ELECTRICALLY DRIVEN PUMPS*
For Small Water Works the Unit Driven by Electricity is Particularly Adaptable— Growing Tendency to Replace Other Forms of Power with Electricity— Advantages of its Use in Small Developments
THE great development in motive power and power pumps within the last half century has placed the possibility of a reliable water works system within the reach even of rural villages. The period 1885 to 1905 saw the installation of public water works in many small towns. At that time about the only pumping power available was the familiar steam plant, consisting of a steam boiler and a reciprocating steam pump. Many of these plants have lived out their allotted days and the question of replacement is becoming a rather common one.
Within the last fifteen years several new types of pumps have been developed to a high state of perfection. This is true also of the electric motor. A study of waterworks statistics reveals a rapidly growing tendency to replace other pump motive powers with the electric motor. The subject which forms the caption of this paper, therefore, is of pertinent interest not only in an academic way to engineers, but also in a highly practical way to municipal authorities, electric central station utilities, and manufacturers of pumping and power equipment.
Advantages of Electric Motive Power
As practically all forms of so-called “power” pumps— centrifugal, rotary and displacement—can be adapted to electric drive, the questions arising relative to the application of electric motors to municipal pumping are those pertaining to first cost and operation rather than mechanical design. In brief, the advantages commonly claimed for electrically driven pumps are:
Lower first cost.
Lower operating expense.
Greater compactness and simplicity.
Requires less attendance and are more convenient than other forms of drive.
Where water is pumped from widely separated wells electric drive is particularly advantageous. Pumps so driven may readily be arranged for automatic operation, starting and stopping automatically at predetermined pressures on the discharge mains.
The energy required by electrically driven pumps is usually expressed in kilowatt-hours per 1,000 gallons of water pumped. This is not a very satisfactory duty unit as pumps work between wide ranges of such lift and discharge head and their energy requirements, therefore, differ greatly. For large low head pumps the energy requirement may be as low as 0.4 kw.-hr. per 1,000 gallons, and for small high head pumps as high as 10 kw.-hrs. For average conditions 1.25 kw.-hrs. per 1,000 gallons seems a fair figure. The energy cost, of course, depends on the electric rates and they, too, vary through a wide range being dependent on size, kind and location of the central power station. These rates usually contain a demand charge and an energy charge both graded as to magnitude of the customer’s requirements. A rough average flat figure for small stations is of the order of five cents per kilowatt-hour. For larger stations two cents per kilowatt-hour is probably nearer the correct average.
Paper read before the Annual Convention of the Indiana Engineering Society.
Possibly the greatest objection that has been urged against electric drive is lack of reliability. This objection is based largely on the fact that the electric motor is not a prime mover but is dependent for its energy supply on transmission lines and the central station. As a matter of fact, however, for a small pumping plant consisting of but a single unit the electric drive is the most reliable of all power drives. The motor itself, if properly applied, is more reliable than either steam or internal combustion engines and the transmission line and central station are more reliable than is a single boiler.
The engineer who applies an electric motor to a given drive must have a very accurate knowledge both of the characteristics of the motor and pump, and the requirements of the drive if the application is to be successful. Too often the electric drive has been condemned as inherently defective, whereas, the defect lay in the application and not in the drive itself. The requirements in waterworks service are rather numerous, although in the main they are not difficult to meet.
As far as power driven pumps are concerned, water works systems may be classified as (1) direct pumping, (2) pumping to storage, (3) combination of the two systems. With the latter system it is not uncommon to shut off the storage line during a fire and pump directly into the mains at some higher pressure.
Several types of pumps are available and in general pumping can be classified into deep well and surface pumping. The latter class includes all of the suction lifts low enough to permit the pump being placed at the ground surface.
For deep well pumping displacement, centrifugal, propeller and air-lift pumps are in common use. No attempt is here made to discuss in detail the problem of pump application to a given service; rather it is to limit the discussion to the requirements of different types of pumps from the standpoint of the application engineer. For the displacement pumps, that is those using a plunger or a piston, a pumping head to change the rotary motion of the motor to the reciprocating motion of the pump is necessary. This head must be geared to the motor because the lowest economical motor speed would still be far too high for the pump. Centrifugal and propeller pumps for deep well pumping are driven by direct-connected vertical shaft motors. Obviously the motor and pump speeds must be mutually adapted and this requires care on the part of the pump manufacturers. Alternating current motors are most commonly used in small municipal plants and their speeds are fixed depending on the number of poles of the motor and the frequency of the alternating current. For the common frequency of 60 cycles the standard small motor speeds are 1,800, 1,200, 900, 600 and 450 revolutions per minute. In connection with air-lift pumps motors may be used to operate the air compressors.
For surface pumping, displacement, centrifugal and rotary pumps are used. Until recently displacement pumps had the field pretty much to themselves. Because of lack of understanding of its characteristics, the centrifugal pump has gained, in some quarters, an undeserved reputation for trickiness. The modern centrifugal pump correctly applied is really an extremely reliable piece of machinery and because of its high speed and favorable operating characteristics is particularly well adapted to motor drive. Its efficiency is usually somewhat lower than that of well designed displacement pumps. In a line of modern pumps ranging in capacity from 200 to 2,000 gallons per minute at 100 feet discharge head, the efficiency ranges from 60 to 77 per cent. These figures are much higher than those secured with some of the older designs.
The output rating of an electric motor is limited by the heating of the motor windings. The rating assigned by the manufacturer is that output which the motor can carry continuously without overheating the windings. Because of this heating limitation, some care must be taken in selecting a centrifugal pump which is to be mated to a motor.
If the head against which a centrifugal pump is discharging is gradually increased, the discharge lessens until finally at a head fixed by the design of the impeller, it ceases and the rotating part of the pump simply churns the water in the casing. Under these conditions that pump requires only a fraction of the power required when delivering rated discharge. The valve in the discharge line of such a pump may be closed, therefore, without endangering either the pump or the motor.
If now the head be slightly decreased the pump will start to discharge and the power required by it will increase. For a pump of suitable design for operation with an electric motor the curve showing the relation between the head and rate of discharge should be comparatively flat until rated discharge is reached. Thereafter the head should decrease rapidly. Such a pump is said to have a drooping characteristic. As the power required to operate the pump decreases as the head decreases the possibility of seriously overloading the motor, in case of a break in the water main, is avoided.
As protection to the motor where displacement pumps are used, suitable safety valves must be provided in the discharge line to prevent the pressure from being built up to a dangerous value when the demand on the pump is low. Displacement pumps are not as well adapted to direct pumping as the centrifugal types are.
The motors best suited for pumping service are the three-phase, squirrel cage induction motors for alternating current and the shunt motor for direct current. The alternating current motor is extremely rugged, simple and reliable, has no moving electrical contacts and may be secured with moisture proof windings. Both types of motors and all well designed high speed centrifugal pumps are equipped with ring oilers and will run continuously for long periods with only occasional inspection.
In addition to type of pump the application engineer must know the required discharge, the total pumping head, the kind of electrical energy available, whether direct or alternating, and if the latter, the frequency. The total head is fixed by the suction lift, the pipe function, and the discharge head. Working pressures vary widely in different towns. For ordinary service the common range is from 30 to 50 pounds per square inch, while for fire service pressures ranging from 50 to 100 pounds per square inch, are in common use.
The following actual example illustrates in a general way the motor application problem. The figures are of the pre-war period, the example chosen being considered rather more typical than one in which the present fluent cost and labor conditions would be involved.
A town of 2,500 population has a steam pumping plant which has been in service for 20 years and is now about ready for the scrap pile. The direct pumping system is employed and funds are not available for the erection of a stand pipe or tank. Three-phase 220-volt, 60 cycle power of high relaibilty is available. The ordinary working pressure is 40 pounds per square inch, but for fire protection purposes, 65 pounds per square inch, with enough capacity behind it to furnish two 1 1/8-inch nozzles is desired. At this pressure two such nozzles will discharge about 400 gallons per minute.
Meter readings of the domestic requirements were taken during two summer periods, one of six days in August, the other of fifteen days in September. For the first period the following data were taken:
The average for the second period was 168 gallons per minute. The maximum discharge noted was 240 gallons per minute. It was only of short duration. The suction lift and pipe friction were such that for ordinary pressure the total head was approximately 110 feet and for the fire pressure 165 feet.
For ordinary service the data indicate that a 200 gallon per minute pump is large enough and for fire service a 500 gallon per minute pump. In case of fire the larger pump would be started and the smaller one shut down. Had it been a standpipe installation, a larger low head pump would have been advisable as it would have to run fewer hours per day to maintain the water level. Also, automatic starting and stopping would have been desirable. For the conditions in hand centrifugal pumps best fit the requirements and they were selected.
The sizes of motors necessary to drive these pumps may be calculated:
Hp = Gallons per minute X total head 4,000 X pump efficiency
From the case in hand the figures are 9 1/4 and 31 horsepower respectively. The nearest commercial motor sizes are 10 and 35 horsepower and these were selected.
The estimated cost of changing to electrically driven pumps, including changes to the pumphouse and some suction line alterations, was $3,000, the pumping units themselves costing $475 and $850 respectively. The cost of similar units today would be about twice that, the costs of a line of well designed modern centrifugal pumping units, capacities up to 2,000 gallons per minute, for 100 feet head being given by
Cost = $850 $34 n where n = number of hundreds
of gallons per minute rating
For 200 feet heads the cost = $1,600 + $107 n
The pump costs alone can be figured from cost = $540 + $7 n on 100 foot heads, and cost = $1,200 + $53 n for 200 foot heads.
The fixed charges on the new installation at 15 per cent, a year amount to $450 per annum. The salvage of the old steam plant actually decreased the investment a small amount, but this was not considered in the above estimate.
The estimated operating expenses of the new plant were $1,500 per year. This included one-sixth of the time of a water works employee who spent the rest of his time on other work. Energy for the operation of the pumps was obtainable at an average figure of 1.85 cents per kilowatt hour, the actual rate being of the familiar sliding scale type. The estimated energy consumption per 1,000 gallons was 0.8 kw.-hrs. and the energy cost 1.48 cents.
The average operating cost of the old steam plant for three years preceding the time in question was in round figures $2,900 per year. Of this amount nearly 50 per cent was fuel cost. The entire time of two men was necessary for the operation of the plant.
For the last year of steam operation the average fuel consumption was 14.7 pounds of coal per 1,000 gallons. The average fuel cost was 1.8 cents, with the average cost of attendance 1.27 cents per 1,000 gallons. With the present prices of fuel and labor the saving in operating expenses would be nearly double the $1,400 per year indicated by the above figures.