FIGURE 1 (Left). STEEL T A N K 5 LINED WITH RUBBERAND SHEATHED WITH ACID-PROOF BRICK F O R B I L L E T PICKLING: SERVICE IN A STEEL MILL A steel rinse tank (unlined) is in foreground. The methpd of mounting makes all exterior aurfaoes readily accessible apd allows free circulation of air.
FIQURE2 (Lower Left).
ROLLING: A RUBBERLINING:TO ENSURE FIRM CONTACT WITH THE ADHESIVE FILMIN A STEEL TANK
'ED 'T Fundamental Principles of Design FIQURE3 (Below). T w o SE c T I oN RUBBER-LINED STEELTANKENTERINQ A LARQE STEAMVULCANIZER The flanged oonstruction of the
asnemblv. tank halves The is designed vuloenizar for isfield 1.6 feet in' diameter -and - - ~ p p - & ~ mately SO feet long. it is thou ht to be the largest h the word.
J. R. HOOVER AND H. C. KLEIN The B. F. Goodrich Company, Akron, Ohio
I
N 1924 a process known as Vulca-
lock, discovered by chemists of The B. F. Goodrich Company, made possible the first successful steel tank car for hydrochloric acid service. The tank was lined with acid-resisting vulcanized rubber bonded to the steel shell with adhesion e x c e e d i n g 500 pounds per square inch. A new material of chemical construction was thus made available in practical form. The resulting widespread and rapidly increasing use of rubber-lined equipment in the processing industries is well known. Basically, the value of such construction lies in properly combining the unique corrosion- and abrasionresistant properties of rubber with the rigidity, strength, and adaptability of steel or other structural materials. It is essential, therefore, that chemical e n g i n e e r s be familiar with certain principles of design, upon which the successful use of rubber-lined equipment depends. No attempt will be made in this paper to define the broad field of usefulness 394
or the limitations of rubber linings. The fact must be emphasized, however, that an extremely wide range of chemical and physical properties is available in commercial rubber compounds and that these compounds, like metals and alloys, are designed for specific uses. I n considering the application of rubber to any chemical project, unless previous experience clearly points the way, the engineer would do well to obtain the cooperation of reliable rubber manufacturers who maintain research facilities and who are constantly studying and improving the usefulness of rubber compounds for specific conditions of service. Too much emphasis cannot be laid upon the necessity for accurate, complete definition of service conditions and requirements. Only through cooperation between the engineers using rubber and the engineers designing it can the best advantage be taken of this unique and versatile engineering material. While unvulcanized fine Para and pale crepe rubbers have been, and to a very limited extent are still, used as corrosion-resistant linings, the great bulk of rubber applied to chemical service is vulcanized. The present discussion will be limited, therefore, to equipment lined with vulcanized rubber.
Function of Lining It is fundamental to bear in mind that the function of rubber linings in chemical equipment is protective, not structural. Such linings are useful only to the extent that they (1) protect equipment from corrosion or abrasion and (2) protect contents from contamination. Structural requirements, such as resistance to pressure or vacuum, must, therefore, be taken care of wholly in the design of the vessel or other part to be lined.
Choice of Materials Rubber linings can be applied to many different structural materials. There are few cases, however, in which steel is not the logical choice because of low cost, ease of fabrication, strength and rigidity, adaptability to a wide variety of structural and process requirements, adaptability to requirements for successful application of rubber lining, and ease with which alterations and repairs can be made. For corrosive service, iron and steel vessels possess the further advantage that, in case of accidental rupture of the rubber lining, leakage will show up quickly through the shell and (provided proper bonding of rubber to metal has been obtained) directly behind the point of damage to the rubber. Detection can thus be made before the damage becomes extensive. Repairs are made quickly and easily. Rubber-lined metal tanks should be located so that all exterior surfaces are readily accessible for inspection, painting, and repairs (Figure 1). I n cases where rubber is required to be bonded to alloys, the rubber manufacturer should be consulted for recommendations. There are some limitations in this field. Alloys of high copper, lead, manganese, or silica content are generally to be avoided. Wood or concrete tanks and other equipment can be satisfactorily rubber lined where structural and process requirements indicate the advisability of such construction.
Application of Lining A brief description of how rubber linings are generally applied will facilitate an understanding of certain principles which must be followed in designing the equipment. Essential steps in the case of steel are as follows: (1) Preparation of the metal surface. Areas to be rubbercovered are cleaned and roughened, usually by sand or steel grit blasting.
(2) Application of special a d h e s i v e s . Generally such materials are ? read over the freshly %lastedsurface in liquid form, like paint. A number of coats are used, often of v a r y i n g composition. Each coat must be thoroughly dry before the next is applied. (3) Application by hand of unvulcanized rubber compound in sheet form. The rubber is sticky and semi-plastic at this stage, permitting adjacent sheets to be made integral at the seams. Thorough hand-rolling ensures firm contact with the adhesive film over the entire area (Figure 2). The rubber sheets which are commonly 8 / ~ a to inch thick, depending upon service requirements, are built up of a large number of very thin layers, plied in a calender. (4) Vulcanization of the rubber. Best and most uniform results are obtained when steam (or a combination of air and steam) under p""8ure can be used. This is accomplished by either of the fol owing two methods: (a) Placing the entire rubber-lined part inside a large autoclave (Figure 3). (a) In the case of closed rubber-lined vessels, internal steam under pressure may be introduced, up to the safe limit. Steam pressures of 30 to 75 pounds per square inch are generally emP d , with the period of exposure varying from 2 to 8 houra. ere neither of the above methods is applicable, such as in the case of large, open tanks lined in the field, a satisfactory cure can be obtained by more prolonged exposure to boiling water or exhaust steam. Temperature control in this method is likely to be less uniform and accurate than when steam pressure can be used. In order to obtain the advantages of pressure cure it is often advisable to fabricate extremely large tanks in flanged sections so that they may be accommodated in vulcanizing equipment available at the rubber manufacturer's plant. Such flanged sections are assembled in the field by bolting together with suitable gaskets.
t
_ _ ______ ~ c-
ri
DESIQNOF RUBBER-LINED VALVE FIQURE 4. STANDARD SUITABLE FOR BOTH PRESSURE AND VACUUM SERVICE Nominal Size A
The dimensions, in inches, are as follows: Size Length Opening Clearance Height Opening Face to Face at Seat C D E B
T
396
INDUSTRIAL AND ENGINEERING CHEMISTRY
Labor obviously constitutes a very large proportion of the total cost of rubber linings applied to industrial equipment. I n many cases, therefore, not only a higher factor of safety but also better value can be obtained by specifying more than the standard 3/l~-inchthickness of lining in large permanent installations. This is especially true where severe service conditions are anticipated.
VOL. 29, NO. 4
for their own inspectors to approve each metal part a t fabricator's plant before acceptance. Likewise, tanks for pressure or vacuum service should be tested before lining with rubber and should be made perfectly tight under the specified test conditions. An air leak in the steel tank will cause failure of the rubber lining if the vessel is used under vacuum.
Accessibility Fundamental From the foregoing it is obvious that one of the basic design principles for equipment to be rubber lined is that all surfaces to be covered with rubber must be readily accessible for handwork. There is only one important exception to this rule, and that is in the case of round pipes, where the lining procedure used is somewhat different. I n pipe lining, a flattened tube of unvulcanized rubber is drawn through the pipe, which has been previously blasted and cemented. The rubber tube is then brought into firm, uniform contact with the adhesive film on the pipe walls by applying air pressure inside,
Design of Tanks With the exception of railroad tank cars, practically all rubber-lined metal tanks today are of welded steel plate construction. Modern welding technic has made this method of fabrication by far the most economical and adaptable. Welded construction, owing to the comparative smoothness and uniformity of the surfaces, is particularly suited to the process of lining and should, therefore, be given first consideration in designing rubber-lined equipment. The following precautions are necessary on the part of the fabricator: (1) Joints over which rubber is to be applied must be solid welded. (2) Porosity in the finished welds is not permissible. Porous welds may be peened. Pits, sharp depressions, or porosity are likely to trap air or solvent from the ad* I hesive, causing blisters to form under the liniIIIli ing during cure. (3) Burrs, e x c e s sive weld spatter, and sharp edges must be removed by grinding. Many fabricators have develo ed their welding tectnic t o the point where a very d e n s e smooth bead is laid on, requiring almost no peening or grinding. (4) All sharp edges : of sheared plates must be removed. (5) Corners should be rounded to approximately l/S-inch minimum radius. (6) Nipple outlets should be made flush with plate inside and corners rounded. Flanges o n n i p p l e s must be screwed or
w e l d e d flush w i t h e n d o f pipe (no exposed threads), and corners must be rounded. When screwed flanges are used, they must be peened in place t o prevent loosening in service. Tanks should be closely inspected for finish at the place of fabrication and all defects corrected before shipment is made to the rubber manufacturer. Rubber manufacturers will gladly accept the responsibility of furnishing complete equipment, metal and rubber, and in such cases usually arrange
FIGURE5.
RUBBEREXPANSION JOINTFOR PIPE LINES
Practically integral bonding of the soft rubber slab t o the steel rings is required for success of this joint.
Sectional tanks for field assembly should be provided with joining flanges of ample size, drilled for bolts (Figure 3). The rubber lining is brought over the faces of these flanges, and suitable soft rubber gaskets are provided to make a tight seal when the flanges are bolted together. In large installations it is advisable further to specify that a rubber sealing strip about 8 inches wide be applied over each joint after assembly. This is simply an added factor of safety. All outlets in tanks should be of flanged type, with the rubber carried out over the flange faces. This construction readily permits connection to pipes, valves, and other fittings without danger of leakage or contact with metal. Minimum size nipple for best results is 1 1 / 2 inches. The reduction in internal diameter owing to rubber lining should be borne in mind. If the workmen applying the rubber lining must enter the tank through manhole, this should be not less than 18 inches in diameter in the interests of safety.
Physical and Thermal Protection Owing to the natural limitations of rubber in respect to service where mechanical abuse or high temperatures are encountered, protective sheathings are frequently installed within rubber-lined tanks. Tongued and grooved wood linings often provide the required mechanical protection in such cases, but offer little or no thermal advantage. The best practical method of avoiding the adverse effects of high temperatures is to install a sheating of suitable acid-proof brick inside the rubber lining. The maximum reduction in rubber temperature is obtained only when a steel tank is used, with reasonably free circulation of air past all exterior surfaces (Figure I). This construction is successfully and widely used in the metal industries where pickling operations are normally conducted a t temperatures exceeding 175' F. and where, owing to the nature of material handled, mechanical abuse is unavoidable.
Castings A safe rule to follow in designing equipment to be rubber lined is to avoid the use of large or complex castings when-
Y
ever possible. Small simple castings, such as standard fittings and flanges, offer no difficulties, provided the following precautions are closely adhered to : (1) inIron castings must beforofstrength.) close-grain gray iron. (Steel scrap the mix is permissible (2). Surface to be rubber-covered must be free from excessive porosity. Blow holes or sand holes on the surface may be cleaned out and edges rounded t o ‘/a-inch radius or greater. 00casional small porous spots exposed on the surface may be drilled, tapped, and plugged (with steel or iron only) in such a manner that the plug does not trap air or leave exposed cracks, and cannot work loose. The use of lead, babbitt, [‘smooth on,” brazing, or other iron fillers or cements is not permissible. (3) All corners to be rounded to at least l/,-inch radius. (4) Surface must be free from all sharp projections and fins. Inasmuch as undesirable porosity often does not become apparent until after blasting, it is well t o specify in the case of large castings that the fabricator shall, a t the time of inspection, sandblast areas to receive the rubber covering. This will not eliminate the necessity for further blasting a t the rubber manufacturer’s plant just prior to lining. ~
Pipe, Fittings, and Valves Flanged construction is best suited to the majority of rubber-lined pipe installations. Each job is necessarily “tailormade,” since pipe lengths cannot be altered conveniently after lining. Engineers not thoroughly familiar with this type of equipment should pursue either of the two following courses: (1) Draw up details of proposed pipe lines and connections, showing all center-to-face dimensions, and submit such drawings to the rubber manufacturer who will gladly prepare a bill of materials. ( 2 ) Consult the rubber manufacturer who will recommend type and thickness of lining to be used and will also advise proper allowance t o be
made for rubber a t joints. The user will then be in a position to list his own bill of materials. Fittings are commonly of standard cast-iron flanged construction, selected from manufacturer’s stock to be sure they are suitable for rubber lining. Pipe is usually standard wrought steel, with cast-iron flanges screwed on and peened in place. Care must be taken that flanges are made flush with the DiDe ends. If steel flanges are required, they may be eithk; screwed on and peened, or welded in place. Bolt holes must be lined up so that pipe will fit properly into the final assembly. It is advisable in most cases to permit the rubber manufacturer to furnish pipe and fittings complete in order that metal parts will conform to his specifications covering suitability for lining. Valves must be specifically designed for rubber lining. It is best, therefore, t o consult the manufacturer for recommendations. Figure 4 shows one of the standard rubber-lined valve designs, suitable for both pressure and vacuum service. I n any long pipe line, expansion and contraction are a problem. Short lengths of rubber hose can be installed a t intervals to take care of these changes, or, preferably, special expansion joints can be specified, as shown in Figure 5 . This has proved to be a very simple and efficient construction. Figure 6 shows a typical assembly of rubber-lined pipe, fittings, and valves a t the top of a rubber-lined steel storage tank for hydrochloric acid.
Flexlock Construction A new type of flexible coupling for pipe lines has recently been made available and seems particularly adapted to the larger sizes of rubber-lined pipe in low-pressure or gravity-flow installations. I n this construction, plain-end steel pipe is lined on the inside and the rubber is carried back about 5 inches on the outside a t each end. A special ribbed, rubber gasket, known as Flexlock, is slipped over each end of two adjoining pipe lengths, and a split-steel rubber-lined sleeve is bolted over the two ends to make the seal. About 1/1 inch of free space is allowed between pipe ends so that expansion and contraction in long lines are readily absorbed at the joints. The coupling assembly is sufficiently flexible to permit small angularities in the line without special fittings. Likewise, misalignment of pipe lengths can be compensated for within reasonable limits. Figure 7 shows an Winch pipe line of this construction installed a t one of the large steel mills to handle hot acid wastes from pickling operations. The pipe is lined with 6/le-inch rubber of a special construction designed for severe service. Sudden variations in temperature, which caused previous pipe lines to fail, have had no adverse effect. Practically all t h e worthwhile developments in this highly specialized field of rubber-lined equipment were brought about primarily as a result of far-sighted engineers in processing industries discussing their problems freely with rubber technologists. Frank and serious discussions between men of different industries cannot fail to bear fruit in new ideas. RECEIVED March 2, 1937.
