How Cyclic Operation Causes Fatigue in Tower Internals

Tower internals can crack even when no single operating load exceeds the calculated static limit. Batch columns, pressure-swing units, solvent-recovery systems and towers with frequent startups repeatedly change temperature, pressure, liquid level and flow. Each change produces a stress range. After enough cycles, cracks can initiate at weld toes, sharp corners, perforations or restrained joints and then grow with every operating campaign.

Fatigue design therefore asks a different question from static design. Static analysis asks whether the internal survives the largest load once. Fatigue assessment asks how often the load changes, where local stress is concentrated and how much damage accumulates over the intended service life.

Define the Real Operating Cycle

“Cyclic service” is not a sufficient design basis. Describe each repeating phase: heat-up, pressurization, feed introduction, full-rate operation, liquid drawdown, depressurization, washing, steam-out and cooldown. State the expected number of cycles per day or year and the target equipment life.

Record minimum and maximum metal temperatures, pressure differences across decks, liquid levels, gas and liquid rates, heating and cooling rates and the time spent at each condition. Include irregular but recurring events such as regeneration, grade changes, filter backwash, pump trips or rapid reflux restoration.

One full production batch may contain several damaging subcycles. A distributor branch can warm and cool whenever intermittent feed enters, while the vessel shell remains comparatively stable. Counting only vessel startups can therefore underestimate the component’s true cycle count.

Separate Low-Cycle and High-Cycle Mechanisms

Large temperature or pressure changes generally create relatively slow, high-amplitude stress cycles. These can cause low-cycle fatigue at restrained supports, welded attachments and regions with strong temperature gradients.

Flow-induced vibration creates many smaller cycles. Vapor jets, valve motion, liquid impact, vortex shedding and pulsating two-phase feed can produce high-cycle fatigue in thin panels, baffles, distributor branches and cantilevered brackets. A component may experience both mechanisms, and the damage locations may differ.

Do not treat a vibration problem as solved merely because the static stress is low. Millions of small cycles at a weld toe can be more damaging than a few high-load events.

Locate the Fatigue Hot Spots

Fatigue cracks usually begin where nominal stress is amplified by geometry or fabrication. Typical tower-internal hot spots include:

ends of intermittent attachment welds;

sharp internal corners and unblended cutouts;

holes close to panel edges or welds;

abrupt changes in plate thickness or stiffness;

cantilevered inlet baffles and spray headers;

beam-seat keepers with impact clearance;

tray-panel joints that slip during each cycle;

thermally restrained downpipes and distributor arms;

repairs that end beside an existing crack location.

Corrosion pits, weld undercut, grinding marks and erosion reduce fatigue resistance further. Inspection should target these details rather than distribute effort equally over broad unstressed surfaces.

Calculate Stress Range, Not Only Maximum Stress

The relevant fatigue input is the change in local stress between operating states. Determine how pressure differential, gravity load, thermal expansion and support movement vary through the cycle. A member that remains highly loaded but nearly constant may have less fatigue demand than a lightly loaded bracket that reverses direction repeatedly.

For thermally driven cycles, calculate the relative movement between the shell and internal. Fixed and sliding points should allow predictable movement. Friction that alternately sticks and releases can create local shock loads not represented by a smooth expansion calculation.

Where project rules require formal fatigue analysis, use the specified fatigue curves, weld-detail categories, stress-concentration treatment and cumulative-damage method. Consider loss of thickness over time because the end-of-life section may experience a higher stress range than the new component.

Design Details That Reduce Fatigue Risk

Improve the load path before simply increasing thickness. Use smooth transitions, generous radii and balanced attachments. Avoid terminating a stiff weld or reinforcement at the point of maximum bending. Short unsupported tabs, long cantilevers and abrupt changes from flexible plate to rigid bracket deserve special review.

Allow thermal movement where the process function permits it, but capture the component so it cannot impact repeatedly. A flexible connection may reduce thermal stress; an uncontrolled loose connection may introduce fretting and impact fatigue. The correct detail balances flexibility, stability and hydraulic sealing.

Weld profile and workmanship matter. Undercut, lack of fusion, arc strikes and forced fit can create crack starters. Grinding a weld smooth is beneficial only when performed to an approved profile without removing required thickness or leaving new grooves.

Understand the Failure Consequences

A fatigue crack may first appear as unstable tower performance rather than complete collapse. A cracked distributor branch can change liquid allocation. A detached baffle can obstruct vapor flow. A broken tray clamp can allow panel lifting and bypass. Loose fragments may damage packing, block a downcomer or travel into downstream equipment.

Evaluate whether a local crack remains detectable and stable or can trigger progressive damage. The inspection interval should reflect both the predicted initiation life and the consequence of a released component.

Inspection and Monitoring Strategy

Establish baseline photographs and dimensions at known hot spots. During turnarounds, use suitable surface or volumetric examination where visual inspection cannot reveal small cracks. The selected method must match the material, weld geometry and likely crack orientation.

Look for secondary evidence such as polished contact marks, fretting debris, elongated holes, repeated gasket extrusion, broken locking devices and local discoloration. Operating evidence may include new rattling, pressure-drop fluctuation, asymmetric temperature response or performance that changes between cycles.

When a crack is found, identify its cause before welding it closed. A repair that restores the original high-stress geometry can crack again quickly. Review adjacent identical details because they have experienced the same cycle history.

Data Required for Procurement

The RFQ should state cycle descriptions, expected lifetime cycles, temperature and pressure ranges, ramp rates, liquid-level changes, vibration sources, corrosion allowance and inspection requirements. Vendor calculations should identify fatigue-sensitive locations, movement philosophy, assumed cycle count and any required field measurement.

Engineering Takeaway

Cyclic tower service must be designed from stress range, cycle count and local detail—not only maximum static load. Reliable internals combine controlled movement, fatigue-resistant geometry and targeted inspection.