September, 1923
INDUSTRIAL r l N D ENGINEERING CHEMISTRY
929
Pumps for Industrial Purposes By W. F. Traudt TABER P U M PCo., BUFFALO, N. Y.
r
THE pump is a vital part of many industrial processes and yet there is probably no piece of equipment which is purchased with so little consideration as to its application. There are an almost infinite number of combinations of mechanical elements which can be assembled into a pump design. It therefore becomes possible, by marshaling such elements properly, to build a pump which will meet an industrial problem more satisfactorily than “just a pump.”
I
advantage of delivering a steady flow without any pulsations or shocks through the filter medium. It also has the advantage that as the filter becomes clogged, the rate of flow will automatically adjust itself to the increased filter resistence, and no undue pressure will accumulate on the filter with the possibility of rupturing the cells or the filter itself. A plunger pump will deliver a relatively small volume of fluid against a high head more economically than either a centrifugal or a rotary pump. A rotary or plunger type pump will handle viscous materials which would be impossible with a centrifugal. A rotary pump designed for the purpose will handle materials which are too viscous for either a plunger or piston type pump (materials like white lead paste, etc.). It must, however, be borne in mind that such materials cannot be handled by an ordinary rotary pump. I t must be designed for the purpose in order to assure success. The fact must not be lost sight of that there is no universal rotary pump. In fact, there is no universal pump of any type. It follows that each type or design of pump is limited to certain uses. These limitations are governed by a variety of conditions, but fundamentally the viscosity, temperature, volume, head, and chemical properties of the fluid are most important,
EFFECT OF VISCOSITY OF FLUID
PISTON TYPEPUMP
There are many types of pumps, but such as are of industrial interest can, in a general way, be placed in four groupsnamely, piston, plunger, centrifugal, and rotary. There are, however, many variations in design and construction in each group. There are pumping problems for which a piston or plunger type pump is better suited than any other. There are also conditions where nothing will so satisfactorily fulfil the requirements as a centrifugal. Then again, there is a combination of circumstances surrounding a pumping problem which cannot be better taken care of than through the medium of a rotary pump; thus, applying the pump to the job. If a fluid has the consistency of water or is no more viscous than light motor oil, then a centrifugal pump can be applied with success. Whether the centrifugal pump should be of the single or double-suction type, or equipped .with open or enclosed impellers, depends upon circumstances surrounding the installation, such as volume in gallons per minute, head in feet, and the method available for driving the pump. In general, it can be stated that for large volumes at low head a single-suction, open impeller type pump is usually satisfactory, while for high heads the double-suction, enclosed, impeller type centrifugal pump is more efficient. A small volume, say 50 gallons per minute, and a high head, 50 to 75 pounds per square inch, is a combination of conditions which can be better met with a piston or plunger type pump than a centrifugal, unless there is some very special reason why a centrifugal pump should be considered. Such a reason might be found in passing certain kinds of fluids through filter presses, because the centrifugal pump has the
When using the term “fluid,” every substance is included which is plastic enough to permit it to flow through a pipefor example, commercial white lead and oil, cereal mash, chilled lard, tooth paste, paper pulp, pitch, tar and asphalt, chocolate paste, glucose, molasses, etc. Good practice would dictate that viscous materials must either gravitate or be fed to the pump inlet. Such materials
TRIPLEX P L U N G E PUMP R
do not respond readily to suction lifts, as they have a high coefficient of friction; therefore, one must avoid trying to impart an excessive velocity of flow to them through any part of the pumping system. A pumping system here refers to the whole equipment from the source of supply to the point of discharge, including the vessel, agitator or equivalent in which the source of supply originates, and the
930
INDUSTRIAL A N D ENGIhTEERING CHEMISTRY
container into which it is to be delivered. If a paste or viscous material can attain a normal (efficient) velocity of 0.5 feet per second the pumping system should be so arranged that in no part of it a greater velocity is attempted than the ideal condition would include, not even through the pump, its ports or valves. Thus, a 3-inch pump might efficiently handle 300 gallons per minute of a fluid of low viscosity, and yet not more than 10 gallons per minute could be successfully handled when the material is highly viscous.
IL R I N G DCPRINGS
O P E N IMPELLER T Y P E CE”CR1FUGAL P U M P
h pump speed or capacity must be synchronized with the fluid’s economical flowing capacity, and all pipes and fittings used in the pumping system must also be of such sizes to accommodate the economical flowing capacity of the material pumped. If a pump has a capacity too large for the normal rate at which a viscous fluid will flow, there will be a tendency to “wire dram” the fluid in the suction pipe and set up a high vacuum in the same. This will result in having the pump’s displacement only partly filled and under vacuum, causing shocks and setting up serious vibration and noise in either piston, plunger or rotary type pumps. The reason for this condition is obvious. It will be seen, therefore, that it is imperative to avoid all attempts to pump a T‘ ’l S C O l E fluid faster than its viscosity will permit it to flow. EFFECTOF PERFECT V~cuuar Too few people realize that even if a pump were able to produce a perfect vacuum on the suction side, the only head pressure forcing the fluid to the pump would be equal to that of the atmosphere, provided the source of supply was on the same level as the inlet to the pump. It would be considerably less, however, with a vertical suction lift, and as much less as the static head resulting from a lift below the level of the pump inlet. A practical ekample might be suggested by assuming 100 feet of straight 1-inch pipe connected to the discharge of a pump. This line of pipe would require a head equal to approximately 12 pounds per square inch to force water a t the rate of 20 gallons per minute through it, because the velocity set up in the pipe requires this pressure to overcome the frictional resistance. Should it be required of a pump to draw water to the inlet, a vacuum of approximately 25 inches would have to be created by the pump to cause a flow of 20 gallons per minute through a 1-inch straight pipe 100 feet long. If the water were warm or saturated with air, it is obvious that such a vacuum could not be maintained, and thus the capacity would be less. If the suction pipe line were longer, the rate of flow could not be established because of the increased friction resulting. It follows that the pump cylinder would then only be partly filled with water while the balance of the space would be under vacuum. Such a condition would cause shock
Vol. 15, No. 9
within the pump as well as the pipe line, resulting in noise and vibration which might cause serious difficulty. This being the condition with water, how much more care must be exercised when given a viscous material to handle, the coefficient of friction being many times greater than that of water. RELATIOX OF PUMPEFFICIENCY TO PLAXT EFFICIEXCY Efficiency is always an important item to be considered in the selection of pumps or any other equipment, but the meaning of efficiency must he applied in its broadest sensethat is, the ultimate efficiency as related to other equipment in the process or plant-namely, freedom of interruptions, etc. 4 pump may h a w a very high mechanical efficiency in itself and yet be the cause of great inefficiency in the plant or process as a whole, while a pump of low mechanical efficiency may bring up the plant efficiency to an exceedingly high level. Some concrete experiences will illustrate this statement. In a plant handling liquid foodstuffs an excellently made and efficient pump was used and would have been correctly applied if the fluid had been of a kind where the element of cleanliness was not important. But the pump was of such construction that accessibility to the interior, ports, etc., was impossible, thus permitting the breeding of bacteria, which seriously impaired the flavor and quality of the product. The difficulty was overcome by building a special type of open impeller centrifugal pump in which the features making for higher efficiency had to be sacrificed in favor of obtaining the essential sanitary features. Thus, a higher efficiency was established within the plant by the application of a pump of lower efficiency, but sanitary in construction. In a certain chemical indusbry in which a great many pumps were installed, continual interruptions were experienced because a standard open impeller type pump had to be used, and since the service was continuous some of these pumps would wear out within from four days to a week. The material to be handled was not only of an acid nature, but held in suspension materials which were highly abrasive. The pumps were operated at the most efficient speeds considered strictly from a hydraulic standpoint. The pressure heads were relativelv high and reauired high meeds to obtain good mechanical efficiency. The writ& was consulted on the matter and advised that iniprovement could only be obtained by using larger pumps and operating them at Y
Y
ROTARY P U M P BELT DRIVEN
slower speeds, which in some instances reduced the mechanical efficiency of the pump unit to half, with the result that shutdotvns and interruptions were reduced. Thus, the total plant efficiency was improved. In one case, with a pump which had to be replaced every week, the time was extended to a month’s operation before repairs or replacements had to be made; and in another
September, 1923
IATD CXTRIAL A.VD ENGINEERING CHEMISTRY
instance, by changing the alloy from which the pump was made the service was extended from five weeks to eleven months. Therefore, it is very evident that the application of efficiency must receive the broadest consideration. Thus, again, the importance of careful selection and application is obvious. USUSUALFORMS OF PUWPS
If the subject of pumping fluids is given a little earnest thought, it is amazing to find how dependent upon some form of pumping operation the world is. A siphon is a pump. Capillarity is a form of pumping and is frequently the only means by which line shafts are lubricated. Vegetation itself depends upon the pumping operation such as is accomplished through the rootlets of trees conveying moisture to the smallest twig and leaf. In short,
931
any process which will cause a fluid to be raised from one point to another a t the same or higher level can be considered a pumping system. Those who are interested in familiarizing themselves with the historical development of pumps will find a brief outline of the subject as practiced in the earliest stages of civilization in a book bv Arthur M. Green on “P;mping Machinery.” It follows, therefore, that if one will consider industrial pumping problems with more care, and examine into all phases of the proposed pumping system, never losing sight of the physical and chemical properties of the fluid to be handled, there will result incalculable savings, not only in fuel or power, but through fewer interruptions in manufacturing processes. Too often when laying out an industrial process, one thinks of the pump a t the “eleventh hour,” and then only in terms of “just a pump.”
Detector for Water Vapor in Closed Pipes’ By E. R. Weaver and P. G. Ledig BUREAUOF STANDARDS, WASHINGTON, D. C .
A simple deuice is described for determining the approximate concentration of water uapor in a gas. A glass tube is coated with platinum and the coating diuided by etching into two electrodes. Platinum wires sealed through the glass connect the electrodes to a measuring circuit. The resistance to alternating current of a thin film of a hygroscopic electrolyte bridging the gap between the electrodes is used as the measure of the water uapor in the atmosphere
with which the film is in contact. Sulfuric and phosphoric acids and uarious hygroscopic salts can be used in forming the conducting film. The detector is simple, rugged, and easily adapted for use in high-pressure piping and other situations in which the determination of water uapor is usually attended with diflculfy. Laboratory experiments showing the reliability. method of application, and limitations of the deuice are described.
HIS device was suggested by a description of the invention of Todd and Bousfield2 for the same purpose. The apparatus described in the patent consists of two gauze electrodes separated by a layer of granular calcium chloride or other hygroscopic salt enclosed in a tube through which the gas is passed. The contents of the tube remain nonconducting so long as the calcium chloride is dry; but in the presence of much water vapor the granules become coated with a continuous film of solution which establishes electrical connection between the electrodes and gives an indication of the moisture present. A somewhat different arrangement was used by Paul A n d e r ~ o n ,d~i o dipped the ends of two wires into the fused salt and exposed the bead of adhering salt to the gas to be tested. Two objections to these arrangements which it seemed possible to overcome are (1) the time and the amount of water required to produce a definite effect, and ( 2 ) the difficulty of restoring the detector to its original condition for further testing. In addition the device of Todd and Bousfield requires a definite circulation of gas through the tube. These objections are largely eliminated in the present apparatus (Fig. l), which is made of a piece of straight glass tubing sealed at, one end. Near the closed end two platinum wires are sealed through the glass and fused against the outer surface of the tube, which is then frosted with a paste of barium sulfate and ammonium fluoride. The ends of the platinum wires inside the tube are soldered or fused to copper leads. The outside of the cell is carefully platinized by applying a colloidal solution of platinic chloride in lavendar oil, as described by McKelvey and Tay10r.~ This process
is repeated until the layer of platinum is entirely smooth and continuous, care being taken that the platinized surface is in good contact with the electrode wires. The tube is then coated with paraffin and a line etched around it between the two electrodes with hydrofluoric acid to break the continuity of the platinum surface. On this narrow surface of separation and extending over the platinized electrode surfaces on both sides is painted a dilute solution of some hygroscopic electrolyte. In order to prevent motion which might break the leads, and a t the same time to assure good insulation, it is well to fill the tube with melted paraffin and allow it to solidify.
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1 Receiwd July 7 , 1923. Published by permission of the Director, U. S. Bureau of Standards. 2 British Patent 137,547 (1920). a J. Chem. Soc ( L o n d o n ) , 131, 1153 (1922) 4 J. A m . Chem. S o c . , 43, 1366 (1920).
The tube is then mounted in any suitable way (by passing through a paraffined cork stopper for ordinary laboratory work at atmospheric pressure) with the prepared end of the tube exposed to the gas to be tested. The resistance to an alternating current of the film bridging the “scratch” between the electrodes is a measure of the concentration of water vapor present,
Lh e €fched Around Tube.
FIG. DETECTOR FOR WATERVAPOR. THESIZEOF THE TUBEIs OF LITTLECONSEQUENCE. THOSE EMPLOYED IN THE LABORAHAVE USUALLYBEEN MADE FROM AHY CONVENIENT LENGTH, HAVI~G ABOUT 8 MM. OUTSIDEAND 5 MM. INSIDB DIAMETER TORY
TUBESOF
