Turbine service temperature

High strength and toughness at service temperatures up to 1200°F. Steam turbine blades, fasteners. 

Figure 7.43 NASA scenario of development trends in in-service temperature of aerospace gas turbines (BMFT, 1994).

C, which is considered typical of the metal temperature that components would experience in gas turbine service. Hot eor-rosion tests for up to 100 h at 700 °C, involving oxidation in air of samples coated with sea salt, indicated an increase in metal consumption rate by a factor of up to 20 compared with air oxidation. The mechanism of hot corrosion described by Nicholls et al. (1997) involved interactions among the deposited salt, the predominantly Ti02 scale and the alloy substrate to form volatile metallic halides, which were subsequently oxidized (pyrohydrolyzed) to form non-protective oxides on the outer surface of the scale. In addition, the salt deposit became enriched in chlorides so that its melting temperature was lowered, allowing it spread laterally over the surface and increase the area susceptible to attack. 

The main alloying elements in these Ni- or Ni(Co)-base alloys are Cr, A1 and Ti. Depending on the specific applications, the alloys contain additional /-stabilising elements such as Ta, Nb, and/or solid solution strengtheners such as W, Mo, and Re. The alloys are mainly used as blade and vane materials in stationary gas turbines and aero engines. Typical service temperatures are 850-1100°C. As a reference material in COTEST the material CM247 was tested. 

To achieve higher efficiency and performance of gas turbine engines, the inlet gas temperature should be inaeased. Currently, the operating temperature on high pressure turbine engines is approaching 1150°C, which is very close to the maximum melting point of the most advanced nickel-based superalloys ( 1350°C). The service temperature of intermetallic compounds based on Ni-Al and Ti-Al cannot meet the requirements for use in the hot part of... 

Appllca.tlons. The principal appHcations of nickel-base superalloys are in gas turbines, where they are utilized as blades, disks, and sheet metal parts. Abcraft gas turbines utilized in both commercial and military service depend upon superalloys for parts exposed to peak metal temperatures in excess of 1000°C. Typical gas turbine engines produced in the United States in 1990 utilized nickel and cobalt-base superalloys for 46% of total engine weight (41). However, programs for future aerospace propulsion systems emphasize the need for lightweight materials having greater heat resistance. For such apphcations, intermetallics matrix composites and ceramic composites are expected to be needed. 

Polyol ester turbine oils currendy achieve greater than 10,000 hours of no-drain service in commercial jet aircraft with sump temperatures ranging to over 185°C. Polyol esters are made by reacting a polyhydric alcohol such as neopentyl glycol, trimethylol propane, or pentaerythritol with a monobasic acid. The prominent esters for automotive appfications are diesters of adipic and a2elaic acids, and polyol esters of trimethylolpropane and pentaerythritol (34). 

The rate of heat-transfer q through the jacket or cod heat-transfer areaM is estimated from log mean temperature difference AT by = UAAT The overall heat-transfer coefficient U depends on thermal conductivity of metal, fouling factors, and heat-transfer coefficients on service and process sides. The process side heat-transfer coefficient depends on the mixing system design (17) and can be calculated from the correlations for turbines in Figure 35a. 

Chemically Functional. Refractory coatings are used for corrosion-resistant high temperature service in gas turbine and diesel engines, components such as cmcibles, thermocouple protection tubing, valve parts, etc. 

In the last chapter we said that one of the requirements of a high-temperature material - in a turbine blade, or a super-heater tube, for example - was that it should resist attack by gases at high temperatures and, in particular, that it should resist oxidation. Turbine blades do oxidise in service, and react with H2S, SO2 and other combustion products. Excessive attack of this sort is obviously undesirable in such a highly stressed component. Which materials best resist oxidation, and how can the resistance to gas attack be improved ... 

It is not known to what extent each of the previous mechanisms contributes to turbine blade degradation during service. It is also probable that each alloy will respond differently to a particular temperature/stress combination. Figure 21-12 shows the typical variation in stress/rupture life determined at 1350°F (375 °C) with service time for forged Inconel X-750 blades. 

For locations in the Middle East subject to wide fluctuations in ambient temperature it has been common to utilize multiple gas turbines which are sized to meet the plant demands during the hot summer months and the cold winter. As the gas turbine output increases substantially, there is sufficient spare capacity to allow for outage of machines without affecting the electrical power export. However, this situation is unique to the environmental conditions and type of equipment in service. 

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