The New Zealand Railways Magazine, Volume 3, Issue 1 (May 1, 1928)
Turbine running in the Otira power station is, in many respects, much different from other power station practice. In the first place the loading factor of this station is very small compared with other power houses—especially those of both lighting and traction.
What is meant by the station loading factor is the ratio of the average power to that of the maximum power during a certain period of time.
For the Otira plant takes as an average 1750 tons pulled by seven trains of 250 tons trailing load on each, and the total time of actual turbine running will be about thirteen hours.
Since each up trip takes half an hour, the total full load time therefore will be 3 1/2 hours. The rest of the time the turbines are practically running on no load.
The uptrip of a train takes 580 units in the actual time of 28 minutes. The unloaded period takes 20 units. In other words, the round trip of a train requires 580 + 20 = 600 units. Total units per day = 7 × 600 = 4,200. Therefore average power per hour 4200/13 = 323. Heaviest loading = 580 × 60/28 = 1243 per hour. Loading factor = 323/1243 = .26 approx.
The boilers are specially designed with a large heating service to cope with very sudden loading. They have nests of four tubes in place of the usual one large tube opposite each hand hole door.
Otira, fortunately, has an abundance of water for most of the year. This gives ample opportunity of carrying a high vacuum in the condensers, thus saving fuel in the boiler room. The circulating water is supplied from a reservoir placed about 40 feet above the level of the power floor, and therefore, unlike most of the city power stations, a circulating pump is not required. Circulating pumps consume quite an amount of power, and there is always a critical point beyond which it is more costly to supply the extra cooling water than the value of the power thereby gained. This is due to the higher vacuum in the condenser.
At Otira, the air is extracted from the condensers by what is known as Worthington's hydraulic vacuum pumps. These pumps are worked (as the name implies) by water (under a head of about 90 feet) which is derived from another reservoir placed further up the mountain side. These small and efficient appliances have rendered excellent service since they were placed in commission five years ago.
Otira usually has a water shortage over a small period of each year, then it becomes necessary to return the circulating water back to the bottom reservoir by means of a centrifugal pump. Sometimes the reservoir water can be made to supply the requirements for a day's run by carrying a reduced vacuum in the condensers. This, of course, is a point for the power station operative to decide. It is often cheaper, however, to use a little more coal in the boiler house to generate extra steam (on page 39 account of the lower vacuum in the condensing plant, thus saving the reservoir water) than it is to pump the used circulating water back into the dam, which requires extra steam for the centrifugal pump employed for that purpose, as well as the extra labour in giving attention to the water pump.
Below are given two sets of figures, one taken with an abundance of circulating water, and the other during a shortage period. Those who are conversant with turbine operation realise what an inch of vacuum means to a turbine (especially on the higher values), and those who are not familiar with this line of engineering will, by comparing the two lists, soon observe what a bearing the vacuum has on the power output from turbines.
|Steam Consumption Tests for Trains of 280 tons.|
|For Plenty of Water.|
|Turbine Exhaust Hg. Column||27.6in.|
|Absolute Pressure||.65lb. sq. in.|
|Pressure at Throttle||135lb. sq. in.|
|Temp. Fah. of Steam||530 deg.|
|Press. after first expansion||31.5lb. sq. in.|
|Temp. Circulating Water Inlet||38deg. Fah.|
|Temp. Circulating Water Discharge||64deg. Fah.|
|Average K.W. on Chart||1275|
|Rate of flow of Condensate through Venturi Meter, lbs. per hour||19,000|
|Condensate Temperature||62deg. Fah.|
|British Thermal Units taken from each lb. of steam||370|
|Circulating water taken through Condenser, gals. per hour||77,900|
|For Shortness of Water.|
|Turbine Exhaust Hg. Column||25.8in.|
|Absolute Pressure||1.5lb sq. in.|
|Pressure at Throttle||130lb. sq. in.|
|Temp. Fah. of Steam||520 deg.|
|Press. after first expansion||36.5lb. sq. in.|
|Temp. Circulating Water Inlet||41deg. Fah.|
|Temp. Circulating Water Discharge||90deg. Fah.|
|Average K.W. on Chart||1260|
|Rate of flow of Condensate through Venturi Meter, lbs. per hour||21,200|
|Condensate Temperature||86deg. Fah.page 40|
|British Thermal Units taken from each lb. of steam||330|
|Circulating Water taken through Condenser, gals. per hour||45,580|
(N.B.—Those who have been trained in steam power calculations will readily select the required items for the computing of the number of British thermal units taken out of each lb. of steam by the turbine, and also the number of gallons of circulating water required to condense the steam. Those who have not had dealings with this line, will have to accept the figures as being correct).
It will be observed that a loss of under two inches of vacuum requires more than 10 per cent. more steam to carry the same load, viz., to propel a train of 280 tons trailing load over this ruling grade of 1 in 33.
As the loading on the main traction generators is only for half an hour and the light running for about 1 1/2 hours, they are able to be loaded to 125 per cent. of their full rated capacity with perfect safety as far as the electric heating is concerned. It will also be observed how low the temperature of the inlet water for the condenser circulation is, compared with what could be expected in a city power station using sea water for condenser circulation.