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Spare Parts Inventory
The problem of spare parts is an inherent phase of the maintenance business. The high costs of replacementparts，delivery， and in someinstances， poor quality， are problems faced daily by everyone in the main-tenance field. The cost of spare parts for a major power plant or refinery runs into many millions of dollars.
The inventory of these plants can run into over20，000items， including over 100 complete rotor systems. The field of spare parts is changing rapidly and is much more complex than in the past. A group of plants have gotten together in a given region and formed ""Part Banks.""
Many pieces of equipment are made up of unitized components from several different vendors. The traditional attitude has been to look to the packaging vendor as the source of supply. Many vendors refuse to handle requests for replacement parts on equipment not directly manufactured by them. More and more specialty companies are entering the equipment parts business; some are supplying parts directly to OEM companies for resale as their ""own"" brand. Others supply parts directly to the end user. The end user must develop multiple sources of supply for as many parts as possible.
Gaskets， turbine carbonpacking， and mechanical seal parts can be pur-chased from local sources. Shafts， sleeves， cast parts can be purchased from local sources.Shafts，sleeves， cast parts such asimpellers， are becoming increasingly available from specialty vendors. All this competition is causing the OEM"s to alter their spare parts system to improve service and reduceprices， which is definitely a bright spot in the picture. The quality control ofboth OEM and some specialty houses leaves much to be desired. Inturn， this causes many plants to have an in-house quality control person checking allincoming parts， a concept highly recommended.
.ondition and Life .ssessment
Condition and life assessment is significant for all types of plants， and especially Combined Cycle Power Plants. The most important aspect of aplant is high availability， and reliability， in some cases even more significant than higher efficiency.
The availability of a power plant is the percent of time the plant is available to generate power in any given period. The reliability of the plant is the percentage of time between planed overhauls.
The availability of a power plant is defined as
A二 P ι S ι F (21-1)
P
where:
P二Period oftime，hours， usually this is assumed as oneyear， which
amounts to8，.60 hours
S二 Scheduled outage hours for planned maintenance
F二 Forced outage hours or unplanned outage due to repair
The reliability of a power plant is defined as
二 P ι PF (21-2)
Availability and reliability have a very major impact on the plant econ-omy. Reliability is essential in that when the power is needed it must be there.When the power is not available it must be generated or purchased， and can be very costly in the operation of a plant. Planned outages are scheduled for non-peak periods. Peak periods is when the majority of the income is gener-ated as usually there are various tiers of pricing depending on the demand. Many power purchase agreementshaveclauses， which contain capacitypayments， thus making plant availability critical in the economics of the plant.
Gas turbines with the new technology， higher pressure ratio and higherfiring temperature， has led to the building of large gas turbines producing
96 85
45
35
Below 100 MW Above 100 MW
Figure 21-5. Comparison of availability and efficiency for large frame type gas turbines.
nearly 300 MW and reaching gas turbine efficiencies in the mid forties. Theavailability factor for units with mature technology， below 100 MW， arebetween 94-9.%， while the bigger units above 100 MW have availabilityfactors of 85-89%. The bigger units produce twice the output， but the avail-ability factor has decreased from 95% to 85%. A decrease of .-10 points for all manufacturers. Part of this decrease may be related to larger machinery taking more time to repair. It is also due to the high temperature and pressure.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(79)
