The development of modern-day, high-efficiency vapor turbines has actually generated a rise in deposition, erosion, and deterioration issues. Close tolerances into the turbines, the use of high-strength steels, and impure steam all donate to these problems.
Although a number of factors manipulate the forming of deposits on turbine elements, the general result is the same whatever the cause. Adherent build up form in the vapor passageway, distorting the original form of turbine nozzles and blades. These deposits, usually rough or uneven at the area, enhance resistance to your circulation of vapor. Distortion of vapor passages alters vapor velocities and pressure drops, reducing the capacity and efficiency for the turbine. Where conditions tend to be serious, build up could cause exorbitant rotor thrust. Uneven deposition can unbalance the turbine rotor, causing vibration dilemmas.
As build up gather on turbine blades, stage pressures enhance. Figure 18-1 shows the result of gradual deposit accumulation on-stage pressure. The deposits were due to the usage of contaminated water to attemperate the steam. In a fouled condition, this 30-MW turbine destroyed over 5% of its creating ability.
Turbine build up can build up really small amount of time when vapor purity is bad. The turbine shown in Figure 18-2 had been required off-line by deposition just 3 months after it was put in operation. Carryover of boiler liquid, resulting from inadequate steam-water separation gear in boiler, caused this turbine deposit issue.
The type of silica build up entirely on turbine blades differs. Dining table 18-1 lists several silica substances which were identified in a variety of studies of turbine blade deposition. Of those, amorphous silica (SiO2) is the most predominant.
Dining Table 18-1. Silicate build up within vapor turbines.
Reasons for Turbine Deposition
Entrainment. Some mechanical entrainment of small falls of boiler liquid in the steam always happens. When this boiler water carryover is extortionate, steam-carried solids create turbine blade build up. The accumulations have a composition like the mixed solids in the boiler water. Priming and foaming are typical factors that cause large amounts of boiler liquid carryover. Because of the high quantities of carryover usually experienced, these circumstances usually trigger superheater tube failures as well.
Attemperating Water Impurity. Turbine build up will also be due to making use of impure water for steam attemperation by leakage in closed temperature exchangers utilized for attemperation. If a boiler produces pure steam and turbine deposits still take place, the attemperating system is examined as a possible supply of contamination. Attemperating water must be of the identical purity given that vapor. Any chemical therapy into the attemperating liquid should really be volatile.
Vaporization of Boiler Water Salts. Another supply of turbine deposition is the vaporization of salts within boiler water. With the exception of silica, vaporization of boiler water salts is normally perhaps not significant at pressures below 2400 psig. Silica can vaporize to the steam at operating pressures as low as 400 psig. This has triggered deposition dilemmas in several turbines. The solubility of silica in steam increases with additional temperature; for that reason, silica gets to be more dissolvable as vapor is superheated. As vapor is cooled by growth through turbine, silica solubility is paid off and deposits tend to be created, frequently where in fact the vapor temperature is below that of the boiler water. To reduce this dilemma, the number of silica when you look at the steam needs to be managed. Silica deposits are not problematic in many turbines where in fact the silica content into the steam is below 0.02 ppm. Therefore, it offers become customary to limit silica to under 0.02 ppm in the steam. Sometimes, because of the much more strict operating problems of certain turbines, sellers specify that steam silica be maintained at significantly less than 0.01 ppm.
Erosion of turbine blades causes rough, uneven areas that alter steam circulation paths. This decreases turbine performance and certainly will in addition limit ability. Erosion during the high-pressure end of a turbine is generally caused by solid particles (usually iron oxide) contained in the steam. Iron oxide particles can be found should they were not removed by vapor hits during system start-up. They may be able also be a consequence of exfoliation of superheater or primary vapor header oxides or is introduced in to the vapor by contaminated attemperating liquid.