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REPORT ON CENTRIFUGAL PUMPS FOR RAILWAY
C. R. KNOWLES, Chairman;
E. H. OLSEN,
Historical The first centrifugal pump was invented by Denis Papin of Hesse, Germany, about 1703 or 1704. This was followed by a centrifugal pump described by Ewbanks as “a straight tube attached in an inclined position to a vertical axis and whirled around by the handle." The invention of this pump is credited to M. LeDemour, who sent a description of it to the French Academy in 1732.
While the description of these earlier pumps is rather vague, it is evident that they bore but little resemblance to the modern centrifugal pumps.
There is no record of their being used to any great extent, except perhaps as mechanical novelties.
There was but little development of the centrifugal pump for over a hundred years after it was invented or until about 1818 when what was known as the Massachusetts pump was brought out. This pump while crude as compared to modern pumps showed great improvement over the earlier pumps and it appears to have been used satisfactorily for a number of years.
Further development followed the production of the Massachusetts pump. The principal improvements being those of Andrews in 1839; Bessemer in 1845; Appold in 1848 and those of Gwynne in 1851. There followed considerable interest and some further development for a few years, after which there was but little improvement in design until the beginning of the Twentieth Century when engineers and pump designers began to realize that a pump capable of much higher heads and greater efficiency than those in general use could be developed. The development was then quite rapid as compared with the former history of the pump and within the past few years the centrifugal pump has been recognized as a formidable competitor of the reciprocating pump.
Principles of Operation A centrifugal pump consists essentially of an outer shell or casing within which revolves an impeller mounted on a shaft. The water enters the impeller at its center and passes out through the vanes into the casing and from the casing to the discharge pipe. The water enters the impeller at a low velocity but passes out from the impeller at an increasing velocity until it reaches its maximuin as it passes out of the impeller.
The velocity at which the water leaves the impeller depends upon the speed and diameter of impeller, form of vanes and area of openings. As the water enters the casing and discharge the velocity is decreased c: account of the increased area and is changed into pressure.
The velocity of the water upon leaving the impeller determines the theoretical head against which it may be delivered by the pump. The efficiency and actual head against which the pump will deliver the water will depend upon the design and construction of the pump, particularly with regard to the impeller and diffusion space.
Impellers Impellers are of two general forms known as the open and closed type. An open impeller consists of radial vanes attached to a hub and disk; the vanes are open at the sides and revolve between the two fixed sides of the pump. In a closed impeller the vanes are carried between two disks. It is apparent that the friction loss must be greater with the open type of impeller as the water is revolved against the sides of the pump at approximately the same velocity as the impeller With the closed impeller the frictional loss is confined to the water passing through the vanes and to the impeller revolving in the surrounding water. Therefore, it is possible to obtain much better efficiency with the closed impeller and it is generally used in types of pump adapted to railway water service.
The principal friction loss in any type of centrifugal pump is that of the impeller and the friction loss by the open impeller is twice that of the closed impeller. This loss together with the leakage loss causes it to be less efficient than the closed impeller type as far as the handling of clear water is concerned. It is therefore, recommended only for handling large volumes of water at low head and where conditions are such that debris might cause trouble by reason of clogging the smaller passageways of a pump of more efficient design.
The open impeller, volute type was about the first successful design of centrifugal pump and until the year 1900 it was practically the only pump of this kind in use. Since that time theory and practice have developed more efficient types of centrifugal pumps, particularly the enclosed impeller volute and the turbine type.
Turbine and Volute Pumps For convenience centrifugal pumps may be divided into two general classes or types: Turbine and Volute pumps.
Turbine pumps have circular casings and diffusion vanes surrounding the impeller. The function of the diffusion vanes is to reduce the velocity of the water upon leaving the impeller through gradually enlarging passages, efficiently transforming the velocity head into pressure head.
Volute pumps have no diffusion vanes, but have a casing of spiral form, which gradually reduces the velocity of the water as it leaves the impeller, thus serving the same purpose as diffusion vanes in turbine pumps.
The turbine pump is adapted to and used for heads greater than 150 ft. A head of 100 ft. per stage is figured for most satisfactory results with this type of pump, although it is possible to operate against heads as high as 300 ft. per stage by operating pump at a high rate of speed. This, however, is not recommended as good practice.
The volute pump is a low head pump and is adapted to all classes of pumping service for which centrifugal Pumps are used. Where the total head is not in excess of 150 ft. it is quite as efficient as the turbine type for low heads and has the advantage over the turbine type, that there are no diffusion vanes to become clogged up or require renewal.
Single and Double Suction Pumps A further classification of centrifugal pumps may be made by dividing them into single and double suction pumps, depending on whether the water enters the impeller from one or both sides. Theoretically, the impeller in a double suction pump is balanced by reason of the water entering from both sides of the impeller, but it has been found in practice that this is not always true owing to variation in moving parts, and other conditions, and it is always advisable to have a thrust bearing.
With the single suction pump there is, of course, always an unbalanced condition of the impeller creating an end thrust which should be taken care of by thrust bearings.
Advantages of Centrifugal Pumps
average pumping equipment.
on the discharge will not effect the pump.
pumps of same capacity.
water with the least injury to working parts.
charge line or vacuum chamber on suction line are
type of pump.
Disadvantages of Centrifugal Pumps
suction lifts more successfully.
variation in the pumping head.
Proper Size and Stage of Pump to Be Used In order to determine the above, the following information must be obtained:
Gallons per minute required.
much sediment and grit. Class of power. From the above can be determined the total net head under which the pump is to operate and this in turn will determine the size of pump and the number of stages for the unit in question.
The number of stages to be used in a pump size to perform a given amount of work under ordinary circumstances will depend on the total net head under which the pump is to operate, the amount of water to be delivered in gallons per minute, the diameter of the impeller and the speed at which the pump can be operated, based on the power to be used. As a general rule, it is safe to say that with heads ranging from 10 ft. to 100 ft. we can select a single stage pump; for heads ranging from 100 ft. to 200 ft., a 2-stage pump and for heads ranging from 300 ft. to 550 ft., 3 or more stages will be necessary, depending on the size and speed of the pump.
Power to Be Used
1. Electric motor.
4. Steam engine. An electric motor may be either direct connected to pump or belt drive used. A centrifugal pump direct connected to an electric motor makes an excellent pumping unit and will meet practically all pumping requirements owing to the fact that motors of practically any speed can be secured. A flexible coupling should be used in direct connecting a centrifugal pump to an electric motor. The belt drive is better suited to small centrifugal pumps than to large units.
Centrifugal pumps may be operated by squirrel cage motors instead of slip ring motors as the starting torque is not great. This shows a saving in the cost of power unit as the squirrel cage motor is not as expensive, nor as heavy as a slip ring motor.
The advantage of direct connected motor driven centrifugal pumps is in the saving in floor space and power transmission losses. High-speed motors are cheaper than low-speed motors and as the centrifugal pump is a relatively high speed machine there is consequent saving in total first cost.
Internal Combustion Engines direct connected or with belt drive are particularly adapted to operating centrifugal pumps, especially for railway water service. Considering power costs alone it is undoubtedly the most economical power that can be used, except where cost of attendance may be eliminated through automatic electric drive.
The Steam Turbine has proven a satisfactory method of operating centrifugal pumps in some cases but there are but few points where it could be used in railway water service. By an adjustment of the governor, the speed of the turbine can be readily altered to suit the varied demands made upon the pump. The steam turbine is efficient under partial loads.
The use of the steam engine for driving centrifugal pumps used in railway water service is limited and could be operated economically only where the exhaust steam was required for other purposes or perhaps where steam was available without the necessity for constructing a boiler plant.
Efficient Methods of Installation To obtain efficient operation a special study must be made of each particular installation. Conditions under which pumping units operate vary to considerable extent and no set rules may be laid down to fit all conditions. The following suggestions are offered as applicable to general conditions covering centrifugal installations.
A certified characteristic curve of the pump at the speed recommended by the manufacturers should be furnished and studied carefully, special attention being given the following points:
(1) The unloading or non-overload feature in which the
power is not increased materially should the head be
reduced. (2) The efficiency at various heads. The horizontal, split casing type with enclosed impeller is recommended where conditions are adapted to its use as it presents the most accessible form of construction for inspection and repairs.
The bearings should be of the ring oiling type and provided with drip pockets connected to a drain to carry away the slight leakage that should be permitted through the stuffing boxes and glands.
A flexible coupling should be used on all direct connected units to connect the pump with the prime mover as this type of coupling permits for slight inaccuracies in the alinement of the shafts and insures cool bearings.
A check and gate valve should be placed on the discharge line near the pump. The check valve is to protect the pump from water hammer; the gate valve to regulate the discharge of the pump and to permit its removal without emptying the discharge line.
If a centrifugal pump is required to draw water from a single driven well, the suction pipe should be dropped down inside the well casing and the top of the casing should not be closed but left open, so that the air can have free access to the surface of the water.