PUMPS AND ACCESSORIES
Selection of Equipment Requires Careful Consideration, as There is no One Best Type for all Conditions—Pumping by Compressed Air
PROBABLY no line of water works machinery, or equipment has been subjected to such a continual change during the past few years as that of pumping apparatus. It was not so very long ago that by water works pumps was meant only the reciprocating type. Although for many years the centrifugal pump had been used for the elevation of large volumes of water to moderate heights, under which conditions it had given satisfactory results, it did not secure much of a following in the water works field.
In its earlv use, before the theory of the design of the centrifugal pump was properly worked out, it was the general opinion that its use was limited to low lifts. In the last decade the placing on the market of reliable high speed motors and steam turbines adapted to be directly connected to this type of pump has resulted in great improvements in the design, and consequently in the efficiency of the centrifugal pump. Centrifugal pumps are now employed where previously only reciprocating plunger pumps were successfully used. Like many other lines of manufacture, there are many types and styles of centrifugal pumps. This is largely due to the fact that experience teaches that the proper type of centrifugal pump for any location must be chosen for the particular work it is to do if most satisfactory results are to be accomplished. At present there are designs of centrifugal pumps for high pressure, for low lift, for well service, or for inclined shaft use. Centrifugal high pressure fire pumps are approved by the Underwriters, while low lift irrigation pumps required to handle water with plenty of sand and dirt in it are doing their work in most satisfactory manner. The favor gained by the centrifugal pump in water works will not wane but instead may be expected to increase rapidly.
Reciprocating plunger pumps of large capacity and also well pumps will remain the choice of those who look for high pump efficiencies. The high state of perfection reached in the manufacture of these pumps has left little room for improvement, so that changes in design during the past few years have been confined to those of a minor nature.
The use of compressed air as an agent for raising and transmitting water and other liquids is a comparatively new departure; but such rapid strides have been made in its application and so many systems devised and presented that the past few years have experienced many air lift installations in water works, particularly in the smaller plant where the source of supply consists of a number of wells. The principal advantage derived from the use of the air lift pump, and the one that makes it so readily adaptable to well systems, is the ease with which compressed air can be transmitted over great distances and the slight losses encountered in so doing. One of the most serious handicaps to the air lift is the high percentage of submergence necessary to proper operation. On this account, installation in shallow wells with comparatively high lifts is impracticable, a fact which limits the use of air lift pumps to wells in which the depth of water is from 50 to 70 per cent, of the depth of the well. While the actual pumping efficiency of the air lift is admittedly low, still the over-all efficiency figured from the power end of the compressor to water delivered in the
reservoir and taking into account upkeep and repairs, compares most favorably with any other means of deepwell pumping. The necessity of a compressor, in addition to the power unit, makes the air lift less suitable for very small water plant installation than well pumps requiring only the power unit in addition to the pump. Nevertheless. there are numerous cases in which the air lift is the most desirable and in view of the recent developments in this type of pumping apparatus prospects for the future seem particularly satisfactory.
The rotary pump, whose chief use is for fire service, will continue to be one of the favorites in this branch of water service. Positive displacement along with light weight are its desirable features.
The value of proper packing in piston pumps and air compressors is too well known to require emphasis here. The varieties of packing and gaskets are numerous and are made to accommodate every special type of service.
Pump slippage due to faulty or defective valves is rapidly diminishing in progressively operated water plants, for valves are now manufactured which have both long life and the ability to seat tightly. Furthermore, the false notion of economy in refusing to junk inefficient valves has been quite completely swept aside by the almost universal campaign for better plant management. Valve replacement at definite intervals, rather than when pump slippage becomes pronounced, is becoming the usual pumping station practice, and particularly in water works where the uniform service makes it possible to secure high pumping engine efficiency.
Pumping by Compressed Air
There is no one best type of pump for all conditions and localities so that in the selection of pumping equipment careful consideration must first be given to all the requirements to be met. Such apparently simple questions as the location of the water supply, the available quantity, the purity of the water itself and its depth below the surface all influence the selection of the type of pumping equipment that will take care of the conditions most satisfactorily. In this particular connection, however, we are limited to water supply obtained from artesian or socalled “deep wells,” which often form the only available source of supply of a great many villages and smaller cities.
For years, the only practical method of drawing water from an artesian well was the deep well pump in which the sucker or piston of the pump is placed either at the bottom of the well or a point sufficiently far down in it to insure at least partial submergence at all times. 1 here are thousands of pumps of this type in use and where the water is free from sand or other gritty impurities, the valves and piston cups have a comparatively long life while the pump itself is not apt to give trouble. The great length of sucker rod usually necessary, and the heavy weight to be moved confine the application of this type to cases where a comparatively small supply is required. They are particularly adapted to cases where there is a very small depth of water in the well.
For lifts not exceedingly 150 feet and where the tubing of the well is sufficiently large to permit of its installation, either a deep well centrifugal or an impeller type pump is often employed, some of these being illustrated in the section on Electric Motors and Oil Engines. Both of these types are also particularly adapted where the supply in the well does not rise more than a short distance in the tubing. Where there is an ample supply of water so that it rises to a point which permits of the required degree of submergence and the flow into the well from below is such that pumping does not lower this submergence level over much, the air lift has demonstrated that it has several important advantages over practically every other type for deep well pumping.
Briefly outlined, the principle of pumping by compressed air consists of the admission of air under pressure below a column of water, the control of the air admission to secure economy and a means of separating the air from the water. It will be apparent that the movement of the column of water is by pulsations, corresponding in a way to the strokes of a deep well plunger, but these pulsations follow one another so closely that they are not perceptible and the discharge stream is continuous. The need for ample submergence will be plain from the fact that unless there is sufficient weight of water over the air inlet, the stream of air under high pressure would “blow” right through it and escape without moving the water.
All air lifts or deep well displacement pumps which operate by compressed air, are based on the same fundamental principles, but each particular make is naturally distinguished by special features of its own in their application. Instead of attempting to give the reader a description of these various patented features of each make of air lift, several typical installations are briefly described instead. From a comparison of the latter with your own conditions, it will be much easier to determine whether an air lift is the most suitable for your purpose or not than it would be from reading a description of the apparatus itself.