It is difficult to overestimate the importance, in connection with a
steam plant, of the appliance which supplies water for the boiler, not
only, but a hundred other uses. Upon the steady operation of the pump
depends the safety and comfort of the engineer, owner and employee, and
indirectly of the success of the business with which the “plant” is
connected. Hence the necessity of acquiring complete knowledge of the
operation of a device so important.
[Illustration: Fig. 102.]
Pumps now raise, convey and deliver water, beer, molasses, acids,
oils, melted lead. Pumps also handle, among the gases, air, ammonia,
lighting gas, and oxygen. Pumps are also used to increase or decrease
the pressure of a fluid.
Pumps are made in many ways, and defined as rope, chain, diaphragm,
jet, centrifugal, rotary, oscillating, cylinder.
Cylinder pumps are of two classes, single acting and double acting.
In single acting—in effect is _single ended_—in double acting, the
motion of the cylinder in one direction causes an inflow of water and
a discharge at the same time, in the other; and on the return stroke
the action is renewed as the discharge end becomes the suction end. The
pump is thus double acting.
A _direct pressure_ steam pump is one in which the liquid is pressed
out by the action of steam upon its surface, without the intervention
of a piston. A direct acting steam pump is an engine and pump combined.
A cylinder or reciprocating pump is one in which the piston or plunger,
in one direction, causes a partial vacuum, to fill which the water
rushes in pressed by the air on its head.
NOTE.—A _suction valve_ prevents the return of this water on the return
stroke of the piston, and a _discharge valve_ permits the outward
passage of the fluid from the pump but not its return thereto or to the
reservoir through the suction pipe.
The force against which the pump works is gravity or the attraction of
the earth which prevents the water from being lifted. This is shown
by the fact that water can be led, or trailed, an immense distance,
limited only by the friction, by a pump.
NOTE.—It may be noted that the difference between a fluid and _liquid_
is shown in the fact that the latter can be poured from one vessel to
another, thus: air and water are both fluids, but of the two water
alone is liquid: air, ammonia, etc., are _gases_, while they are also
fluids, _i.e._, they flow.
The idea entertained by many that water is raised by suction, is
erroneous. Water or other liquids are raised through a tube or hose by
the pressure of the atmosphere on their surface. When the atmosphere is
removed from the tube there will be no resistance to prevent the water
from rising, as the water outside the pipe, still having the pressure
of the atmosphere upon its surface, forces water up into the pipe,
supplying the place of the excluded air, while the water inside the
pipe will rise above the level of that outside of it proportionally to
the extent to which it is relieved of the pressure of the air.
If the first stroke of a pump reduces the pressure of the air in the
pipe from 15 pounds on the square inch to 14 pounds, the water will be
forced up the pipe to the distance of 2-1/4 feet, since a column of
water an inch square and 2-1/4 feet high is equal in weight to about
1 pound. Now if the second stroke of the pump reduces the pressure of
the atmosphere in the pipe to 13 pounds per inch, the water will rise
another 2-1/4 feet; this rule is uniform, and shows that the rise of
the column of water within the pipe is equal in weight to the pressure
of the air upon the surface of the water without.
There are pumps (Centrifugal) especially designed for pumping water
mingled with mud, sand, gravel, shells, stones, coal, etc., but with
these the engineer has but little to do, as they are used mostly for
wrecking and drainage.
The variety of pattern in which pumps are manufactured and the still
greater variation in capacity forbids an attempt to fully illustrate
and describe further than their general principles, and to name the
following general