Turbine case

Larger tubular, or single-can, units usually have more than one nozzle. In many cases a ring of nozzles is placed in the primary zone area. The radial and circumferential distribution of the temperature to the turbine nozzles is not as even as in tubo-annular combustors. In some cases, high stresses are exerted on the turbine casing leading to casing cracks. 

By considering the flow as stopped but the turbine casing full of liquid, it is intuitively obvious that to rotate the wheel or impeller in either direction power will have to be put in. As the flow increases... 

At constant displacement, creep causes stresses to relax with time. Bolts in hot turbine casings must be regularly tightened. Plastic paper-clips are not, in the long term, as good as steel ones because, even at room temperature, they slowly lose their grip. 

Gas versus steam turbines can involve a major side study. The result can be different for the process and utility sides of the plant as shown in Reference 13. For the gas turbine case, simple cycle versus waste heat boilers can be studied. Usually, waste heat boilers will win out unless the plant is in a cheap gas country. If gas turbines are selected for power generation, black start capability is usually a good investment. 

A safety valve is usually needed on the steam exhaust side of the turbine to protect against high pressure on shut down. Most turbine case designs will not safely handle inlet steam pressure on the exhaust side, as the case is not designed to withstand intake pressure throughout. These valves are normally rated for 110% of the design steam rate. 

Air-cooled surface condensers. Figure 8.11 shows a surface condenser elevated above the steam turbine. This creates an additional problem, in that moisture from the turbine exhaust steam will accumulate in the bottom of the turbine case. A special drain line from the turbine s case is needed to prevent condensate backup from damaging the spinning wheels. 

One such turbine, in a refinery near London, would not drain properly, In order to push the condensate out of the turbine case, the operators were forced to raise the surface condenser pressure from 100 to 250 mm Hg (i.e., 20 in of mercury vacuum, in the American system). Note that the balance line shown in Fig. 8.11 keeps the pressure in the turbine case and the condensate drum, into which the turbine case is draining, both equal at the same pressure. 

The turbine case pressure was increased by raising the pressure in the air-cooled surface condenser. This was accomplished by shutting off several of the air fans, which, in turn, increased the condensing temperature of the exhaust steam. But why would raising the turbine case pressure drain the turbine, anyway After all, increasing the surface condenser pressure also increased the pressure in the drum that the turbine case drained to. 

The answer is revealed when the available data are converted to consistent units. The 250 mm Hg turbine case pressure is equal to a height of water of 11 ft. Atmospheric pressure in London was 14.5 psia that day, which is equal to 33 ft of water. The difference between atmospheric pressure and the pressure in the turbine case, expressed in feet of water, is then... 

The 300-psig steam next passes through the steam nozzle. This is an ordinary nozzle. It screws into a hole in the wall, which separates the steam chest from the turbine case. The nozzle is shaped to efficiently convert the pressure of the 300 psig to steam velocity. The pressure of the steam, as soon as it escapes from the steam nozzle, is already the same as the exhaust steam pressure (100 psig). 

We could shut down the turbine and unbolt the steam chest, to expose the nozzle block, which is the wall that separates the steam chest from the turbine case. We could unscrew the existing nozzle, and replace it with a smaller nozzle. A nozzle of 20 percent less diameter would reduce the nozzle cross-sectional area by 36 percent ... 

Having replaced the loop seal piping, (some units use a steam trap instead of this loop seal), I started steam flow to the turbine. But the vacuum in the surface condenser, which had started out at an excellent 27 in Hg, slipped down to 14 in Hg. This loss in vacuum increased the backpressure in the turbine case. The higher pressure in the turbine case reduced the velocity of the steam striking the buckets on the turbine wheel, which reduced the amount of work that could be extracted from each pound of steam. 

The gas that accumulates inside the surface condenser is called the noncondensable load to the steam jets. Some of the noncondensable load consists of C02 accidentally produced when the boiler feedwater is vaporized into steam. Air leaks through piping flanges and valves are other sources of noncondensable vapors. But the largest source of noncondensable vapors is often air drawn into the turbine case, through the shaft s mechanical seals. To minimize this source of leaks, 2 or 3 psig of steam pressure is ordinarily maintained around the seals. However, as the turbine s shaft seals deteriorate, air in-leakage problems can overwhelm the jet capacity. This will cause a loss of vacuum in the surface condenser. 

TURBINE ROTOR - The rotating assembly enclosed within the turbine casing... 

As mentioned above, the missile due to plant turbine case burst is covered by the design basis for the aircraft impact. This event is also made unlikely by the radial placement of the turbine axis with reference to the important plant buildings. 

Such an extended description is in line with the approach to risk analysis described by Aven (2008a 2008b). Figure 3 illustrates the main features of the alternative approach, applied to the ageing turbine case. 

The results from the uncertainty assessment in the turbine case are presented in Table 1. 

Piping, tubing, boiler drums, turbine casing, pressure vessels, turbine rotors, pumps Turbine rotors, high-temperature piping and tubing, cast and forged pressure vessels... 

A hot-gas-path inspection includes disassembly of the turbine casing. A major inspection includes a disassembly of the compressor casing as well as the turbine casing. A major inspection essentially returns the gas turbine to its new, or zero time, condition. For an MS7000 operating on natural gas or distillate, combustion, hot-gas-path, and major inspections occur at 8,000-, 24,000-, and 48,000-fired-hour intervals, respectively. 

Uncontrolled extraction An opening in a steam turbine casing between two stages. The pressure of the extraction steam available is uncontrolled and is a function of the steam flow to the following stage. 

If a turbine disc were to fail and if a large portion of the disc were to be ejected from the turbine casing, it would be possible for the turbine missile to strike and cause damage to components or systems with safety functions. Depending on how the turbine is situated, a missile could also cause dzimage to the control room. 

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