The Importance of Pumping Unit Tests
Enumeration of Various Classes of Tests—Design and Actual Conditions Apt to Vary—After First, Subsequent Tests Made Quickly and Cheaply
HENRY W. TAYLOR
154 Nassau Street, New York City
WITH the present high cost of coal, electric power and the present cost of increasing the capacity of a pumping plant, the power which any pumping station is consuming is of prime importance. The work which a pump is performing and the amount of power required by it to lift a given number of gallons against a given head can be accurately determined by test.
Three Different Classes of Tests
Three different classes of tests should be made in connection with new machinery. When purchasing new apparatus, the specifications should call for a witnessed test at the factory, so that the efficiency of the pump can be determined before the apparatus is shipped from the manufacturer’s plant. Inspection of bearings, impellers and all working parts can at the same time be made, so that the owner will be advised of any defects in the apparatus before it leaves the manufacturer’s hands.
After the apparatus has been installed in the pumping station, an operating test should be made to determine the efficiency of the apparatus in terms of the prime mover, or in terms of those power units for which the owner pays. This second test provides a second opportunity to determine the efficiency of the apparatus and its conformity to the specifications before final payment is made.
The third set of tests is absolutely necessary and applies both to new and old apparatus. In many existing pumping stations, little or no attention is paid to the efficiency of the apparatus after it is once installed and, in fact, the operator seldom knows exactly what work his pump actually performs from day to day. It often happens that an apparent increase in water consumption can be accounted for by a decrease in pump efficiency.
Renewing Worn Impeller and Other Parts
Especially in the case of centrifugal pumps, it is probable that, with a gritty water, the impeller or diffusion veins become worn and this wear immediately affects the amount of power consumed in delivering the required quantity of water to the required elevation. One of the advantages of the centrifugal pump relates to the ease and low cost of renewing these worn parts and practically rebuilding the pump on the ground. A new impeller, new wearing rings, and with some types of pumps, new diffusion veins, which can be readily installed at the pumping station, will bring the pumping apparatus back to its original efficiency or even increase this original efficiency.
It often happens, moreover, that the hydraulic conditions for which the pumping apparatus was designed, materially change. As the water consumption increases and if the force mains are not enlarged, the average working head largely is increased and it may be that such increase in head imposes conditions on the pump not consistent with those including the maximum efficiency. In such case (and if direct connected), this impeller should be removed and replaced by an impeller of slightly different curvature or diameter and a greater efficiency obtained by the exchange.
Difference Between Actual and Design Conditions
With motor driven, direct connected, centrifugal pumps, it often happens that a slight change in cycle, load, load factor, or other electric conditions, results in a change in speed of the motor under full load. It also often occurs that the actual average motor speed is not what was expected and the design of the pumping unit is therefore interfered with. As an instance of this difference between actual and design conditions, we may mention a case where the full load speed of the motor is in excess of that estimated by the manufacturer and, in addition, the cycle of the current was changed from 60 to 61 1/2. This increase in speed generated a higher head for a given quantity of water and it was necessary to throttle the pump in order to keep it from continuously overloading the motor. This throttling of the pump discharge creates a pressure inside the pump which must be generated by the power unit in excess of the actual pressure in the force main and this loss of head represents pure waste.
If these various points have been determined, it is easily possible to accommodate the apparatus to the changed conditions, but it is too often the case that either existing conditions are not known or else they are ignored as being of no importance.
Importance of Operation Tests
Every pumping station of moderate or large size should be given a thorough testing at least every six months to determine the work done and the power consumed and to note any change in electrical or hydraulic conditions. The following table will illustrate the real money value of such an investigation and the saving due to correction of unnecessary conditions.
Table Showing Increased Costs of Pumping at Reduced Efficiencies
The table shows the cost of pumping various quantities of water against total heads of 200 feet with different efficiencies over all and with a cost of power at two cents per k. w. h. and 1 1/2 cents per k. w. h. The total cost is figured over a continuous period for one year and figures are computed to the nearest $100. The first line opposite each head and quantity gives the approximate cost of pumping at the indicated quantity of water against a head of 200 feet for one year. Underneath this total cost are given the differences between lower and higher efficiencies. To illustrate: It costs about $1,000 more a year to pump at an efficiency of 65 per cent. than with an efficiency of 70 per cent., a quantity of 2,000,000 gallons per day against 200 ft. head for a year’s time. A similar excess cost is noted comparing an efficiency of 50 per cent, with an efficiency of 70 per cent, and the amount is $5,300. The table shows that it costs $1,200 more per year to pump for this quantity and head with an efficiency of 60 per cent, than with an efficiency of 65 per cent.
With quantities of six million gallons per day, every 5 per cent. of efficiency represents a considerable amount, the difference between 60 and 65 per cent. efficiency representing $3,600. With a rate of 1 1/2 cents per k. w. h. this extra cost is, of course, reduced to $2,600.
For convenience, the table is figured on the approximate basis that two million gallons per day pumped against 200 feet head at 70 per cent, efficiency requires an even 100 h.p. The table may be used for almost any condition by taking a direct ratio between the head and quantity desired, and the head and quantity given.
The determination of the quantities of water pumped is made according to the most feasible method for each particular case. In case a Venturi meter has been installed on the pumping discharge main, a reading of quantity can be made within about 3 per cent. of the actual flow. In case no meter is available, the flow can be tested by a calibrated flow nozzle, a weir box constructed on the job for such apparatus or by a pitot tube determination.
How Determination of Head Is Accomplished
The determination of head is accomplished on the suction lift by means of a mercury U tube, which is connected by rubber tubing to a tap in the suction inlet of the pump. This column of mercury is reduced from inches of mercury to head of water in feet and the distance from the point of this tap in the suction inlet to the center of a gauge on the discharge of the pump ties this suction reading to the gauge reading on the pressure main. A pressure gauge tapped into the discharge main will read the total head against which the pump operates on its discharge outlet and includes both the static head and the head due to friction loss. If the velocity head is enough to be of importance, this head should be determined and added to the other three dimensions. The pressure gauge used should be a tested gauge which has been recently calibrated and correction to the gauge reading should be made as found necessary.
If the pump is electric driven, the determination of a current used can be made either at the switch board by connecting the watt meter or by portable instruments. In case of steam plants, the pounds of steam consumed can be determined by metering the steam flow or measuring the condensate, or the flow of water to the boilers. Any steam determination should be carried back to the coal consumed during the test so that the efficiency of the boilers can also be included.
Subsequent Tests Made Rapidly and Reasonably
After the first set of tests have been made and the necessary taps and connections installed, subsequent tests can be made rapidly and at a very reasonable cost. From a financial point of view, it is just as important for the water superintendent to test the mechanical efficiency of his pumping apparatus as it is to test from time to time the quality of the water delivered to the consumer. When one takes into account the fact that these tests will bring to light minor changes in operation which will lead to added efficiency, the necessity and value of periodic pumping station tests can be fully appreciated.
There are many stations which keep fairly accurate records of operation, but it has been the writer’s experience that little or no use is ever made of these records. The examination and study of these records will indicate many means of increasing efficiencies if put in the hands of one who can interpret their meaning. In connection with periodic tests mentioned above, the records of a plant should be tabulated and interpreted by the testing engineer and the records will often explain changes which have taken place during the period between tests.
Plans for Topeka Water Supply
Reports of engineers employed to make surveys of the water situation in Topeka, Kan., have been received and all are to be carefully considered before any definite action is decided on, according to a recent statement by Wilbur Stanfield, water commissioner. One report recommends tapping the water bearing sands beneath the surface on the north side of the river, which would necessitate building a pumping plant there at a cost of $150,000 up to $300,000, it is estimated. Other reports suggest the installation of a filter plant at approximate cost of $250,000. Commissioner Stanfield says that there is no intention to wreck the present plant, as the wells will be reserved for water storage, if the plans for a new plant should be adopted.
The city council of Defiance, Ohio, will convert the old city prison, which has long been an eyesore, it is said, into an office building for the water department. It is planned to remodel it by building a handsome front, applying stucco to the brick work and constructing a modern warehouse for storage of pipe, machinery, etc. The grounds will also be beautified and the property converted into something useful and also ornamental.