All About Engines

Steam Turbines 169 the wheels need not run very closely either to the fixed blades or to the casing. But in the reaction turbine, in which the expansive force of the steam is being utilised, leakage past the blades must be, as far as possible, avoided. A slight endwise movement of the shaft, or an increase in the temperature of the superheated steam, may lead to what is called “ strip- ping,” that is, both fixed and moving blades are torn from their grooves and deposited in a heap at the bottom of the casing. While the impulse tur- bine was not free from such accidents in the early days, they were very serious and not infrequent causes of failure of reaction turbines. In fact, the huge drum, weighing often several tons, and the thin blades a few inches in length and a fraction of an inch in thickness, form an unusual combination of strength and weakness, and much ingenuity has been exercised in overcoming the mechanical defects which were so apparent in the early days. The result is that all parts upon which superheated steam plays are now made of steel, and the blades are invariably thinned off at the outer ends, so that if by any mis- chance they do come into contact with the interior of the casing they are rubbed over and not torn out of their sockets. So much for the principle involved. Parsons’ first turbine, constructed in 1884, was of only 6 horse-power. It drove a dynamo, and after run- ning for several years it was deposited in the South Kensington Museum. It was the first turbine in which the drop in pressure was distributed over a series of