_The centrifugal pump raises the liquid to be displaced, by means of a
rapidly revolving fan_ having 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 pump_ cannot 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.
[Illustration: 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 vanes_
that 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.
[Illustration: FIG. 490.]
The centrifugal pump is _the 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 of _ten 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.
489 and its description in the Note on page 214.
[Illustration: FIG. 491.]
[Illustration: 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 letter ^T^ placed
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 letter ^L^
inverted; 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 pumping clean 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, 491 and
492, illustrate these ideas, showing the three progressive grades of
fans for the kinds of work alluded to.
Fig. 490 shows a fan with hollow arms for clean water only.
Fig. 491 shows the disc type of fan for water containing grit, pulp,
etc.
Fig. 492 exhibits a fan used in dredging pumps used for all sorts of
stuff that will pass through the connecting pipes.
[Illustration: 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 a ^U^ shape 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.
[Illustration: 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 in this 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.
[Illustration: 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 conditions are 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.
[Illustration: FIG. 496.—For description see page 216.]
Fig. 495, upon the previous page, is intended to show a _Boggs & 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.