Many different types of pumping machinery are used. For small plants and where steam economy is not important, and where the pump installation can receive attendance from the engine room force, direct-acting steam pumps have often been used in the past, in fact, until quite recent years no other type of pumping apparatus was available, except in the largest sizes. As a result different pumps were added from time to time and the number and variety of pumps to be found in the pumping station of some large blast-furnace plants was remarkable.

The direct-acting steam pump is moderately reliable, but not sufficiently so to run without attendance since the pumps may "stick on center," a valve may become blocked open, and various other contingencies may arise which necessitate inspection at frequent intervals. This type of pump is also extremely uneconomical. In the smaller sizes their steam consumption is probably always in excess of one hundred pounds of steam per horse-power hour. In the larger sizes where compounding and triple expansion are used, and where more refinements can be included in the design, their steam consumption drops to the half of this and often to the quarter, the latter figure being extremely rare except with what may be called pumping engines rather than steam pumps. In all the ordinary sizes the steam consumption is probably never less than fifty to one hundred pounds per horse-power hour.

For larger plants the vertical plunger pump with plungers directly connected to the cross-heads of a triple expansion engine, of the crank and fly-wheel type, with their cranks at one hundred and twenty degrees, are probably as economical as any type of pumping engine driven by steam machinery. They have the great advantage that being vertical, the water cylinders can be placed near the normal water level, while the steam cylinders and driving mechanism are high enough to be out of reach of any possible flood, and in case of a flood the pump cylinders may be submerged and continue to run indefinitely in that condition. These, of course, are necessarily large units and their speed of revolution is limited by the ability of the water to make its way through the valves and follow the plungers which, on account of the inertia of the water, is at a very slow rate. These pumps, therefore, seldom run at more than thirty revolutions, which makes their cost very high in proportion to their output.

In the last ten or twelve years the centrifugal pump with multiple stages has been developed to a high state of efficiency, and instead of being limited to thirty or forty feet lift, as was the old single-stage centrifugal pump some twenty years ago, these pumps are now designed for lifts up to many hundred feet. Their efficiencies with good designs range from seventy per cent. up to eighty in exceptional cases, and while this is considerably lower than the efficiency of a good plunger pump it is maintained through a long period of years.

Questions of leakage scarcely affect these pumps which operate solely by the velocity which their impellers impart to the water, and they are entirely valveless so that under any ordinary circumstances they can run without interruption for very long periods of time.

One word of caution may not be amiss here. The old centrifugal pumps were designed so that they would pass anything up to the size of cabbages, but the modern multiple stage pumps have what may be called a closed impeller with the outlets at its periphery rather narrow, so that sticks of moderately small sizes may lodge in these and cut off the water flow. For this reason screens must be provided with openings small enough to bar anything which could lodge in these discharge ports. The obstructions which will pass these ports are however larger than those which were likely to derange the valves of reciprocating pumps, and centrifugals have therefore the best of the argument in this very important respect. Owing to their extreme simplicity and to the fact that they have but one moving part, and to the entire absence of valves, these pumps are relatively inexpensive, a result which is attained by virtue of the fact that they are run at very high speeds, and pass an enormous volume of water in proportion to their size.

The master mechanic at the plant at which was installed the pumping plant illustrated above, Fig. 235, assured me with great respect, but nevertheless with profound conviction, that those pumps would "never deliver water enough to supply that furnace." When they were put into operation, however, one of them promptly ran away with the supply brought in by the race from the river; he had to bestir himself to increase the supply, and his doubts were overcome with a vengeance.

Centrifugal pumps on account of their extreme simplicity may be operated practically without supervision or at least receive it only at several hours intervals, there being only a few oil cups to look after. These pumps may be driven by any type of motor desired but for large sizes there are practically only two to be considered. One is the electric motor, the other the steam turbine.

When the pumping plant is located at some distance from the main plant, electrical transmission is to be desired, as motors will run for many hours without attention if properly installed and equipped, so that an occasional visit of an attendant is sufficient, whereas when steam machinery is employed it is virtually necessary to have an attendant on duty all the time, though his work may be of the lightest, 95 per cent. of the time.

In small sizes the electric motor driven from an economical central station is probably more economical than the turbine, because a turbine economical in small sizes has not yet been developed, but in large sizes the directly connected turbine furnishes an ideal drive for these pumps. On account of the very high efficiency of the turbine, especially in view of the fact that a surface condenser may be installed in the water main and a very high vacuum attained without much expense, it is probable that the best type of unit of this kind is as economical in steam consumption as the vertical pumping engine earlier described because the greater efficiency of the turbine offsets the smaller efficiency of the pump, and the cost is only a fraction of that of the engine-driven unit, so that where steam-driven plants are to be installed it may be expected that the turbine-driven centrifugal pump will play an increasingly important part.

In one plant of which I formerly had charge the pumping plant consisted of triplex plunger pumps belt-driven from electric motors, and while the pumps were poorly designed and were too weak for the service for which they were furnished, the combination formed (with this exception) a very satisfactory plant, and to my surprise the efficiency of the pumps was very high. A test was made of one of them using a large and accurate weir to measure the water, and although the pump was not tuned up for a test, but tested in its ordinary running condition, the efficiency of pump and motor figured out about 85 per cent. Undoubtedly a plant of triplex pumps properly designed and geared to their driving motors would be an effective and satisfactory type of pumping plant where electric drive was desirable, but of course this type of machine does not lend itself to satisfactory steam drive.

The details of these different types of pumping plants are no different from what they are in ordinary service and therefore need not be especially described and illustrated here.