CENTRIFUGAL PUMPS.

The centrifugal pump raises the liquid to be displaced, by means of a rapidly revolving fanhaving two or more blades straight or curved, fastened upon a shaft and fitting closely into a case having an inlet for water at the end center and an outlet at one side or on top of the case tangent to the circle described by the fan.

Most people are practically acquainted with the principle of the centrifugal pump, viz., that by which a body revolving round a center tends to recede from it, and with a force proportioned to its velocity: thus mud is thrown from the rims of carriage wheels, when they move rapidly over wet roads; a stone in a sling darts off the moment it is released; a bucket of water may be whirled like a stone in a sling and the contents retained even when the bottom is upwards.

The earliest history of the centrifugal pumpcannot be traced, but it is known that centrifugal machines for lifting liquids were in use during the latter part of the seventeenth century. About 1703, Denis Papin, the famous French engineer, designed his “Hessian Suck,” a form of centrifugal pump embodying nearly all of the essential features of the present-day machine. Drawings of this pump are in existence which show that Papin was not only a designer of no mean ability, but that he had a good comprehension of the principles with which he was dealing. After Papin there seems to have been no further development of his ideas until 1818, when the earliest prototype of the present form of centrifugal was brought out in Massachusetts and has since been known as the “Massachusetts pump.” This pump was of the type designated “volute,” and was provided with double suction openings and an open impeller. It was re-invented by Andrews and others in 1846, and was shortly afterwards introduced into England by Mr. John Gwynne.

Note.—The term “volute” so frequently used in connection with these means “a spiral scroll of plate.”

Centrifugal pumps have now attained a degree of perfection, which makes them a serious rival of the plunger pumps. The high-class turbine pumps of to-day are simply machines in which the water is given a velocity which is partially changed to pressure before discharge, and the pump is designed so that the well-known actions, outlined above, proceed along natural lines, which are, to use the common phrase, lines of least resistance. It is simply a question of careful design.

The modern pump causes the water to flow along paths naturally due to the forces acting and to guide the stream in such a way as to avoid the production of eddies and whirls within itself which so enormously cut down the efficiency.

Fig. 489.

The blades now take the form of “impellers” which have warped surfaces whose form is the result of careful calculation, and the water, after leaving the impellers, is guided by vanes of equally and carefully calculated form.It is owing to the correct form of the guide vanesthat the nearly perfect conversion of velocity into pressure head is possible, and this is the principal factor which produces the high efficiency shown in tests.

When pumping against high heads, the units are arranged in stages or series. The discharge from one is led to the inlet of the next in series, the separate units being usually mounted on one shaft, and the whole really forms one machine. In this manner heads approaching 2,000 feet are worked, still preserving the high efficiency which in some cases reaches between 80 and 90 per cent.

The ability to generate such pressures enables them to be used for feeding boilers, and their efficiency particularly commends them for this purpose.

Fig. 490.

The centrifugal pump isthe converse of the turbine water-wheel. Its development has been analogous to that of the steam turbine in that both were abandoned in favor of reciprocating machines before having been thoroughly exploited; the pump because the principles of its action were not clearly understood, and the steam turbine because of mechanical difficulties in its construction.

Opposite the title page of this book can be seen a representation of a centrifugal wheel often thousand horse power; it will repay the careful student to consult also page 133 of part one of this work for the details of this enormous machine; the curious will also be interested in an early form of the centrifugal pump to be seen in Fig.489and its description in the Note on page 214.

Fig. 491.

Fig. 492.

Where large quantities of water are to be moved quickly and more especially in cases where the water is impure and contains floating matter, as well as sand, mud, coal and the like, as in wreckage, the centrifugal pump has its peculiar advantages. It is suited particularly for use in tanneries, paper mills, dry docks, corporation work such as building sewers, sand dredging, and with water that contains large quantities of solid matter held in suspension. Pumps used for these purposes have to be primed on starting, and the suction pipe should be as short as possible. Long suction pipes very much impair the efficiency of this type of pump. They will draw water upwards of twenty feet but nothing like the full capacity of the pump can be realized under such circumstances. It is always better to lower the pump as much as possible and force the water instead of trying to suck it.

Note.—Upon page 212 is represented one of the very earliest types of a turbine pump, an account of which is left by Ewbanks, to whose book on hydraulics credit should be given for the figure. “This pump consists of tubes united in the form of a cross or letterTplaced perpendicularly in the water to be raised. The lower end is supported on a pivot; perforations are made to admit the water, and just above them a valve to retain it when the pump is not in motion. The ends of the transverse part are bent downward to discharge the water into a circular trough, over which they revolve. To charge it the orifices may be closed by loosely inserting a cork into each and then filling the pump through an opening at the top which is then closed by a screw-cap. A rapid rotary motion is imparted to the machine by a pulley fixed on the axis and driven by a band, from a drum, &c. The centrifugal force thus communicated to the water in the arms or transverse tube, throws it out; and the atmosphere pushes more water up the perpendicular tube to supply the place of that ejected. These pumps are sometimes made with a single arm like the letterLinverted; at others quite a number radiate from the upright tube. It has also been made of a series of tubes arranged round a vertical shaft in the form of an inverted cone. A valuable improvement was submitted by M. Jorge to the French Academy in 1816. It consists in imparting motion to the arms only, thus saving the power consumed in moving the upright tube, and by which the latter can be inclined as circumstances or locations may require.”

Centrifugal pumps designed to raise clean water alone should not be used for any other purpose, that is to say, pumps for handling more or less solid matter mixed with the water have much more clearance in the case than those for pumpingclean water. The fan is also made differently so that it cannot be clogged up by lumps of coal, gravel, and sticks of wood. The accompanying engravings, Figs.490,491and492, illustrate these ideas, showing the three progressive grades of fans for the kinds of work alluded to.

Fig.490shows a fan with hollow arms for clean water only.

Fig.491shows the disc type of fan for water containing grit, pulp, etc.

Fig.492exhibits a fan used in dredging pumps used for all sorts of stuff that will pass through the connecting pipes.

Fig. 493.

There are two general types of centrifugal pumps, viz., 1,single suction, Fig.493, in which the suction pipe enters the end of case parallel to, and in line with its center; 2,the double suction, Fig.494, in which the suction pipe is divided forming aUshape and enters the case at both sides of the center.

The single suction is used for clear water only, while the double suction will pass everything that enters the suction pipe—see engravings.

When the pump is located above the water, it has to be primed before it will raise water. For these purposes an ejector, or exhauster, is frequently employed, which will exhaust the air and draw water up from the required depth. The arrangement of the ejector is illustrated at A, in Fig.496, and is the smallest and most convenient contrivance that can be used for this work. It is screwed into the highest part of the pump, and is connected by a separate steam pipe to boiler. In a short time after turning on steam, the pump will be primed, the pump remaining stationary during the operation of priming.

Fig. 494.

To prevent air returning through the discharge pipe, a check valve, B, is used. For larger pumps a gate valve is generally employed here.

A foot valve fitted with a strainer to keep out obstructions likely to clog the pump should be used as it keeps the pump primed and ready for immediate use.

The general form of the blades is of great importance in this type of pump, because the water is driven through the fan partly by the pressure of the blades on the water and partly by centrifugal force. The ratio which each of these forces bears to the other varies in the same pump, depending upon the proportion the speed bears to the height of it. With low lifts and high speed the water is discharged with but little rotary motion, the resistance to the outward motion of the water being so small that the oblique action of the blades is sufficient to effect the discharge without imparting to the water the same speed of rotation as is given to the fan. The principal object inthis type of pump is to effect the discharge of as large a volume of water as possible with the least rotary motion. The power absorbed in imparting the latter motion is not given up later on and consequently is lost, while the rotary motion tends to impede the flow of water.

Fig. 495.

The passage through the pump should be so timed as to produce a gradually increasing velocity in the water until it reaches the circumference of the fan, then a gradually decreasing velocity until it is discharged from the pipe. These conditionsare met by having a conical end to the suction pipe, and a spiral casing surrounding the fan. The form of the casing should be such that the water flowing round the casing will move with the same velocity as that issuing from the fan; the casing enlarges from that locality into the discharge pipe.

A small increase in the number of revolutions of the fan after the pump commences to discharge produces a large increase in the volume of water delivered.

Fig. 496.—For description see page216.

Fig.495, upon the previous page, is intended to show aBoggs & Clarke hydraulic dredging or sand centrifugal pump. This is a heavy strong pump fitted with flap valve,without close fitting joints, but with ample room for the water to wash away the sand from the working parts. The cut shows the pump with ejector for priming and large elbow to discharge through. The pump is lined with sheet steel fitted so that it can be easily and cheaply replaced. The diameters in which these pumps are made run from 4 to 12 inches inclusive.

Table.

Size of PumpPipe Size of FlangesSize of PulleyCapacity per HourCubic Yards SandDischargeSuction44512 × 1030 to 4055612 × 1040 to 6066818 × 1260 to 80881024 × 1280 to 12510101230 × 12150 to 25012121436 × 14250 to 400

Fig. 497.

Smaller sizes for pumping sand and gravel are made with cast chilled linings with chilled piston, and brass covered shaft especially adapted for stone and marble mills to carry the sand to the saws, or for mining where there is a large quantity of sand to be pumped with water.

The table will convey an idea of the capacity, sizes, etc.

For pumping sand or heavy material the speed of pump should be increased 25% more than for water.

Fig. 498.

The engraving Fig.497exhibits asteam-driven centrifugal pumpof an approved design constructed by theMorris Machine Works.

This pump is directly connected to the engine and has a double suction. Pumps directly connected to engines are to be preferred over belt-driven pumps when conditions of elevation, etc., will allow, as they are self-contained, take up less space and are more economical.

The figure 498 shows a 20-inchhydraulic dredge, directly connected to a 450 horse-power triple-expansion engine. A hydraulic dredge consists mainly of a centrifugal pump with its driving engine and boiler, and hoisting machinery for handling suction pipe and boat; the pump in operation creates a strong suction flow in the suction pipe, sufficient to pick up the material and draw it into the pump, from which it is again delivered through the discharge pipe any distance to point of delivery, and can at same time be elevated to reasonable heights. Sand, mud, silt, etc., are readily picked up by the suction force only, but where material is packed it must first be agitated.

Table.

DiameterDischargeOpeningCapacity,Gallonsper MinuteElevationsin Feetup toSize Steam CylinderSizeSteam Pipe,InchesSizeExhaust Pipe,InchesDiameterStroke212025333⁄4121⁄218025333⁄41326025333⁄41447025443⁄4157352555111⁄4610503055111⁄482000308811⁄221⁄2103000106611⁄411⁄210#300040121021⁄2312420020992312420030121021⁄2312#4200401412331⁄21570003014143415*700022121021⁄2318100003015104518*1000020121231⁄220120002014143* Low-Lift Pumps. # Special High-Lift Pumps.

The steam shovel, bucket or elevator dredge will do efficient service in raising material, but none are capable of delivering the material except within a very short radius of the dredging operation. The centrifugal dredge not only raises the material, but also delivers it at the place wanted, at one operation.Besides, it is practically impossible to build any other typewith the enormous capacity that some hydraulic dredges have.

Fig. 499.

Fig.499is a perspective view of a centrifugal vertical pump ofthe submerged type. This pump is used largely by contractors in excavations and coffer dam work and for keeping pits drained.

A double suction centrifugal pump,driven directly by a steam engine, is shown in Fig.497; these are generally and very satisfactorily used for circulating water through surfacecondensers and the cooling pipes in refrigerating systems. The engine and pump thus combined occupy small floor space and consequently little masonry is required for a foundation.

Fig. 500.

The Buffalo centrifugal pumpis shown in Fig.500. These are built by the Buffalo Forge Co. in two types, viz., the submerged and the suction; the latter is the one shown in the cut. The suction type is employed for pumping from mines, pits, etc., and all places where the supply will not allow of a horizontal pump to be used, or in others where the supply is either below the pump,or sometimes above and at other times below. This type possesses merit above the submerged design in that it will work equally well, when set either above or below the liquid to be pumped.

Multi-stage centrifugal pumps.Experience has demonstrated that by placing several pumps together and discharging from one into the other, water can be delivered to almost anyheight. For a long time after the introduction of centrifugal pumps, it was supposed that about sixty feet was the limit for their economical working, owing to the high speed at which they had to be run to accomplish the results desired.

It was a discovery of importance, thatby coupling two or more pumps in series, so that each pump worked against only a part of the total delivery head, water could easily be raised to even two thousand feet or any reasonably high head with satisfactory efficiency. Pumps connected in this way will throw more water at a given speed than when operated separately, and are therefore attended by less wear and tear.

Fig. 501.

Pumps worked in series are built compound, triple or quadruple as required by service either belt driven or directly connected to engines. Owing to the fact that they have no valves or absolutely close-parts, they are able to pump muddy or gritty water with sand in suspension, and are, therefore, especially in the vertical type, the only ones that can be successfully used for draining deep mines.

Fig.501is designed to illustratea four stage centrifugal pump, or a quadruple compound pump capable of lifting water 250 feet.

Fig. 502.

Explanation of diagram page 225.In determining the requirements best suited as regards rotation of shaft and connection with the suction and discharge pipes, in installing a pump, the figure502, will be found a convenience. It is important to run the most direct pipes with the least number of elbows or bends.

The diagram relates particularly to the centrifugal pumps made by the Morris Machine Works of Baldwinsville, N. Y.; the principle is, however, of general application. In making use of the diagram each number represents a particular design. See Note.

How to determine right-or left-hand pumps.If, when standing at the suction end of pump, looking over the pump shell toward the pulley, the top of the shaft revolves from right to left, oragainst the hands of clock, the pump isright-hand, and from left to right, orwith the hands of clock, it isleft-hand.

Directions for erecting and running centrifugal pumps.Place the pump as near suction water as possible, and limit suction lift to 20 feet or 25 feet.

Erect the pump so that the pump shaft is level; in bolting to foundation be careful that the frame is not sprung. See that the bearings are clean and well oiled. The suction pipe and stuffing-boxes must be air-tight.

Never use pipes smaller than those represented by the flanges on the pump; avoid elbows or bends as much as possible; if discharging long distances, use pipes one or two sizes larger than ordinary.

Whether a foot valve is used or not, a strainer should be attached to the suction pipe to prevent large substances from entering, that might choke or clog the pump, but be careful that suction area is not contracted.

Note.—In viewing diagrams on page 225 you are supposed to stand at the outer half of pump shell looking over pump towards the pulley or engine, if directly connected.The pump can be swiveled around the frame, so that, for instance, if you order pump per diagram No. 50, it can after receipt be made Nos. 51, 52 or 53.

Run the pump in proper direction, as indicated by arrows cast on the pump shell.

If the combined length of suction and discharge pipe is more than 50 feet, the speed must be increased to overcome friction.

Before starting, prime the pump so that suction pipe and pump are filled with water.

Warm water can only be raised by suction to moderate heights, and if very hot it must flow to the pump. To prevent freezing in cold weather, drain by unscrewing plug provided in the bottom of the pump shell.

Sometimes a pump when first started will deliver a good stream of water, which gradually diminishes in volume until it stops entirely. One reason for this is a leak in suction pipes or stuffing-box of pump, or, when suction primer is used, in the hand pump stuffing-box. Another reason might be that the pump lowers the suction supply, thus increasing the lift until there is not sufficient speed for the elevation. If the pump works indifferently, delivering a stream obviously too small, it is generally because the pump was not properly primed and some air remains in the top part of pump shell. Unless primed by steam ejector, the pet cock or plug found on top of pump shell should always be open while priming, and the pump must not be started until water flows out of same.

Note.—“One feature or fact in centrifugal pumping that is overlooked or not known to many makers, is that water will not enter a pump when the impeller vanes sweep over the inlet way and are driven at high speed. To illustrate this, one can not thrust a cane or lath through the spokes of a rapidly revolving wheel. European centrifugal pumps with their small impellers and consequent high speed of rotation, are especially liable to this repelling action, and very often are wholly inefficient from this cause. One maker who claims a high duty for his pumps, attaches a screw at the sides of the impeller to coax the water into the pump, and the idea is a good one if the difficulty is not otherwise provided for. In this way a pump can be made of smaller diameter for a given duty, but it is commonly inferior to a larger one for the same work.”—Industries.

A pump with horizontal top discharge and short length of discharge pipe is sometimes difficult to start, especially if suction lift is high, owing to the fact that the water is thrownout of the pump shell before the water in the suction pipe has got fairly started, thus allowing air to rush back into the pump. If the pump is to work under this condition, it is better to use a pump with a vertical discharge and deliver through an elbow, or else lead the discharge pipe upward for a short distance so as to keep a slight pressure, or head on the pump, and after priming as high as possible start quickly.

Generally nothing is gained by running a pump above the proper speed required for a given elevation.

In addition to what is said in connection with the priming device illustrated on page 218, numerous other methods have been adopted to suit pumps of various designs. The accompanying engravings represent those largely used.

Fig.1illustrates a multi-stage turbine pump with ejector for priming. The ejector is connected to the highest point on the pump casing, and either steam, air or water under pressure may be employed in it to produce a vacuum.

Fig.2shows an auxiliary hand pump mounted on top of the discharge casing. When the pump is ready to start, the gate valve on the discharge is closed, and by operating the hand pump a vacuum is produced and water drawn in, filling the suction pipe and casing.

The method of priming shown in Fig.3may be resorted to where a foot valve is used on the suction pipe. Water is allowed to run into the pump until it reaches the discharge flange, when the supply is shut off, and the pump may be started.

After the pump has been properly primed, it should be started before the gate valve on the discharge is opened. When full speed is reached, the discharge gate may be slowly opened, and the pump will perform its work.

Note.—The Worthington centrifugal pumpsare divided into three classes, viz.:Conoidal,Voluteand Turbine.The Conoidal Centrifugals(named from the cone-shaped impeller) are designed especially for low lifts and large deliveries and are adapted to irrigation work, the handling of sewage and similar purposes. They are comparatively inexpensive and operate at high rotative speeds, making possible direct connection to electric motors.For heads up to 30 feetthey are unexcelled in the pumping field.The Volute Centrifugals(illustrated on page 232) are built for medium lifts, but for all capacities. Since they run at moderate speeds, diffusion vanes are not needed, but the volute casing has been carefully designed to obtain high efficiency and 86% has been shown under test. These pumps are recommendedfor heads up to 70 feet, although they will safely withstand 150 feet.

Note.—The Worthington centrifugal pumpsare divided into three classes, viz.:Conoidal,Voluteand Turbine.

The Conoidal Centrifugals(named from the cone-shaped impeller) are designed especially for low lifts and large deliveries and are adapted to irrigation work, the handling of sewage and similar purposes. They are comparatively inexpensive and operate at high rotative speeds, making possible direct connection to electric motors.For heads up to 30 feetthey are unexcelled in the pumping field.

The Volute Centrifugals(illustrated on page 232) are built for medium lifts, but for all capacities. Since they run at moderate speeds, diffusion vanes are not needed, but the volute casing has been carefully designed to obtain high efficiency and 86% has been shown under test. These pumps are recommendedfor heads up to 70 feet, although they will safely withstand 150 feet.

It is always best to use a foot valve in connection with centrifugal pumps where the lift is more than three to four feet, and even under these low lifts where long suction pipes are used to conduct water long distances, foot valves should always be used to keep the pump and suction pipe charged.

Figs. 1, 2 and 3.

Fig. 503.

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