The preservation of the HRSG fluid and steam systems during shutdown periods is an important objective at any power plant. While shutdown the unit is considered to be in “layup”. This is often one of the most neglected aspects of HRSG operations. Improper layup will lead to increased corrosion, which can be the initiator of serious major failure mechanisms that will affect plant availability.
Guidelines for layup practice at thermal power plants have been issued by a number of industry organizations including ASME [1], EPRI [2] and VGB [3]. The ASME guidelines are relatively succinct yet comprehensive. The EPRI layup guidelines are contained in water chemistry guidelines for fossil plants with the view that layup should be a considered an integral part of operating water chemistry. The VGB guidelines focus on case studies from German plants and include a section on gas-side layup. In addition, the HRSG OEM usually provides recommendations on layup in the design documentation provided with the plant.
The goal of unit preservation during layup is achieved by reducing corrosion during idle periods and thereby:
There are five factors that influence the rate of corrosion on carbon steel surfaces:
In layup the goal is primarily to act on the first two factors: reduction of corrosion is accomplished by preventing the simultaneous presence of moisture and oxygen on metal surfaces. This includes surfaces in the feedwater, steam and circulating water piping, steam turbine blades and disc, the condenser and the feedwater heaters.
Pitting, crevice corrosion, stress corrosion and general surface corrosion will occur throughout these systems if metal surfaces are insufficiently protected during the inactive periods. Damage will also occur upon return to service with initiation of corrosion fatigue at pits and deposition of corrosion products on heat transfer surfaces or turbine blades.
Protection of surfaces during layup can be implemented either wet or dry. Wet layup means that water-touched surfaces remain immersed during the shutdown period. Water chemistry is controlled to keep oxygen levels low. Steam-touched surfaces are dried and then protected with a nitrogen blanket, or kept dry using desiccants or by circulating dehumidified air. Dry layup means that all surfaces are dried and then protected with either a nitrogen blanket or with desiccants, or by circulation of dehumidified air. The primary reason for choosing wet layup is that the time to restart the boiler is considerably shorter than with dry layup as the unit does not need to be refilled. Choice of the ideal layup practice depends greatly on the length of the idle period. Most guidelines and industry practice attempt to define these periods. The following is a common approach:
Other factors that influence choice of layup practice include:
The degree to which layup practice at a plant has avoided corrosion can be qualitatively evaluated by monitoring of water quality during HRSG startup. Large increases in feedwater iron and (for mixed-alloy systems) copper concentrations indicate poor layup.
More information:
[1] Consensus for the Lay-Up of Boilers, Turbines, Turbine Condensers, and Auxiliary Equipment, CRTD-Vol.66, ASME 2002.
[2] Cycling, Startup, Shutdown, and Layup Fossil Plant Cycle Chemistry Guidelines for Operators and Chemists, TR-107754, EPRI, August 1998.
[3] Guideline for the Preservation of Power Plant Systems, VGB-R116He, 1981, VGB Powertech.
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