December, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1257
Locating the Obstruction in a Clogged Line of the Compressor1 John Rathbun SIBBBL 1 NSTITUTB
OF
TSCHNOLOGY, CHICAGO, ILL.
PERATING difficulties which are basically very simple are frequently hard to locate, even though the engineer is provided with the latest type of equipment and means for making measurements. Often some petty detail is neglected in the diagnosis, misleading the engineer and resulting in trouble, especially in the case of a faulty suction line. A clogged suction line is capable of many peculiar antics, the symptoms of which seem to contradict an analysis by ordinary methods and thus lead the chase into other parts of the system where the hunting is not, productive of much result. One of the most baffling and puzzling of the symptoms exhibited by a clogged suction line is the apparent reversal of pressure and vacuum that will take place owing to accumulations of liquid ammonia on the high pressure side of the obstruction. The machine gages may show a good vacuum with the compressor running slowly, but when an attempt is made to open the line near the machine it will be forcibly proved that a considerable pressure may exist a t this point. One example of this sort of thing comes vividly to the writer's mind and awakens memories that are far from pleasant. The engineer had pumped down the plant to make a repair on the suction line. After pumping down several times to a vacuum of 25 inches, the steam fitters opened the suction line, only to find there was still a considerable pressure on it. The line was again pumped down and the machine was then kept running slow to hold the vacuum while the steam fitters broke into the line. Even then there was a comparatively high pressure a t points some distance from the machine. A council that lasted well into the evening was held. It should be explained that this was a packing-house plant for operating the pork-house coolers, and during all of this period the cooling-room temperature had climbed steadily, as is usual under such conditions. The pork was fresh and still warm. The outdoor temperature ranged around 90" F. A hurried examination of the system revealed no external evidences of trouble, for the expansion valves were open several turns and no leaks were then apparent nor had been apparent for several weeks. The only conclusion that could be reached was that there was an obstruction in the suction line. Where it was, however, remained a problem. With the machine running slow to hold the suction, another joint was opened in the suction line somewhat nearer to the machine than before. The results were precisely the same, and the pressure a t this point made it highly desirable to close the joint a t once. The next attempt was more successful, for upon opening the line between the suction line trap and the machine it was found that the full vacuum was maintained a t this point as indicated by the gages on the instrument board. That the trouble existed within the suction trap was now apparent, as a difference of pressure existed between the inlet and outlet of the suction trap which was disconnected after closing the line valves. Removed from the line, the entire trap was found to be filled with a mass of congealed lubricating oil, so viscous and hard that it effectively clogged the line and prevented all but a very small quantity of the ammonia from passing to the machine.
0
1
Received August 27, 1929.
The temperature was so low that some time was spent with the steam hose before the mass was sufficiently softened for removal. This was due primarily to neglect on the part of the engineer, for it would not have happened had the trap been blown out regularly. It is the habit in most plants to blow out the oil traps on the high side frequently, but the scale and oil traps on the suction side receive very little attention, if any. There are many plants in this country where the scale traps on the suction line have never been blown out since the plant was started. I n such plants there is always a chance of having the difficulty described. When the engineer started to pump down the plant, there was probably a considerable amount of liquid ammonia in the trap. When a vacuum of 25 inches was reached, this ammonia boiled out and reduced the temperature far below zero, so that the oil was congealed. The difficulty might not have become apparent had it not been for the pumping down and the resulting refrigeration of the oil in the trap. Clogged liquid lines usually are not so difficult to locate, for the reason that there is always a sizzling sound near the obstruction due to the flow of the liquid through a small opening. This sound is very much like the noise a t the expansion valves when they are operating properly. This method, of course, is not of much value with a completely blocked liquid line unless a branch line can be opened. Further, if the opening is much obstructed the liquid line will be frosted near this point, for the obstruction is then giving the same general results as an expansion valve. If the liquid line is not covered this is a more effective method of determining the location of the restriction. A clogged suction line would be indicated by a vacuum on the suction side of the compressor. The evaporating surface would do no cooling, regardless of the expansion valve setting. A partial obstruction in the suction line, however, is far more difficult to locate, for the coils will do some cooling, and a vacuum will not appear in the suction line unless the restriction is great. A restriction will be manifested by the sluggish action of the cooling coils and by a suction pressure somewhat lotver than normal. These symptoms are identical with those experienced when the system is short of ammonia. An inspection of the liquid gage glass on the receiver, however, will eliminate this uncertainty. A further check on the system should be made, provided the foregoing conditions exist, for pipe-line restrictions cause a direct thermal dynamic loss, resulting in an increased power bill. If the suction line or the coils are restricted, however, the suction pressure will remain nearly constant, regardless of the expansion valve settings, provided the compressor is being operated only on that part of the low side in question. Allowing the expansion valve to remain open by a couple of turns for about 5 minutes, to fill the coils with plenty of liquid ammonia, should cause the coils to defrost and sweat up to the point where the obstruction reduces the flow of ammonia. From this restriction frost should extend as far as the intake of the compressor. Great care is necessary to avoid slugging of the ammonia during this test. This can easily occur if the obstruction loosens up. If the obstruction should become dislodged there
I h l D U s T R I A L A N D ENGINEERING CHEMISTRY
1258
will be a heavy rush of liquid ammonia in the compressor line. To prevent damage to the compressor some one should be in readiness to close the valve on this line as soon as slugs of liquid make themselves known by hammering or knocking To sum up, the only true remedy for the situation outlined is to insure proper drainage of the suction line and to make periodical inspections to ascertain that the lines are purged regularly at points where sediment, iron scale, or lubricating oil is likely to accumulate. All gas filters, accumulators, screens, and similar protective devices in both the liquid and suction lines must be provided with effective purge connections and these purge connections must be used for the purposes for which they were intended. By this means, and by this means only, are we protected against the annoyance and monetary losses that inevitably follow clogged lines. It should be further noted that inferior grades of lubricating oil frequently augment troubles due to poorly drained systems. Oils with high setting points congeal rapidly in the suction
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VOl. 21, No. 12
line and build up solid masses that soon obstruct the flow through the protective devices and lines. Similarly, resinous or tarry bodies in poorly refined lubricating oils form a nucleus for congealing masses and serve as a foundation for deposits that would otherwise not form. Under such conditions, mere purging is not sufficient. The separators and screens must be removed and actually scrubbed with kerosene or similar solvents. Paraffin-base oils, unless given a preliminary chilling at the refinery, congeal at higher temperatures, owing to the settling out of the paraffin waxes, and are therefore more likely to cause clogging than the paraffin-free naphthenic base oils with their lower setting point. Most oils are polymerized during the compression stroke, producing certain resinous materials that are carried by the ammonia into the traps and separators, where they have a tendency to adhere and form a base for further deposits. For full protection against such solid matters, the primary precaution is the selection of an oil properly prechilled and fully refined.
Lubrication of Gas Mains b y Means of Oil Fog’ 0. H. Blackwood2and P. G. Exline THEKOPPERS COYPANY LABORATORIES, PITTSBURGH, PA.
A persistent oil fog, suitable for internal lubrication It is seen that in a 45-cm. (18of gas mains, is composed of particles one micron in hydrated city gas, oil-fog inch) main, a fog consisting of diameter, or smaller. Such a fog may be produced by lubrication of the distrip a r t i c l e s of a radius much condensation or by atomization with compressed gas. bution mains has many beneg r e a t e r than a micron will The condensation method produces large quantities f i c i a l r e s u l t s (1, 3). The exhibit little staying power. of fine fog, but is rather difficult of control. Atomizaproblem of producing oil fog I n work to be described in tion offers a simple method, but the quantity of fine which is adequately persistent this paper it has been found particles formed is limited. seems a t first quite simple, that persistent fog particles Oil fog may be detected and its density measured by have radii of less than 0.5 since we are familiar with the aspirating samples through weighed filter papers, or micron (0.00005 cm.) and fall fact that water-vapor clouds the oil may be dyed and the tint produced on a filter persist in the sky for many a t speeds of about 12.2 cm. paper in a tar camera compared with laboratory standhours and dense fog screens per minute. In view of the ards. The persistence of a fog is determined by measa r e e a s i l y produced b y turbulence existing in the flow uring its density at various periods subsequent to its sprayers used for insecticides. of gas through the mains, it formation. is evident that gravitation is However. the tvDical Darticle size for such cloyds is so large n o t t h e sole, nor even the that they would be rapidly eliminated from the stream in a gas principal, factor of elimination. The turbulence causes a main. Moreover, the concentration of liquid in a dense cloud mixing of the gas, tending to offset the settling due to gravity. such as can be easily produced by a commercial atomizer is RADIUS vELOCITY O F FALL relatively small. Such a cloud so thick that it “may be cut 0 . 0 1 cm. 94.30 meters per second 0,943 meter per second 0.001cm. with a knife” may have a density of only 0.065 gram per liter, 0.0001 cm. (1 micron) 0 , 9 4 3 cm. per second 0,00001 cm. 0.0094 cm.-per second the liquid concentration being less than 0.5 per cent by weight.
I
N THE distribution of de-
Factors Affecting Persistence of Oil Fog
The forces which influence the movements of particles suspended in a gas are those external to the gas, such as gravitation, centrifugal forces, and electrical forces; and those depending upon the structure of the gas itself-that is, molecular activity. The velocity with which a particle will fall under the action of gravity has been shown by Stokes (8) to be V = 2r2g(p - p’)/9n where g is the acceleration of gravity, r the radius of the particle of density, p , and n the viscosity of the gas of density p’. The following table calculated from this formula illustrates the velocity attained by oil particles of different radii. 1 2
Received August 31, 1929. Professor of Physics, University of Pittsburgh.
Another factor favorable to elimination is molecular bombardment of the particles, giving rise to Brownian movements. These tend to cause the droplets to collide with one another, bringing about the formation of larger aggregates which quickly fall out. A consideration of this fact emphasizes the desirability of diluting the fog as quickly as possible after its formation. Methods of Producing Oil Fogs
BY CONDENSATION-T~~ physical processes involved in the production of droplets by condensation have been carefully studied (4, 5 ) . Since the vapor pressure at the surface of a liquid sphere is higher than that over a plane surface, to secure condensation we must have supersaturation. Thus, let a small mass of liquid of density d be transferred from a plane surface to form a droplet of radius T . Because of the surface tension s, the energy expended in this process is
