Pressure Vessel Corrosion Case Study in Practice

A pressure vessel corrosion case study is most useful when it shows the decisions behind the inspection finding, not merely the defect itself. In this representative example, a production facility identified local internal corrosion during a planned outage of an ageing compressed-air receiver. The vessel remained functional, external condition appeared satisfactory, and routine operating pressure had not changed. Without internal examination, the loss of wall thickness would have remained concealed until the next inspection interval or a more serious failure.

The case illustrates why pressure equipment integrity cannot be assessed from service history, external appearance or thickness readings taken at a few convenient locations. It requires an inspection scope that reflects the vessel's duty, design, operating environment, previous inspection findings and the requirements of AS 3788.

Asset background and inspection trigger

The vessel was a horizontal steel compressed-air receiver serving continuous manufacturing equipment. It had been in service for more than two decades and operated in a humid environment with a history of condensate accumulation. Compressed air was supplied through upstream treatment equipment, but the receiver had no effective means to demonstrate that condensate was consistently drained during all operating conditions.

The asset register recorded periodic external inspections and pressure-relief valve maintenance. Previous inspection records noted general surface corrosion at the lower shell, but the available documentation did not clearly establish the extent of internal examination or provide a reliable baseline for local thickness monitoring.

A planned shutdown created an opportunity for a condition-based internal inspection. This was the appropriate trigger. The purpose was not simply to obtain a certificate, but to establish whether the receiver could safely remain in service and whether its inspection interval remained justified.

Before entry, the vessel was isolated, depressurised, drained, cleaned and made safe under the site's confined-space and isolation arrangements. Access conditions matter. Poor cleaning can obscure pitting, deposits and weld details, while incomplete isolation creates an unacceptable risk for inspection personnel.

Pressure vessel corrosion case study: the inspection findings

Internal visual examination found a band of adherent deposits along the lower quadrant of the shell, concentrated near the condensate outlet. Once deposits were removed, localised pitting was evident beneath them. The corrosion pattern was materially different from the light, broadly distributed oxidation expected from ordinary atmospheric moisture.

Ultrasonic thickness measurements were then taken on a defined grid, with tighter spacing around the affected area and at weld-adjacent locations. Several readings were below the nominal shell thickness, as expected for a vessel of this age. More significantly, the local minimum reading within the pitted zone was substantially lower than adjacent readings taken only a short distance away.

This distinction is critical. A small number of readings can produce an apparently acceptable average while missing a local area of severe metal loss. For pressure-containing equipment, remaining wall assessment must address the governing local condition, not the average condition.

The inspector also identified restricted drainage at the low point and evidence that the drain arrangement had been operated inconsistently. Water and contaminants had collected beneath the deposit layer, creating conditions favourable to under-deposit corrosion. The damage mechanism was therefore not treated as a generic ageing issue. It was linked to a credible service-related cause that required correction.

No crack-like indications were identified during the examination, and the external shell and supports showed no obvious evidence of distortion. Those findings were positive, but they did not remove the need to assess the reduced wall thickness against the vessel's design and operating conditions.

Engineering assessment, not a pass-or-fail judgement

The facility's immediate question was whether the receiver could be returned to service after the shutdown. That answer could not be based on a visual opinion alone. The assessment required confirmation of vessel design data, maximum allowable working pressure, material details where available, corrosion allowance, measured thicknesses and the extent of the affected area.

Where original documentation is complete, the evaluation can proceed from known design information. Where records are incomplete, additional engineering review may be required to establish conservative assumptions and define what further examination is necessary. This is one reason defensible pressure equipment records have operational value well beyond compliance.

In this case, the local minimum thickness did not support unrestricted continued operation at the existing maximum working pressure without further action. However, the condition did not automatically require disposal of the receiver. The available options included engineered repair, replacement, or temporary operation at a reduced pressure subject to a documented fitness-for-service assessment and appropriate controls.

The selected path was a planned repair during the existing shutdown. This avoided a speculative return to service and limited production disruption. The repair scope included removal of the affected shell section, examination of adjoining material, a qualified repair procedure, and the required inspection and testing activities before the vessel was returned to operation.

Repair work on pressure equipment requires careful control. The repairer, procedure, material traceability, examination method and post-repair testing requirements must be appropriate to the equipment and the work being performed. Depending on the vessel's construction, jurisdictional requirements and applicable code of construction, this may involve competent welding supervision, non-destructive examination and pressure testing. AICIP-accredited inspection and relevant ASME or National Board expertise can be particularly valuable where repair documentation or international equipment codes apply.

Correcting the cause of corrosion

Repairing the metal loss alone would have addressed the symptom, not the cause. The facility therefore reviewed the air-treatment and condensate-management arrangements. The drain point was modified to improve low-point drainage, and a documented routine was introduced to verify drain operation. Condensate discharge was also considered as part of the site's environmental and waste-management obligations.

The inspection plan was revised to include targeted internal examination and repeat ultrasonic thickness monitoring at the affected lower-shell locations. The first follow-up inspection was set earlier than a standard interval to establish the actual corrosion rate after the drainage changes. This is a practical example of condition-based inspection: the interval is informed by evidence, not selected merely because a calendar date has arrived.

There is a trade-off. More frequent inspection carries cost and may require plant downtime. Yet an interval that ignores a demonstrated local corrosion mechanism can create a far greater cost through unplanned outage, loss of containment, injury risk or regulatory action. The appropriate interval depends on the measured condition, rate of deterioration, consequences of failure and effectiveness of the controls in place.

What the case demonstrates for asset owners

Several lessons apply across compressed-air receivers, steam plant, heat exchangers, process vessels and other pressurised equipment. First, corrosion mechanisms are often location-specific. Low points, dead legs, drain connections, support interfaces, crevices and areas beneath deposits warrant focused attention because general examination alone may not reveal the governing defect.

Second, inspection records need to support comparison over time. A report that records only an overall judgement provides limited value at the next outage. Useful records identify inspection locations, thickness readings, photographs where appropriate, damage mechanisms, recommendations, operating limitations and the basis for the next inspection interval.

Third, independence matters when decisions affect repair scope, operating pressure or replacement timing. The inspection recommendation should be based on the condition of the equipment and applicable requirements, free from pressure to minimise repair work or sell a particular outcome. Independent advice gives maintenance and engineering managers a clearer basis for decisions that must withstand internal review, insurer scrutiny and regulatory attention.

Finally, certification should follow evidence. AS 3788 provides the framework for in-service inspection of pressure equipment, but compliance is not achieved by paperwork detached from vessel condition. A competent inspection considers the equipment's service history, design, access limitations, examination findings and any engineering assessment required to support continued safe operation.

For facilities operating ageing pressure equipment, the useful question is not whether corrosion will occur. It is whether the inspection plan is capable of finding the relevant corrosion mechanism early enough to manage it safely, economically and with a defensible record.