US3110610A - Coating method - Google Patents

Description

Nov. 12, 1.963 R. H. GooDELL 3,110,610

COTl-.NG METHOD Filed March 30. 1961 United States Patent O 3,110,610 COATING METHD Richard H. Goodeil, Baton Rouge, La., assignor to Ethyl Corporation, New York, N .Y., a corporation of Virginia Filed Mar. 30, 1961, Ser. No. 99,503 1 Claim. (Cl. 117-49) This invention relates to applying protective coating. More particularly, the invention relates to a nevl and improved method in applying heat-setting or drying resinous coatings to tanks for containing and transporting liquids. Tanks of this character, of generally circular cross section, and relatively long compared to diameter, are ernployed as the tanks in railroad or highway trailer tank cars or trailers.

Railway tank cars and highway motor truck trailer tank units have long lbeen used for the bulk transport of many industrial chemicals. The growth and diversication of this mode of transport has given rise to a substantial variety yof specially designed tank car units and materials employed in said units. In many cases, it has been found that improved service is provided, in transport of certain chemicals or commodities, by applying a coating on the interior of the tank proper. Various cotatings can be employed, some of the most successful being `coatings of the thermosetting resin type, for example, coatings including an epoxy resin base, or a phenolic resin base.

In applying such coatings, it has been customary to dry a tank by passing warm combustion gases through the tank for a sutlieient period of time to substantially evaporate any moisture or residual quantities of previous liquid contents, to get substantially a clean metal surface. The `coating is then sprayed on the interior surface of the tank, and a drying operation is again performed, at least to the point of dry-ing the coating material to a fairly hard, non-tacky surface. This coating and drying is repeated to provide a plurality of coats, such as 4 to 6 coats, to provide a heavy -nal iilm of Well dried coating. The final step has been a thorough curing and drying operation with Warm gases or )gaseous combustion products.

This general procedure has worked reasonably well for most tanks, but considerable difliculty has been encountered in tanks of a certain character and especially for certain services. It is found that -it is diicult to provide an adequate coating for tanks employed for the transport of liquid antiknock mixtures, having organometal constituents and halogen-ated hydrocarbon constituents therein. The previous use of unlined tanks for this service has been found to result in a certain amount of etching or surface corrosion, which corrosion is accompanied by retention of trace quantities of decomposition .products, probably the result of corrosion occasioned by halogen `compounds Within the liquid antiknock mixtures, which trace amounts apparently are ditlicult to remove from the metal surface, and subsequently result in failure of the lining applied. Further, the tanks for the indicated service are normally jacketed with a fairly substantial covering of insulating material, particularly cork, which unfortunately has a relatively low charting or ignition temperature. Hence, conventional efforts to adequately pre-condition the metal, before coating, and to apply a satisfactory final relatively high temperature heating and curing operation have been unsatisfactory. The attempts to apply conventional coating methods for tanks of the indicated service, and especially those with the described type of insulation, have been unsatisfactory in that certain portions of the interior tank services have not been adequately coated and the coating applied rapidly deteriorates, blisters, or disappears entirely.

3,1103610I Patented Nov. 12, 1 963 Mice It is therefore an object of the present invention to provide ia new and substantial improvement in the mode and procedure for coating tank units of the character described, viz., railway or trailer unit tanks of substantially cylindrical cross section and elongated dimensions, having a centrally positioned manhead or access opening. A further object is to provide an improved method of pretreatment of such tanks, prior to coating `and as a step of the entire coating operation, and to apply a more effective nal curing method for the coating process.

In carrying out the improved method, a new and improved heating method has been found and devised, which substantially overcomes certain weaknesses which have been discovered to be associated with the prior techniques. The process employed involves a preti-eating operation, consisting of heating the interior of the tank by the flow of warm gases into the car tank and out of the access point. The successive coatings are then applied by spraying and a similar heating with Warm gases after each coat, and applying Ia final curing and heating operation after the last coat has been applied. It is found that the peculiarities of construction of an insulated car requires a combination of operating conditions in the several heating operations or gaseous fluid heating operations, viz., the pretreating operation and the nal curing operation, requiring .a critical set of operating conditions or variables. Specifically, it is -found that the gaseous heating fluid should be supplied as two streams subs-tantially parallel to the longitudinal axis of the tank, and entering the car or tank interior at points Icentrally positioned, longitudinally, not more than about 1/7 of the tank vlength apart, the stream centers being 1A to 1/3 of the tank diameter from the bottom. Further, the transverse area of each of the said `gaseous uid streams, upon entry into the tank space, should not be less than about 1/70 of the transverse area of the tank. The rate of feed of the gases is such that a complete change of atmosphere in the tank is provided in less than a minute, preferably from about 1/2 to 4about 5710 of a minute. The feed temperature of the lgases is not highly critical, but generally should not exceed about 450 F., a preferred feed temperature being from 350 to 400 F. By maintaining the foregoing interrelated operating conditions the tank walls are rapidly brought to la relatively uniform temperature condition. By la relatively uniform temperature condition is meant that the maximum temperature spread, at various points in the tank walls, is not over about 35-40 F. Further, by providing the :described interrelated operating conditions, measurement of the wall temperature at :only one point is adequate as a control temperature. When the desired temperature level is reached, steady state operation is maintained for various periods, depending on the tank condition and the type of coating being applied. Thus, if the heating operation is for preconditioning a tank, previously used in service, a heating period of 2 to 6 hours is customary. When the heating is to provide -a nal cure of a coating, the 1duration of the operation will vary with the type of coating material, but is usually from 6 to 20v hours at steady state conditions.

The imode of carrying out the present improvement be readily understood from the detailed description and example given hereinafter and from the figure, which is a schematic sectional view of a tank of a railway tank car, showing apparatus in place for carrying out the improvement process.

Referring to the figure, a tank mounted on a railway car chassis is shown. In this instance the tank includes the tank vessel proper 13, being a relatively heavy-Walled vessel, having a wall thickness of the order of about 3% to about one inch. The tank proper is covered by a layer of about four inches cork insulation 14, the insulation in turn being surrounded by a relatively light sheet or plate metal jacket 15. A dome or access opening 12, is provided at the center-top of the tank, the dome being provided With a variety of feed and discharge lines, valves, and relief devices not shown. The weight of the tank is supported by two saddles or bolsters 16a, 16h, near the ends of the tank above the trucks of the car. VThe tank 4rests :on these saddles, not being permanently attached thereto. Attachment to the car chassis is made by a bracket or plate 18, at the center lof the car, which is engaged with and fastened to a similar bracket or plate 22, which is permanently attached to the center sill 21 of the car body. It is seen that the orientation of the tank with respect to the car chassis is assured by the attachment of said brackets 18, 22, this attachment normally Ibeing made by rivets or similar fastenings. The brackets themselves, however, are normally permanently welded to the tank `13 and the car sill 21, respectively. It is thus seen that expansion or contraction of the .tank 13 results in a slight degree of sliding movement on the supporting bolsters or saddles 16a, 16h. The insulation layer is relatively continuous, except for gaps at the areas of contact with the bolsters 16a, 16h, and at the attachment brackets 18, 22. Similarly, the access dome 12 is not surrounded by insulation.

Shown in position is a typical feed mechanism for the hot gases employed according to the present process, the mechanism including Va feed line 33 projecting through the top of the tank 11 through the dome 12. The supply mechanism includes a horizontally positioned discharge por-tion 31, having discharge ends 32a, 32b. Normally the feed line 33 and the discharge section 31 are cylindrical, sheet metal or light plate metal conduits. Frequently the feed line 33 is a flexible conduit, to expedite insertion of the apparatus generally.

In operation, hot gases are fed from a combustion device, not shown, by positive fan pressure, through the feed line 33, and substantially uniformly distributed by the two branches of the discharge section 311, being discharged through the terminal openings 32 321,. The gases 35 follow generally the ow pattern indicated and leave the tank 13 through the access dome 12.

To illustrate a typical application of the improved process, the following example is typical.

In this operation, the car tank treated was a generally cylindrical vessel having an internal diameter of about 7 feet, and a length, at extremities, of about 312 feet. The internal volume of the car tank 13 was then about 1150 cubic feet. The discharge section of the apparatus for inducing warm gases 31 was a tube of ten inches diameter at the discharge ends 32a, 32h. The center line of the discharge section 31 was about 23 inches from the bottom of the car, hence the distance of the center of the outflowing gas streams leaving the discharge ends, was in the proportion of 1:3.7 with respect to the tank diameter. The transverse area of each of the outlet streams, relative to the transverse area of the tank, was in the ratio 1:71. The discharge tube 31 was approximately four feet long, or about one-eighth of the length of the tank 13. In addition to the circular openings in the ends of the discharge tube 31, several narrow slots were provided near the bottom of said tube, adjacent each end. These are desirable, but yoptional openings.

To demonstrate 4the effectiveness of the operation, with respect to uniformity of temperature levels attained in the wall 13 of the tank, thermometers or thermocouples were installed at numerous points on the wall of the tank. Such installations of temperature measurement devices were made by drilling a small hole through the light metal outer shell 15 and the cork insulation 14 until the temperature responsive element could be positioned at the surface tof the metal Wall 13 of the tank. Thus, a temperature measuring element 4S was positioned at the top of the tank. Thermometers 47, 44 were also positioned at each end of the tank, near the upper extremity of the ends. Two thermometers 45, 46 were positioned on one side of the tank, near the upper extremities of the holsters 16h, 16a. It had been previously discovered that the areas at and adjacent these locations were particularly difficult to maintain `at a temperature consistent with the desi-red average. Similar thermometers, not shown, were inserted on the opposite sides of the tank at corresponding elevations. An additional thermometer 42 Was provided to determine the tank temperature in the bottom region adjacent the mounting bracket 18, this area, as in the case of the areas adjacent the bolsters, being difficult to provide at a uniform temperature consistent with the desired temperature levels.

Operation of a pretreating treatment was started, using a flow of hot gases of approximately 1380 cubic feet per rninute, thus providing a complete gas change in the order of 0.8 minute per complete change. The temperature level of the inlet gases Was initially about 420 F., and shortly after the temperature Was reduced to 3504GO F. Measurement of the temperatures at the various points on the tank 13 walls were made at intervals with typical results being given below:

lemperatures-U F.

Time Sec- Bot- First Side First ond Top tom Second Spread end end 48 42 Side In the above operation, it will be noted that a reasonably low temperature differential, between the maximum and minimum temperatures measured at the tank shell, of about 35, was experienced. When the heating operation was discontinued after fl2 hours, this spread of temperature levels, and a minimum temperature level of about 270 F. for the lowest temperature, had been maintained for about two hours.

To illustrate the importance of the above described combination of operating conditions, the following describes an operation wherein these criteria were not observed. A similar tank of a railroad tank car, having the same dimensions, Was heated, using a heating gas supply system having the same ygeneral configuration as in the above described embodiment. However, in this instance, the discharge tube 31 was appreciably longer, amounting to about one-fifth of the length of the tank. Further, the heating gas rate Was at a relatively low level, corresponding to change in atmosphere only once every 1.2 minutes. The temperature of the entering streams of hot gases was, however, the same as in the preceding example. After a short period of operation, using a hot gas supply similar to that employed in the preceding example, it was found that the cork insulation had become ignited at a zone around the temperature point 44. Thus, although the rate of gas flow inV this operation was low, nevertheless hot spots developed and the curing or pretreating operation was obviously a failure.

, Generally, in the preconditioning of a tank prior to applying any coatings, the heating is continued :for a eriod of at least two hours. After the tank has cooled down sufficiently to temperatures permitting spray or brush application of coating materials, a series of coats are applied from four to eight coats, and each coat is dried to lat least a moderate degree, or sutiicient to allow application of a succeeding coat.

The drying operation at this point is not highly critical, although again the same operating conditions are preserved, with respect to the magnitude, size, and location of the entering wlarm gas streams. However, the temperature of the tank walls is not raised to the same level as in the pre-treating operation or in the final curing operation.

After the nal coat is applied, the operation illustrated by the precedi-ng example is repeated, but in this instance the curing is continued for a period of at least 6, and preferably up to hours at the indicated temperature levels, that is with maximum temperatures of at least about 270 F., and a maximum temperature range of about 35 F.

The coatings applied to the tanks according to the present method may be any of a substantial number of commercially available materials which are desirable for resistance to chemical attack, absence of pin holes, and formation of du-rable iilm. Epoxy resin containing materials, phenolic resin based coatings, urea formaldehyde coatings, polyesters, urethanes, and the like are illustrative. Generally, it should be understood that the preferred coatings are those undergoing a condensation or cross linking polymerization reaction duri-ng a heat treating drying operation as applied according to the present `improved coating process.

The control criteria described herein are equally applicable to other tanks in addition to those used on railroad tank cars. Thus, in compartmentalized cars, the same criteria are applied in coating the individual compartments of the tank. Similarly, tanks for highway trailer tank use can be eifectively coated.

The heat source in the foregoing example, as described, was the product of combustion of natural gas, consisting predominantly of carbon dioxide, canbon monoxide, and water vapor and nitrogen and a small amount of entrained air. In certain instances, it will be desirable to apply la reducing flame to generate a combustion product gas lfor the heating which is a reducing character. In most lcases, however, the presence of small quantities of free oxygen in the combustion products are not objectionable. In certain unusual situations, instead of .the direct products of combustion, it may be ldesinable to preheat dry air, or even an entirely inert gas such as nitrogen, as the cu-ring medium for the operation. This expedient is necessarily more expensive, as indirect heat exchange is required.

Having fully described the invention and the best mode of performing the improvement thereof, what is claimed In a method of lapplying heat setting resinous coating to a railway car tank or similar elongated tank having a cork insulating covering which is susceptible to charring, said method consisting of pre-drying the interior of said tank with Warm gases for a period of two to six hours,

applying a series of four to eight liquid resin coatings and ydrying each coat with warm Igases to a non-tacky state,

then applying a iinal baking treatment with warm gases yfor a period of from six to twenty hours,

the improved procedure of supplying hot gases for the pre-drying, drying, and final baking in the form of two streams discharged from conduit means, each of said streams being discharged toward a tank end in a direction parallel to the axis of said tank, and being approximately equidistant from the center of the tank and separated by a distance of not more than about 1/7 of t-he tank length, the centers of saidl streams on discharge being from about 1A to 16 of the tank diameter 'from the bot- `tom of the tank, land each stream having a transverse area yof not less than about 1/70 of .the transverse area of the tank, the said gases being fed at a temperature above the charring temperature of the cork insulation of from about 350 :to 400 F. `and at a rate suiicient to provide a complete change in a period of from about 1/2 to W10 of a minute, whereby final temperatures, during lthe pre-drying and baking, of 300 to 310 F. are achieved at at least a portion of the tank wall, with all tank wall temperatures above 270 F., but below the chanring temperature, of the cork insulation.

References Cited in the file of this patent UNITED STATES PATENTS 2,108,017 Lithgow Feb. 8, 1938 2,290,038 Folmsbee Iuly 14, 19'42 2,520,397 Greer Aug. 29, 1950 2,719,093 Voris Sept. 27, `1955 2,881,091 Schulze Apr. 7, 1959 2,901,454 Stewart Aug. 25, 1959 2,974,060 Dettling Mar. 7, 1961

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