Centrifugal Pumps for Motor Fire-engines.
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The reciprocating type of pump has had its day. It has been effectively challenged by the centrifugal type, and even the staunchest advocates of the older form of fire pump have been forced to offer fire-engines in which the centrifugal or turbine type has been adopted. The reason for this development is that the latter type possesses so many advantages over the former, whilst its practical efficiency is as high as, and in many cases even higher than, that of any plunger pump. One of the greatest advantages attending the use of turbine pumps is the steady flow, with consequent. absence of .shock within the pump chamber and delivery pipes. The intermittent delivery of a plunger pump sets up very-great shocks, and leads to heavy cost for maintenance of the delivery mains, etc. In every fire brigade where turbine pumps have been adopted, there has been a considerable reduction in the cost of maintenance of the lengths of hose, as a direct result of the steady and maintained delivery. The periodic deliveries of pumps of the plunger type cause surging in the hose pipes, and consequent friction with the road surface or any other supporting body on which they rest ; this friction leads to excessive wear of the expensive lengths of delivery hose. '
Another great advantage of the turbine pump is that the water to any one or all of the delivery nozzles may be shut off, whenever required, without the necessity for stopping the engine or disconnecting the pump spindle; such interruption of the water's flow may be made without the slightest danger of bursting the pump casing. When the delivery is interrupted, the suction automatically ceases, and the contents of the pump chamber are simply churned round. The dimensions and weight, compared with its capacity for delivery, are both low in a pump of this description. These are points of great importance to the designers of motor fire-engines, and they have proved to be powerful factors in helping the development of modern fire-fighting equipment ; petrol-enginedriven machines are now mostly in favour, and are likely to hold the field, and for use with such engines the turbine type of pump is most suitable.
When the simplicity of construction and small cost of centrifugal pumps are considered, it seems remarkable that their adoption should have been so long delayed ; this is perhaps due to the impression which has prevailed, until recent years, that these pumps were useless at other than quite short lifts. Now that their adaptability for, and satisfactory working at, deep lifts is recognized, there seems every reason to suppose that they will come into use in many other cases besides fireengines, where reciprocating pumps have hitherto been solely employed.
The simplest form of centrifugal pump is that in which the driven member, or impellor, is provided with a number of radial vanes, that is to say, an impellor in which the vanes follow a line drawn from the centre to its circumference ; with such a form of pump, only about one-half of the power put into it is converted into useful work. If, however, the blade be suitably curved, a sustained and uniform push may be maintained en the water as it passes from the centre to the circumference of the driven member. The exact curvature of the blade must, of course, be determined by the speed of rotation for which the
pump is designed to run, and it should be so shaped that the direction of the water's flow is not altered during its passage between the vanes. In order to make clear the necessity of this condition, let us imagine a disc rotating at a definite speed, and that the face of the disc be so prepared that a line may be inscribed upon it by means of a pencil. If we place the point of the pencil at the centre of the disc, and by a rapid movement transfer the point to the circumference, keeping it in contact with the disc during the period of translation; a curve will he traced upon the disc as is shown at A in one of the accompanying illustrations. If, now, the speed of the disc's rotation be increased, and the experiment repeated, the curve will assume a more-spiral shape, as is shown at B in the same figure. With such a form of blade, the water, like the point of the pencil, travels in a direct line from the centre—to which position it has been led through the suction pipe—to the tips of the impellor, and thence into the spiral vortex chamber in which the radial velocity which has been imparted to the water by the rapidly-rotating vanes is greatly reduced, and converted into pressure.
The question of the efficiencies ef pumps of this class has often been de-. bated by hydraulic engineers, because, at the outset, it may be somewhat difficult to account for the well-known fact that, although a centrifugal pump is merely a turbine reversed, and therefore, should show theoretically the same efficiency as the turbine, yet, in practice, the efficiency of the pump is considerably below that of the turbine. The effective work done in lifting water by centrifugal pumps varies from 55 per cent. to 80 per cent, of the work which is put into the shaft. The explanation of this difference seems to be that, whereas the potential energy
of the water in the turbine is converted into kinetic energy with very little loss, in the pump there is considerable waste of energy in converting the kinetic energy of the water leaving the wheel into the potential or pressure form, even when the greatest care be taken to avoid deflection of the course of the water and sudden enlargements in the vortex chamber. The efficiency, in all but the lowest lifts, is generally above 60 per cent., and, by careful design and good workmanship, 80 per cent, efficiency has been obtained. The lift of a pump having a single impeller is limited by the circumferential speed at which it is driven, and, when very-deep lifts are required, it is usual to put two or more such pumps in series, although they may be so designed that two, three, or four of them are built into a single casing If a single pump can deliver water to a height of 40 ft., the addition of a second one—the suction pipe of No. 2 being connected to the delivery pipe to No. 1—will increase the height of delivery to 80 ft., whilst, if a third one be connected, the jet could be thrown to a height of 120 ft. In a test which was made a few years ago by Messrs. Greenwood and Batley, of Leeds, on a duplex pump running 2,200 r.p.m., an efficiency of 70 per cent. was obtained under the following conditions :depth of suction, 19,7 ft.; height of
delivery, 137.8 ft.; eater delivered per minute, 73S gallons; power absorbed in driving the pump, 50.3 b.h.p.
Various experiments have been made from time to time and these go to show that the efficiency of a centrifugal pump increases rapidly with the quantity of water discharged_ This, of course, might have been predicted with a tolerable amount of certainty, since, for a given head and speed of wheel, the slip and friction will be nearly constant, so that an increase in the discharge will only affect the work supplied to the pump by an amount practically equal to the net work done, and so the efficiency will thus be considerably improved.
The old trouble about inducing a turbine pump to lift has now been entirely eliminated, and there is no longer any need to prime the casing before the pump can do useful work. The Halley Co. makes use of a singlecylinder single-acting air-pump, the cylinder of which is 5 in. in diameter and the piston stroke is of like dimension. This pump, which is chain driven from the extension shaft from the engine to the turbine, sucks air from the turbine easing, and quickly exhausts both the casing and the main water-suction pipe. The valves of the air-pump are so arranged that they cannot be damaged by accidental admission of water into the cylinder, and tests have shown that the Halley or
rangement is quicker at lifting than is any other combination against which it has competed.
The Dennis Co.' now, also employs an air-piimp for drawing the initial charge of water into the turbine casing, although, previously, this makers engines each carried a small tank, from which the pump drew water, anti returned it again to the tank by way of an ejector ; the ejeetcr was connected to the suction hose, and a suitable arrangement of valves allowed the tank to be cut out of action, and the suction hose to communicate direct with the pump chamber, In all recent Dennis engines, however, the lift is effected by means of a two-cylinder air-pump, the driving gear for which mar be caused to engage with the turbine spindle by simply depressing a foot-lever ; so long as this lever is held down, the air-pump will continue to work, but, as soon as water shows at a sight-hole, the pedal should be released, when the air-pump is then placed out of action. Like the Halley engine, the Dennis machines are also provided with means to prevent any damage to the air-pump cylinder and valves by the presence of water. In the Dennis engines, two separate. single-acting pumps are employed, an arrangement which is not without its advantages, as, if one pump be disabled, a sufficient vacuum to keep the. lift may be maintained by the other.