Comparison of impulse and reaction turbines

Applications of the principles generally called reaction and impulse often appear in the same turbine, and either is adopted without hesitation as may best suit a particular design, size of unit, or place in a unit. Impulse turbines of low output are less costly and more efficient than reaction turbines, but with increase of size these differentials disappear, and in the large sizes there is little to choose as to efficiency or as to cost for equal efficiency. High efficiency is expensive in either type, but it cannot be said that either has any absolute superiority.

COMMENTARY

comparison – порівняння

hesitation – коливання

Comparison of steam turbines with steam engines

The advantages of large steam turbines over large reciprocating engines are very great. They occupy less space, permit smaller buildings and foundations, their original and maintenance costs are less, their economy is greater (in part owing to their ability to use high vacua and high superheats), and they may be built in units of far greater output, reducing greatly the cost of attendance. Engineers engaged in designing small plants often find, however, that is there preferable to use reciprocating engines, on account of the better economy of small engines than small turbines and the disadvantages of the reduction gearing usually necessary in applying small high-speed turbines to industrial uses. The reverse is usually true, however, and reciprocating engines of all sizes have been scrapped and replaced by turbines long before their useful lives were over. When it is admitted therefore that certain difficulties still meet the designer, the builder, and the operator of steam turbines, it is not be understood that the overwhelming superiority of the turbine to the reciprocating engine for most power-plant purposes is in any way questioned.

COMMENTARY

attendance – супровід

scrap – шматочок

Steam turbine operation and maintenance

The operator is usually as much interested In reliability as he is in economy, and the removals of turbines from service are due more to “rubs” or their effects than to any other cause lying in the turbines themselves. The speed of wheel rims relative to the adjacent stationary parts varies from 400 to 1,500 or more feet per second; if contact does occur, so much heat is generated almost instantaneously that the deflection which caused the initial rub is apt to be increasing owing to warping of the unevenly heated metal, sometimes resulting in the complete destruction of the unit. These rubs may result from one of several causes, even in a turbine which is properly designed in the first place: (a) improper adjustment of the rotor in its axial position; (b) unbalance of the rotor, producing excessive vibration and deflection; (c) sudden wear of the thrust bearing which holds the rotor in axial location or of the bearings supporting the rotor, caused by failure of the lubricating system; and (d) rapid changes in temperature produced by sudden changes in load conditions, causing unequal expansion.

In improperly designed or constructed turbines, the relative expansion of the rotor and casing may be so much that with the clearance originally provided, it is impossible to have any adjustment that will surely prevent rubbing under all operating conditions. Also the discs or diaphragms may be mechanically weak and may deflect under centrifugal force or steam pressure, respectively. The wheels particularly will also vibrate under certain conditions which must be avoided.

Corrosion and erosion of the blades and nozzles will usually necessitate their replacement after a period of service whose length depends on the conditions of operation. In some cases, this has not happened within the commercial life of the machine; i. e., the machine has become obsolete. The economy of units seems to hold up fairly well through years of service, corrosion and erosion causing a small reduction with age as does the increased leakage due to increased clearance.

Mechanical losses are small; hence the mechanical efficiency is high; and while perfection of the lubricating system is practically essential for successful turbine operation, when once it is attained, bearing troubles and adjustments are infrequent. Care of the unit resolves itself largely into careful watching of the oil temperatures and proper care in cleaning and renewing the oil.

With the steady increase of the initial pressure and temperature of the steam supplied to turbines have come difficulties due to clogging of blade and nozzle passages by solids carried over from the steam generators. More or less effective remedies have been found, but reliable preventatives are still being sought.

COMMENTARY

removals – видалення

adjacent – суміжний

uneven – нерівний

infrequent – рідкісний

clog – груз, перепона