| HRSGs and boilers
requirements and preservation equipment can be prepared while the unit is still operating.
Available preservation methods Long term preservation can be done either wet or dry. Dry preservation requires draining the unit whilst wet preservation entails keeping the system filled with water and adjusting the chemistry before shutdown. There are several options available depending on the plant’s design and resources available, Tetra Engineering has
Figure 5. Corroded external tube surface
What to consider when deciding on a strategy? Deciding when standby should move into short-term preservation can be difficult and is highly dependent on the configuration and operating constraints of a given plant. If the unit is put out of service for more than one week then long-term wet or dry layup should be considered. At some plants, based on TETRA experience, the move from short to long term is often made several weeks or more after shutdown. While this is not ideal it is the reality, generally driven by operating constraints or availability of resources. A preservation procedure should be based on general principles that are suitably adapted to the design and operating constraints of the specific site under consideration in keeping with the resources (time, money, manpower) available. Guidelines for layup practice at thermal power plants have been issued by a number of industry organisations including ASME1 EPRI2
, and VGB3 , as well as OEMs.
These guidelines often provide useful high- level guidance, including on options such as wet and dry layup. However, they rarely give enough detail to define the best approach for a specific plant. The main challenge is therefore moving from the general guidance into a practical preservation strategy tailored to each given plant.
The choice of appropriate method is
influenced by several factors, including: ● duration of shutdown; ● restart constraints; ● operating water chemistry and water
treatment plant capabilities, availability of nitrogen or dehumidified air;
● piping layout and materials used in steam/ water piping and tubing;
● condition of unit, particularly cleanliness of heat transfer surfaces;
● local environmental regulations concerning disposal of chemicals; and
● local climate, particularly humidity and likelihood of freezing over the year. Various guidelines provide aids for choosing a layup strategy, whether wet or dry, given the particular plant operating constraints, design and local climate. Finally, whatever the strategy chosen, planning should begin before shutdown. For example, any isolation points, temporary connections, drainage paths, monitoring
categorised typical methods as follows: ● dry nitrogen blanketing (DNB); ● active dehumidification/drying (ADD); ● passive dehumidification/drying (PDD); ● wet preservation (WP); and ● chemical surface protection (CSP).
These are each described in detail below.
Dry nitrogen blanketing (DNB) Dry nitrogen blanketing can be used for steam-side and water-side areas, as well as other fluid systems. It involves draining the components and filling the protected volume with nitrogen, which significantly reduces steel corrosion. Complete drying is not essential, but it is recommended that the volume is dried as far as is practicable before introducing nitrogen.
For larger protected volumes, inherent leakage usually means that an on-site nitrogen generator is required. A purity of 99.5% at a positive pressure of 0.5 barg is generally sufficient for effective protection; higher purities typically require more costly generators to deliver the same volume flow rate. Safety considerations limit use to areas where any entry into enclosed space is strictly controlled.
This is not to be confused with nitrogen blanketing in wet preservation, where nitrogen is used only to fill sections of the boiler that are not filled with water (such as the drums above waterline).
Active dehumidification/drying (ADD) Steel corrosion rates are strongly influenced by the relative humidity, as shown in Figure 6. Maintaining relative humidity below 60% is essential and reducing it to 40% or lower greatly decreases the risk of corrosion. The presence of airborne chlorides and other contaminants, such as sulphates, will increase corrosion rates for a given level of humidity. This implies effective humidity control in coastal or industrial sites is particularly important.
Protection with dry or dehumidified air, generated by industrial rotating desiccant dehumidifiers or similar, offers the advantage of relative simplicity and good corrosion protection if done correctly. The key is to have a flow path through the protected systems from the outlet of the dehumidifier through the system and back into the unit, which operates continuously. This requires installation of temporary ductwork and selection of suitable inlet and
outlet points for the ducts into the system to be preserved (such as HRSG waterside).
Passive dehumidification/drying (PDD) For less critical systems, or where ADD is difficult to implement, potentially because of limited flow paths, equipment distance, or the cost of additional dehumidifiers, protection can be achieved by drying the unit. The required level of effort can be adjusted according to available resources and the importance of the equipment. The most comprehensive approach involves hot draining the unit on the waterside, then drying with a temporary dehumidifier and sealing after desiccants such as quicklime or silica gel have been placed in the protected areas. Monitoring of the relative humidity can be performed and if required, further drying can be achieved with another temporary dehumidifier installation. The benefit of hot draining is that water is removed while the metal temperatures remain high, allowing residual moisture to evaporate and internal surfaces to dry. With fewer resources available, the system can be hot drained and then sealed after the use of desiccants or a portable heater to blow hot/warm air into the space until the relative humidity reaches the 40% target. The simplest method involves hot draining and then leaving the system open to air where possible to allow natural circulation. This is often the only practical option for complex, widely distributed systems such as piping.
Wet preservation
Wet preservation involves keeping the water circuits and vessels filled while adjusting the chemistry prior to shutdown. This typically includes increasing boiler water pH to approximately above 9.6 and maintaining a sufficient oxygen scavenger residual or applying a film-forming substance (FFS). Areas that cannot be kept filled are generally protected with a nitrogen cap.
To ensure mixing and uniform chemistry, the water should be recirculated at regular intervals, for example weekly, with sampling and additional dosing as required. This is difficult to implement for certain sections of the boiler such as evaporators. Installing temporary jockey pumps or injecting nitrogen into the downcomers are two methods that can be used in many cases to create a circulating flow. The preservation
www.modernpowersystems.com | October 2026 | 17 0 20 40 60 Humidity of the air (percent)
Figure 6. Corrosion rate of steels vs humidity in air4
80 100
Corrosion rate
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