Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/549

533 MACHINERY.] which the amount of fall can be regulated. At o is a delivery valve, opening outwards, which is often a ball-valve but sometimes a flap-valve. The water which is pumped passes through this valve into the air vessel a, from which it flows by the delivery pipe in a regular stream into the cistern to which the water is to be raised. In the vertical chamber behind the outer valve a small air vessel is formed, 533 PUMPS. Fig. 201. and into this opens an aperture inch in diameter, made in a brass screw plug 6. The hole is reduced to T ^- inch in diameter at the outer end of the plug and is closed by a small valve opening inwards. Through this, during the rebound after each stroke of the ram, a small quantity of air is sucked in which keeps the air vessel supplied with its elastic cushion of air The discharge valve d is of greater weight than the statical pressure of the water on its under side. When, therefore, the water is at rest in the supply pipe this valve opens. In conse quence of the flow through this valve, the water in the supply pipe ac quires a gradually increasing velo city. The upward flow o the water, towards the valve d, increases the pressure tending to lift the valve, and at last, if the valve is not too heavy, lifts and closes it. The forward momen tum of the column in the supply pipe being destroyed by the stoppage of the flow, the water r g- 202. exerts a pressure at the end of the pipe sufficient to open the delivery valve o, and to cause a portion of the water to flow into the air vessel. As the water in the supply pipe comes to rest, the valve d opens again and the operation is repeated. Fart of the energy of the descending column is employed in compressing the air at the end of the supply pipe and expanding the pipe itself. This causes a recoil of the water which momentarily diminishes the pressure in the pipe below the pressure due to the statical head. This assists in opening the valve &amp;lt;/. Mr W. Anderson states that the recoil of the water is sufficiently great to enable a pump to be attached to the ram body instead of the direct rising pipe. With this arrangement a ram working with muddy water may be employed to raise clear spring water. Instead of lifting the delivery valve as in the ordinary ram, the momentum of the column drives a sliding or elastic piston, and the recoil brings it back. This piston lifts and forces alternately the clear water through ordinary pump valves. 186. The different classes of pumps correspond almost exactly to the different classes of water motors, although the mechanical details of the construction are somewhat different. They are properly reversed water motors. Ordinary reciprocating pumps correspond to water-pressure engines. Chain and bucket pumps are in principle similar to water wheels in which the water acts by weight. Scoop wheels are similar to undershot water wheels, and centrifugal pumps to turbines. Reciprocating Pumps are single e or double acting, and differ from water-pressure engines in that the valves are moved by the water instead of by automatic machinery. They may be classed thus : (1.) Lift Pumps. The water drawn through a foot valve on the ascent of the pump bucket is forced through the bucket valve when it descends, and lifted by the bucket when it reascends. Such pumps give an inter- mittent discharge. (2.) Plunger or Force Pumps, in which the water drawn through the foot valve is displaced by the descent of a solid plunger, and forced through a delivery valve. They have the advantage that the friction is less than that of lift pumps, and the packing round the plunger is easily accessible, whilst that round a lift pump bucket is not. The flow is intermittent. (3.) The Double-acting Force Pump is in principle a double plunger pump. The discharge fluctuates from zero to a maximum and back to zero each stroke, but is not arrested for any appreciable time. (4.) Bucket and Plunger Pumps consist of a lift pump bucket combined with a plunger of half its area. The flow varies as in a double-acting pump. (5.) Diaphragm Pumps have been used, in which the solid plunger is replaced by an elastic diaphragm, alter nately depressed into and raised out of a cylinder. The variation of velocity of discharge would cause great waste of work in the delivery pipes when they are long, and even danger from the hydraulic ramming action of the long column of water. An air vessel is interposed between the pump and the delivery pipes, of a volume from 5 to 100 times the space described by the plunger per stroke. The air in this must be replenished from time to time, or continuously, by a special air-pump. At low speeds not exceeding 30 feet per minute the delivery of a pump is about 90 to 95 per cent, of the volume described by the plunger or bucket, from 5 to 10 per cent, of the discharge being lost by leakage. At high speeds the quantity pumped occasionally exceeds the volume described by the plunger, the momentum of the water keeping the valves open after the turn of the stroke. The velocity of large mining pumps is about 140 feet per minute, the indoor or suction stroke being sometimes made at 250 feet per minute. Rotative pumping engines of large size have a plunger speed of 90 feet per minute. Small rotative pumps are run faster, but at some loss of efficiency. Fire-engine pumps have a speed of 180 to 220 feet per minute. The efficiency of reciprocating pumps varies very greatly. Small reciprocating pumps, with metal valves on lifts of 15 feet, were found by Morin to have an efficiency of 16 to 40 per cent., or on the average 25 per cent. When used to pump water at considerable pressure, through hose pipes, the efficiency rose to from 28 to 57 per cent., or on the average, with 50 to 100 feet of lift, about 50 per cent. A largo