Technical Guide to Inspection and Maintenance in Distillation Towers

Distillation towers are fundamental components in thermal separation operations within the Oil & Gas sector. Their proper performance depends not only on operational parameters such as temperature, pressure, and flow composition but also on the physical and mechanical condition of their internal components—such as trays, packings, distributors, and structural supports.

Periodic inspection of these internal components is essential to maintain optimal system performance. These internals are responsible for maximizing phase contact and ensuring efficient mass transfer, making them key to productivity, process safety, and compliance with product specifications. They also contribute directly to the operational reliability of process units.

This article offers a comprehensive approach to best practices for inspecting the internals of distillation towers—from risk assessment and pre-cleaning to the most advanced non destructive testing methods—with the goal of ensuring efficient technical control aligned with international standards in the energy sector.

Basics of distillation towers

Function and principles of operation

Distillation towers allow complex liquid mixtures to be separated into simpler fractions, using the differences in boiling points. They are essential in refineries and petrochemical plants.
The process is based on selective vaporization and condensation: the vapor rises and meets the descending liquid, generating liquid-vapor equilibrium stages that enrich the upper fraction with light compounds and the lower with heavy ones.

Common types of distillation:

• Fractionated: At atmospheric pressure, to separate products such as naphtha and gas oil.
• Vacuum: For heavy fractions, avoids thermal decomposition.
• Azeotropic: To break azeotropes with entropic agents.
• These methods optimize energy efficiency and final product quality.

Main components of a distillation tower

Towers include internals designed to maximize mass transfer between phases. The main ones are:

• Feeder plate: Introduces the feed at the appropriate point.
• Trays: vapor-liquid contact; they can be bell, valve, or grid.
• Filling (packing): Alternative to trays, ideal for lower pressure drop.
• Distributors and collectors: Guarantee uniform flow over the packing.
• Structural internals: Include downcomers, redistributors, and supports.
• Sensors and nozzles: Allow monitoring and maintenance.

Proper design and periodic maintenance of these elements ensures separation efficiency, operational safety and compliance with technical standards.

Importance of the Inspection of Interns

The inspection of internal components in distillation towers is a critical practice in the maintenance of separation units within the process industry, particularly in refineries, petrochemical plants, and fine chemical facilities. These internal components—such as trays, packings, distributors, and collectors—play an essential role in the efficiency and stability of the process. Their periodic evaluation is key to ensuring operational continuity and preventing critical failures or severe malfunctions.

Role in operational reliability and process safety

Prevention of mechanical failures and efficiency losses

Poorly installed, worn, or damaged internals can disrupt the ascending-descending flow pattern of vapor and liquid inside the tower, leading to channeling, entrainment, or liquid accumulation. These effects reduce separation efficiency and increase energy costs. A structured inspection allows for the identification of these defects before they impact product quality or create unstable operating conditions.

Early detection of corrosion, fouling, or deformation

Severe temperature conditions, the presence of acidic compounds, flow variations, and contaminants can cause localized corrosion, including galvanic reactions in metallic materials, fouling buildup, or mechanical deformations in metallic and polymeric internals. Visual inspection, complemented by non-destructive techniques (such as UT or industrial endoscopy), enables early detection of these issues to take corrective actions before irreversible damage occurs.

Reduction of unplanned downtime

Undetected failures during planned shutdown windows can lead to process deviations that force unplanned outages, resulting in significant economic consequences. To explore strategies that enhance operational reliability through maintenance, refer to the article Improvements in Operational Reliability Through Maintenance. Timely inspection of internals ensures that functional integrity criteria are met, reducing the likelihood of emergency shutdowns and improving overall asset reliability, especially when supported by digital predictive maintenance tools.

Applicable standards

Internal inspections of distillation towers must be conducted in accordance with established national and international standards that define the safety requirements, mechanical integrity criteria, and acceptable non destructive testing (NDT) methodologies. Adherence to these standards ensures not only the reliability of the inspection process but also the safety of personnel and the continued fitness-for-service of critical equipment. The following are key references that govern inspection practices:

• API 510 – Pressure Vessel Inspection Code
  Provides minimum requirements for the inspection, repair, alteration, and rerating of pressure vessels, including distillation towers operating under pressure. It outlines qualifications for inspectors, inspection intervals, and assessment protocols.
• API 579 – Fitness-for-Service (FFS)
  Offers methodologies for assessing equipment that has sustained degradation or defects, such as corrosion, cracks, or mechanical deformation. It supports decision-making on whether a vessel can remain in operation safely under current service conditions.
• ASME Section V – Nondestructive Examination
  Specifies the principles and practices for NDT techniques, including ultrasonic testing (UT), eddy current testing (ECT), radiographic testing (RT), and liquid penetrant inspection (PT). It ensures accurate detection of flaws without compromising the structure.
• OSHA 29 CFR 1910.146 – Permit-required confined spaces
  Defines the safety requirements for entering and working within confined spaces, such as the internal volume of distillation towers. It mandates gas testing, ventilation, permitting, rescue procedures, and personnel qualifications.
• NFPA 350 – Guide for Safe Confined Space Entry and Work
  Complements OSHA by offering best practices and additional guidance for hazard recognition, atmospheric monitoring, and work planning in confined spaces.
• ASTM and ISO Standards
  Provide globally recognized procedures for calibrating NDT instruments, qualifying inspection personnel, and documenting inspection results for traceability and quality assurance.

By aligning internal inspection programs with these standards and with risk based inspection frameworks, operators ensure compliance, minimize risk, and support informed maintenance decisions based on validated data and internationally accepted practices.

Preparation prior to internal inspection

La inspección interna de una torre de destilación es una actividad de alto riesgo que debe planificarse rigurosamente. La preparación adecuada no solo garantiza la seguridad del personal involucrado, sino que también permite una ejecución eficiente y conforme a los estándares regulatorios. La fase previa es tan importante como la inspección misma, ya que errores en el aislamiento, limpieza o autorización pueden comprometer los resultados o generar accidentes graves.

Risk Assessment and work permits

Before allowing access to the tower, a comprehensive risk assessment must be conducted regarding confined space entry, the presence of hazardous atmospheres, and the residual operational conditions of the system.

• Confined space entry permit
  A specific work permit for confined space entry must be issued, in accordance with local and international standards such as OSHA (Confined Spaces), NFPA, and NIOSH. This document must specify the personnel responsible for each task, the permit’s validity period, required equipment, and available rescue procedures.
• Hazardous atmosphere evaluation (H₂S, hydrocarbons, inert gases)
  Towers may retain toxic gases such as hydrogen sulfide (H₂S), volatile organic compounds, or atmospheres with displaced oxygen due to nitrogen or steam. Before entry is authorized, atmospheric testing is mandatory to measure oxygen levels, lower explosive limits (LEL), and the presence of toxic gases using certified multi-gas detectors.
• Coordination with the industrial safety team
  The entire procedure must be planned and supervised in conjunction with the industrial safety department, which will validate the risk analyses, approve work permits, and define the necessary personal protective equipment (PPE) and rescue tools. It is advisable to hold a pre-job safety briefing before each inspection day.

Sequence of pre-inspection activities

Safe execution of the inspection depends on strict adherence to a technical sequence of tower preparation, including system isolation, depressurization, and subsequent cleaning.

Isolation and depressurization

• Line locking and tagging
  Physical isolation (blind flanges, slips, double block and bleed) must be installed on all inlet and outlet connections of the tower, according to an approved isolation matrix. Each point must be labeled with a visible tag (LOTO) indicating the responsible person and the date of isolation.
• Draining of liquids and pressure release
  All residual liquids must be completely drained to designated tanks, avoiding spills or hazardous exposure. The tower should then be depressurized in a controlled manner to eliminate any remaining internal pressure and prevent sudden ejections of gases or liquids when opening flanges or access points.

Tower cleaning

• Steam-out
  Plant steam is used to purge any remaining volatile hydrocarbons. This step helps reduce the concentration of flammable gases and facilitates subsequent mechanical cleaning. Steam is injected through upper or lower nozzles, depending on the tower’s design.
• Chemical or mechanical washing
  Depending on the nature of the contaminants (coke, polymers, sludge), specific chemical agents or mechanical tools such as high-pressure water jets are used. This step must remove all substances that could interfere with visual or instrument-based inspections.
• Forced ventilation and gas testing prior to entry
  Once cleaning is complete, axial fans or blowers are installed to continuously renew the internal atmosphere. Ventilation must remain active throughout the inspection process. Atmospheric testing is conducted just before the first inspector is authorized to enter.

Internal inspection process of the tower

The internal inspection of a distillation tower must follow a precise technical sequence that enables a thorough evaluation of the condition of the internals, the mechanical integrity of the shell, and any signs of deterioration that could compromise operational performance. This requires the use of appropriate tools, trained personnel, and a clear methodology for recording and documentation.

Required tools and equipment

A well-prepared inspection team must have certified tools and safety equipment suitable for potentially hazardous environments, in compliance with industrial safety standards.

• Explosion-proof flashlights, inspection cameras, and thickness measurement tools
  Flashlights must meet ATEX or IECEx standards. Industrial cameras (manual or endoscopic) facilitate inspection in hard-to-reach areas. Conventional ultrasonic testing (UT) devices and depth gauges allow for accurate measurement of wall thicknesses, plates, and supports—even under difficult access conditions.
• Safety equipment: Tychem suits, lifelines, multi-gas detectors
  Tychem suits or equivalent are ideal for protection against residual chemical substances. Lifelines must be anchored outside the confined space. Multi-gas detectors are used to continuously monitor explosive or toxic atmospheres during the inspection.

Step-by-step methodology

Entry and primary visual assessment

• Verification of general conditions: Upon entry, the inspector must evaluate lighting, atmosphere, internal access, and stability of the working area.
• Coating condition, corrosion signs, and accumulations: Areas with coating loss, generalized or localized corrosion, accumulated sediments, solidified hydrocarbons, or fouling should be identified.

Detailed Inspection of Internals

• The detailed inspection of internal components includes the following critical elements:
• Trays: Verify alignment with the design plane, presence of deformations, perforations, damaged supports, or detached parts.
• Packing: Evaluate displacement, collapse due to compaction, fouling, or scaling that may impact mass transfer efficiency.
• Distributors: Check uniformity of perforations and presence of obstructions or deposits that may disrupt flow distribution.
• Structural supports: Inspect beams, support rings, and guides to confirm the absence of cracks, severe corrosion, or structural weakening.

Measurement and Documentation

• Wall thickness (UT): Using conventional ultrasonic testing, take measurements at representative locations and visually affected areas.
• Measurement of deformations and settlement: Use calibrated rulers, laser levels, or templates to evaluate deviations from the original design.
• Photographic documentation, use of software for traceability and predictive maintenance analysis: Capture photographic evidence and associate it with sketches or digital models using asset management software (CMMS or IDMS). These tools are increasingly integrated into condition-based maintenance (CBM) strategies, where smart sensors enable continuous monitoring and early diagnostics.

Applicable inspection methods for internals

The selection of the appropriate inspection method depends on the type of internal components, the accessibility of the area, and the resources available. An effective inspection combines direct observation with advanced non-destructive evaluation techniques, and should ideally be part of a broader risk based inspection approach. In this context, artificial intelligence applied to industrial inspections is revolutionizing how data is interpreted and hidden failures are detected.

Direct and indirect visual inspection

• Use of industrial remote cameras (telescopic or endoscopic) and certified drones in tall towers or areas with restricted access. These technologies enable safe, preliminary indirect visual inspections. However, their results must always be validated through direct visual inspection or complementary techniques, as specified by API 510 and best practices in the Oil & Gas sector.
• Comparison with historical records or isometric drawings: Deviations in internal configurations are identified by comparing with original drawings or previous inspection records.

Ultrasonic Testing (UT) and Eddy Current Testing (ECT)

• Wall thickness verification: Conventional ultrasonic testing allows measurement of residual wall thickness in internal surfaces and structural components without disassembling the internals.
• Non-invasive evaluation of inaccessible areas: Eddy current sensors (ET or ECT) are effective for detecting surface corrosion or subsurface defects beneath coatings, especially in conductive materials.

Advanced techniques

• Phased Array UT in critical sections: This technology provides cross-sectional imaging of the inspected component, detecting internal cracks or laminations in high-stress zones.
• Acoustic emission for leak or active crack detection: Passive sensors are installed to detect sound waves generated by the growth of cracks or leaks under pressure during dynamic testing.
• Complementary thermographic techniques: Infrared cameras detect temperature differences associated with buildup zones, active corrosion, or internal leaks.

Most common damage mechanisms

During continuous operation of a distillation tower, internal components and structural elements are exposed to physical and chemical conditions that, over time, may compromise their integrity. Recognizing the most frequent damage mechanisms is essential to focus inspections on high-risk areas and prevent operational failures.

Corrosion from aggressive chemical compounds

• In distillation towers—particularly in crude, visbreaking, or alkylation units—it is common to encounter corrosive compounds such as organic acids (e.g., naphthenic acid) or chlorides resulting from residual salts or poorly controlled chemical injections. These agents can severely affect trays, packings, and internal supports:
• Formation of organic acids: Leads to localized corrosion, especially in low-flow or mid-temperature zones.
• Chloride attack: Promotes pitting and stress corrosion cracking (SCC), particularly in stainless steels.

Mechanical or thermal fatigue

• Distillation towers are subjected to frequent changes in load, temperature, and pressure, which create expansion and contraction cycles. These cycles can lead to cracking in welds, misaligned trays, or stress concentration points:
• Frequent thermal cycling: Induces thermal fatigue cracks in attachment areas or between trays and support rings.
• Fatigue from vibration: Can occur due to mechanical vibrations linked to hydraulic pulsations or sudden flow changes during unstable operation.

Fouling

• Fouling refers to the accumulation of solids, polymers, salts, or coke on the internal surfaces of components, which significantly impairs tower performance:
• Deposits that block flow: Cause excessive pressure drop and disrupt pressure profiles.
• Reduced separation efficiency: Fouling impairs phase contact (liquid-vapor), leading to lower fractionation performance.
  Critical fouling zones include lower trays, poorly irrigated packing areas, and distributors located beneath contaminant-rich streams.

Damage from poor flow distribution or design

• Poor design or improperly executed modifications may result in non-ideal flow patterns within the tower, reducing capacity and accelerating internal deterioration:
• Channelling: Occurs when liquid or vapor concentrates in specific flow paths through the packing, creating areas with little or no effective contact.
• Hydraulic shocks on faulty trays: Obstructed, misaligned, or poorly sealed trays can experience repeated impacts that deform or fracture components, posing a critical safety risk in high-pressure operations.

Final recommendations for an effective inspection

An internal inspection goes beyond visual assessment; it requires thorough planning, analysis, and detailed documentation to generate long-term value. The following are two key pillars for enhancing the effectiveness of such programs.

Importance of historical records and lessons learned

• Accumulated knowledge is one of the most valuable assets for operational reliability. Systematically documenting each inspection enables the development of intelligence to support future maintenance decisions.
• Use of databases for predictive analysis: Recording findings, photographs, repairs, and operational conditions helps feed digital platforms that forecast deterioration trends.
• Correlation between inspection results and operational performance: Cross-referencing variables such as pressure drop, separation efficiency, and process alerts with field data strengthens diagnostics and helps prevent recurring incidents.

Integration with RBI programs (Risk Based Inspection)

• The risk based inspection (RBI) methodology allows inspection resources to be prioritized based on the likelihood of failure and the potential operational or safety consequences.
• Prioritization of inspections by criticality: Towers operating under severe conditions, containing toxic products, or posing high environmental or economic impact if they fail are inspected more frequently.
• Cost reduction and increased reliability: By focusing inspections where they are most needed, maintenance budgets are optimized, and unnecessary personnel exposure to hazardous environments is reduced—reinforcing a predictive maintenance approach.

Conclusions

Internal inspection of distillation towers is a critical operation that directly impacts the reliability, safety, and efficiency of processing units. From proper preliminary preparation —including isolation, cleaning, and atmospheric control— to the application of techniques such as conventional ultrasonic testing (UT), eddy currents, or detailed visual inspection, each stage contributes to extending the service life of internal components and optimizing process performance.

Identifying and understanding the most common damage mechanisms—such as acid corrosion, thermal fatigue, fouling, or design-related failures—allows inspection efforts to be focused on priority areas. Furthermore, integrating inspection findings into asset management platforms and Risk Based Inspection (RBI) programs strengthens decision-making processes, guiding them toward predictive maintenance and operational reliability.

In an industrial environment where energy efficiency, product quality, and personnel safety are fundamental, internal inspection should not be seen as a mere shutdown requirement, but rather as a strategic approach to ensuring operational continuity and reinforcing the culture of industrial safety in high-risk settings. Applying the best practices, supported by standards and technical expertise, is essential to maximize profitability and minimize risks in the Oil & Gas sector.

References

1. American Petroleum Institute (API). API 510 – Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration. 10th Edition. Washington, DC: API Publishing Services, 2014.
2. American Petroleum Institute (API) and ASME. API 579-1/ASME FFS-1 – Fitness-For-Service. 3rd Edition. Washington, DC: API Publishing Services, 2016.
3. American Society of Mechanical Engineers (ASME). Boiler and Pressure Vessel Code, Section V – Nondestructive Examination. New York: ASME, 2021.
4. U.S. Department of Labor, Occupational Safety and Health Administration (OSHA). 29 CFR 1910.146 – Permit-Required Confined Spaces. Washington, DC: OSHA, 1993. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.146
5. National Fire Protection Association (NFPA). NFPA 350 – Guide for Safe Confined Space Entry and Work. Quincy, MA: NFPA, 2021.
6. ASTM International. ASTM D2892 – Standard Test Method for Distillation of Crude Petroleum (15-Theoretical Plate Column). West Conshohocken, PA: ASTM International, 2020.
7. Center for Chemical Process Safety (CCPS). Guidelines for Risk Based Process Safety. New York: American Institute of Chemical Engineers, 2007.
8. American Institute of Chemical Engineers (AIChE). “Inspecting Distillation Towers Part 1: Turnarounds.” CEP Magazine, Vol. 114, No. 9, September 2018.
9. American Institute of Chemical Engineers (AIChE). “Inspecting Distillation Towers Part 2: Revamps and Other Inspections.” CEP Magazine, Vol. 114, No. 12, December 2018.
10. Refinery Training. Inside the Tower: Exploring Distillation Tower Inspections [Video]. YouTube, 2022. https://www.youtube.com/watch?v=8bd6Q3XwzqY