US3557437A - Method of fabricating large vessels capable of withstanding high internal pressures - Google Patents

Abstract

HOLLOW THICK WALLED CYLINDERS SUITABLE FOR WELDING TOGETHER TO MAKE LARGE VESSELS THAT WILL WITHSTAND 1000 TO 5000 POUNDS PER SQUARE INCH INTERNAL PRESSURES ARE MADE BY CASTING A HOLLOW CYLINDER, OUT OF STEEL, THAT HAS TWICE THE WALL THICKNESS THAT IS DESIRED. THEN THE CASING IS CUT LONGITUDINALLY INTO TWO HOLLOW CYLINDERS OF THE DESIRED ONE-HALF WALL THICKNESS OF THE CAST CYLINDER BY ELECTROCHEMICAL MACHINING. THIS BRINGS ANY INTERNAL CAVITIES TO THE SURFACE ON THE TWO CUT APART CYLINDRICAL SHELLS AND THE CAVITIES CAN BE CHEAPLY REPAIRED BY GRINDING AND WELDING.

Description

United States Patent 3,557,437 METHOD OF FABRICATING LARGE VESSELS CAPABLE OF WITHSTANDING HIGH IN- TERNAL PRESSURES John C. St. Clair, Box 333, RR. 2, London, Ohio 43140 No Drawing. Filed Jan. 14, 1969, Ser. No. 798,854 Int. Cl. B23k 19/00; B23p 17/00 US. Cl. 29-415 6 Claims ABSTRACT OF THE DISCLOSURE The purpose of this invention is to provide cheap vessels capable of withstanding internally the high pressures that are needed in processes for converting coal into natural gas, fuel oil, gasoline and other products. In practically all processes for producing the preceding valuable products from coal it is necessary to react a gas under high pressure [with coal as the first step of the process. The development of cheap high pressure 'vessels would not only cheapen the cost of this expensive initial high pressure step but it would also allow lower temperatures to be used, which will produce normally higher yields of desired products, since then the longer reaction times and larger high pressure vessels necessary would not be too expensive as they are now. Of course high pressure vessels are used by many other industries and my invention supplies these.

In the usual high pressure process it is economical to design the high pressure vessel used as relatively long as compared with its diameter. This makes the construction of the long cylindrical shell the major item of cost. (I will later show a cheap method for making the ends that even reduces the ends present low relative cost.)

A long cylindrical shell is normally made of several shorter shells or rings welded together by girth welds. The making of these shorter shells has in the past been very expensive. To withstand high internal pressures the thickness of the metal walls becomes too thick to allow the shell to be made out of metal plates bent into cylindrical form, for welding, by cheap conventional methods. Therefore expensive multiple walled constructions have been normally used at internal vessel pressures of over 3000 pounds per square inch or greater.

Casting the vessels out of molten metal, poured into molds, on the surface has looked attractive but there have been encountered severe difficulties in casting high pressure vessels in the past. The main objection is the expense of obtaining sound castings, lwithout internal cavities, at a reasonable cost. For the usual vessel of large size, the vessel has a large external area. This requires a large number of risers to be used to supply more molten metal to the casting while it is solidifying. This is necessary since normally molten metal and particularly molten steel decrease in volume when they solidify and it is necessary to keep supplying molten metal to the casting while it solidifies. Since the molten metal cannot be depended on to flow far when the casting is undergoing the critical stage when it has almost completely solidified there must be a large number of risers under normal casting conditions. Another difiiculty, that arises with the thick walls of a cast high pressure vessel, is what to do if there happens to be a cavity in the final cast walls. These cavities can be readily located by known methods but the objectionable cavity will normally be in the center of the thick wall. The standard procedure of grinding down to the cavity and then welding the hole full of metal becomes very expensive. Also weld metal is never as strong as the original metal and this produces an undesired last minute reduction of the internal pressure that the vessel will stand. Therefore to prevent having to throw any vessel with cavities away it is necessary to overdesign all vessels to start with. Now with smaller outside areas and the resulting smaller number of risers and with only part of the metal to undergo high stress as is encountered in average castings the probability that at least one of the risers will not work well enough can be reduced to a small figure by conservative riser design. However when one has so many risers for one casting and when it is desirable to have all the metal stand the maximum possible stress as it is with high pressure withstanding vessels the expense of obtaining all solid metal by normal methods becomes high.

In my disclosed invention I solve the previous problems of casting thick walled high pressure vessels by initially casting a very thick walled cylindrical shell or shape and then longitudinally cutting the very thick walled cylindri cal shape into at least two concentric cylindrical shapes. As for example I might cast a cylindrical shell 10 feet in diameter on the outside and 9 feet in diameter on the inside. The wall of the large cylindrical shape would be constant throughout and would be 6 inches thick. Then I would cut the large cast cylindrical shape into two concentric desired shells. The larger shell would be 10 feet diameter on the outside and 9 feet-6.25 inches in diameter on the inside. The smaller cylindrical shell produced would be 9 feet-5 .75 inches in diameter on the outside and 9 feet in diameter on the inside. Both cylinders would each be equal in length to the originally cast larger cylinder.

The method used for cutting the larger hollow cylinder originally'cast into two or more smaller cylinders is conventional. Probably the best method would be to use electrochemical machining. This method is described in Standard Handbook for Mechanical Engineers by Baumeister & Marks, 7th ed., McGraw-Hill, New York, pp. 13-103 and 104. Very briefly this method consists of a reverse electroplating process. That is the direction of the flow of direct current is reversed, from its direction of travel in electroplating, so that the metal is eaten away from a metal object instead of plating on the metal object.

In electrochemical machining water is supplied to the area where the metal is eaten away both to carry a desired electrolyte to carry the electrical current but more particularly to carry away the heat of the reaction. The rate of eating away of the metal is carried out at a fast rate so that the distance between the electrode (carrying the current to near the metal that is eaten away) to the metal is the controlling factor rather than the reaction on the surface that eats away the metal. Therefore by keeping the electrode very close to the metal, the velocity that the metal is eaten away at any relatively long distance from the electrode is small and with insulation the over cut of the process to the sides is normally of the order of 0.015 inch though this can easily be made larger if desired. In my immediate case the electrode and the duct carrying the water solution of the electrolyte would be in the form of a very thin walled hollow cylinder. The electrode would be covered with plastic paint to limit the eating away of the metal to at only the end of the electrode. The cutting device would look like an enormous biscuit cutter.

Another method that is cheap is to make the casting out of easily machined metal and then with sharp tools on the bottom of walls of a very thin walled hollow metal cylinder cut or machine the large cast cylinder into several thinner walled concentric cylinders. In this case the cutting device would look like an enormous biscuit cutter that is provided with means for rotating while it cut the biscuits.

Instead of sharp tools on the bottom of the very thin walled hollow cylinder used for cutting, pieces of bonded abrasives could be used. Then the metal removed in cutting the meal cylinder would be removed by grinding. This latter grinding procedure would be preferably preformed by my copending patent application Ser. No. 761,539 in which the abrasive would be bonded by cast copper and the abrasive material would be periodically sharpened by removing dulled grit by contacting the copper bonded abrasive with an aerated water solution of ammonia which dissolves copper without corroding iron. In this case the cutting device would look like an enormous biscuit cutter with means to rotate the biscuit cutter while cutting.

The preceding process of casting a thicker walled hollow cylinder first and then cutting it up into a plurality of concentric thinner walled hollow cylinders has the advantage that it only takes the construction of one mold and one core to make a plurality of final hollow cylinders. But of more importance is that the number of risers needed for the single mold is only a fraction of the number needed for each mold required if the thinner walled cylindrical shells were cast separately. The number of risers needed for a casting is inversely proportional to the wall thickness cast. That is if a conventional casting process for a single thinner walled hollow cylinder required 8 risers, then to cast the same diameter and height but with a wall twice as thick there would only be needed 4 risers. And besides with a wall twice as thick there would be, after cutting them apart, two thinner walled cylinders made. In other words in this example the required number of risers per final shell produced would be reduced 75% by my method.

However, the most important advantage of my invention is that when the thicker walled cast cylinder is cut up into a plurality of thinner walled hollow cylinders any cavities in the walls of the original casting become very easy to repair. This is because, in a casting of the preceding type, cavities about always occur in the center of the wall of the casting. Therefore when the casting is cut in just two equally thick thinner walled cylinders the cavities are on the surfaces of the cut-apart cylinders. Therefore the cavities can be easily welded full of metal. And there will not be the usual large amount of grinding out of metal for holes with its resulting filling in of large amounts of weld metal that somewhat weakens the casting.

In case the thinner walled cylinders are not cut apart near the center line of the walls of the thick walled cast cylindrical shell in all cases the cutting procedure will place the cavities nearer to a surface and facilitate repair of the cavities.

As I have said before, the big expense normally for making thick walled high pressure vessels is the cost of making the cylindrical shell. However, due to recent developments even the cost of the ends have been made cheaper. In this case an end of the vessel is cast with its wall over twice as thick as is required. Then the outer surface is ground down to the proper thickness by my method of grinding with copper bonded abrasive as I have described previously in this application. The abrasive will be in the form of a grinding wheel of course instead of a giant biscuit cutter. In this grinding method all cavities will be removed from the cast end of the vessel and the metal ground off can be recovered unharmed for remelting and reuse.

In conclusion I may say that I have disclosed a method for casting cylindrical shells or shapes by casting a thicker walled cylindrical shell or shape and then cutting the thicker walled cylindrical shape into a plurality of thinner walled cylindrical shapes. My method greatly decreases the cost of the molds and risers required. It also greatly cheapens the cost of repairing cavities and greatly reduces the amount of overdesign necessary to prevent rejection of castings on the account of cavities.

The longitudinal axis of a cylinder is the straight line joining the centers of the two ends of the cylinder.

In addition it may be said that with a relatively small investment that a manufacturer of steel castings may go into the business of casting rings and heads for thickwalled vessels that normally sell for as much as $100,000 apiece. All the company has to do is to purchase the electrical equipment and the etching liquid handling equipment for a 3000 ampere electrochemical machining apparatus which costs less than $30,000. The so called biscuit cutter shaped electrode or tool assembly can be home made and made by spacing copper tubes placed longwise, and spaced for example 3 inches apart, on an expendable drum form. Between the copper tubes are placed copper strips laying lengthwise. Then the sides of the copper tubes are brazed to the edges of the copper strips and with brazing on a large flat end, to the formed cylinder, thus the so called biscuit cutter shape is made. The drum around which the biscuit cutter shape is made is removed and the biscuit cutter shape is covered with an electrical insulating paint over all but the ends of the copper tubes that do the actual electrochemical machining. The biscuit cutter shape or tool is slowly rotated around its longitudinal axis while the tool cuts a large cast hollow cylinder into two hollow cylinders. The ends of the copper tubes that do the actual electrochemical machining can have plastic guards put in front of them in the path of rotation so that the copper tube ends wont actually touch the metal cut and cause short circuits which are harmful.

Heads for the thick walled vessels can be made two at a time by cutting a head cast with twice the desired thickness of metal. The cast twice-as-desired thickness head is cut into two heads using the above biscuit cutter shaped tool that is modified. In this case the biscuit cutters cutting edge is cut itself into longitudinal strips that are given an angle inward towards the biscuit cutters longitudinal axis. The longitudinal strips are made with a clearance between them and are made movable in relation to the flat end of the biscuit cutter but with the angle of the strips with the flat end of the biscuit cutter being held constant. Then by moving the longitudinal strips of the cutting edge of the biscuit cutter slowly inward at the angle above mentioned, and which is by moving the longitudinal strips relative to the fiat end of the biscuit cutter, a cone shaped surface can be cut. If the longitudinal strips are bent in the shape of an arc of a circle, a vessel head in the form of a hemisphere can be cut.

I claim:

1. The method for casting hollow cylindrical metal shells for the construction of vessels that withstand internal pressures which comprises: casting a large hollow cylindrical metal shell whose wall thickness is sufficiently thick from which to make a plurality of desired cylindrical shells, cutting said large metal shell into a plurality of metal shells each having wall thicknesses in excess of one inch by cutting said large shell at a constant distance from its longitudinal axis, repairing the cavities which exist in the cut-out metal shells, and using each of the plurality of cut-out metal shells in the construction of a vessel that is capable of withstanding internal pressures in excess of 200 pounds per square inch gage pressure.

2. A method according to claim 1 in which the metal is an alloy of iron.

3. A method according to claim 2 in which the cutting is done by electrochemical machining.

6 4. A methodaccording to claim 2 in which the cutting References Cited is done by grmdmg- UNITED STATES PATENTS 5. A method according to claim 3 in which said large metal shell is cut into two cylindrical shells in such a manner that neither cut-out shell has a wall thickness that is more than 20% greater than the wall thickness of the z d d l h JOHN F. CAMPBELL, Primary Examiner met 0 accor ing to c aim 4 in w ich said large metal shell is cut into two cylindrical shells in such a man- REILEY Asslstant Exammer ner that neither cut-out shell has a wall thickness that 10 5 CL th 20 t h l ghlrggrscilenan grea er t an the wal thickness of the 29 402, 527.6; 164 69 1,606,282 11/ 1926 Witter. 2,184,183 12/1939 Fykse 29-416 5 3,142,115 7/1964 Schaming 29416