Abrasive Cleaning of Sand slurry circulation at moderate to high velocities economically and effectively removes soft scale from the tube side of process condensers
BASIC REQUIREMENT for
the
op-
eration of the gaseous diffusion process for the separation of the isotopes of u r a n i u m is the controlled removal of the heat of compression generated in p u m p i n g the gas. The bulk of this heat load is discharged to a treated, open, recirculating water system using induced draft cooling towers for ultimate h e a t disposal. T h e heat is transferred to this cooling water system from fluorocarbon refrigerant vapors condensing on the shell side of multipass shell a n d tube condensers in which the cooling water is circulated t h r o u g h the tubes. T h e system includes several h u n d r e d condensers with heat transfer areas ranging from 8000 to 14,000 square feet each. Each unit contains from 16,000 to 27,000 linear feet of 6 /s inch, B W G 19, integrally lowfinned copper tubes. D u r i n g the course of the evaluation of recirculating water treatments for corrosion a n d scaling inhibition over the past 5 years, severe localized corrosion of the copper tubes causing pits filled with h a r d deposits a n d deposition of a soft phosphate scale have been observed on the water side of the condensers. T h e pitting causes outleakage of coolant to the recirculating water, a n d the scale inhibits water flow t h r o u g h the tubes a n d reduces condenser heat transfer coefficients. Recently, the need for recovering
1 Present address, Thiokol Corp., Brigham City, Utah.
58 A
Chemical
heat transfer performance has become acute. T h e pitting has been arrested by recent improved water treatments, a n d several methods for the removal of scale were tried including air blasting, water jet cleaning, acid cleaning, steam j e t cleaning, a n d the use of a commercial mechanical tube cleaner. Air blasting a n d water jet cleaning were ineffective, a n d acid cleaning proved too severe, causing excessive leakage due to chemical action in the area of the pits where the t u b e wall was thinnest. Steam j e t cleaning a n d mechanical cleaning were effective, but the mechanical method scarred the tubes, a n d both methods were tedious, did not clean the hairpin bends, a n d necessitated removal of the condenser to a convenient work area. T h e cleaning m e t h o d developed consisted of p u m p i n g a n abrasive slurry t h r o u g h the condenser tubes at m o d e r a t e to high velocities to remove the soft scale by gentle, controlled, erosive action.
Laboratory Scale A p p a r a t u s
In order to evaluate the slurry cleaning process, a laboratory scale test a p p a r a t u s was constructed which was capable of circulating the slurry at velocities u p to 12 feet per second in a 6-foot length of fouled condenser tubing. This a p p a r a t u s consisted basically of a slurry p u m p , a 55-gallon slurry t a n k with two portable agitators m o u n t e d on it for m a i n t a i n i n g a homogeneous
INDUSTRIAL AND ENGINEERING CHEMISTRY
slurry, r u b b e r tubing connections, a glass sight tube, a n d a pinch c l a m p which was used as a throttling valve. T h e suction to the p u m p was connected to an 8-inch standpipe in the barrel to allow the sand to settle in the tank prior to stopping the p u m p , thus preventing sand from blocking the suction line. W i t h o u t the agitators in operation, the velocity t h r o u g h the tank was sufficiently low to prevent sand carryover into the recirculation lines, and this type of operation preceded a n d followed slurry circulation to keep sand from settling in the lines. T h e success of this type of operation later led to the incorporation of the settling c h a m b e r in the mixing tank of the prototype a n d mobile units. Slurry Velocity Studies
T h e l a b o r a t o r y a p p a r a t u s was used primarily to study the action of different sands in removing the soft scale from the t u b e samples. T h e extremes tested ranged from coarse river sand, which was largely — 8 to + 4 0 mesh, to a graded O t t a w a , 111., sand ranging from - 4 0 to + 1 6 0 mesh. While the coarse river sand did a fair j o b , it was too heavy for the small p u m p a n d was difficult to keep evenly suspended. A —30 to + 1 0 0 mesh sand did a good cleaning j o b , but it, too, was difficult to keep evenly suspended. T h e coarseness of the sand has a great effect on the uniformity of suspension in water at any given velocity.
I/EC
Equipment
& Design
Shell and Tube Heat Exchangers
by R. F. Battistella, J. L. Powell, R. A. Yealcey,1 and S. Bernstein, Union Carbide Nuclear Co.
Effect of Slurry I m p i n g e m e n t on Tube Sheet Facing
One further test was conducted in the laboratory to determine quantitatively the effect of abrasive slurry impingement on the leaded (50-50, lead-tin solder) tube sheet. The tests indicated that a penetration of about 0.002 inch per hour could be expected using fine grade sand. This effect was considered negligible since the duration of slurry circulation was not expected to exceed one-half hour. The process proved to be extremely effective in the removal of the soft scale, while slurry action on the hard deposits found in the pits removed only that portion which projected into the tube without disturbing the deposit recessed below the tube surface.
were used throughout. The pump was a medium duty unit of the type used in sand dredging operations. When valved to the upper section of the tank with the agitator inoperative, relatively sand-free water was pumped for startup or flushing, and when valved to the lower section with the agitator in operation, the slurry was circulated. Reinforced rubber hose sections were used for connecting the condensers to the system to provide sufficient flexibility to obviate the need for repiping for the different condensers. A 5-hp., turbine bladed mixer was centrally mounted on the
Although the scale-up factor was immense, the urgency of the problem combined with the straightforward nature of the process made this risk both necessary and feasible. The unit was designed as a minimum cost prototype, stationary system, but, wherever possible, consideration of future portability was incorporated into the design so that conversion to a mobile unit would be possible with minimum expense and effort. All of the equipment purchased for the prototype was commercially available as standard equipment. Quick connecting pipe couplings
Plant Scale Prototype
The parameters for successful operation were developed in the laboratory and the data derived were used for scaling up to a plant scale prototype. The basic factors used to size the prototype were slurry velocity, particle size of the abrasive, abrasive concentration, and duration of exposure to slurry. The average values for these- parameters were established. Slurry velocity Abrasive particle size Abrasive concentration Duration of exposure
10 feet per second, minimum - 4 0 to +160 mesh sand 20% by volume in water 15 minutes
Upper Reservoir (Sond Free) Baffle-
Lower ReservoirtSand Slurry)
Condenser
Sand slurry cleaning system
VOL. 52, NO. 72
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DECEMBER 1960
59 A
EQUIPMENT A N D DESIGN top of the slurry tank and was utilized, in conjunction with appropriate baffling, to ensure uniform sand suspension in the slurry. During start-up and shutdown, sandfree water was circulated through the system, and during cleaning, slurry was circulated. A baffle was placed horizontally, approximately midway in the slurry tank, to divide the slurry tank effectively into two sections—one of which was used to supply sand-free water for starting and for flushing, following cleaning; the other of which served as a slurry reservoir during the cleaning portion of the cycle. Smaller quantities of sand can be used if the effective slurry volume is limited to the below-baffle volume of the tank. This arrangement also eliminated the need for a large, clean water supply to flush the condenser at the cleaning location, a very important consideration, especially for a mobile unit. The first condenser cleaned contained 510 hairpin tubes arranged in four-pass configuration. From start-up to shutdown the cleaning cycle required 49 minutes, only 15 minutes of which were used for sand slurry circulation. The effect of the cleaning was to remove approximately 375 pounds of material (dry basis) from the condenser tubes without causing leakage in the already pitted tubes. Some difficulty was experienced obtaining even flow during this run due to air entrapment under the upper suction line antivortex baffle and the angled horizontal sand baffle. Sand settled in large quantity on the top of the horizontal baffle because of insufficient slope, and this effectively reduced the concentration of the slurry during cleaning to 14.5% by volume from the expected value of 20%. Despite these drawbacks, a great improvement in tube condition was accomplished, and the operation of the unit in this initial run was considered successful. Flow during cleaning was calculated to be approximately 3300 g.p.m. based on pressure drop as. flow curves for both the condenser and the pump, both of which were in close agreement. This flow corresponds to a velocity of 18 feet per second through the tubes. Pressure tests of the tubes before 00 A
Condenser tubes before and after cleaning. sive slurry. Scaled tube is shown below
and after cleaning were made using 100 p.s.i.g. air on the shell side, corking one end of each hairpin tube, and soaping the other end. No tube leaks were found before or after cleaning. (One hundred pounds per square inch pressure was maintained on the shell during the entire cleaning cycle to preclude the possibility of getting water in the shell in the event of a leak.) The amount of material removed from the tubes was determined by taking a sample of the sand-free water after cleaning, filtering the suspended solids, evaporating to dryness, and weighing. The dry residue was analyzed and the weight of sand subtracted leaving the weight of solids removed in the sample, which was extrapolated to the volume of water used. Prior to cleaning another condenser, the prototype was modified to correct the faults discovered during initial operation. The horizontal baffle slope was changed from 10° to 35° to slightly exceed the angle of repose of the wet sand and so prevent deposition on top of the baffle in the upper section of the tank. The antivortex baffle over the upper tank section, pump suction connection, and the horizontal baffle were vented to prevent the trapping of air and the bumping caused by it during the first run. The system was charged to contain 2 0 % sand by volume during the cleaning portion of the cycle. The second condenser cleaned contained 430 hairpin tubes in fourpass configuration. Following cleaning, the condenser was pressure tested revealing two leaking tubes, both of which were removed and leak tested to pinpoint the leaks.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Upper tube was cleaned with a b r a -
Inspection of the leaking sections indicated the scale build-up over the holes was still intact and had not been removed below the tube surface by slurry circulation. It is to be noted that this condenser was not leak tested immediately prior to cleaning and it was concluded that the small leaks probably existed before cleaning. Since 4 3 % of the first and second pass tubes were plugged, the total volume of slurry pumped through the inlet passes was small due to the small flow path, while the slurry velocity was approximately 18 feet per second because of the high pressure drop across the inlet passes. The velocity in the third and fourth passes, which were only 3 % plugged, was calculated to be 10 feet per second. Incomplete cleaning was considered to be a function of insufficient time of slurry circulation at the lower velocity, since laboratory data indicated that sand suspension at 10 feet per second was adequate for effective cleaning. The effect of the relatively high slurry velocities in the inlet passes was to scour the tubes slightly in the short radius bends. The effect of the reduced velocities in the outlet passes was residual scale in these tubes. The third run with the prototype was made to evaluate cleaning efficiency in a six-pass condenser containing 755 hairpin tubes. The main purpose was to ascertain flow rates in this size unit at the higher pressure drops anticipated. Flow rates of the order of 14 feet per second were obtained and, although the particular unit had been previously cleaned by the steam cleaning method, substantial quantities of scale were removed. The results obtained with the
EQUIPMENT AND
prototype were comparable to those achieved with the laboratory scale unit. Virtually all of the soft scale was removed from the con denser and no tube leaks were caused by cleaning. The hard, adherent, protective film formed by water treatment chemicals was not penetrated except on the outer radius of the sharper bends of the hairpin tubes. The removal of leading (50-50 lead-tin solder) from the face of the tube sheet was neg ligible, and general operation of the unit was simply and easily controlled. Following the prototype runs, preparations were made for the construction of the mobile unit to be used in the plant. The data ob tained indicated that a cycle pro viding a 5-minute flush followed by 15 to 30 minutes of slurry circula tion followed by a 10-minute flush would clean any condenser in the plant. Approximately 2 to 3 % sand by volume could be expected in the final flush, and the settling of this in the condenser was not significant since a high velocity onstream flush would remove it. During operation of the prototype, it was the practice to reuse the sand following a simple washing using the slurry agitator. The sand would settle rapidly when the agitator was turned off and the lighter scale which remained suspended could 170
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Effect of cleaning on heat transfer per formance
be decanted. This procedure seemed feasible for mobile unit operation and the purchase of sand was limited to an adequate supply to replace anticipated losses.
Basic E q u i p m e n t I t e m s a n d P e r f o r m a n c e Characteristics Pump
Medium duty d r e d g e pump r a t e d at 2 5 0 0 g.p.m. at 1 5 0 p.s.i. total h e a d . Eightinch suction and dis charge. 8 6 0 r.p.m.
Mobile Unit The decision to make the clean ing unit portable was prompted by the difficulty and expense of removing the bulky and heavy con densers and transporting them to a convenient work area. The pump, tank, agitator, pipe couplings, and valves of the prototype were all utilized in the construction of the mobile unit. The pump and drive unit was mounted on a rubberwheeled dolly, and the slurry tank was equipped with lifting lugs so that it could be easily moved by crane. Suitable drain, fill, and sampling connections were added. Packing problems were solved by the addition of a booster pump to supply the shaft seal with clear water at a sufficient pressure to keep sand from entering the packing gland. Between runs the sand was washed. Complete changes of the sand charge were not necessary, and losses were approximately 300 pounds of sand per run. The operation of this unit has been entirely satisfactory for the 3 years that it has been used to clean fouled condensers. To date, several hun dred condensers have been cleaned with heat transfer recovery closely approaching that of a newly tubed condenser in all cases. The graph shows typical heat transfer recovery as a result of the cleaning process. The actual costs involved in cleaning a condenser at this plant average approximately $280 in cluding the cost of disassembly, moving to a new location, setup, operation, maintenance, and sand usage, but not including overhead or utilities. Since the unit size and configuration, as well as the disassembly, moving, and set-up time would vary widely from plant to plant, no set cost can be established to cover these factors. Operating time would be substantially the same for any case, and 8 manhours may be assigned for the entire op erating cycle including preopera tional testing, cleaning, and sand washing following cleaning. The process was, in this case,
DESIGN
Drive
Heavy duty chain
Slurry Agitator
5 h p . motor; 34-inch diameter, six-curved, bladed turbine; 6 8 r.p.m.
Slurry Tank
6-foot diameter X 1 0 f e e t high; horizon t a l l y b a f f l e d , filled to approximately 1 600 gallons
Pipe
8-inch aluminum, g r o o v e d f o r Victaulic couplings
Couplings
Victaulic, neoprenegasketed, two-bolt type
Valves
Ο ring seated butterfly, ceramic lined
tailored to remove a soft scale, but it is entirely feasible to adjust the variables to achieve effective clean ing action to remove harder scales. Proper selection of the abrasive particle, abrasive mesh size, con centration, slurry velocity, and time of exposure, should adapt this proc ess to a wide variety of pipe and tube bundle cleaning jobs. Al though the sand used in these tests was an uncrushed, smooth, round particle sand, crushed rock, or other sharp grits might be used to advantage on harder scales. Background Reference Powell, J. L., "Corrosion of Copper in O p e n Recirculating Water Systems," I / E C 5 1 , No. 3, 75A (March 1959). This document is based on work per formed for the U. S. Atomic Energy Com mission by Union Carbide Corp., P a d u c a h , Ky.
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly. VOL. 52, NO. 12
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DECEMBER 1960
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