Keeping Down the Operating Cost of Standby Pumping Units
How to Choose the Best and Most Serviceable Pump for This Type of Service—Pump Speed Characteristics—Comparative Operating Costs
THE installation of standby pumping units in water works systems for fire protection, for emergency and for extra demands on the system has become a necessity for many water works. The following article will be found of considerable value in giving suggestions toward choosing the type of standby engine most suitable for the particular needs of water works of various sizes:
Before the days of hydro-electric energy the water works pumping station was operated by steam power. With the spread of hydro-electric energy throughout Ontario, which was much cheaper for power purposes than steam, nearly all water works systems have converted their plants from steam operation to electrical. With the installation of electric equipment these steam plants were faced with the necessity of either keeping fires banked in their boilers and their steam pumps in readiness for operation, or the installation of some other standby unit. It has been found that the cost of banking coal fires amounts to such a large item that most plants have abandoned steam equipment entirely and have installed pumping units which do not require a continued operating expenditure. It will be the purpose of this paper to outline briefly the various standby units together with their operation.
Gasoline Engine Driven Units
For the small water works plant pumping below 4,000,000 gallons of water a day, the gasoline engine driven centrifugal pump has become standard equipment. These gasoline engines are built to operate at speeds ranging between 1.000 and 1.400 r. p. m. and are directly connected to the centrifugal pumps. They are comparatively cheap and when equipped with storagebatterv electrical starting equipment can be started almost immediately and have proved very satisfactory. The operation of these units must not be considered in any other light than as emergency units. Their fuel consumption is about one pint of gasoline per h. p. per hour and when in best running condition for compression and properly adjusted for timing and carburetor supply, the consumption may be brought down to nearly Y pint per h. p. per hour. If we assume that the average consumption is .7 pints per h. p. per hour and the cost of gasoline is $0.31 per imperial gallon, the cost of-operating the gasoline engine is 2.71c. per h. p. per hour. A pump supplying 1.000 gallons per minute at 100 lb. pressure will cost $2.71 per hour or 4.5c per 1.000 gals, of water for gasoline alone. This is admitted to he a very high cost of pumping, but when one takes into consideration the capital costs of the various units as well as the comparatively short time which this pump will be called upon for operation, the operating costs of the gasoline driven pump are. without doubt, the least.
Pump Speed Characteristics
One feature in connection with the design of standby units which is deserving of careful study is the behavior of centrifugal pumps under varying speeds, and the designer should go carefully into the question of the speed of the pump which will be required for his work. The speed of the engine can be varied by changing the amount of gasoline fed to the carburetor. The engines are designed for operation between 1,000 and 1,400 r. p. m. and if one stands beside one of these engines and notes the difference in vibration and laboring of the engines for speeds between 1,200 and 1,400, there is no question that he will readily come to the conclusion that the speed of 1,200 for a gasoline engine is quite high enough. Having this in mind the pump should be designed for an operating speed of 1,200 r. p. m. which to give a certain capacity will call for a much larger pump than at 1,400 r. p. m. In tendering on this class of equipment the manufacturers top often figure on the smallest sized pump and the highest speed of the engine, and the operator consequently gets a unit which will only do the specified work when operating at its highest speed.
In Fig. 1 are shown the various characteristic curves for a given pump at various speeds. It can be seen on this chart that a 1,000 g. p. m. pump at 100 lb. pressure at 1 ,300 r. p. m. has its pressure reduced to 65 lb. if operated at 1,100 r. p. m. These curves show that the efficiency of the pump below the design discharge increases for a drop in speed and above the design discharge decreases for a drop in speed. The gasoline consumption per h. p. per hour ranges the same for varying heads for constant pressure. The consumption per h. p. per hour decreases with the increase of speed.
There is a wide field for investigation of the proper type of pump to be operated at varying speeds as the standard centrifugal pump is designed for the induction motor which has a constant speed.
When the capacity of the plant is 4,000,000 gallons per day or upward, the question of standby units become a much larger problem than for the smaller plant; because, to take care of the capacity of the plant, it is necessary to place in the pump house several gasoline engine driven units and these units to start and to keep in operation require the close attention of an operator; and it is doubful if it is good practice to put more than two or three such units in the pump house and expect one man to look after their operation. About the largest pump which can be driven by the largest commercial gasoline engine is one to give 3,000 g. p. m. against 100 lbs. pressure. This unit would require an 8-cylinder gasoline engine of about 250 h. p. at 1,300 r. p. m.
Standby Units for Large Plants
In order to take care of standby units in these larger plants either one of the following methods must be adopted:
- —Steam driven reciprocating pumps or steam turbine driven centrifugal pumps.
- —Steam driven generating set to generate electricity to drive the electrical equipment.
- —Crude oil engine of the diesel or semi-diesel type driving an electrical generator to supply power for the electrical equipment or directly connected to centrifugal pumps by means of gearing.
If steam equipment is used, the question of banking fires is the big operating expense. If the boilers are of the water-tube type, this can be very easily taken care of by installing a small donkey boiler which will act as a water heater and keep tlie water temperature in the water-tube boilers around 200 deg. so that in case of emergency the steam can be quickly raised to the desired pressure by putting fires under the boilers. If this equipment be oil fired, it will not lie necessary for the plant to maintain a staff of firemen, and when the emergency arises the oil burners can be lighted under the boilers and steam pressure secured in a very few minutes. The cost of operation of standby units in this manner can be reduced to a minimum.
The diesel and semi-diesel engines are very economical in operation and consume about pa pint of crude oil per h. p. per hour. This oil can be bought for about 10c per gallon and the engine, if kept in good condition, can be very quickly started. The advantage of this type of equipment lies in the fact that it is not necessary to maintain a continuous consumption of fuel as in the steam plant. The capital cost of the steam and diesel equipment runs so high that the gasoline engine driven equipment can be operated for a long period before the combined operating and capital costs are equal. In figuring a specific case it was found that the gasoline engine driven equipment could be operated for 30 days, and at that time the combined operating and capital cost would be equal for the two types of equipment.
The semi-diesel type engine is considerably cheaper per h. p. than the diesel.
If steam equipment is maintained in the pump house it is found desirable, from an operating cost viewpoint, to keep in continuous operation a small steam driven unit so that the steam which is generated may be used to some good purpose. Most water works plants have certain specific conditions as to capacity of units, size of units, size of boilers, etc., and the economic operating point for a combination of electric and steam units can only be maintained by experimentation.
Comparative Operating Costs
In June, 1921, the National Electric Light Association printed a pamphlet called “Small Steam Movers,” in which the capital cost and operating costs of various units was figured out in detail by a committee Of experienced engineers. The accompanying table, shown in Fig. 2, and which was copied from this report, gives the capital and operating cost of these units and also the operating cost based on load factors of 25, 50 and 75 per cent. It will not be necessary to go into tbe detailed figures of computations.
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Operating Costs of Standby Pumping Units
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They are based on the assumption given in the table and assume a spare unit. These serve to give a comparative statement of the cost of operation of the various units and can be considered as reliable figures upon which to pass an opinion as to the most economical type of equipment. It is not probable that any standby units would be used with as high a load factor as 25 per cent, but the figures will be interesting to show a comparison between the different units when operating at various load factors. No doubt, if one considers these units as idle units, except in the case of emergency, the unit with the lowest fixed charge, namely the semi-diesel engine, would be the cheapest, and it is interesting to note that for a continuous operation the semi-diesel engine is as low as any other unit.
In calculating operating costs of standby units one must assume some basis as to the number of interruptions and the length of these interruptions to the hydro-electric energy throughout the year. With very few exceptions, the interruptions to hydroelectric energy is comparatively short and very seldom runs over a two hour period.
In all water works plants operated by electrical energy there should be an elevated reservoir containing a sufficient supply of water to carry the system over the short periods of interruptions of power and enable the standby unit to be put in operation without interfering with the pressure. A good basis for figuring the size of these reservoirs would be to figure a one-half day supply for the smaller systems and a half million to a million gallons supply for the larger systems. There is a tremendous strain placed upon the distribution system due to water hammer when the power is cut off if there is not an elevated reservoir to maintain a pressure on the system.
Economy of Diesel Engines
Having in mind the comparatively short time that a standby unit would be called upon and the large expenditure which is necessitated due to installation of such units, the question of duplicate power units is a very serious one where there are water works systems which are called upon for large daily supplies of water, and it is very doubtful if one could improve upon an installation of a semi-diesel oil driven engine direct connected to a generator to supply power in an emergency for the motors of the pumps. A plant of this nature has very wide possibilities in the average small city because, during normal periods of consumption there would be a considerable amount of electrical energy available for other purposes, and quite possibly, without too much extra expense, enough energy could be supplied to take care of the street lighting as well as the domestic supply of water; and. if these two problems could be worked out together, the question of heavy charges against the water works plant might be relieved to a considerable extent by the street lighting bearing a portion of the cost.
In order to give an idea of the cost of gasoline engine driven units a centrifugal pump of 4 million g. p. d. capacity against 230-ft. head, connected to a G. R. C.-8 Sterling engine, costs f. o. b. factory about $7,000, or figuring pump and engine together, the cost per horsepower is about $25. The cost per horsepower for a semi-diesel oil engine is about $100.
This paper does not take into consideration the condition where alternate sources of electrical energy are available or where hydraulic power is possible. These are ideal local standby units and those water works plants so situated are fortunate.
(Excerpts from paper read at the meeting of the Canadian Section of the American Water Works Assn., Windsor, May 19, 1923.)
Tiffin, Ohio, May Remove Water Tanks—It is proposed by the Tiffin Water Works Company, of Tiffin, Ohio, of which C. H. Wetter is superintendent, that if the city would remove the six watering tanks which have been decorating the city streets for a number of years the company would install non-freeze drinking fountains in their place and would not charge the city for the water consumed. The acceptance of the company’s offer would mean a saving of $150 a year to the city and also a saving to the company, for about $1,500 worth of water is lost to the company a year through wastage of water in the tanks. No action as yet has been taken by the city council in the matter.