R
L. PORTER
UNUSUAL PRESSURE RELIEF aeiection and use of rupture assemblies to protect equipment nberated under extreme conditions are large& a matter of experience. But here are workable designs for some specz@c cases
outlined the fundamentals of Manyreliefstudies .devices.havefor commercially operated equip-
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ment at conditions normally encountered in the chemical process industries, but new problems arise at higher temperatures and pressures. The term, extreme conditions, as defined by Gasche and Sliepcevich, covers the range of pressures beyond 2000 p s i . or temperatures above 1000" F., and various modifications of these conditions. At present, only a few commercial processes fall within this categorymanufacture of polyethylene by the high pressure process, manufacture of ammonia, production of alcohol by the Oxo process, and a few others. Most work is done in the research laboratory. Therefore, the frame of reference for this study must be defined to encompass the work performed in the high pressure research laboratory. There are many difficulties in selecting proper protective devices for this type of activity Therefore, in addition to the relief devices to be discussed, additional protection should be required, such as extremely well qualified, trained personnel and well engineered barricade systems. By far, the more important of these is highly qualified operating personnel. The rupture disk is not the only protection given the high pressure research worker. With high pressure equipment, it is usually quite important that rupturr disks be placed as close as possible to the interior of the pressure vessels to allow little opportunity for build up of solids and to ensure that the pressure drop ahead of the rupture disk be as low as possilde.
AUTHOR R . L. Porter is a chemical engineer who has worked as Production Engineer at the Celanese Corp. of America, and
as Research and Development Engineer'at the Spencer Chemical Co. Presently he is Engineering Sales Manager for Autoclaue Enlineen, Inc., Erie, Pa. ' I
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I N D U S T R I A L A N D P k G l N E E R l N G CHEMISTRY
Figure 1 shows an exploded view of the most commor relief device for h i s type of operation. It consists es sentially of the rupture disk body, the disk itself, a method of sealing the disk against leakage, and a means for carrying the exhausted products away from the equipment. ..
Extremely High Pressures
Rupture disks have become indispensable to the chemical process industry. There are some cases, however, where the range of work being carried out in research is beyond that covered by commercially available rupture disks. For instance, at extremely high pressuresabout 100,000 p s i. or above-it is impossible to purchase reliable rupture disks. To solve this problem, the reverse intensifier has been developed. A pressure intensifier is a common piece of equipment, used to
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raise prrssures to high values by means of a diffexntial area piston. For extremely high pressure work, it has been found satisfactory to reverse this process, placing a rupture disk on the low pressure side of the differential area piston. The high pressure side is exposed to the process under study. As an example, Figure 2 shows this principle applied, with a tenfold difference in piston areas. By this method, a reliable rupture disk rated at 10,000 p.s.1. can protect a 100,000-p.s.i. system.
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Venting Explosive Reactions
Frequently, high pressure research operations are carried out in the explosive range of certain chemicals. Sometimes, reactions take place which go through the explosive range during the course of reaction. In cases of this type, it is necessary to protect against extremely rapid rises in pressure.
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. . 2. A rzw~lseinfern& u r d a relid d o ' For example on piston P,the +&,of the low pre A can be 70 times urca 8. r f high pIesJurc ( p.&i.) is applied to m a B, only 10,0#, p.ewatEd on the low brcsnrre side. which. @&re
Figure 3 shows an autoclave with a rupture disk built as the bottom of the autoclave. With this type of device, the objective is to have a rupture disk with as large an area as possible. Because the rupture disk is in the bottom, the autoclave can become a rocket in case of an explosion. It is necessary, therefore, to mount this equipment with heavy-duty reaction springs so that after relief, it will not shoot through the roof. Avoiding Plugging
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A ticklish problem is associated with operations where the object is to grow a solid material. How can one prevent the solids from covering the rupture disk? An example of such a problem is found in polymer production. I n this type of reaction, chemicals are allowed to polymerize in a continuous flow reactor. T o protect the equipment, the rupture disk was introduced into the interior of the reactor (Figure 4). I n this way, even though polymers may form over the rupture disk, protection is still provided. This type of protection is useful in such operations as the polyethylene reaction. Another example of a process which would plug a relief assembly is the growth of synthetic quartz in high temperature, high pressure autoclaves. The novel rupture disk assembly designed to prevent plugging is described in the article beginning on page 16 of this issue. limited Space
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Fisurc 3. A
rupture disk of fhis to handle explosim reactions
Fig th soli& 26
tyfie may be called for
disk instolled aspmt of the si& of ly to be plugged by formtion of
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I n chemical research, a reactor which has been irequently used, and still finds some occasional uses, is the rodting type. I n this operation, a rocking mechanism agitates the contents of a high pressure reactor. Since the equipment is generally small and is continuously moving, position of the rupture disk is a problem. The solution is shown in Figure 7. This compact assembly includes a valve which shuts o f f the flow of gases to the reactor, a pressure gage indicating pressure in the reactor, and a rupture disk to provide relief for the vessel.
Figure 5. Temporary c a m of excess presnue can be raliemd by a valvc, which will mt emptj the vessel. A mflture disk should still be included in thssydm, howevn
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I n the close-up of the valve, note that thc flow of gases can be shut off without blocking the passage between the vessel and the rupture disk. Quite frequently, in high pressure research, very expensive or very toxic chemicals are used. Therefore a rupture disk, which is the final safety device, should be supplemented with a primary device which will relieve excess pressure temporarily, but will not empty the contents of a vessel. I n Figure 5 there is an air-operated valve, which is actuated by air on the valve diaphragm. The air is controlled by a solenoid valve which in turn is actuated by an electric contact gage. In this manner, the electric contact gage can be set to operate a t a pressure just slightly above normal operating pressure. If pressure reaches this point for any temporary operational reason, the solenoid valve is actuated. This opens the air-operated valve and relieves the pressure, but only temporarily-as soon as the pointer falls below the set point, the valve closes and the operation continues. Exhaust should be carried to a safe area. TEMPERATURE, "F.
High Temperature Procedures
Figure 6.
Yield strength of Inconel at elevated temperatures
Only two methods have been used to solve the pro'nlem of relief devices on such equipment:
-A
cooled relief device. Quite often, water circulation around the rupture dzsk 7s used -A rupture disk make of high temperafur? metal. To date, this is the preferred solution, and Inconrl, which has good strength at high temperatures, has been f i equently used. Also, Inconel has low reduction of strength with increase in ternperalure. (Fzgure 6 ) As more work is done in the high temperature field, other solutions to the safetv problem will be found.
Figure 7 (below). A rocking reactor, equibped with an assembly which incorporates a value for shutting of the jow of gases to the reactor and a pressure gage, as well as a rupture disk.
7 and 2, hexagonal nuts, steel; 3, handle, aluminum; 4, sleeve, ,410 stainless steel; 5: insert, 304 stainless steel; 6, gland nut, 4 7 6 stainless steel; 7, thrust washer; 8, stem assembly, 316 stainless steel t$, 476 stainless steel shank; 9, packing washer, bronze; 70, packing, T e j o n ; 77, bottom washer; 72, cozier, 416 stainless steel; 73, hold down ring, tool steel; 14: rupture disk, 78-8 stainless steel; 75>body, 316 .stainless steel
VOL. 54
NO.
1
JANUARY
1962
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