« PreviousContinue »
is absorbed by a rotary water friction brake mounted in place of the generator and operated precisely as the ordinary prony brake. The machines are first run under load to adjust the governor and determine the speed regulation, after which economy tests are conducted under various loads stipulated by contract. These are run in the presence and under the supervision of the purchaser, or his representative. Every machine must show results within the guarantees before final inspection for shipment.
In adhering to this individual testing, Westinghouse practise may be considered as standing alone. Efficiency tests involve considerable time, expense and plant investment, yet the builders have considered themselves well paid in the knowledge that every machine shipped has conformed to the necessary specifications. Other builders rely upon the chance of the purchaser running his own tests. In the great majority of cases, this is never done, aud even if so-called tests are made, there is usually room for discussion, owing to the scant facilities for carrying out such work. Great stress is often laid upon the value of a test in which power is measured entirely at the generator terminals. A moment's reflection, however, will prove the fallacy of this and reveal the simple fact that the vertical turbine cannot be readily tested by itself. Modern generator practice is so thoroughly standardized and the variation in generator efficiences is confined within such a small limit that the only uncertainty involved is the efficiency of the turbine. Moreover, with the present rate of turbine manufacture, electrical tests would be practically prohibitive, owing to the time involved in the setting up and providing a load for the generator, whereas brake tests can readily be made with the necessary facility and precision.
A typical test log is illustrated in Figure 18, covering all necessary observations on pressure, temperature, vacuum, quality of steam, speed and power. This test involves five observers. with readings at five-minute intervals. As to the character of results obtained, the curves show in Figure 19 would best
indicate the general results. The turbine (a standard 1,000 horse power machine) was tested under saturated and superheated steam, non-condensing and condensing, and over a range of load from 25 to 200 per cent, of rating. Note in the change of curvature of total water line, the effect of the overload or secondary valve. When this valve has come fairly into action, the efficiency of working, although slightly decreasing at first, undergoes gradual improvement with heavier loading, as shown by the reversal of the water rate curves. Here we have a machine working from one half load to twice full load, with a variation in steam consumption of but 1.5 pounds per brake horse power hour, a little over 10 per cent. Although designed for condensing service, the machine developed when running non-condensing a brake horse power on 28 pounds of satuated steam, or 22.8 pounds with steam superheated 100 degrees F.
Without discussing this economy further, suffice it to say that the Westinghouse-Parsons turbine today holds the record in this
country for efficiency when operating under conditions for which the machine was designed. Under 177 pounds pressure, 96 degrees superheat and 27.3 inches vacuum, one of these turbines has given a water rate of 14.72 pounds per kilowatt hour for two consecutive hours. This is equivalent to a little over 10 pounds per brake horse power hour, or between 9 and 9.5 pounds per indicated horse power hour, assuming a mechanical efficiency of 90 per cent., and this under very reasonable operating conditions.
In more or less confirmation of the above economies, Table 2 will be of interest.