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PUMP 



PUMP 



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made, it is, however, essential that it should be perfectly straight and 

 true in its bore ; even when the piston does not work in close contact with 

 it. The power of a pump, of whatsoever description it may be, depends 

 xipon the inner diameter of the cylinder. It is considered to be small, 

 when the diameter is less than from 5 to 6 inches ; it is considered to 

 be large when it exceeds 1 foot in diameter ; and it is only in the great 

 mining operations, or in municipal water supplies that the diameter 

 of pump cylinders exceeds 2 feet. The length of the cylinder only 

 exceeds that of the stroke of the piston by a small quantity. 



The piston is the most important part of the machinery of a pump ; 

 it is the part which requires the greatest skill in its manufacture, and 

 upon whose correct action the success of the whole machine depends ; 

 under these circumstances, as might have been expected, it has been 

 modified in the greatest possible variety of manners. If the piston 

 raise the water during its ascent, or its descent only, its action is said 

 to be single ; if on the contrary, it should raise the water by both those 

 motions, the action is said to be double. The ordinary suction pumps 

 have wooden pistons, working hermetically against the sides of the 

 cylinder, but bearing a clack in the centre, through which the water 

 rises on the down stroke, tcr be lifted on the up stroke to the point of 

 discharge. A more complicated description of piston for the purpose 

 of raising the water on the up stroke is used in the lifting pumps, and 

 they have to support the weight of the column of water above them. 

 In the ordinary forcing pump, however, the piston only forces the water 

 forward on the down stroke ; but in the double action forcing pumps, 

 the water is raised by both the up and the down strokes, by a 

 peculiar arrangement of the clacks and delivery pipes, to be noticed 

 hereafter. 



The pistons of common pumps are made ordinarily of a piece of 

 hornbeam, to which the rod is attached by a kind of stirrup-handle, 



Fig. 1. 



Fig. 2. 



and whicli ia pierced by a circular opening, a (fig. 1), whose diameter 

 Nil to about half that of the piston. In the simpler forms of tho 

 suction and forcing pumps, the piston is made solid, and the water 

 raised into the barrel or cylinder by the up stroke of the piston is 

 forced on the down stroke to pass through a foot-valve placed upon 

 the rising pipe fixed by the side of the cylinder, l> (fej. I); there is a 

 valve also placed at the top of the arrival pipe, and both these valves 

 are made so as only to open upwards. Under whatsoever circum- 

 stances the pistons may work, they are made to fit closely against the 

 sides of the cylinder, by means of a species of packing, in order to cut 

 off any connection between the air above and below them ; but of 

 , whilst thus excluding the passage of air, they must not give 

 rise to such an amount of friction as to cause any serious waste of 

 labour in pumping. The packing may consist either of leather, spun- 

 yarn, or of some anti-friotion metal, acted upon by an internal spring. 



The valves of pumps are known under the names of clack-valves, or 

 of spindle-valves. The former are in fact flat plates of wood, metal, or 

 leather, working on a hinge in only one direction, a and b (fye. 2 and 3), 

 fitting closely upon the seating or face of the valve, and sufficiently 

 stiff to resist deformation by the weight above them. The latter are 

 either portions of cones, or small globes kept in their positions by 

 means of spindles traversing their axes ; upon the spindle a stop is 

 placed to regulate the dimensions of the passage way. There are, 

 however, some modifications of the spindle-valves, in which lateral 

 guides are substituted for the central spindle, so that perhaps the term 

 leat- valves would be more generally applicable to the class ; and in the 

 pumping-engines on the Cornish system a very complicated system of 

 machinery is introduced, under the name of the equilibrium-valves, to 

 regulate the admission of steam to the cylinders. The consideration 

 of these steam valves hardly comes under this portion of our subject, 

 but it has been thought necessary to allude to them in passing, as 

 being connected with pumping machinery. When very foul water 

 has to be pumped, or when a very heavy column of water has to be 

 ABTS ASD SCI. DIV. VOL. VI. 



raised, there are also introduced some special forms of valves, such as 

 the ball-and-socket valves, &c., a (%. 4), which it would be impossible 



to describe in detail. It may suffice here simply to add that, inasmuch 

 as the valves are precisely the portions of a pump which are the most 

 likely to get out of repair, it is essential that means should be pro- 

 vided for obtaining easy access to them. This is usually effected by 

 swelling out the pipes immediately above them, and by placing a 

 moveable cover over the increased space thus enclosed. 



The suction and rising mains do not require any special description ; 

 for all that is required in the former, in addition to the ordinary con- 

 ditions of pipes, is, that they should have some provision made at their 

 lower end for keeping out large extraneous substances, and that the 

 valve-seats and chambers of the upper end should be provided for. In 

 the rising mains, the junction pieces, the valve-seats, and the delivery 

 cisterns, are the only portions which require special castings. The 

 suction pipes can rarely be made of more than 28 feet in length, and 

 their diameter should not exceed more than or A of that of the 

 cylinder, without however falling below the latter dimension. The 

 diameter of the ascending main should bear about the same propor- 

 tions ; its length need only be regulated . by the power able to be 

 applied to it; in some mines, ascending pipes of 750 feet in length 

 are used. 



In D'Aubuisson's admirable treatise, ' De 1'Hydraulique,' the reader 

 will find (at page 511, edition of Paris, 1840), the various conditions of 

 the respective kinds of pumping machinery discussed in the most 

 elaborate and philosophical manner. In this cursory notice it may 

 suffice to state that D'Aubuisson shows that an ordinary suction pump 



j cannot work efficiently at a greater distance, measuring from the 



'. underside of the piston to the surface of the water at the lower level, 

 than 126, in which expression 4 will equal the height of the barometer 

 at the place observed. This distance will vary between 30 feet and 

 26 feet 8 inches, according to the elevation ; but the effective height 



I will be further diminished by the interval between the bottom of the 

 stroke and the foot valve ; the interval in question should be made as 

 little as possible, and should not exceed one-tenth of the diameter of 

 the cylinder. 



There is in all descriptions of pumps a certain amount of power 

 required to be exerted, in addition to that which is necessary to raise 

 the quantity of water to be delivered. In an ordinary suction pump 

 this additional power is, on the. ascending stroke, composed of the 



' weight of the water above the piston, of the weight of the piston itself, 

 and of the friction of the various parts. D'Aubuisson calculates that, 

 in well-made pumps, the sum of these elements is equal to about /, h 



! more than the real statical weight resting on the piston. On the 

 descending stroke the piston meets with a resistance arising from 

 the contraction of the fluid vein, when the water passes through 

 the clacks, and from the friction of the packing against the sides of the 

 cylinders ; as this resistance is, however, considerably less than that 

 which is overcome on the ascending stroke, little attention is paid to it. 

 In cases where large quantities of water have to be raised by pumps of 

 this description, it is customary to couple them, in order to equalise 

 the power exercised during the upward and downward strokes, by 

 making the motions of the respective pistons reciprocally alternating. 

 The two pumps pour their water into the same head, which thus gives 

 nearly a constant discharge. 



The resistances involving loss of power are much more numerous 

 and more complicated in the case of force pumps, for in them not 



