Fast-starting combined-cycle plants are designed for a certain operating life based on a customer-specified set of operating scenarios. During that design phase, periodic inspection and maintenance procedures to benchmark equipment actual wear and tear should be developed, but seldom are. Without an accurate assessment of remaining equipment life for components subjected to fast and frequent start-ups and shutdowns, the real operation and maintenance cost is only a guess.
Combined-cycle plants installed in the mid-1990s to the early 2000s were largely designed to operate at base-load, with relatively few planned starts each year. However, deregulation of the electricity markets and the rising cost of natural gas relegated many of those plants to daily cycling to shave load peaks. Some owners were even “two-cycling” their plants: covering the weekday morning peak, shutting down, and restarting to cover an afternoon or evening peak. These unplanned operational modes caused much anxiety among plant owners and operators because daily cycling increased the wear and tear on components, further driving up their cost to generate electricity. Few plant owners have quantified the effect of increased cycling on the remaining life of components and the “hidden” economic penalties.
The design of every heat-recovery steam generator (HRSG) begins with a life-cycle analysis (LCA) using very specific plant operating procedures and owner-specified operating profiles that include number of starts, start-up ramp rates, number of load swings and shutdowns, and operating pressures (Table 1). In actuality, few owners are capable of accurately projecting these statistics over a 30-plus-year life cycle. The assumed inspection and maintenance programs in the conventional LCA for the typical combined-cycle plant are based on fixed periodic intervals and statistical information derived from baseloaded units. But an inspection and maintenance program for any unit should take into account the actual operation of the HRSG, not merely the theoretical operating profile. Deviate from those design assumptions, and the actual life of critical components may be severely compromised.
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| Table 1. The customer specifies the plant operating profile. The heat-recovery steam generator (HRSG) purchaser will normally specify the operating profile of the equipment purchased. A typical operating profile of a 2 x 2 combined-cycle plant with supplemental HRSG firing is illustrated. Source: Vogt Power International Inc. |
A rational approach for determining remaining equipment life is to first develop a methodology that will identify where damage will likely occur in the plant, quantify the impact of that damage on equipment life, determine new operational limits to minimize equipment damage, and then estimate the economic impact of those measures. The purpose of this article is to demonstrate this analysis approach on the HRSG portion of the combined-cycle plant and show that this approach produces data that can be used to optimize operation and maintenance (O&M) costs on a life-cycle basis.
Cycling and Fast Starts
Cycling occurs when units are required to be brought online and shut down to meet grid demand and to provide the owner/operator with the most cost-effective operating profile. When cycling, units are generally kept online for short durations, usually a few hours or a few days. For example, the units may be brought online to meet excess demand during peak hours and then shut down overnight. This is regular cycling in the sense that the time and duration of operation is well defined and the schedule is predictable.
Sometimes the units meet electricity demand when a baseload unit is down for maintenance or repairs. If the maintenance is regularly scheduled, then the cycling unit’s operation is also predictable. However, there may be times when the regular baseload units are shut down for a forced outage. Forced outages, by definition, are unpredictable in both occurrence and duration.
Another type of cycling occurs when owners want to take advantage of a market price opportunity. Power grids buy power daily, based on the market price. Power generators may want to start a unit on very short notice if the market price will result in what they believe will return a quick profit.
Start-up time is the most important, and most controllable, statistic for cycling units. If the start-up time is predictable, then units can be started with well-defined and optimized operating procedures in advance of the demand. However, unscheduled start-ups may be tied to a particular power market opportunity offering potential extra revenue. If the unit starts up more quickly, then more revenue can be generated by supplying the power sooner. Thus, faster starts are desirable to reduce start-up costs or maximize revenue potential.
Cycling and faster starting may generate additional revenue, but doing so also increases the life consumption of an HRSG. Any baseload-designed unit can last the predicted life in cycling mode if it is started slowly enough. The pressure in the HRSG can be ramped up at a specified rate that ensures all components are not adversely affected. However, these specified ramp rates may be too slow and thus take too much time for starting, or they may simply be too slow to be practical for today’s fast-moving electricity markets. Fast starting damages the unit because rapid ramping produces much higher stresses, causing faster unit deterioration.
It is not practical to operate baseload-designed units under cyclic conditions without a greater, and unknown, level of deterioration of HRSG components. Few owners know how much additional life is consumed during such an event, and fewer still add into their market bid price an amount of money that represents the amortized value of the equipment loss of life. The question becomes not whether there is an unknown amount of accelerated HRSG damage occurring by cycling any combined-cycle plant but how much and where the damage occurs.
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