Boiler tubes play a critical role in both heat transfer and in maintaining the pressure integrity of marine boilers. When leaks occur, careful evaluation is required to prevent secondary damage.
And DNV has shared guidance on class acceptance criteria, including the assessment of repair feasibility and the follow-up of plug repairs on defective boiler tubes.
Boiler tubes are often exposed to high temperatures, corrosive environments and fluctuating operating conditions, making them susceptible to various forms of degradation. In case of leaks, plugging is often used as a rapid method to restore boiler integrity.
Common causes of leakage
- Corrosion. Corrosion is one of the most common causes of tube failure. Water‑side pitting corrosion develops due to poor boiler water management, contamination or insufficient chemical treatment. On the gas side, cold corrosion occurs in exhaust gas boilers, particularly during slow steaming, when low exhaust temperatures allow acidic condensates to form and attack tube surfaces.
- Overheating. Overheating results from conditions that impair heat transfer. Scale deposits, oil contamination, high firing rates, low water levels or flame impingement can cause localized temperature spikes. These conditions weaken tube material, reduce wall thickness and, eventually, lead to rupture.
- Workmanship. Manufacturing and installation defects also contribute to tube leakage. Poor tube expansion, inadequate seal welding, and root pass corrosion at tube terminations create potential weak points. In addition, stress corrosion cracking may occur near welds where high residual stresses are present, further increasing the likelihood of failure.
- Soot-related mechanisms. In fin-type or pin-type exhaust gas boilers, soot accumulation combined with poor water circulation can lead to smouldering deposits and, ultimately, soot fires. These fires generate intense heat capable of melting uncooled tubes, damaging adjacent components, and potentially triggering hydrogen fires if nearby tubes rupture.
Why plugging repairs are used
Plugging is commonly used because it provides an immediate way to stop leakage and restore pressure containment without requiring full tube replacement. It allows the boiler to remain operational until permanent repairs can be arranged and is often the only practical option during voyages. However, plugging does not address the underlying cause of the defect and may introduce new risks if not properly assessed.
Impacts of plugged tubes
Plugging alters the thermal and hydraulic behaviour of boilers. In water tube boilers, it disrupts natural circulation, causing uneven heat distribution and increasing the risk of overheating or burnout of uncooled plugged tubes near the furnace.
In exhaust gas boilers, uncooled plugged tubes significantly increase the likelihood of soot fires and increased exhaust gas back pressure which can affect main engine turbocharger efficiency.
In all boiler types, the remaining tubes are subjected to higher thermal loads, and the steam generation capacity may be reduced.
Class acceptance criteria for plug repairs
Tube plugging is generally considered a deviation from the boiler’s approved design, and class acceptance of a plug repair is given on a case-by-case technical assessment of the boiler condition and the consequences of the plugged tubes.
Principles for class acceptance:
- DNV evaluates the number and location of plugged tubes, their impact on circulation and heat transfer, the associated risk of burnout or soot fire, and the overall condition of the boiler.
- Acceptance is conditional upon repairs being carried out in accordance with evaluated and accepted procedures, using qualified welders, suitable materials, satisfactory nondestructive testing (NDT) and pressure testing, in line with DNV requirements and the manufacturer’s instructions.
- Mitigating measures may be required, such as derating the boiler by reducing firing rate and pressure, or applying refractory protection.
- Where justified, plugging may be accepted as a permanent repair and recorded as a Memo to Owner (MO) in the vessel’s class status.
Recommendations
All plug repairs shall be carried out in accordance with the boiler manufacturer’s instructions, by qualified personnel and in compliance with applicable classification requirements.
Early involvement of DNV is essential. The proposed repair plan shall be submitted to DNV for evaluation prior to commencement of the repair. This evaluation may be carried out by DNV technical support or by the responsible local surveyor. For further details, refer to Section 2 of DNV-CG-0456.
Download the full guidance: DNV – Boiler tube plugging class requirements