Present Limits of Speed and Power of Single-Shaft Steam Turbines

This paper will be restricted to a discussion of some of the factors which influence limits as applying particularly to turbines of the reaction type. With the employment of high vacua the limit of power will be determined largely by the area obtainable through the last stage. Limiting factors include: 1. Chosen maximum values of steam speed through the blades in order to keep the leaving losses within permissible limits. For highest efficiency the steam speed should be about 25 per cent greater than the blade speed, but in the last stages it is sometimes made 100 per cent greater as a compromise between efficiency and cost. Similarly the outlet angle of the blades is increased from 20 deg. to 35 deg. 2. Physical characteristics of materials employed and chosen limits to which these may be safely stressed. By varying the form of rotor construetion stresses in it may usually be kept within necessary limits up to the point at which limiting stresses in the blades or blade fastenings are reached. For any given rotative speed and blade angle the steam capacity is directly proportional to the stress at the base of the blades regardless of the diameter and blade height. 3. Capacity limits of manufacturing facilities, increased bracing and clearances necessary to insure requisite rigidity and reliability, and capacity loss due to outages for inspection and repair. Fig. 6 shows maximum capacity at various speeds which are physically possible without exceeding present limits of stresses.