A critical issue facing many fossil and gas power plants operating in the United States today is that they are being pressured to operate substantially different from their original base load design. Most traditional power generation facilities were designed on the assumption that they would be operated in a baseload mode or infrequently cycled. However, in response to renewable integration, many power plants are now cycling their units more frequently than designers had intended. Ultimately this results in greater induced thermal stresses, more pressure cycles, more cyclic fatigue damage and overall faster wear and degradation to the critical components. The increased exposure to thermal, mechanical and chemical damage mechanisms as a result of cycling dramatically reduces to life expectancy of a baseload designed power generating facility.
When discussing this topic, I often will use the analogy of driving in a city versus driving on the highway. We could look at two identical brand new vehicles, each right off of the factory floor, same tires, same engine design, same mileage, etc. If we then were to drive one car 100,000 miles in New York City (NYC) and the other 100,000 miles on the highway in Nebraska, chances are the NYC driven vehicle will have incurred much higher maintenance and operation costs over the course of those 100,000 miles than the later. Simplified to this level, even a layperson could conclude that the added “wear and tear” that the NYC driven vehicle sustained could result in the premature failure of critical components, and thereby affecting the safety and reliability of the vehicle’s operation. Not to mention the increased likelihood of operator error.
Similarly, baseload designed generation units responding to the variable renewable energy integration, have incurred “wear and tear” conditions that are increasing the cost of maintenance and operation and reducing the longevity of reliable and safe service hours of the unit. Just as each plant is dispatched to serve the demand of the power grid, each plant can also be expected to adjust output in response to other events on the grid. Should a large power plant go offline, another plant may be dispatched for increased production to account for the loss. Likewise, if wind and solar installations go offline, other resources must be dispatched to adjust to demand. It is this unpredictable fluctuation of renewables that has put pressure on the traditional baseload units to operate in a cyclic mode.
Many different factors play into the increased costs for operation and maintenance due to cycling. However, for purposes of this discussion, let’s take a look at a few of the most significant damage mechanisms driving the increased maintenance cost and premature failure of critical pressure components of a traditional baseload power generating unit.
Thermal Fatigue. The most common problem resulting from cycling is thermal fatigue damage. When a unit reduces load or is brought offline, the systems carrying steam begin to cool which causes a contraction of the piping systems, boiler tubing, and welded attachments, etc. When the unit is brought back up to operating temperature and pressure, these systems experience thermal expansion. The temperature swings associated with the cycling operation, particularly at notch locations, restraint locations and locations with differing wall thickness and materials, are subject to increased thermal fatigue damage susceptibility.
Mechanical Fatigue. Mechanical fatigue in pressure vessels and piping can be caused singularly or in a combination of by the following:
- Pressure cycling: changes in the internal pressure
- Variations in flow: non-uniform steam flow
- System changes: expansion differentials between components and systems attached to them
- External factors: external vibrations produced by valves, compressors, pumps, etc.
During cycling operation, all of the above factors come into play, thereby significantly increasing the mechanical stresses these components and systems are subjected to.
Corrosion. Cycling operation challenges the ability of the plant to maintain proper water chemistry controls which can lead to increased corrosion and accelerated component failure. This mechanism can manifest itself as increased problems with corrosion-fatigue of waterwall tubing, flow accelerated corrosion in piping systems, and stress corrosion cracking in steam turbine components.
Creep-Fatigue. When evaluating the effects of thermal and mechanical fatigue working in conjunction with one another, we should consider the progression of creep damage. Creep is the deformation of a material under load or stress. Under baseload design conditions, the rate at which creep occurs at a given stress is dependent on time and temperature. Increasing time and temperature increases creep rate. On the other hand, under cyclic conditions, the stresses are substantially more considerable, and thereby the progression of creep damage is exponentially influenced. Creep strain differs from plastic strain in that it can occur at stresses well below the yield strength of the material and does not deform the actual metal grains. Creep produces cavities or voids at the boundaries between the grains. As more of these creep voids are produced, they join up and form cracks in the material. These cracks weaken the material and can eventually lead to failures. Figure 1. defines creep by classes depending on the extent of damage to the material. The Wedel-Neubauer damage parameter of creep classification is generally accepted as an accurate correlation between creep damage and remaining service life.
The severity of the impact of these damage mechanisms can be partially mitigated through improved plant operation and process controls, however, it is impossible to avoid the reduced life expectancy of critical components caused by the cycling operation. It is imperative to conduct routine non-destructive examinations of the metallurgical components in your facility that are exposed to high temperatures and pressures to determine if creep damage has begun, and if so what stage it is in. Establishing the stage of creep provides insight into the remaining useful life of the equipment, giving you the ability to be proactive in preventing failures caused by creep damage.
For more information or for questions pertaining to managing the damage of your cycling units, please contact Peter Kennefick at Pkennefick@thielsch.com.