

Do Not Apply More Torque to a Seizing Fastener

When a stainless nut begins to seize, the natural reaction is often to use a longer wrench or a stronger impact tool.

That usually makes the problem worse.

Once adhesive damage has started, increasing torque can:

• tear thread material;
• deform the bolt;
• damage the nut;
• lock the joint permanently;
• make later removal much more difficult.

The correct response is to stop.

If the fastener can still be backed off without further damage, it should be removed and inspected.

A severely galled fastener should normally not be reused.

Replacing one bolt and nut is far easier than cutting a completely seized joint inside a tower after surrounding internals have already been installed.

Fastener Material Pairing Can Influence Galling Resistance

Using exactly the same stainless grade for both mating components is common, but it is not the only possible approach.

In applications where galling control is particularly important, engineers may consider differences in material grade, hardness, surface treatment or approved fastener coating.

The objective is to reduce the tendency of the two mating surfaces to adhere under sliding contact.

However, material changes must not be made only to solve galling.

The complete service environment still needs to be considered, including corrosion resistance, temperature, galvanic compatibility and project material specifications.

A fastener that assembles easily but cannot survive the process environment is not an acceptable solution.

Tower Internals Create Special Installation Conditions

Thread galling becomes particularly inconvenient inside tower equipment because assembly often occurs under constrained conditions.

Segmented internals may require installers to work through a manway and then assemble sections at the final elevation.

Access to the bolt may be limited.

The installer may be working above or below a support beam, beside distributor troughs or close to the vessel wall.

If a bolt seizes in an exposed workshop, cutting and replacing it may take only a few minutes.

If it seizes deep inside a partially assembled column, the same repair can require additional tools, access arrangements and dismantling.

This is why galling prevention should begin before components enter the tower.

A Practical Fastener-Control Procedure

For stainless tower internals, the installation team should treat bolting as a controlled assembly activity rather than simply tightening until the connection feels secure.

A practical procedure should ensure that threads are clean and undamaged, nuts engage smoothly before load is applied, approved lubrication is used where specified, tightening speed is controlled, the required torque or preload method matches the lubrication condition, and any fastener showing abnormal resistance is stopped and inspected rather than forced.

Where many identical bolts are required, testing the assembly method on several representative fasteners before entering the tower can also reveal problems early.

This is especially useful when the project uses a new fastener supplier, coating system or anti-seize compound.

Avoid Mixing Used and New Damaged Fasteners During Reassembly

Tower internals may be dismantled during maintenance and later reinstalled.

Previously used stainless bolts should be inspected carefully before reuse.

Threads that have already experienced adhesive damage may look acceptable from a distance but behave poorly when tightened again.

Look for displaced metal, flattened thread surfaces, rough engagement or local tearing.

If the nut does not travel smoothly during hand assembly, the pair should not simply be returned to service because it was previously installed successfully.

Maintenance work should not turn questionable fasteners into future removal problems.

Think About Future Disassembly

Tower internals are not always permanent.

Liquid distributors may need cleaning. Demister sections may need replacement. Packing support components may need inspection during major shutdowns.

A joint that can be installed today but cannot reasonably be dismantled several years later creates another maintenance problem.

Good fastener practice therefore supports both:

installation reliability and future maintainability.

Galling prevention is not only about getting the nut tightened during construction.

It is also about preserving a removable joint for the next tower shutdown.

Final Engineering Consideration

Stainless-steel thread galling is a small mechanical detail compared with tower diameter, packing height or distributor hydraulics.

But during actual installation, small details can control how smoothly the project proceeds.

The most important principle is simple:

A stainless fastener that begins to tighten abnormally should be investigated, not forced.

Clean threads, appropriate material combinations, controlled tightening speed, approved lubrication and correct torque practice can greatly reduce the chance of seizure.

For segmented liquid distributors, support grids, hold-down grids, collectors and other bolted tower internals, controlling thread galling before installation is far easier than removing a seized fastener inside an assembled column.