Packing the Centrifugal Pump F. C. THORN The Garlock Packing Company, Palmyra, N. Y.
INCE the chemical industries are, for the most part, concerned with the handling of liquids, pumps occupy a prominent place in their paraphernalia, and in the affections, or vice versa, of their operatives. I n particular, the ubiquitous centrifugal pump has come in for a large measure of both approval and condemnation. The condemnation is only too often directed toward the stuffing box and its contents and has led to a quite understandable, and to some extent justified, urge toward designs in which the stuffing box is eliminated. The conventional stuffing box, however, can be made to function much better than it usually does, if packing user and packing manufacturer can be brought to a clearer understanding of each other’s problem. This discussion is an attempt to present the case of the packing manufacturer. Although the following remarks are applicable primarily to the centrifugal pump, many of them are equally true of rotary displacement and turbo pumps, and to other stuffing-box app l i e a t i o n s i n v o 1v i n g rotary shafts. The most important and yet the least understood fact regarding the centrifugal pump packing is that it inevitably leaks. If this sounds like heresy to the packing user, let him give some thought to the nature .of the problem. A liquid by its very nature penetrates any kind of a crevice, no matter how minute. If liquids are to be excluded from the stuffing box, it follows t h t the packing material and the shaft must be in intimate, in fact, molecular, contact. Packing materials are so plastic that such a degree of intimacy is easily attained; all one has to do is to pull up on the gland nuts. But this sort of contact is incompatible with highspeed relative motion. The only possible effect of such motion in the presence of this
tightness is to destroy the packing or the shaft or both. Such needless destruction unfortunately is taking place in almost every chemical plant every hour of every day. If destruction is to be avoided, a crevice must be left, and through this crevice fluid will pass in accordance with the lams of fluid flow. Whether this flow proceeds in accordance with Poiseuille’s lam or some other law may be answered at some later date. The main thing is to recognize the necessity for the leak and design accordingly.
S
Liquid Seal The function of a packing on a high-speed shaft is to throttle, not to eliminate, leakage. The situation is not hopeless, however. The operator has two weapons a t his disposal. The first and most important is that he can decide what liquid is going to leak. If he does not like the idea of any particular fluid escaping through the packing, he can substitute a more desirable one. The most common instance is that of the pump running with inlet p r e s s u r e below atmospheric. 3 Under these conditions air leaks through the packing, and this is particularly undesirable since it may cause the pump to lose its priming and may introduce other undesirable effects in the fluid being pumped. T o stop the air leak by tightening the gland is to invite the ruin of the packing. The problem is neatly solved, however, by injecting a suitable liquid into the center of the packing and allowing it to flow both ways from the point of admission. This arrangement FIGURE1. LANTERNRING FOR INTRODUCING A is known as a liquid seal, and LIQUIDSEAL the H-shaped metal ring which is introduced into the stuffing FIGURE 2. TANDEM GLANDFOR INTRODUCING .4 box (Figure 1) opposite the LIQUIDSEAL point of liquid injection is known as a lantern ring. The FIGURE 3. FLOATING STUFFING Box tandem gland (Figure 2) offers an alternative method for in929
930
troducing a liquid seal. The liquid may be the pump fluid itself, tapped from the discharge line, if this fluid is reasonably free from dissolved or suspended gases or solids; otherwise it should be water or oil from an independent source. In the latter case the compatibility of the independent fluid with the fluid in the pump must be given careful consideration since some admixture is unavoidable. The amount of such a d m i x t u r e , h o w e v e r , is often grossly overestimated. If properly adjusted, most centrifugal pump packings can be made to run cool on leaks of a few drops a minute, or even a few drops an hour. The ratio of this quantity to the normal delivery of the pump is negligible. The lack of a water seal on pumps handling a fluid designed for subsequent evaporation is often excused on the grounds that the re-evaporation of “gland water” is uneconomic. The reply may be made that the destruction of a packing set every day may prove to be far more uneconomic than the evaporation of an insignificant additional amount of water. The use of the liquid seal is not limited to applications a t negative pressures but should be employed in general when the pump fluid is a t positive pressure, but is costly, volatile, poisonous, corrosive, or full of dissolved or suspended solids. I n such cases the seal should be supplied with an independent fluid, usually clean, cold water. Many engineers carelessly refer to a water seal as a “water-cooled” stuffing box. A little reflection will show that the amount of heat removed by such an unimportant trickle may be disregarded. The real function is to make possible a slack adjustment of the packing and thereby avoid heat formation. The injected fluid functions, not as a cooling agent, but as a lubricant. There are water-cooled stuffing boxes also, but they are water-jacketed (Figure 3). Where the pump fluid is of such a nature that not even a trace of water may be allowed to mix with it, it is often possible to substitute oil under pressure. For instance, large refinery gasoline pumps are frequently designed for the continuous injection of lubricating oil into the packing, traces of which may be allowed to mix with the gasoline without
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 31. NO. 8 harm. Another excellent arrangement is the continuous introduction of a light grease by means of a spring-loaded or weight-loaded grease cup. It is only rarely that some sort of suitable sealing liquid cannot be found, if the will exists to find it. But where no admixture can be tolerated, there is nothing left but to accept a certain amount of leakage of the pump fluid through the packing.
Other Means for Beeping Leakage at a Minimum This leads us to the second weapon of the user; it is the means available other than overtightening, for keeping leakage a t a minimum, whether operating with a seal or without. These means are four in number: i iR 1. Elimination, as far as possible, of pressure differentials, since leakage is directly proportional to pressure difference. In the case of multistage centrifugals which would normally develop several hundred pounds pressure against the packing, the only way that leakage can be k pt within reasonable limits is to i&ert a metal seal ring ahead of the packing and tap the area between this and the packing to some point of low pressure, such as the pump inlet. The same rinciple is r i t e generally employed’ in rotary isplacement and turbo pumps, where the ressure, if not bled to inlet, woulfrise to about half that of the pump discharge. Such devices cause a certain loss of pump efficiency but make up for this in the packing trouble they avoid. To avoid excessive leakage from a liquid seal, the pressure in the seal should be adjusted so that it is only slightly in excess of that in the pump. 2. Elimination, as far as possible, of shaft gyration. An eccentric movement of even 0.001 inch is enough to make leakage impossible to control with almost any packing. Gyration arises from unbalanced impellers, irregularitiesin the stream being pumped, or defective belt or coupling drives; the remedy lies in sturdy shafts and in properly spaced bearings with close clearances. The single suction pump is especially prone t o gyrate, and great attention should be paid t o rigidity in design. Where a certain amount of gyration is unavoidable, a floating stuffing box, as in Figure 3, is helpful. 3. Use of packing of sufficient depth, the leakage being roughly inversely proportional to depth. 4. Selection of the proper type packing.
s
V
I
FIGURE 4. FIGURE 5. FIGURE 6. FIGURE 7. r
NESTEDCONEPACKING NESTEDV-PACKING HATPACKING ROTARY SEALPACKING
Selection of Packing The last is where the packing manufacturer steps into the picture, and it will be readily understood that his task has been simplified if the designer and user
AUGUST, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
have done their parts properly. Too often, unfortunately, the packing manufacturer is asked to make up for all the errors and omissions of both designer and user. The packings which the packing manufacturer is prepared to offer may be classified broadly into three categories: (a) automatic (viz., tightened by the fluid pressure itself), (b) spring-actuated, ( c ) stuffing-box types. The first two categories will be dealt with briefly since their field of usefulness is rather in connection with pure water and oil than with the miscellaneous collection of fluids encountered in the chemical plant. The nested cone packing (Figure 4) and nested Vpacking (Figure 5) are automatic; the hat packing (Figure 6) and rotary seal (Figure 7) types are spring-actuated, Since these devices are often employed without gland nuts or other external evidence of their presence, they are often but erroneously referred to as “packless.” (The term “rotary seal” is used to describe almost any packing, but the writer prefers to use it in the restricted sense of a stationary collar making contact with a revolving collar or shoulder on the shaft.) The overwhelming majority of packings used in the chemical plant are of the type intended for installation in a stuffing box and for intermittent adjustment by a gland. The requirements for an ideal stuffing-box packing may be summarized as follows: 1. It should be sufficiently plastic to conform closely to the shaft under the influence of gland pressure. 2. If drawn into too intimate contact with the shaft, it should possess means of freeing itself without production of destructive friction and temperature. 3. It should contain nothing that is dissolved, emulsified, swelled, or weakened by the sealing liquid, or by the pump liquid if no seal is being used or if the seal should become inoperative. . 4. It should not abrade or corrode the shaft. 5. It should be sufficiently elastic t o absorb whatever shaft gyration is not eliminated by design. 6. It should lose volume slowly so as not to require frequent renewal. Nothing has been said about normal running friction and temperature since, as previously noted, these are essentially functions, not of the packing, but of the thickness and vis-
931
as packings for a high-speed shaft because of requirement 2. I n other words, they are easily overtightened, whereupon they promptly seize, since they have no means of freeing themselves; on the contrary, because of their high rates of thermal expansion, they keep on growing tighter. Rubber-fabric types with the cloth plies a t right angles to the rod are better, because the voids within the cloth offer a slight opportunity for harmless expansion of the rubber layers. The principal use, however, for rubber and rubber-fabric is as an end ring in
FIGURE 8. EIGHT-STRAND BRAID a set of braided packing and then only occasionally (Figure 3)-for instance] when the fluid contains grit and it is desired to filter this out of the liquid escaping through the packing. Even in this instance the effect is better secured by running clean water backward through the set.
Braided Packing Braided packings are practically standard for the centrifugal pump. They have many advantages. They are cheap and lend themselves well to making rings in the field and thereby avoid the necessity for carrying a large stock of molded parts. Yarns used for braided packings are primarily flax, cotton, and asbestos. Lead wires are sometimes included in the
BRAID FIGURE 9. MUI,TIPLE-STRAND cosity of the escaping fluid film. Any packing will run hot on a small leak and cool on a large leak. However, some packings are better adapted than others to withstand the temperatures associated with small leaks. Certain types of packing will be discussed in the light of the above requirements.
Rubber and Rubber-Fabric Packing Although rings of vulcanized rubber are capable of making a very close fit to a shaft, they are of little use by themselves
braid to assist in the formation of smooth wearing faces, and centers of vulcanized rubber are sometimes added to improve elasticity for service on gyrating shafts. I n Europe other kinds of yarn and strips of leather are often employed, and elaborate combinations are achieved. There are three general types of braid, all of which are constructed on the familiar “grapevine” principle : the eight-strand braid (Figure 8) in which the cross section is built up from a single braid comprising eight thick strands; the multiple-strand braid (Figure Q), with twelve, sixteen, or twenty-four strands,
932
VOL. 31, NO. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
0 FIGURE 10.
100 200 300 DURATION OF TEST IN HOURS L O G OF
400
TESTRUNS FOR AN EIGHT-STRAND ASBEST08 BRAID
producing a rounded cross section which, for all but the smallest sizes, is built up by successive braid applications; and modifications of these two with special strands which bind the regular strands together in various ways. The second type can be made in square cross section by the introduction of longitudinal yarns a t appropriate points in the outer braid; similar longitudinal yarns are sometimes employed with the first and third types. A braided packing would be worthless as a packing without a binder of some sort. Rubber in the form of cement is sometimes employed, and the resultant rubber-bonded braids are quite durable but are open to the same objection that has been noted in the case of other rubber-bonded styles-via., a tendency to heat up and seize. The usual type of binder is a semisolid fat or wax, with a liberal dusting of graphite on the outside and sometimes between yarns. The mixture is quite commonly referred to as a “lubricant” but not entirely correctly, since the true lubricant is usually the escaping liquid. The function of the binder is to fill the interior of the packing and prevent leakage a t this point; along the
“ - 0
IO0
200
DURATION
OF TEST IN HOURS
300
400
FIGURE 11. LOG OF TESTRUNSIN POUNDS OF FRICTION PER INCHOF SHAFT PERIMETER
wearing face it serves as a surface leveler, producing a smooth face parallel to the shaft but more or less completely separated from it by a film of the escaping fluid. If a packing is overtightened, this fluid film is pinched out, which brings the binder everywhere into contact with the shaft; the effect is usually to raise the temperature, thus cause some of the binder to melt and exude from the stuffing box, and thereby restore the fluid leak as before. This self-correcting or foolproof feature is the most important property which the braided packing possesses for high-speed service. The mechanism of the performance of a braided packing can be best understood from the log of test runs shown in Figure 10, reprinted from a recent article by the writer.‘ This test was obtained in a double stuffing-box apparatus resembling a centrifugal pump, except for the absence of the impeller. The packing was a conventional eight-strand asbestos braid consisting of five rings, each 1.75 X 2.75 X 0,5 inch (45 X 70 X 12.5 mm.) saturated with a blend of petrolatum and an animal fat, with graphite throughout. The 1
Am, Soo. Testing Materials Bulletin, Dec., 1938.
FIGURE 12, PLASTIC COIL
AUGUST, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
conditions were: speed, 1800 r. p. m.; fluid, water a t 25 pounds per square inch (1.75 kg. per sq. cm.) pressure and 66” C. (150.8” F.). I n test A the gland was retightened every time a visible water leak appeared; i. e., it ran throughout the test on a water leak so slight that it flashed to steam on reaching the atmosphere. The log shows clearly how the packing constantly lost volume under the influence of repeated tightening, eventually failing in 185 hours when further gland pressure failed to bring a fresh supply of binder to the surface. I n test B, a leak up to 10 cc. per minute was permitted; inasmuch as this rate was not exceeded during the test, there was no occasion to retighten and the life of the packing was accordingly prolonged. The apparatus was also arranged for torque readings which were translated into pounds iriction per inch of shaft perimeter and plotted in Figure 11. The steep rises in friction following upon each retightening in test A are clearly revealed, as well as the return to normal friction after a certain portion of the binder melted out. Test B was free from any such difficulty. It is interesting to note that abrasion of the shaft occurred only in the latter part of test A . A packing running on a reasonable leak rarely, if ever, causes abrasion unless there is grit in the escaping fluid. The variety of braids and binders which have been employed for centrifugal pump service is truly enormous, and each construction has its advocates. Perhaps the most common type for general service is the one described consisting of eight strands of chrysotile asbestos yarn with a binder of petrolatum alone, or a petrolatum-animal fat blend plus graphite. For acid pumps, where leakage of acid through the packing cannot be avoided, it is customary to substitute crocidolite (blue asbestos) ; the binder in such packings is generally pure petrolatum to avoid any interaction with nitric acid in case it should be used to pack against this material. The chief disadvahtage of braids is a tendency to lose binder when employed a t low leakage rates. This volume loss also makes the proper location of lantern rings, etc., difficult to maintain. Braided packings should, in general, be operated with liberal leaks.
Plastic Packings The term “plastic” is used in the packing industry to describe a mixture of loose asbestos fiber, graphite, and a binder with the inclusion of metal particles optional. The ingredients are essentially the same as in braids but in a different form. Rubber-bonded and grease-bonded types are both employed and have substantially the same advantages and disadvantages as in the corresponding types of braid; viz., the rubber-bonded type is mechanically stronger but more inclined to seize if overtightened; the grease-bonded type is more foolproof but more inclined to lose volume through the clearances at the ends of the box. T o minimize the latter tendency, it is good practice to employ the grease-type with end rings of metal foil. Constructions of this type are capable of surprisingly long service. Special grease-bonded types of blue asbestos, with or without metal end rings, are employed for acid pumps. Plastic packings are competing strongly with braids, and in some respects they are better suited to operation a t low leakage rates. They are less easy to install, which disadvantage has been largely overcome by preparing them in coil form, with or without a skeleton braid of cotton yarn (Figure 12).
Metal Foil Packings Metal foil packings have also come to the fore in recent years. They consist of metal foil, usually alloy lead, slumi-
933
num, or copper. The two general classifications are the shredded type, consisting of narrow strands of foil bonded with rubber or other solid binder, and the broad foil type, in which a strip of foil several inches wide is twisted, rolled, or folded upon itself, generally with the simultaneous introduction of grease and graphite. Conical or V-cross sections are often employed to give more sensitive response to gland adjustment (Figure 13).
FIGURE 13. METALFOILPACKING
Metal foil packings are very durable and, if properly cared for, may last the lifetime of the pump. They are not quite as close fitting as the previous types, and have been used perhaps more for oil service than for water service. When used on water a t positive pressure, their tightness can be improved by injecting graphite grease via a lantern ring and a grease-cup or grease-gun fitting. This arrangement is sometimes also employed with the preceding types and is known incorrectly as a ‘Lgreaseseal.” It is not, however, a seal a t all because the grease is not under pressure except a t the time of injection. It is simply a means for renewing the layer of grease and graphite on the surface of the packing and thereby maintaining conformity to the shaft without retightening. Metal foil packings are stable as to volume and permit the accurate location of lantern rings, etc. They are not much given to seizure since the thermal expansion is about the same as the surrounding metal parts, and, if made from soft metal, they cannot abrade. Their chief disadvantages are cost and the fact that they do not lend themselves as readily as other types to preparation of rings in the field; therefore, it is frequently necessary to carry molded sets in stock. Furthermore, in handling corrosive substances, it is necessary to consider the possible effect on the metal, which somewhat limits their uses in the chemical plant. It would be possible to launch into a list of detailed packing recommendations for all the different fluids which the centrifugal pump is called upon to handle, but such a list is unnecessary if the underlying principles are properly grasped. Once the necessity for controlled leakage is clearly recognized and proper sealing arrangements are made to take care of those cases where the fluid itself is not a suitable lubricant, the selection of packing is relatively easy. I n case of doubt the packing manufacturer should be consulted.
