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Table 12-10 Typical Manufacturer.s Fuel Data on Total Installed Horsepower
Fuel .nits . . Hours of Operation . Total hp
Natural gas 60 80 90 Dual fuel 22.5 8 4.0 Distillate oil 15 6 0.6 Residual oil 2.0 5 5 Crude oil .2 .5 0.4 Other 0.3 .5 -
treatment wasinitiated. Eventually， the corrosion-causing materials werediscovered， and a fuel treatment system to limit corrosion was developed.
Power plants in both peaking and standby modes achieved30，000 hours between major overhauls. It was during these operations that the depositproblem on the turbine nozzles became apparent. Also， deposits developedon the fuelnozzles， a situation that could cause deviation in the fuel sprayangle and related combustion problems. Therefore， both turbine and fuel nozzles needed frequent cleaning.
As discussedearlier， economic situations heavily dictate fuel selection. Afterthe surge of interest in gas turbines in the early1950s， use in the1960s dwindled because of thecost， problems， and availability of natural gas. The 1990s have seen a tremendous growth in gas turbine usage with theadvent of the high-efficiency gas turbines (40-45%)， being used in CombinedCycle Power Plants， which have plant efficiencies between 55-60%. Most ofthese turbines were all driven by natural gas. In 2000-2001， gas turbines are backordered for the next three to five years. All this growth in the turbine has been fueled by cheap natural gas at .3.50/mmBTU (.3.32/mmkI). Thecost of natural gas in late 2001 is heading to .9.0/mmBTU (.8.53/mmkI)， this will make alternative fuels interesting once again. Table 12-10 is anestimate of the world population of gas turbines， and it reflects the growth of natural gas driven gas turbines in the late 1990s and early 2000s.
Heat Tracing of Piping Systems
As mentioned earlier， heavy fuels need to be kept at a temperature where the viscosity of the fuel is limited to 20 centistokes at the fuel nozzles. Heat tracing is used to maintain pipes and the material that pipes contain at temperatures above the ambient temperature. Two common uses of heat tracing are prevent-ing water pipes from freezing and maintaining fuel oil pipes at high enough temperatures such that the viscosity of the fuel oil will allow easy pumping. Heat tracing is also used to prevent the condensation of a liquid from a gas.
A heat-tracing system is often more expensive on an installed cost basis than the piping systemit is protecting， and it will also have significant operation costs. A recent study on heat-tracing costs showed installed costs of .31/ft (.95/meter) to .142/ft (.430/meter) and yearly operating cost of .1.40/ft (.4.35/meter) to .16.66/ft (.50/meter). In addition to being a majorcost， the heat-tracing system is an important component of the reliability of a piping system. A failure in the heat-tracing system will often render the piping system inoperable. Forexample， with a water freeze protection system; the piping system may be destroyed by the expansion of water as it freezes if the heat-tracing system fails.
The vast majority of heat-traced pipes are insulated to minimize heat loss to the environment. A heat input of 2 to 10 watts per foot (6 to 30 watts per meter) is generally required to prevent an insulated pipe from freezing. Withhigh windspeeds， an un-insulated pipe could require well over 100 watts per foot (300 watts per meter) to prevent freezing. Such a high heat input would be very expensive.
Heat tracing for insulated pipes is generally only required for the period when the material in the pipe is not flowing. The heat loss of an insulated pipe is very small compared to the heat capacity of a flowing fluid. Unlessthe pipe is extremely long (several thousands of feet or meters)， the tempera-ture drop of a flowing fluid will not be significant.
The three major methods of avoiding heat tracing are:
1. Changing the ambient temperature around the pipe to a temperature that will avoid low-temperature problems. Burying water pipes below the frost line or running them through a heated building are the two most common examples of this method.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(76)
