FLUID FLOW T
HE laws and data pertaining to the motion of fluids are
of basic importance in many branches of technology. The civil engineer requires them in his treatment of the flow of water, often in enormous quantities, in open channels. The aeronautical engineer must utilize them in determining the speed, lift, and power requirements for his airplanes. The marine engineer must utilize them for calculating the resistances offered to the movement of vessels. The industrial physicist finds fluid dynamics an important field of research. The mechanical engineer is concerned with the flow of fluids in his power, refrigeration, and other equipment. Fluid dynamics are a t least as important in chemical engineering as in other technical fields. Just so long as his processes involve fluids which must be transported in pipe lines, just so long as he operates continuous processes to which and from which fluids must flow, and just so long as he utilizes physical and chemical processes which require that fluids be brought to or removed from a zone of reaction, will the chemical engineer be deeply concerned with fluiddynamic problems. The chemical engineer is interested in fluid dynamics for several reasons. I n the first place, he is interested in the friction losses that result from fluid flow under a wide variety of conditions. Much of his present information concerns the isothermal flow of single fluids in pipeline systems. He is, however, also concerned with the flow of fluids through beds of packed solids; often this case includes the countercurrent flow of two fluids through a packed solid. Twophase flow, including the case of immiscible liquids, a gas and a liquid, a gas and a finely divided solid, or a slurry consisting of a liquid carrying suspended solids, are all important in chemical engineering. I n the case of the countercurrent flow of two fluids, an important group of problems involves the tendency for one fluid to entrain the other; problems of flooding and holdup arise from this group of effects. On the other hand, the deliberate use of one fluid to entrain and compress another fluid is used in ejectors of all kinds. The engineering problems concerned with the design and selection of pumps are the practical aspects of the fundamental principles of fluid dynamics and fluid flow. Important methods of measuring fluid flow utilize fluiddynamic principles; these methods have been actively studied,
INDUSTRIAL AND ENGINEERING CHEMISTRY-APRIL,
and the results of the investigations are used in practice. Indeed, the importance of fluid dynamics is more far-reaching than a t first sight appears. Fluid-dynamic effects are also of fundamental importance because the rate of heat transfer and of mass transfer in fluid streams is intimately bound up with the mechanism of the fluid flow itself. It has long been appreciated that the resistance to the flow of heat and matter from one fluid phase to a solid, or from one fluid phase to another fluid phase in contact with it, usually includes a strong resistance a t each interface, and the familiar film theories of heat flow and mass transfer arise from the recognition of this fact. More recently it has been appreciated, however, that the turbulent mass of the flowing fluid also presents definite, and in some cases a very important, resistance to these same processes. Accordingly, the study of turbulence and eddying effects in moving fluids is of great importance in the rational approach to processes of heat and mass transfer. On the other hand, study of heat and mass transfer processes can shed important light on the mechanism of fluid flow itself. For this reason it is inadvisable to attempt to separate too sharply heat flow, fluid flow, and mass transfer in turbulent fluid streams; important results obtained in the study of one type of process can be valuable in understanding the others. For many years the chemical engineer has carried his weight in research and study of those portions of fluid dynamics of most interest to him. H e should not overlook, however, the fact that the technical physicist, the aeronautical engineer, the mechanical engineer, and the hydraulic engineer, are all working in the field of fluid dynamics and that the results of researches in these other groups are of importance in his own work. The papers which follow (pages 408 to 486) cover many of the subjects mentioned above. Papers are presented by workers other than chemical engineers, and these are important contributions to the literature of chemical engineering principles. It is hoped that these papers will in their aggregate comprise an important advance in the knowledge of fluid dynamics as applied to chemical engineering theory and practice. WARREN L. McCABE
1939
SELECTION OF PUMPS FOR CHEMICAL SERVICE WARD E. PRATT Worthington Pump & Machinery Corporation, Harrison, N. J.
E
VERY conceivable kind of pumping problem exists in
inch expansion of the shaft (thrust bearing to impeller). This would cause binding of the impeller on the suction head and consequent damage to the pump. It would be better for such a plant to use a more cheaply constructed, less efficient pump, in which the clearances are permanently set so large that rubbing of the moving parts cannot take place under any conditions. The desirability of a complete understanding of the p u m p ing problem between both user and manufacturer will be best illustrated by the following points that are involved in any pumping problem.
the process industries, from the simplest transfer job to those involving the handling of hazardous liquids a t extremely high temperatures and pressures. The discussion here will be limited to the types of pumping problems that occur with considerable frequency in the chemical and allied industries. Such problems can be classified roughly as follows : 1. Transferring from one open tank to another. 2.
Filter pressing.
3. Circulating for mixing, agitation, absorption, spraying,
heating, cooling, and crystallization. 4. Discharge from vacuum evaporators and filters. 5. Closed systems involving pressure or vacuum.
The scope of this paper will allow discussion of only a few of these typical problems. When they involve the handling of clear cold water, the selection of a suitable pump is relatively easy; there is such a wealth of information and experience available that most pumps give a satisfactory performance for this ordinary type of installation. When the problem involves extremely high pressures and temperatures, then the pump selection requires engineering of the highest orderfor instance, a high-pressure boiler feed pump. However, when the liquid to be handled is highly corrosive and perhaps hazardous and costly, the simplest pumping installation requires the most careful consideration from many angles. These are the special problems we are most concerned with here. Specifications and the selection of a pumping unit for any given installation are sometimes handled entirely by the user's engineering department. More often, the pump manufacturer is requested to make recommendations with little or no indication of what may be desired by the user. Probably neither procedure is as satisfactory as when there is complete cooperation between the user and the manufacturer in arriving a t the selection of a unit that will give satisfaction under the local plant conditions. By this we mean, for example, that certain refinements in pumps and accessories may be furnished that will prove an asset to the operators in a plant which maintains a corps of intelligent mechanics; on the other hand, such refinements may prove to be disadvantageous if certain adjustable features that are provided are improperly used. As an illustration, a certain type of ball bearing chemical pump has an adjustable thrust bearing, which provides the operator with a method of varying the position of an open impeller to allow for expansion, and adjustment for compensating for wear on the impeller vanes and suction head. (A '/16-inch clearance between the impeller vanes and suction head results in approximately an 18-foot loss of head in a pump with a 101/2-inch diameter impeller running a t 1750 r. p. m.) It is possible that a careless mechanic could set the impeller with a lo/looo-inchclearance a t room temperature and operate the pump a t a temperature which would cause 1 6 / 1 ~ ~ ~ 408
Materials of Construction Where highly corrosive or corrosive-abrasive liquids are t o be handled, the selection of the material is of the first importance. A high-grade efficient pump would be a total loss if i t was furnished in a material that would go to pieces in a few days or even weeks.
The selection of chemical pumps involves special problems that can only be met by the close cooperation of buyer and seller. Types of pumps and their applications are reviewed. The importance of the correct selection of material and type of pump and a thorough understanding of the operating conditions are discussed and illustrated by specific installations. Typical data to accompany inquiries for chemical pumps are shown in the form of a questionnaire. The items to consider in the final choice of a pump are summarized. Both buyer and seller can benefit by giving more consideration to a pumping job before an inquiry is sent out, before a quotation is made, and before an order is placed.
Most of the larger plants, which operate large numbers of noncorrosive metal pumps and other types of apparatus, are in a position to know what general type of material should be used for any given condition. For certain conditions the operators of such plants are the only ones who have any basis for the proper selection of a material. This may be due t o the obscure character of the liquid or process about which little is known except by the user. Again, certain elements
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409
CROSSSECTION THROUGH CHEMICAL PUMPEQUIPPED WITH WATERSEAL CAGEIN STUFFING Box Pressure points at different places inside the pump are shown in the chart as a percentage of the pump discharge pressure. The pressure at the suction inlet, A , is assumed as 8 per cent of the discharge pressure, or 8 feet positive head with 100-foot discharge head. Leakage past the rear wearing ring increases this pressure back of the impeller, G, to 10 feet which is the pressure on the stuffing box packing (no fluid seal) tending to cause leakage a t the gland. With seal cage (lantern ring) in position as shown and sealing water introduced, J, at 30 feet (13 pounds per square inch) head, the pressure in the stuffing box will show a reduction from the seal cage pressure to the pressure at each end of the stuffing box. These pressures are zero at the gland, K , and 11 feet (4.75 pounds per square inch) at the bottom of the box, H . Thus there is little tendency for the sealing fluid t o enter the pump, and the liquor or slurry pumped does not enter the stuffing box.
may poison a process or cause objectionable catalytic reactions which can be known only by] the operator of the process. The pump manufacturer who furnishes a pump made of a material specified by the buyer is largely free of any responsibility as to the corrosive effect on the material. The pump manufacturer is responsible for the mechanical suitability of any equipment he furnishes in the specified material. If his equipment is not of suitable design for construction in the material specified, he should not accept the order. For the more ordinary corrosive conditions involving sulfuric, nitric, hydrochloric, acetic, phosphoric, and many organic acids, a large number of materials can usually be considered. For instance, in a compilation by Chemical and MetaZZurgical Engineering of materials recommended (by the manufacturer) for resisting dilute sulfuric acid, eighty-six metals and alloys and many nonmetallic materials were listed. B y being more specific as to the exact conditions, this long list of materials shrinks considerably. As soon as a certain type of pump is to be considered, many of these materials are automatically eliminated because of their physical characteristics. Also, when the acid is defined as 10 per cent sulfuric acid a t 175" F., many more of the possible materials are eliminated. If the specifications are even more explicit and state that the acid may contain some abrasive material in suspension, then the number of materials decreases to a relatively short list. Further information regarding the process might enlarge or reduce the number of available materials. For instance, it might be stated that the pumped acid would contain an appreciable percentage of copper sulfate from a brass pickling process. Copper sulfate acts as an inhibitor on some grades of stainless steels and allows them to be used whereas they would be worthless for sulfuric acid alone. Or the buyer might specify that the pump must handle a cold wash water after the hot acid. This would make hazardous the use of certain materials which might break as a result of the sudden change in temperature. Another limiting condition in the
selection of a suitable material might be a requirement for the pump to handle a strong alkali alternating with acid. This dual service has a drastic action on some alloys. Pump nianufacturers who have not specialized in building noncorrosive pumps generally prefer to have the buyer specify the material of which the pump is to be made, since this limits the responsibility, However, assuming that the inquiry is to be sent only to those pump manufacturers who are known to have had a wide experience in building acid pumps, the best procedure is to give the pump manufacturer all pertinent information and get his recommendation for a pump suitable for the conditions. Recommendations by any one pump manufacturer are usually limited to those materials on which he specializes. Hence, a manufacturer specializing in lead pumps will offer only lead pumps, and another manufacturer building only rubber or rubber-lined pumps mill offer a pump of those materials. Such pumps may be suitable for the conditions but may cost more per year of service than pumps of some other material. Therefore, unless the user is prepared to select the materials of construction, he should assure himself that the manufacturer regularly builds chemical pumps of an adequate variety of materials and in suitable types. The question of economics also enters the picture, as well as the character of the plant in which the pump will be used. Thus, we would not recommend a certain nickel-copper alloy pump for 10 per cent sulfuric acid, not because i t is not suitable but because i t would have t o be built specially, whereas ' we keep pumps of other equally suitable materials in stock and can therefore furnish them a t a lower price. Although
most types of these pumps are handicaps to the builder and bring about new problems in determining the life the parts will give under corrosive conditions. Inquiries are quite frequent for rotary pumps built in some of the stainless steel alloys. The well-known galling action of these alloys when they rub together under pressure makes the construction of a rotary or screw type pump an almost impossible job, if long life is an important factor. For highly viscous, clear liquids successfully operating rotary pumps have been built with fairly liberal internal clearances. One such rotary pump is handling a hot sulfonated oil containing sulfuric acid and highmolecular-weight fatty acids that readily attack Monel metal. The viscosity was too high to use a centrifugal pump. The body, rotors, and shaft are of Worthite,' the side plates and bearing sleeves are of one of the Hastelloys. Monel and pure nickel rotary pumps are relatively easy to build because of the nongalling properties, and large numbers of rotary pumps in these alloys,are furnished for handling various rayon dopes, etc. Reciprocating pumps of the plunger type are made in many of the machinable alloys and are economical where small capacities and high pressures are necessary. This company has built many stainless steel triplex pumps for handling various rayon and cellulose acetate dopes of high viscosities, some of them for filter pressing at pressures up to 1000 pounds per square inch. Reciprocating pumps may also be justified where it is essential to avoid the churning action produced in a centrifugal pump. However, it is sometimes possible to use a large-casing, volute centrifugal pump running a t low speed for handling a product where crystal breakup has been objectionable with a small centrifugal pump operated at high speed and a t a condition point where considerable churning action is experienced. Reciprocating and rotary pumps are also usually favored for handling foaming liquids, particularly where high viscosity is also encountered. The lower cost of centrifugal pumps built of special materials makes them attractive for such services, but foaming is usually promoted by the action of a
either an alloy steel pump or a high-silicon iron pump might be offered for 10 per cent sulfuric acid, if the pump was to go to an oil refinery or a steel mill, the alloy steel pump would normally be recommended. Maintenance crews in such plants would not be familiar with the brittle qualities of highsilicon iron, and the hazard of breakage would not be present with an alloy steel pump. Perhaps material specifications for some of the more common corrosive conditions will some day be written which will give physical characteristics and rate of penetration under a definite type of laboratory corrosion test and thus avoid naming proprietary alloys. But when a 1 per cent variation of a constituent of a certain type of alloy may change the rate of corrosion by 500 per cent, and when the actual foundry technique used on the same analysis alloy may cause even more variation in the properties of the alloy, this desirable procedure seems to be a long way off.
Types of Pumps The type of pump for any given condition depends on so many factors that it is a difficult matter to discuss in a few paragraphs. There is a choice among a great many types of centrifugal pumps, as well as of reciprocating and rotary pumps. For small capacities and relatively high heads, the reciprocating and rotary pumps are more efficient, but they are usually much more costly to build in special materials. Many of the desirable noncorrosive materials have limitations in physical properties that put reciprocating and rotary pumps entirely out of the picture. Reciprocating and Rotary Pumps The necessity of close clearances or actual rubbing contact in piston and rotary pumps and the difficulty of machining
1 Worthite is a high-nickel, high-ohromium, molybdenum alloy steel, with t h e following typioal analysis (in per cent): nickel, 24; ohromium, 20; molybdenum, 3; silioon, 3.5; carbon, 0.07 maximum; other elements, 2; balanoe, iron.
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centrifugal pump. Where the condition is not too bad, or the beating action on the foamy liquid is not disadvantageous, some types of self-priming centrifugal pumps may be used. Possibly this problem could be approached by manufacturers of separating apparatus. If a relatively low-cost piece of equipment could be installed ahead of a centrifugal pump, which would separate the “foam” from the liquid, then, p?ssibly, the centrifugal pump would function satisfactorily. Diaphragm pumps have a certain field of usefulness, but their construction requires a nonmetallic diaphragm, which cuts down the field for their application. They are costly to build and a broken diaphragm usually means a high repair bill. For handling abrasive slurries they are the most satisfactory type of reciprocating pump if they can be built of suitable materials. A recent inquiry specified centrifugal pumps for handling a very small volume of an acid product a t 380 feet head. This looked like an obvious application for reciprocating pumps, since the lower operating cost would soon offset the higher first cost. Accordingly, we spent a lot of time in working up costs on reciprocating pumps only to be told that certain operating characteristics of the centrifugal pump were so desirable for their process that they must use a centrifugal pump. We then quoted on horieontal-split-case centrifugal pumps as the most efficient type of centrifugal, and were then told that only vertical-split end-suction pumps could be used in the particular location. This brings out the desirability of giving information in the inquiry of what is wanted and what is not wanted and why. It would have saved money to have sent an engineer to the customer’s plant in the first place, but i t was several hundred miles away and we thought we were going to save time and money.
Centrifugal Pumps Centrifugal pumps are used in about 90 per cent of the a p plications involving the handling of corrosive liquids. This is due to the hydraulic characteristics of the centrifugal pump and to the much lower cost. Even for the small-capacity high-head jobs, where the efficiency is much lower than the reciprocating pump, i t is usually more economical to use a centrifugal pump because of the small total power involved per year of service. The usual filter pressing job, involving hot corrosive-abrasive slurries, is a frequent application for a hard acid-resisting, alloy centrifugal pump. Such alloys are not adaptable for construction of other types of pumps, except a t enormous cost. The performance characteristics of the centrifugal pump are especially suitable for filling plateand-frame filter presses. It gives a large volume a t the start to coat the leaves quickly and evenly, and the pressure builds up gradually without pulsations. Reciprocating pumps cause pulsations which pack the cake and interfere with filtration. Many plants have found that certain pigments can be filtered in about half the time and a t much lower pressures than were formerly required with reciprocating pumps. I n handling abrasive slurries where some wear is bound to result on the pump parts, the selection of the material and the mechanical features of a centrifugal pump warrants special and careful consideration. Rubber, which resists abrasion very well, is useful only with certain acids and becomes too soft a t high temperatures. This leaves only the hard or very tough corrosion-resisting materials from which to choose. Hazards of breakage of stoneware and high-silicon irons dictate the alloy steels and high-strength nonferrous alloys for the conditions where the rate of corrosion and wear is not too high. However, if the rate of corrosion is a factor in the life of the material, i t usually lasts an exceptionally short time when abrasion is also present. Also, if the resistance to corrosion of a certain metal is dependent on a relatively thick
411
soft coating, such as ferrous sulfate on steel, which forms in concentrated sulfuric acid, an abrasive condition will cause exceedingly rapid loss of metal. This explains why highsilicon irons are so widely used for handling abrasive acid slurries, in spite of their fragile characteristics. Some slurries are thought to be much more abrasive than is actually the case. Sometimes previous failures of materials have been due more to a moderate rate of corrosion than to the abrasive character of the solids in suspension and have led to the use of high-silicon iron pumps without sufficient investigation. I n one case where we furnished a high-silicon iron pump as specified by the buyer, repair orders for parts brought about an investigation of the difficulties. It was found that the pump was washed out a t the end of a run by water supposedly preheated, but that a t times it was much cooler than the pump. This tended to put the quicker cooling sections in tension and resulted in cracked castings. Stainless steel pipe was used on this job, which indicated that an alloy steel pump could have been used to withstand the corrosion. The customer feared that the solids in suspension would cut out alloy steel too quickly, but a Worthitepumpwas substituted and an inspection after one year indicates that the impeller will give two to three years of life. Of course, the breakage stopped. I n another case a Worthite pump was used for circulating weak hot sulfuric acid on a dust-recovery job. Bronze pumps had given but one week’s service, as a result of the cutting action of the dust in suspension. The Worthite impeller lasted two q o n t h s and was thought to be satisfactory until a high-silicon iron pump was used in which the impeller gave over 12 months of service. Details of construction of various makes and types of centrifugal pumps cannot bediscussed here, although they may be of great importance to the pump operator. Since acid pumps may require more frequent repairs and servicing than water pumps, construction details affecting ease of servicing mre worthy of carefu! consideration by the plant engineer-buyer. Some more or less important features of construction may prove advantageous for some processes and of no advantage to others. Operators in one plant may have a definite preference towards some detail of design while the same construction may be rejected flatly in another plant. One such controversy may be over sleeve bearings us. ball bearings, although most plant engineers now prefer ball bearings. Another is over the close-coupled type of pump us. the so-called frame-mounted pump with independent motor. There is a tendency towards specialty constructions in centrifugal pumps, such as self-priming pumps, vertical submerged pumps, boot pumps, and pumps without stuffing boxes. All of these types have their applications.
Self-priming Pumps The self-priming pump came into existence to fill the demand for a method of starting centrifugal pumps which could not be conveniently located with flooded suction. It is not difficult to prime water pumps, but the same procedure on an acid pump is not so simple and many times is impossible. Many other advantages of the self-priming pump became apparent-notably, less trouble from leakage a t the stuffing box and the excellent application for pulling a suction on vacuum flters or discharging from a vacuum receiver where the liquor leg becomes empty a t times. Self-priming pumps are also convenient for handling “gassy” liquids, for mixing liquids with gas for absorption, and for decantation operations. Where self-priming pumps have been located above the liquor level, and particularly where automatic controls have been employed, some difficulties have been experienced with failure to prime due to air leakage into the stuffing box. This
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is not difficult to overcome, but too frequently the gland has been run too tight to try to avoid leakage, and has thus caused burned packing and a scored shaft. Self-priming pumps are also subject to troubles if they are used on liquors carrying materials that can cause clogging. Many self-priming pumps have probably been installed where other types of pumps could have been used with less trouble and at lower cost. A simple lift job out of a sump can frequently be accomplished automatically by means of an enlarged trap in the suction line which can be built by the user of the pipe material. It is usually more foolproof than some self-priming pumps since the pump is always under flooded suction. Any standard pump can then be used and will probably operate a t higher efficiency.
Vertical Submerged and Boot Pumps Vertical submerged and boot pumps are justified for certain conditions, but their record for trouble-free service is not good unless very costly construction is used. Theoretically it appears good practice to submerge the liquid end of a centrifugal pump, and then all problems of a stuffing box disappear. Also the problem of handling very dangerous liquids is simplified. The difficulties start with the problems of materials of construction that must be submerged. The usual bottom or guide bearing is a source of trouble in practically all corrosive liquids. Materials that act as good bearings are usually worthless when running in some acids. Construction that avoids the bottom bearing increases the cost materially, and the total allowable overhang is relatively short. Our experience has been that the majority of inquiries calling for vertical pumps are for conditions that can be handled by standard horizontal pumps if properly installed and operated. I n one problem, for which we quoted on both types, the vertical pumps of suitable construction were about five times more costly than suitable horizontal pumps with suction line priming chambers. A recent order for five chemical pumps (2000 gallons per minute capacity) for circulating copper sulfate electrolyte specified standard horizontal pumps located 27 inches above the supply tanks with steam siphon priming arrangement. Vertical boot pumps were formerly used for this service. Occasionally acid storage tanks are employed which are too deep to empty by means of a suction lift from the top. Where city ordinances prohibit air pressure on acid tanks and also the use of bottom or side outlets, such tanks create a pumping problem. The deep-well type of pump is not desirable, as explained above. Air lift pumping is costly to install and to operate, and air is not desirable for many conditions. One practical method to avoid the use of submerged moving parts is to use a standard chemical pump hooked up in the so-called Jetflo arrangement, which is fairly common for some house water systems. This system by-passes a portion of the liquid discharged by the pump through an injector or jet located in the suction line near the bottom of the tank. The high-velocity jet forces a much larger volume of liquid up the suction line to the pump.
Stuffing Box Problems That stuffing boxes in acid pumps have been the greatest source of operating troubles is undoubtedly true; and certain treatments of centrifugal pumps, to avoid or minimize this source of trouble, are now seen-for example, the floating stuffing box, centrifugal fluid seals, and the pump without a stuffing box. Since such devices have their enthusiastic advocates, their success in overcoming difficulties in some plants is obvious. However, such devices require extra gadgets which cost money, and if these devices fail a t any time,
VOL. 31. NO. 4
the service problem is more acute than on a standard type of pump which most plant mechanics understand. Any maintenance man can pack a standard centrifugal pump, but if repacking does not make the pump work right, or if the trouble is in some mechanical detail of a special device, it usually means that the pump manufacturer has a complaint to satisfy. In the meantime the customer may be without a pump unless he has gone to the expense of installing spare units. Experience has shown that the simplest type of standard horizontal pump sells in the greatest volume and requires the least amount of servicing. It is true that the packing in a n acid pump may be the source of considerable trouble, even if the best grade of packing is used and the box is properly packed. An analysis of packing troubles brings out the fact that most troubles come because the operator expects too much from the packing. Acid-resisting packings may be impregnated with suitable lubricants, but these lubricants do not remain in the packing forever under corrosive conditions, especially if the packing is subjected to heat and particularly to abrasive material from the pump. Water pumps are built to force leakage of water to keep the packing in good condition. Acid leakage, on the other hand, may deteriorate packing materials and be objectionable from the standpoint of cost and hazard. The troubles, therefore, start when the operator tries to stop the acid leak by tightening the gland. This may temporarily stop the leak but eventually results in charred or hardened packing, a scored shaft, and even more leakage. The long life of packing in water or oil pumps, therefore, points the way towards equally satisfactory results from packing in acid pumps. It is a problem only of feeding lubricant to the packing continuously under sufficient pressure to prevent leakage of the acid being pumped and to provide a flow, where necessary, through the packing to prevent overheating. This can be done by fitting the stuffing box with a suitable seal cage or lantern ring connected to a supply of suitable lubricant at the proper pressure. Water is the cheapest lubricant and easiest to handle and can be used in most cases. Where water is objectionable, owing to a chemical reaction or to the possibility of a slight amount of water entering the pump, then a suitable insoluble grease can be fed to the seal cage in the stuffing box under continuous pressure. Lowcost devices for this purpose are available. Since a t most, there is usually only suction pressure, on the packing, this pressure rarely amounts to more than a few feet or pounds. Therefore, relatively low fluid seal pressure is required in the stuffing box. If this is suitably regulated, the amount of sealing fluid that will enter the pump is negligible in a properly designed stuffing box. On a pump that handles 100 gallons per minute, if there is a leak of sealing water into the pump of as much as 10 gallons per day, this will amount to only about 70 parts per million. On one pump handling acetic acid a t about 40 gallons per minute against 180-foot head, which used sealing water a t 5 pounds gage pressure above the suction pressure, the amount of water eptering the pump was measured a t only 24 p. p. m. I n another pump where full city water pressure was kept on the stuffing box seal cage, 500 p. p. m. entered the pump. On a 3-inch Antaciron pump emptying a sulfuric acid ore digester 30 feet deep, an automatic grease seal was employed with success for several months. The chemists would not allow water to be used for sealing purposes. However, water was tried one night when they ran out of grease, and titration could not show a trace of additional water in the product. Water for sealing has been used ever since. The photographs show several pumps which are operated with water seals. These pumps handle hot abrasive acid slurries. Before water sealing was used, they had to be packed every day because the abrasive solids cut the packing
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INDUSTRIAL AND ENGINEERING CHEMISTRY
to pieces. Since water sealing has been used, the packing lasts from 4 to 9 months. This represents thousands of dollars saving per year to this plant since they now use water sealing on all acid pumps. With such infrequent packing of the stuffing boxes, the old objection to the nuisance of removing the seal cage for repacking will not stand up in view of the tremendous savings effected by its use. It should be pointed out, however, that this method of sealing stuffing boxes is not satisfactory if a pump is used that has a whipping shaft. Large-diameter shafts operating in heavy ball bearings and accurate workmanship throughout are necessary to get good results.
Conditions of Service The type of pump to use and the best method of operation is a t times dependent on the conditions of service. If a pump is to handle molten lead or fused nitrate, it is obvious that a submerged pump is the best type. On the other hand, diphenyl a t 500' to 700" F. is well handled by suitable horizon-
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acid flow to the pump a t a rate that keeps the stuffing box under negative pressure. At times air leaking into the stuffing boxes has proved troublesome. When automatic float switch control is used, there is no setting of the pump that will avoid leakage during shutdown periods unless the more expensive and less efficient self-priming pump is used, and even these pumps may leak during the run-back periods. A standard pump with a suction-line priming chamber will operate a t improved efficiency, but slight stuffing box leakage is to be expected. If a standard pump is equipped with a seal cage in the stuffing box and sealed with water or a suitable grease, excellent results are secured without leakage of acid, regardless of pump location or method of operation. In one case a paper mill installed a stainless steel pump to draw sulfite acid from the outside storage tank; a 10-30 foot suction head was on the pump with no shutoff valve except at the storage tank 60 feet away. This was a proper installation for a water pump but not for acid. The stuffing box eventually leaked, and as the sulfur dioxide gas was very objectionable, the operator kept tightening the gland. This scored the shaft sleeve, and it was soon impossible to pack the pump properly. Packing had to be changed frequently, and
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each time 60 feet of 6-inch pipe had to be drained of acid through a leaky stuffing box before the operator could approach the pump. By installing a new shaft sleeve and a water seal in the Ftuffing box, no further trouble was experienced, and relatively cheap packing was satisfactory. Where, for local reasons, the use of stuffing box seals is impossible and the liquid to be handled is a very abrasive slurry, i t is sometimes possible to use a standard horizontal acid pump and save the buyer considerable in first cost and in maintenance. As stated above, the pump may a t times be mounted above the slurry tank and automatically primed by a simple suction-line trap. In operation the packing is under negative pressure, and there is no tendency for the abrasive slurry to enter the packing space and cause trouble. In order for the pump manufacturer to make the best POSsible recommendations to the individual plant operator, it is desirable to give all possible information with an inquiry. However, very few buyers will give complete information since they are either not convinced of the value of most of the data, or they are not available. A fairly complete inquiry to allow a pump manufacturer to get the whole picture of a job would cover the following points: 1. Liquid to be handled 2. Viscosity 3. Temperature
4.
Boiling point
5 S ecific gravity ,6: cpharacter and amount of solids, if any; specific gravity of 7.
8.
9. 10.
solids, liquid, and slurry; rate of settling (01 floating) of solids Suction pressure or lift: a. Static b. Friction loss c. Size, type, and length of Pipe and number of valves and fittings Discharge pressure : a. Static b. Friction loss c. Size, type, and length of pipe and number of valves and fittin s Type of packing tkat will be used Type of process: a. Filter pressing and type of press b. Transfer c. Circulating (open or closed system) d. Discharge from vacuum apparatus Will suction leg pipe always remain full? e.
ll. Is leakage from stuffing box objectionable? 12. Will fluid sealing be allowable for stuffing box? 13. Sealing fluid preferred: a. Water b. Grease c. Other fluid 14. Can any other pump location be permitted to reduce positive suction head if sealing of stuffing box is impracticable? 15. Continuous or intermittent operation (if intermittent, length of usual run) 16. If pump is t o be throttled at times for control Purposes, where is it t o be throttled and how much? 17. willpump beDsubject to sudden temperature changes; if 80, how many F. and in what direction? 18. Type of pump preferred 19. Materials of construction preferred (type) 20. Type of drive 21. Preference for speed 22. If 1750 r. p. m. is too low for a standard pump, will higher speed be allowed by ( a ) belt drive, ( 6 ) step-up speed gear motor? (This assumes that such arrangements will save considerable in first cost as against a larger specially built pump.)
It is perhaps too much to expect all of this information, but it would certainly be desirable for some types of installations t o allow the pump manufacturer to make the selection and recommendation that will best suit the process and the individual operator.
VOL. 31, NO. 4
Results of Lack of Information Since Probabh' no engineer or buyer will agree that it is possible or desirable to give all this information with an acid pump inquiry, it may be of interest to illustrate what has happened in certain INSTALLATION 1. Inquiry specified a certain type of slurry, the specific gravity, the number of gallons per minute, and the head to be used on a stainless-steel open-impeller Pump. A pump was furnished to fulfill these requirements. The first trial run brought a complaint that the apparatus clogged and would not pump the slurry. Investigation disclosed that the suction line was too large between the supply tank and the pump for the capacity handled, that it allowed settling in the suction line and clogging of the whole system with the solids contained in the slurry, and also that the discharge line was too small for the capacity desired, with a resulting higher head than was specified. When the size of suction line was reduced and the size of the discharge line increased, the pump functioned perfectly but the system clogged at shutdown, since no provision was made for draining. This brought about a second complaint, which was more in the nature of a request for help, for the pump had demonstrated it would handle the slurry. An analysis of the process operation showed that clear liquor could be introduced into the suction line after the supply tank valve was closed which cleared the system of slurry before the pump was shut down. The next complaint was on packing. No packing could be found containing lubricants that would stand up for long. With a 10-15 foot suction head, considerable gland pressure was needed to avoid drip from the stuffing box. The operator would tighten the gland too much and cause overheating and disintegration of packing. The abrasive slurry getting into the packing made ordinary greasing or lubrication almost worthless, particularly since the liquid was a solvent for all ordinary greases. The pump manufacturer had recommended water sealing of the stuffing box, but this had been banned because of possible water contamination of the liquor pumped. As a last resort, a water seal (15 pounds per square inch pressure) was tried and found satisfactory. No objectionable amount of water entered the pump, and packing now lasts for months with no attention required for even tightening gland bolts. A complete understanding of the problem and process before the installation was first made could probably have avoided the subsequent complaints, delay to production, and expensive changes. INSTALLATION 2. A stainless steel open-impeller pump was furnished against a specification calling for a given capacity and head. After installation a complaint was registered that it would not prime. Investigation disclosed that the pump was set on top of an agitated slurry tank and priming was being attempted by introducing water into the suction line with no foot valve. A foot valve could not be used with this slurry. An air injector was then connected to the vent plug opening on the pump, and the pump was readily primed but it clogged immediately. A study of the settling rate of the slurry disclosed that part of the solids settled and part floated, and that these solids felted and packed much like snow. What was happening was that, in spite of thorough agitation, the floating solids were forming a plug in the suction line that would not pass the elbow in the suction line. A tee connection and valve were made on the suction line for water and were used for washing back the suction line for a second or two before the pump was started; it then worked perfectly. It is obvious that if the engineer who specified the pump had had to determine the data regarding the liquid handled for the questionnaire, operating recommendations could have
APRIL, 1939
INDUSTRIAL A N D ENGINEERING CHEMISTRY
been given which would have avoided the difficulties encountered. INSTALLATION 3. A high-silicon iron pump was furnished to handle 75 per cent sulfuric acid a t 90°C. for a given capacity against a specified head. It operated a t lower capacity and a complaint was made. Although the pump was found to have about 10 feet of static suction head, the suction line was extremely long with several elbows and even 180" square return bends. The pressure a t the suction inlet of the pump, due to friction in suction line, was far below atmospheric and this caused air leakage through the stuffing box which cut down the liquid handling capacity. A stuffing box seal cage was installed and connected to an automatic grease sealer. This not only stopped the air inleakage and raised the pump capacity, but the life of the packing was increased from two weeks to about a year.
Safety Features The question of safety features in any equipment has become of paramount importance in most of the larger plants. Equipment for handling hazardous liquids should therefore receive careful attention. Pumps are subjected to considerable pressures, and acid pumps of heavy, rugged construction for the higher pressures are well worth the extra cost. If two or more materials of construction may be used, the stronger material should be selected even a t a higher cost. Construction which avoids bolts through gasket surfaces may be preferable, especially if the bolting material is of commercial steel which would be subject to concealed corrosion. Bolting should be adequate to provide adequate pressure on gaskets to prevent blowing. Pipe line strains should be avoided since they can promote leaks a t joints. Vibration of the unit should be avoided, particularly where lead lines are connected to the pump. Vibration will eventually crystallize lead pipe, and a disastrous break is possible. Guards over gaskets under high pressure and over a stuffing box that is not sealed or flushed and over the coupling are obvious first principles for safety. Operating pumps on a foundation that has been
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badly deteriorated by leaking acid or that has a cast iron frame partially destroyed by acid splash or leak is not good safety practice. I n one midwestern plant where acid leakage had been allowed for a number of years, the concrete floor, supporting tanks, and pumps caved in for a depth of about 4 feet. Acid had removed the organic matter in the soil under the floor, and instead of upheaving the floor as is the usual case, the material was washed away through a large sewer. Serious injuries to workmen would have resulted if the cave-in had not occurred on Sunday.
Conclusions The selection, installation, and operation of a chemical pump require expert and intimate knowledge of all the special problems involving materials, engineering, and hydraulics. Only those pump manufacturers who have made a specialty of furnishing pumps for chemical service are in a position to give adequate recommendations to the user who may not have technical personnel capable of solving the special problems that exist in the handling of corrosive and corrosive-abrasive liquids. In buying a pump built of special materials, the following items should determine the final selections: (1) satisfactory material for the conditions; (2) mechanical design, with particular reference to ease of servicing, adjustability, interchangeability of parts, safety features, etc.; (3) hydraulic performance; (4) availability of pumps and parts from stock; (5) reliability of the manufacturer; (6) efficiency; and (7) first cost. If this paper results in more complete information being given to the pump manufacturer with inquiries for chemical pumps, and if it causes some plant engineers to give more thought and study to the hydraulics involved in the installation and operation of acid pumps, it is believed that both pump users and pump manufacturers will be mutually benefited.
Courtesy, G r i n n d Company. Ino. PREFABRICATED PIPING U S E D AT THE PLANT OF THE
BARBER ASPHALT
COMPANY
