HRSGs and boilers |
Method DNB
ADD PDD Wet CSP
Summary of typical boiler/HRSG preservation methods Best suited to
Boilers, HRSG pressure parts, piping Turbines, gas paths, large volumes Less critical/complex systems Shorter outages/rapid restart Local/small, difficult areas
chemistry is managed using the same basic controls applied during operation, including oxygen removal, feedwater and boiler water pH control, treatment of insoluble material, and blowdown to control dissolved material.
Chemical surface protection (CSP) Chemical surface protection during preservation can be achieved using a vapour phase inhibitor (VPI) or, for the water/steam side, film-forming substances. The idea is that these chemicals stick onto the metal surfaces and form a protective film which inhibits corrosion. VPI products are typically supplied in bags or pouches that are suspended or mounted within the equipment to be protected. The required quantity depends on the enclosed volume and equipment geometry. For larger spaces or long piping runs, certain VPI products may instead be applied by fogging. VPI is generally most practical for relatively small, enclosed volumes, such as tanks and vessels, or where other preservation methods are impractical; however, its cost per unit protected volume or surface area can be relatively high. FFS products are typically introduced during operation as part of the water chemistry treatment programme to establish a protective film on internal surfaces. Prior to restart, compatibility of any VPI product with the normal operating water chemistry should be confirmed.
Main consideration Requires sealed envelope
Requires airflow path Less positive control Chemistry & recirculation Compatibility & cost
Choosing the right method In practice, there is no “one size fits all” solution and choosing the most appropriate preservation strategy is dependent on several factors. Wet preservation is generally better suited to shorter outages where rapid return to service is important, while dry methods become more attractive as shutdown duration increases or when systems must be opened and drained for repair purposes. Among dry methods, DNB offers particularly robust protection for steam and waterside systems, including complex circuits, but depends on maintaining nitrogen pressure and becomes less attractive where leakage is a concern, although this can be factored into the strategy. ADD is more flexible for turbines, gas paths and other spaces where a suitable airflow path can be established, with the additional advantage that its effectiveness can be readily monitored, whereas PDD is generally reserved for less critical or complex systems where the cost or complexity of active protection is not justified. CSP can work well for smaller areas or where drying or nitrogen protection is not practical, but the cost can make it less suitable for larger systems. One important factor, particularly for the ADD and PDD approaches, is whether it is possible to achieve good draining of all sections targeted for preservation. Residual standing water significantly delays achievement of the target humidity and may leave local areas inadequately protected.
The various methods, area suitability and considerations are summarised in the table, left. Ultimately, the preferred method is therefore site specific, primarily influenced by shutdown duration, restart time as well as component size, available equipment and the level of protection justified for the equipment to be preserved. Whatever method is selected, the effectiveness should be verified, meaning that regular monitoring of relevant parameters (relative humidity, nitrogen pressure or oxygen concentration, water chemistry) should be an integral part of the preservation activity.
Case study
Tetra Engineering has worked with numerous power plants to develop site-specific preservation strategies. In one recent case, a strategy was developed for a large coastal combined cycle plant facing a six-month shutdown, where local high humidity and airborne chlorides increased the risk of corrosion. Rather than applying one method throughout, Tetra Engineering assessed each component separately and selected the approach that best matched its preservation needs. Steam and gas turbines were assigned active dehumidification (Figure 7) because their high value justified a high level of protection and existing dehumidification equipment was already available. HRSG pressure parts were assigned dry nitrogen blanketing because the interconnected waterside volumes could be easily drained and sealed. It was more practical to use active dehumidification for HRSG gas paths because of their large volumes and accessible ducting routes. Steam, feedwater and condensate piping were also assigned nitrogen blanketing where continuous sealed envelopes could be established, with passive drying or local chemical protection used where the use of nitrogen was impractical due to leakage, access or geometry. It was suggested that certain electrical systems continue operation at minimum load rather than full layup because maintaining the system online reduced condensation risk and simplified the restart, which was a key consideration of the client. The recommended strategy also established preservation limits and monitoring requirements for each system, allowing operators to confirm that acceptable conditions are maintained throughout the shutdown.
The result was a practical mixed method strategy in which the resources were focused where equipment value and corrosion consequences were highest, while keeping restart constraints at the forefront.
References 1
66, 2002. 2
ASME, Consensus for the lay-up of boilers, turbines, turbine condensers, and auxiliary equipment, CRTD Vol
EPRI, Cycling, startup, shutdown, and layup fossil plant cycle chemistry guidelines for operators and chemists,
TR-107754, 1998. 3
VGB, Guideline for the preservation of power plant
systems, VGB-R116He, 1981. 4
Figure 7. Dehumidifier ducting to steam turbine for preservation. Photo: Tetra Engineering 18 | October 2026 |
www.modernpowersystems.com
R. Winston Revie (ed), Uhlig’s corrosion handbook (ECS series of texts and monographs 51), 3rd edition, Wiley, May 2011.
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