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The effect of firing temperature is very predominant-for every 100 0F
(55.5 0C) increase in temperature， the work output increases approximately 10% and gives about a 1-1坦2 % increase in efficiency. Higher-pressure ratios and turbine inlet temperatures improve efficiencies on the simple-cycle gas turbine. Figure 1-6 shows a simple cycle gas turbine performance map as a function of pressure ratio and turbine inlet temperature.
Another way to achieve higher efficiencies is with regenerators. Figure 1-7 shows the effects of pressure ratio and temperatures on efficiencies and work for a regenerative cycle. The effect of pressure ratio for this cycle is opposite to that experienced in the simple cycle. Regenerators can increase efficiency as much as 15-20% at today"s operating temperatures. The optimum pres-sure ratios are about 20:1 for a regenerative system compared to 40:1 for the

1800 2000 2200 2400 2600 2800 3000

 

Figure 1-.. Performance map of a simple cycle .as tur.ine.
50.00
45.00
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
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Figure 1-.. Performance map of a re.enerative .as tur.ine.
simple-cycle at today"s higher turbine inlet temperatures that are starting to approach 3000 0F (1649 0C).
High availability and reliability are the most important parameters in the design of a gas turbine. The availability of a power plant is the percent of time the plant is available to generate power in any given period. The reliab-ility of the plant is the percentage of time between planed overhauls.
The Availability of a power plant is defined as
A二(P -S -F)/P (1-1)
where:
P二Period oftime，hours， usually this is assumed as oneyear， which
amounts to 8760 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
R二(P -F)/P (1-2)
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 nonpeak periods. Peak periods are when the majority of theincome is generated， as usually there are various tiers of pricing depending onthe demand.孔any power purchase agreements haveclauses， whichcontain capacity payments， thus making plant availability critical in the economics of the plant.
Reliability of a plant depends on many parameters， such as the type offuel， the preventive maintenance programs， the operatingmode， the controlsystems， and the firing temperatures.
To achieve a high availability and reliabilityfactor， the designer must keepin mind many factors. Some of the more important considerations， which governthedesign， areblade and shaft stresses， bladeloadings， materialintegrity， auxiliary systems， and control systems. The high temperatures required for high efficiencies have a disastrous effect on turbine blade life. Proper cooling must be provided to achieve blade metal temperatures between 1000 0F (537 0C)， and 1300 0F (704 0C) below the levels of the onsetof hot corrosion. Thus， the right type of cooling systems with proper blade coatings and materials are needed to ensure the high reliability of a turbine.
Serviceability is an important part of anydesign， since fast turnarounds result in high availability to a turbine and reduces maintenance and oper-ations costs. Service can be accomplished by providing proper checks suchas exhaust temperature monitoring， shaft vibration monitoring， and surgemonitoring. Also， the designer should incorporate borescope ports for fast visual checks of hot parts in the system. Split casings for fast disassembly，field balancing ports for easy access to the balanceplanes， and combustorcans， which can be easily disassembled without removing the entire hot section are some of the many ways that afford the ease of service.
　
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