US3238027A - Pressurized flash arrester - Google Patents

Description

March 1, 1966 H. c. DELLINGER L 3,238,027

PRESSURIZED FLASH ARRESTER Filed Nov. 20,.1964

INVENTORS HARTLEY c, DELLINGER RUDOLF F. HEUER A TTORNEV 3,238,027 PRESSURIZED FLASH ARRESTER Hartley C. Dellinger, Tonawauda, and Rudolf F. Heuer,

Getzville, N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed Nov. 20, 1964, Ser. No. 414,509 5 Claims. (Cl. 48192) This is a continuation-in-part of application Serial No. 238,388, filed November 19, 1962, now abandoned.

This invention relates to pressurized flash arresters, and more particularly to the dry type for acetylene and similar reactive gases for high density service.

In a conventional dry bed arrester, packed uniformly with small size bodies, the particles on the surface of the bed facing the flash are heated by the subsequent flow of hot gases to a temperature which is often above the ignition or reaction temperature of the reactive gas. When the pressure surge resulting from the flash has subsided, fresh reactive gas again flows into the arrester. Upon reaching the hot particles on the face of the packed bed, the fresh gas reacts and releases more heat within the hot zone of the bed. By conduction and continued gas reaction, the hot front gradually penetrates through the bed and reinitiates the flash on the oposite side of the bed. Thus, the fiash continues along the pipe, having stopped only momentarily at the flash arrester,

It is therefore the main object of the present invention to prevent re-ignition of acetylene or reactive gas following a flash.

This invention employs a gradated packed bed consisting of layers of particles arranged with the coarsest layers toward the entering flash. The flash is gradually quenched without any particles becoming hot enough to permit re-ignition of acetylene when it continues to flow after the flash has occurred.

The rates of heat transfer from the gas to the packing have been studied. These studies have shown that the major heat transfer resistance for quenching a flash lies Within the solid particle and not across the gas film. Since the time for arresting a flash is of the order of one millisecond (.001 sec.), only that small portion of the solid which is very near the surface can act as a heat sink as the flash passes. Following the flash, the higher the thermal conductivity of the packing material, the more quickly the temperature at the surface of the packing will drop, thus better preventing re-ignition.

Smaller particles possess more heat transfer area per volume of packed bed and thus produce more effective flash arresters for initially quenching the flash. However, small particles at the face of the arrester remove heat so efliciently from the gases that they become heated above the surface ignition temperature of acetylene and will sustain decomposition if the acetylene flow continues. Since larger particles are not as effective heat exchangers as the small particles, the larger particles at the face of the bed will only partially cool down the hot gases and carbon, thus they remain below surface ignition temperatures. The internal layers of small particles will finish cooling the gases and will also remain below ignition temperatures since the gases reaching these particles are reduced in heat content.

The total heat content of the gases is thus dissipated throughout the bed rather than primarily at the surface initially confronting the flash. The gradated packed bed flash arrester uses this approach for extinguishing flashes. In selecting the size of particles to be used in adjacent layers of the gradated packed bed flash arrester, the gradations used must be such that extensive intermixing of the layers is prevented. Intermixing will be avoided nited States Patent 0 if the relative particle size contained in each adjoining layer does not exceed a diameter ratio of about 1.7 to 1.

According to the invention, there is provided a quenching zone comprising a first packing of heat resistant bodies creating a multitude of inter-body flow passages smaller than the critical quenching dimension for the reactive gas. This zone contains the smallest packing bodies in the bed provided to quench the detonation waves resulting from the flash.

There is also provided a cooling zone comprising a second packing of heat resistant bodies at least a portion of which, remote from the quenching zone, has a surface to volume ratio less than one tenth that of the first zone. This zone acts as a heat exchanger and cools by heat absorption the flow of hot gases passing through the arrester. Such heat exchange can be safely effected in the second zone by selecting relatively large packing bodies having a considerably lower surface area-to-volume ratio than in the first zone, so that their absorbing capacity is high in relation to their heat absorbing rate. By the time the hot gases reach the fine particle zone, they will have been cooled by the second zone well below the ignition temperature of the reactive gas.

When assembling the gradated bed flash arrester, the packing is pre-loaded by imposing axial compression on the packing while the end plates are secured in place. The object of pre-loading is to maintain a tight bed and to avoid shifting, intermixing, and abrasion of the packing when in service or when called upon to stop and confine a flash. To be effective, some part of the assembly subject to the pro-loading compression must be elastically deformed so that the compression will be maintained despite any slight change in volume of the packed bed. If the packing is an elastic material, such as steel, stainless steel, or nickel, the packing itself will serve as the element which is deformed elastically. If the packing is an inelastic material such as alumina or silicon carbide, springs or other such means must be provided.

Pre-loading is now recognized as an important step in the manufacture of arresters. As standard practice, we now weld both end plates of our nickel bed arresters in place while applying about 5,000 psi. axial compression. In assembling single-ended arresters, at least the end plate covering the coarsest material should be loose when compression is applied. Preferably for both double-ended arresters and single-end arresters, both end plates should be loose for the compression step. Compression applied against the face of the packed bed is dissipated laterally to the walls of the flash arrester at progressive depths through the bed rather than being exerted uniformly in a direction parallel to the axis of the bed. If either end plate of a double-ended arrester is welded securely prior to the compression step, then an appreciable part of the compressive force will be transmitted from that end through the walls of the arrester rather than through the packing. As the result, the packing at the fixed end of the bed will not be compressed elastically to the desired high degree and will be prone to shift or intermix during service.

Some degree of axial compression is required for the successful assembly of any gradated bed flash arrester, with or without springs, in order to maintain a tight bed during normal service and installation of the arrester; otherwise, the packing will loosen and intermix as a result of handling, vibration, and normal gas flow through the packed bed. Such light compression, designed only to secure the bed during normal handling and operation (not during a flash itself), is an absolute minimum requirement for all gradated bed flash arresters. Applying such pre-loading to an elastic bed without springs may result in a bed suitable for stopping only a single flash following which the arrester must be replaced for dependable future protection. Spring-loaded beds require light compression, e.-g., 500 p.s.i., in order to precompress the spring elements during assembly.

In preferred practice, we use much heavier axial loading for holding elastic packing by pre-loading alone. Preferably, gradated beds secured by preloading alone are compressed to much higher values of pressure than described above. The preferred result accomplished by pre-loading is to prevent any extreme change in compression on the packing proper during the momentary absorption of a flash. To achieve this objective, it is necessary that the preloading compression approach and preferably exceed the differential compression exerted across the end plate at the moment the arrester receives the impact of the flash. Such pre-loading compression may be on the order of 5,000 p.s.i., many times greater than the relatively light compression necessary for packing stability during normal service.

Pre-loading alone is not entirely effective for friable packings such as alumina and silicon carbide. Due to continuous breakage, such materials cannot be depended upon to maintain throughout long service a compressive force which is stored entirely within the walls of the arrester. Furthermore, friable materials are apt to be crushed by the pre-loading compression.

Spring-loading was proposed and tested as a means of holding a bed of inelastic packing tight despite some shifting and particle breakage.

In tests, spring elements were inserted at one end of the arrester between the welded end plate and a second loose slotted plate covering the packing. The arrester was assembled with suflicient axial compression to compress the spring elements about inch. Split lock washers were conveniently used as the spring element and were held in place between the two plates by compression alone.

In the drawings:

FIGURE 1 is a horizontal axial section through a double-ended gradated bed flash arrester according to the preferred embodiment of the present invention, employing nickel shot as the heat resistant packing material;

FIGURE 2 is a vertical section through an end plate of the flash arrester shown in FIGURE 1; and

FIGURE 3 is an inner end view of the end plate shown in FIGURE 2.

Referring to FIGURE 1, the flash arrester consists of a body 10, preferably a cylindrical shell made from, for example, nominal five inch double extra heavy steel or stainless steel pipe. This shell is capped by two identical end plates 12 and 14 centrally drilled as at and slotted on their inner sides with, for example, one-sixteenth inch Wide saw slots 16 which intersect a transverse hole 18 of, for example, three-quarters of an inch diameter.

These slots also serve as a support to keep the packing in place. A shock wave entering the end plate inlet is evenly dispersed throughout the diametral hole before passing through the saw slots. This prevents the shock from being transmitted en masse to the packing. This in turn protects the weld holding the end plates from being subjected to the full detonation pressure.

The flash arrester is packed with, for example, five pounds each of five different sizes of cast nickle shot arranged in transverse layers and having the largest size shot at each end of the arrester and the smallest size at the center of the arrester.

Nickel shot was choosen for this service because of its ready availability, its resistance to corrosion, and its ability to withstand pressure and shock loadings without significant change in packing density or volume. Although greater surface area is available when refractory packings such as aluminum oxide or silicon carbide is used, the friable nature of refractory type material allows the bed to loosen up when hit by a flash, thus making the effectiveness of this type of material very poor for high-pressure service.

SPECIFICATION FOR NICKEL SHOT Size A On 4 mesh percent max 0.0 Thru 4 mesh on 7 mesh percent min 90.0 Trus 7 mesh on 8 mesh percent max 8.0 Thru 8 mesh on 12 mesh percent max 2.0 Thru 12 mesh Trace Size B On 8 mesh percent" 0.0 Thru 8 mesh on 12 mesh percent min 85.0 Thru 12 mesh on 14 mesh percent max 13.0 Thru 14 mesh on 18 mesh percent max.- 2.0 Thru 18 mesh Trace Size C On 12 mesh percent 0.0 Thru 12 mesh on 14 mesh percent max 5.0 Thru 14 mesh on 18 mesh percent min 80.0 Thru 18 mesh on 20 mesh percent max 14.0 Thru 20 mesh on 40 mesh percent max 1.0 Thru 40 mesh Trace Size D On 18 mesh percent 0.0 Thru 18 mesh on 20 mesh percent max 10.0 Thru 20 mesh on 35 mesh percent min 85.0 Thru 35 mesh on 40 mesh percent max 4.0 Thru 40 mesh on 60 mesh percent max 1.0 Thru 60 mesh Trace Size E On 35 mesh percent" 0.0 Thru 35 mesh on 40 mesh percent max 5.0 Thru 40 mesh on 60 mesh percent min 80.0 Thru 60 mesh on mesh percent max 14.0 Thru 80 mesh on mesh percent max 1.0 Thru 100 mesh Trace All mesh sizes are from US. Standard Fine Sieve Series.

The flash arrester can be assembled, by first welding the end plate 14 in place to a nominal 5 inch double extra heavy steel or stainless steel .pipe. The nickel shot is then placed in the arrester in layers as described. The remaining end plate 12 is positioned and a total load of about 120,000 lbs. (about 9,250 p.s.i.g.) is applied to compress the bed. The end plate 12 is then welded in place While a load of about 80,000 lbs. (about 6,150 p.s.i.g.) is maintained on the bed. The nominal 5 inch double extra heavy steel or stainless steel pipe is constructed and arranged to maintain the packing under a loading from about 25,000 to about 120,000 lbs. (from about 1,930 p.s.i.g. to about 9,250 p.s.i.g.).

Zone E contains the smallest packing bodies in the bed provided to quench the detonation waves resulting from the flash. The dimension of the inter-body passages through this zone of the bed must be smaller than the critical quenching dimension characteristic of the gas being handled.

Zone A acts as a heat exchanger and cools by heat absorption the flow of hot gases passing through the arpester. This zone has relatively large packing bodies having a considerably lower surface area to volume ratio than zone E, so that their heat absorbing capacity is high in relation to their heat absorbing rate. By the time the hot gases reach the fine particle zone E, they will have been cooled by zone A well below the ignition temperature of the reactive gas. At any point in zone A, the surface to volume ratio of the packing must be compatible with the temperature of the hot gases at that point so that overheating will not occur. Preferably, the surface area to volume ratio of the coarsest material in zone A is less than a tenth the ratio of the finest material in zone B.

What is claimed is:

1. A dry type flash arrester comprising: a hollow cylinder; end plates attached to each end of said hollow cylinder, each end plate constructed to provide for gas to flow therethrough; and a packing of heat resistant elastic material filling said hollow cylinder between said end plates, said material of such size that substantially all of said material passes through a No. 4 size screen of US. Standard Fine Sieve Series, said material arranged in at least three discrete contacting layers of respective particle sizes, said layers being disposed transversely to the longitudinal axis of said cylinder, the particles in each of said layers being approximately uniform in size but different in size from those of adjacent layers, the pant-icle size difierence between adjacent layers being sufficiently limited not to exceed a diameter ratio of 1.7 to 1 to prevent intermixing of the particles between said adjacent layers, the layers being arranged in decreasing order of size from the end of the arrester nearest the source of flash, and said packing being of sufficient volume before assembly in said hollow cylinder that when assembled in said hollow cylinder said packing is deformed and compressed under a pressure of from about 1,900 p.s.i.g. to about 9,250 p.s.i.g.

2. A dry type flash arrester as claimed in claim 1, which is double-ended to extinguish a flash from either end of the arrester with the finest particle size layer in the center and coarse particle size layers at both ends, and intermediate layers of gradated particle size interposed therebetween.

3. A dry type flash arrester as claimed in claim 1 in which at least one of said end plates has an axial hole in the end opposite the packing opening intoa transverse hole disposed substantially perpendicular to said axial hole, said transverse hole opening into multiple slots which open into said cylinder.

4. A dry type flash arrester as claimed in claim 1 in which said particles are of at least one metal of the group consisting of nickel, steel, and stainless steel.

5. A dry type flash arrester as claimed in claim 1 wherein said packing is nickel shot.

References Cited by the Examiner UNITED STATES PATENTS 2,810,631 10/1957 Kanenbley 48192 3,148,962 9/1964 Dellinger et a1 48192 MORRIS O. WOLK, Primary Examiner.

Claims (1)

1. A DRY TYPE FLASH ARRESTER COMPRISING: A HOLLOW CYLINDER; END PLATES ATTACHED TO EACH END OF SAID HOLLOW CYLINDER, EACH END PLATE CONSTRUCTED TO PROVIDE FOR GAS TO FLOW THERETHROUGH; AND A PACKING OF HEAT RESISTANT ELASTIC MATERIAL FILLING SAID HOLLOW CYLINDER BETWEEN SAID END PLATES, SAID MATERIAL OF SUCH SIZE THAT SUBTANTIALLY ALL OF SAID MATERIAL PASSES THROUGH A NO. 4 SIZE SCREEN OF U.S. STANDARD FINE SIEVE SERIES, SAID MATERIAL ARRANGED IN AT LEAST THREE DISCRETE CONTACTING LAYERS OF RESPECTIVE PARTICLE SIZES, SAID LAYERS BEING DISPOSED TRANSVERSELY TO THE LONGITUDINAL AXIS OF SAID CYLINDER,THE PARTICLES IN EACH OF SAID LAYERS BEING APPROXIMATLEY UNIFORM IN SIZE BUT DIFFERENT IN SIZE FROM THOSE OF ADJACENT LAYERS, THE PARTICLE SIZE DIFFERENCE BETWEEN ADJACENT LAYERS BEING SUFFICIENTLY LIMITED NOT TO EXCEED A DIAMETER RATIO OF 1.7 TO 1 TO PREVENT INTERMIXING OF THE PARTICLES BETWEEN SAID ADJACENT LAYERS, THE LAYERS BEING ARRANGED IN DECREASING ORDER OF SIZE FROM THE END OF THE ARRESTER NEAREST THE SOURCE OF FLASH, AND SAID PACKING BEING OF SUFFICIENT VOLUME BEFORE ASSEMBLY IN SAID HOLLOW CYLINDER THAT WHEN ASSEMBLED IN SAID HOLLOW CYLINDER SAID PACKING IS DEFORMED AND COMPRESSED UNDER A PRESSURE OF FROM ABOUT 1,900 P.S.I.G. TO ABOUT 9,250 P.S.I.G.

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