HRSG Preservation During Shutdowns: A Key Issue Often Neglected

Tetra Engineering in Modern Power Systems

Misha Gadher, Mechanical and Thermal Process Engineer, and James Malloy, Senior Consultant, both of Tetra Engineering, have written on HRSG preservation for the October 2026 issue of Modern Power Systems. The article looks at one of the most neglected parts of HRSG operation: protecting the plant while it isn't running.

Why preservation matters more now

Owners face more uncertainty about when their units will be needed. Some plants have moved from baseload to cycling. Others sit on standby for long periods before being called back. Every idle period is a chance for corrosion to start.

What goes wrong

Without proper layup, both the gas side and the water side are exposed. The damage can be severe:

• Pitting needs stagnant water, so it happens almost only during shutdown. It can progress fast enough to penetrate a tube wall in a few weeks.
• In cycling units, a pit that forms in layup can act as a stress riser on restart. A corrosion-fatigue crack grows from it and can eventually fail the tube.
• General corrosion produces large amounts of iron oxide. On restart, it's carried onto heat transfer surfaces, where it cuts heat transfer and gives impurities a place to concentrate.

Tetra recently assessed an HRSG that had been relocated and left without any preservation for several years. Advanced corrosion was evident, including on pressure parts.

Five preservation methods

• Dry nitrogen blanketing: suits boilers, HRSG pressure parts and piping. Needs a sealed system.
• Active dehumidification: suits turbines, gas paths and large volumes. Needs an airflow path.
• Passive drying: suits less critical or complex systems, with less positive control.
• Wet preservation: suits shorter outages and rapid restarts. Depends on water chemistry and recirculation.
• Chemical surface protection: suits small, local or hard-to-reach areas. Check compatibility and cost.

Humidity is the key number for dry methods. Keeping relative humidity below 60% is essential, and getting it to 40% or lower greatly reduces the risk of corrosion.

No single method fits a whole plant

The right choice depends on shutdown length, restart needs, equipment size and the resources on site. If a unit will be out for more than a week, long-term wet or dry layup should be considered, and planning should start before shutdown. Whatever the method, its effectiveness should be checked by monitoring humidity, nitrogen pressure or water chemistry throughout the outage.

In one recent case, Tetra developed a strategy for a large coastal combined cycle plant facing a six-month shutdown, where humidity and airborne chlorides raised the risk of corrosion. Each component got the method that suited it. The steam and gas turbines and the HRSG gas paths were dehumidified. The HRSG pressure parts were blanketed with nitrogen. The piping used nitrogen where it could be sealed, with passive drying or chemical protection elsewhere.

Read the full article

HRSG preservation during shutdowns: a key issue often neglected (Modern Power Systems, October 2026)

Planning a shutdown?

Tetra Engineering develops preservation strategies tailored to the expected shutdown length, equipment condition and site infrastructure. The one-day course Methods and Strategies for Effective Power Plant Preservation can also be run on site.

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