ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Design of Reactors and Closures FRED GASCHE Autoclave Engineers, Inc., Erie, Pa.
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INCE World War I1 there has been a marked increase in
chemical processing under elevated temperatures and pressures. The ever-growing demands of the consumer for newer, better, and greater varieties of products have spurred the researcher, particularly in the field of petrochemicals. The practical realization of the fruits of this research is becoming more feasible as a result of the development of superalloys which are pushing the metallurgical limit higher and higher. For many years, industrial operations a t high pressures were confined t o the manufacture of only a few products, such as ammonia and methanol, by those companies who had developed a backlog of experience in the “art” of high pressures. Gradually, however, the appearance of a number of excellent papers on high-pressure technology in the published literature reduced the closely guarded trade secrets to a more rational and scientific basis. With it came the growth of several industrial concerns who specialize in the design and fabrication of equipment for service a t elevated pressures and temperatures. Although the factor of experience is not to be taken lightly, the lack of it need no longer be a deterrent from entering this promising field.
Example 1: Effect of Pressure. Consider the design of a 12-inch i.d. vessel for working pressure of 3000 pounds per square inch a t working temperature of 650’ F. The allowable stress as shown in the ASME Code for Type 316 SS is 17,050 pounds per square inch. According t o the code formula, the wall thickness required is
On a 12-inch i.d. vessel, a wall thicknees of 1.18inches is not considered an extreme condition. Example 2 : Effect of Temperature. The conditions are the same as in Example 1 except that the working temperature is 1500’ F. From the ASblE Code the maximum allowable working stress is 1500 pounds per square inch. According to the code formula, the required wall thickness is
Extreme Conditions The terminology “extreme conditions’’ is obviously arbitrary because it is somewhat dependent on the limits of one’8 experience. For this reason, it is difficult to define “extreme conditions,” although i t may be poseible t o establish what are not extreme conditions. Perhaps the easiest place t o start the defining process is the ASNE Code for Unfired Pressure Vessels which covers pressures up t o 3000 pounds per square inch. Table AHA-23 in the code lists the allowable stresses for most of the common materials of construction for temperatures up to 1500’ F. The code formula for determining the thickness of material required is
where P = working pressure R = inside radius S = allowable working stress t = wall thickness From this formula, it is evident that the wall thickness is a function of the pressure, the allowable working stress (n-hich is dependent on the materials of construction as well as the temperature), and the inside radius (size) of the vessel. In order t o illustrate the influence of pressure, temperature, and size, three examples are given.
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Figure 1.
Comparison of American Standard flange with modern design set-screw closure
INDUSTRIAL AND ENGINEERING CHEMISTRY
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EXTREME CONDITION PROCESSING
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Figure 2.
POVER
Button-type gasket closure
Because of the negative result, this might well be considered a n extreme condition. Example 3: Effect of Size. A small reactor, S/le-inch i.d., is required for working pressure of 30,000 pounds per square inch at 650' F. Without carrying out the detailed calculations, it is immediately obvious t h a t a standard steel nipple and necessary standard steel fittings would suffice. For this reason, this application cannot be regarded as an extreme condition. Therefore, in extreme conditions the size of the unit must be taken into consideration as well as temperature and pressure. At this point it may be worth while t o elaborate on some of the common materials of construction. Several manufacturers offer stainless steel tubing for working pressures of 30,000 pounds per square inch, since stainless steel tubing lends itself t o cold working t o a yield point in excess of 80,000 pounds per square inch. However, it does not follow that a 1-gallon stainless steel autoclave for a working pressure of 30,000 pounds per square inch can be fabricated. T o design for this working pressure, the design stress should be a t least 45,000 pounds per square inch. The yield point of a forging, from which the 1-gallon autoclave could be fabricated, probably would not exceed 35,000 t o 40,000 pounds per square inch, since i t could not be cold worked t o the extent t h a t tubing can. Therefore, it would be impossible to design such an autoclave from stainless steel. Today, in this country particularly, numerous alloy materials are available although some of them are difficult t o obtain without defense priority. For the design of chemical reactors for use at temperatures above 1000° F. and pressures above 1000 pounds per square inch, the superalloys, 19-QDLand 16-25-6, have in recent years found numerous applications. These alloys have
May 1956
properties similar t o Types 347 and 316, but in general they have better creep resistance. For temperatures below 750' F., the Type 400 stainless steels and AIS1 4340 can be used for pressures under 100,000 pounds per square inch, since they can be heat treated t o 150,000 pounds per square inch yield and with 15y0 elongation. Other alloys such as N-155, S-186, and Inconel X have excellent strength even at temperatures as high as 1350' F., but their weldability and machinability by present-day techniques are not entirely satisfactory.
I Figure 3.
Examples of 12-inch reactors
Of utmost importance in selecting materials for extreme conditions is the elongation of the material. Even though i t is possible t o get materials of 200,000 pounds per square inch minimum yield point, they cannot be recommended because of their low elongation. For materials under tension, a minimum elongation of 15% is desirable.
Closure Designs To illustrate the advancement in closure design, a comparison between present-day practice and t h a t of 25 years ago is illustrated in Figure 1. The autoclave in dotted lines shows a 5gallon unit designed 25 years ago using 2500-pound American Standard flanges. This design calls for twelve 2*/2-inch diameter bolts and requires a torque of approximately 2500 foot-pounds on each bolt t o ensure a tight seal. The unit shown in solid lines is a present-day, modern design. Eighteen 1-inch set screws are utilized t o make the joint. This modern unit, complete, weighs less than just the bolts and nuts on the old design. Another design which is frequently used is the button-type
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT l
tightened when the unit is under pressure. Careful handling is required since highly polished surfaces are necessary for the tight seal. The cover, the seal ring, the main nut, and the thrust washer and the lock nut should be assembled loosely, so that the main nut will rotate freely against the seal ring. The main nut is tightened by using a tommy bar, and a slight tap with the bare hand will suffice. The set screws then are tightened with a moderate torque.
P O V E R
r C O V E R
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NUT
-REACTOR BODY
LREACTOR BODY Figure 4.
Modified Bridgman closure - " O " RING
gasket-closure s h o r n in Figure 2. The design of this closure is quite simple, and it can be used for working pressures up t o 30,000 pounds per square inch and for inside diameters up to ll/pinch. A solid gasket is used. The closure consists of four main parts-the gasket, the cover, the thrust washer, and the nut. Two types of self-sealing closures which have been used extensively in industry are the Delta and full-Bridgman. A comparison of the two closures for a 12-inch reactor is given in Figure 3. The Delta closure requires a larger outside diameter forging in order to accommodate the twelve 21/2-inch bolts, required to seal the cover, as compared t o the twelve 1-inch set screws on the full-Bridgman. I n the Delta closure, the bolts must carry the full end load in addition to the initial sealing load, whereas in the full-Bridgman the set screws are used only t o make the initial seal while the threaded plug carries the hydrostatic end load. Both the Bridgman and the Delta closure8 are quite satisfactory for service which requires t h a t they be opened frequently. I n Figure 3, the gasket shown for the full-Bridgman closure is usually a hard metallic material. I n some applications a pliable gasket such as rubber or braided metallic packing can be used. A close-fitting metal backup washer is placed on the top and bottom of the pliable gasket. The set screws are then tightened only moderately to obtain the initial seal; the internal pressure produces a tight seal. This design is suitable for pressures over 100,000 pounds per square inch, but its primary limitation is the availability of pliable materials for the working temperatures involved. For extreme pressures, the modified Bridgman closure, as shown in Figure 4, has been used successfully. This closure utilizes the principle of unsupported area. It has been used for working pressures of 100,000 pounds per square inch and over, for inside diameters up to 2 inches. The material of construction is usually either 410 heat treated, 4340, or any other heat treatable alloy steel with good physical properties. I n this design, the seal is dependent on metal-to-metal contact, which is achieved by machining the seal ring and reactor-body seat at slightly different angles. The seal is accomplished by the set screws which are tightened with a moderate torque. As the pressure increases, the internal pressure is utilized to make the seal. Under no circumstances should the set screws in the lock nut be
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Figure 5.
O-ring closure
A closure which has gained considerably in popularity since World War I1 is the O-ring type of which there are numerous variations. One design which has given almost foolproof service is shown in Figure 5. By employing an unequal, tapered, metal-to-metal seat just above the O-ring, it is not essential to machine t o as close tolerances as otherwise would be required. There is no question that if the materials from which the O-ring is fabricated are suitable for the intended service, this type of closure is superior t o any t h a t has been devised.
Safety Factors No discussion of designs for reactors and closures rould be complete without some mention of safety factors. Safety factors are entirely dependent on the pressures and temperatures involved. Although the ASBIE Code bases its safety factor on tensile strength of the material, the writer has based the safety factor on the yield point of the material when designing for extreme conditions. The higher the pressure, the lower the safety factor. It is virtually impossible t o make a definite recommendation regarding safety factors. However, a suggested rule might be: when designing for 20,000 pounds per square inch, use a safety factor of 2.5 based on the yield; when designing for 50,000 pounds per square inch, use a safety factor of 2.0 based on the yield; when designing for 90,000 pounds per square inch, usc a safety factor of 1.5 based on the yield, But if a t the same time the temperature should exceed 1000" F., there is no safety factor as such. In this case, the design should be based on hours of life. RECEXVED for review Ootober 13, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED March 16, 1956.
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