US3148962A - Gradated bed flash arrester - Google Patents

Description

Sept. 15, 1964 H. C. DELLINGER ETAL .GRADATED BED FLASH ARRESTER Filed March 20, 1961 INVENTOPS HARTLEY, C. -DELLINGER BRUCE L. FAYERWEATHER JR.

A T TOPNE V United States Patent Office Patented Sept. 15., 1964 3,148,962 GRADA'IED BED FLASH ARRESTER Hartley C. Dellinger and Bruce L. Fayerweather, Jr.,

Tonawanda, and Martin L. Kashohm, Snyder,

assignors to Union Carbide Corporation, a corporation of New York Filed Mar. 20, 1961, Ser. No. 96,919 4 Claims. (Cl. 48192) This invention relates to gradated bed 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 re-init-iates the flash on the opposite side of the bed. Thus, the flash 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-ignit-ion of acetylene or reactive gas following a flash.

According to the present invention, the packing comprises discrete layers of granular heat resistant or heat stable material, the particles in each layer being approximately uniform in size, but different in size from those of adjacent layers, the particle size difference between adjacent layers being sufliciently limited to prevent intermixing, the layers being arranged in decreasing order of particle size from the end of the arrester nearest the source of the flash.

Preferred non-metallic packings are alumina and silicon carbide while preferred metals are nickel and stainless steel. Ordinary steel is suitable when the gas is dry and corrosion is not a problem. In selecting a size progression, the important factor is the ratio of particle diameters in the adjacent layers. Intermixing will be avoided if the diameter ratio does not exceed about 1.7 to 1. For example, particle sizes for respective layers may be selected consecutively from the following progression of grit sizes: 2, 4, 6, 10, 16, 30 and 54. Other progressions such as 3, 5, 9, 12, etc. may be equally as good. Single and double ended beds may be employed.

In the drawings:

FIG. 1 is a vertical axial section through a single ended gradated bed flash arrester according to the preferred embodiment of the present invention; and

FIG. 2 is a horizontal axial section through a modified double ended form of such flash arrester.

Referring to FIG. 1, the flash arrester consists of a body 10, which is a heavy-walled cylinder of a metal such as steel or stainless steel. This cylinder is capped at the inlet end with a flange 12, which receives the end of the cylinder in gasketed grooves, and at the discharge end with flange 13. The entire assembly is secured together by means of four bolts 15 extending the full length of the arrester. As shown in this particular design, discharge flange 13 is welded integrally with the cylindrical body and a discharge port 17 is drilled in the flange 13 at right angles to the axis of the body. The uppermost flange 12 at the inlet end has a circular recess in its outer face to receive a bell-shaped inlet 18 welded therein.

' Each of the end flanges is drilled with a large number of closely spaced in. diameter holes 23 for distributing normal flow of acetylene through the arrester.

The new packing consists of several discrete layers, each of which has an average grain size quite different from that of adjacent layers. A layer of relatively coarse particle material 24 is used at least at one end of the bed and subsequent layers are composed of progressively smaller particle size material. For example, in FIG. 1, items 24, 26, 27, 28 and 29 are 1 inch thick layers, respectively, of four grit, six grit, ten grit, sixteen grit, and thirty grit alumina. This gradated bed flash arrester not only quenches an initial flash should it occur, but it also provides positive protection against re-ignition of the acetylene when used within the design limits of pressure and temperature.

The material chosen for the packing should be a high temperature resistant material such as a refractory or a metal. Suitable refractories are alumina and silicon carbide, and suitable metals are nickel and stainless steel. Ordinary steel is satisfactory if the gas is dry. Characteristics of a good packing material are high density, high heat capacity, high melting point, high thermal conductivity, and sufficient strength to resist being pulverized or crushed when a flash occurs. Of course, chemical inertness with respect to the gas being handled is also required.

The layer of coarse particle packing 24 is installed at the end of the bed nearest the section of piping to be protected. If a flash occurs, a high temperature wave travels rapidly along the pipe and is quenched in one of the fine grit layers of packing in the flash arrester. Subsequently, the surge of hot reacted gases pass through the drilled holes in the retaining flange and are distributed into the layer of coarse particle packing. Thus, the hot gas is broken up into a number of smaller streams which pass in heat exchange relation with the packing material. The gases are thereby partially cooled before reaching the next layer of reduced particle size material.

The partial-cooling function performed by the initial layer of coarse particle material is carried out safely and without danger of re-ignition because the particle size is selected to have relatively high mass and low surface area.

The primary purpose of the initial layer of coarse particle packing is to accomplish the described partial cooling of the hot gases and in so doing to absorb heat without itself attaining a surface temperature sufiicient to re-ignite the acetylene.

The partially cooled decomposed gases reaching layer 26 of intermediate particle size material are again broken up into still finer streams which pass in heat exchange with the packing. By virtue of the cooling eflect in layer 24, the gases reaching layer 26 are at reduced temperature and can be safely cooled against smaller size particles without danger of reaching reignition temperatures Within the packing bed.

In another zone of the flash arrester remote from the layer of relatively coarse particle packing material 24, there is installed another layer of relatively fine particle size packing material 29. Layer 29 is composed of particles having low mass/ surface area ratio, as for example, number 30 grit alumina. This layer consists of the finest material employed in the bed, and its particle size is selected to provide positive assurance that the flash will be stopped and will not pass through the bed.

Between the coarse particle layer 24 and the fine particle layer 29 a suflicient number of layers 26, 27 and 28 of intermediate size particles are employed to provide gradual cooling of the hot gases and to prevent intermixing of the particles between adjacent layers. Intermixing will not occur if the diameter ratio of particles in adjacent layers does not exceed 1.7 to 1. Additional intermediate layers of still smaller gradation may be employed, if desired,

but are not necessary either to avoid intermixing or to control the flash. Refractory type materials are usually specified in grit or mesh sizes. The grit sizes referred to herein are standard commercial specifications as defined, for example, in US. Department of Commerce publication entitled Simplified Practice Recommendation No. 118-50. This publication specified grits 6, 10 and 16 as shown in Table I.

TABLE I Specifications for Grit Sizes The screen sizes given in Table I refer to US Standard Fine Sieve Series. The specifications for No. 6 grit read as follows: of the material is retained on a No. 4 screen; no more than 15% passes No. 4 and is retained on 5; at least 45% passes 5 and is retained on 6; at least 80% passes 5 and is retained on 7. Thus, it is seen that each grit size specification provides a material of reasonably uniform particle size.

Metallic particles are normally specified according to SAE shot numbers. A suitable progression of metallic shot for the five layers in FIG. 1 are shown in Table II together with the approximate equivalent grit sizes for comparison.

Grit Size Screen Size Sample Fraction zero (max). 15% (max.). 45% (min). 80% (m1n.). zero (max). 15% (max.). 45% (min). 30% (mill). zero (max.). 15% (max). 45% (min). 80% (min).

TABLE II Metallic Shot Sizes Approx. equiv- SAE metallic shot number: alent grit size While a good grade of refractory particles such as alumina meets all the essential requirements for a packing material, a metallic shot such as nickel is an excellent alternative which possesses several advantages. Compared With alumina, nickel has a higher heat capacity per unit volume and a greater thermal conductivity. It is also less friable and more resistant to abrasion. Furthermore, its thermal expansion matches more closely that of the metal body 10, so that the bed is free from any tendency to loosen up at elevated temperature. Due to its excellent characteristics, a nickel bed may use smaller particles in the coarse layer facing the flash. In other words, if an alumina arrester requires a progression of 4, 6, 10 etc. grit sizes, an equivalent nickel bed can usually omit the No. 4-size layer.

It is important to avoid appreciable intermixing of particles between the layers; particularly, it is important to avoid mixture of fine materials into the coarse layer 24 which is first contacted by the flow of hot products of decomposition. Should fine particle materials be exposed to the flow of hot gases at the face of the bed, these particles may easily be overheated due to their low mass-tosurface ratio and thereby cause re-ignition of the reactive gas. In practice, it is preferable to avoid appreciable intermixing between any adjacent layers comprising the gradated bed flash arrester. Tests indicate that while the gas temperature may be reduced uniformly from end to end of the packed bed, the packing surface temperature may reach a maximum at some intermediate layer Within the bed.

Limiting the particle diameter ratio to 1.7 to 1 (maximum) will permit virtually no intermixing of particles between adjacent layers in the gradated bed flash arrester. That intermixing does not occur was proved by test on a packed bed gradated 2, 4, 6, 10 and 16 grit materials. This bed was vibrated for 22 hours at a frequency of 710 cycles per minute with a vertical amplitude of A; in. The finest particle layer was at the top of the bed. At the end of the tests, the bed showed no effect due to vibration; the material was still tightly packed in discrete layers Without intermixing or settling. In another test, a nickel bed gradated with shot in accordance with Table II was vibrated for 36 hours under more than 6,000 p.s.i. compression without discernible intermixing.

In FIG. 1, the end connection nearest the source of the flash is designed to provide a right angle entrance into the bed. This is thought to be beneficial since the hammer-like impact of the detonation wave dead-ends against the bottom of the hole drilled in the flange and the intensity of the wave is thereby reduced. However, both the inlet and discharge connections may be designed for axial or angular flow as desired.

As shown in FIG. 2, instead of the flange 13 and outlet 17, the double ended flash arrester has a flange 20 identical with the flange 12, and an outlet 22 identical with the inlet 18. This arrester will extinguish a flash from either end of the arrester. Thus, the finest grit layer is disposed in the center of the arrester while coarse particle layers are employed at both ends. Intermediate layers of gradated particle size are interposed between the central, fine grit material and each end layer.

The finest and coarsest grit materials are selected to suit the particular service for which the arrester is designed. The density of the gas being handled in the piping system and also the exothermic characteristics of its decomposition will determine to a large extent the diificulty of confining the effects of a flash. Higher density gases liberate more intense heat which must be absorbed by the packing in order to quench the flash and cool the products of decomposition. The density of the gas will be determined by both temperature and pressure; for example, acetylene at 50 p.s.i. and -65 P. will have a density approximately equal to 125 p.s.i. acetylene at normal room temperature. The high rates of heat release obtained for example with an oxygen-acetylene mixture are more diflicult to quench and require gradation to smaller grit size than a slower burning propane-oxygen mixture.

A flash arrester designed for one service may be entirely inadequate for a diiferent gas or a higher density gas. For example, a gradated bed of alumina similar to FIG. 2 consisting of a 1 /2 inch depth center layer of 16 mesh alumina grit, with successive 1 inch layers of 10, 6 and 4 mesh grit was found by test to stop the following flashes without re-ignition:

TABLE III Flash Conditions Imposed on a Double-Ended Alumina Bed Gradated 4, 6, 10 and 16 Grit Gas Mixture Pressure, Temperap.s.i.g. ture 100% Acetylene 150 Ambient. 40% Methane-60% Oxygen 90 Do. 50% Methane50% Oxygen 90 D0. 20% Propane% Oxygen- D0. 50% Propane-50% Oxygen 75 Do.

5 for combustible gas, and a packing of granular heat resistant material all of which passes through a No. 4 size screen of U.S. Standard Fine Sieve Series, said packing comprising at least three discrete contacting layers of re spective particle sizes, said layers being disposed transversely to the passage of said gas therethrough, the particles in each layer being approximately 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, the layers being arranged in decreasing order of particle size from the end of the arrester nearest the source of the flash.

2. In a dry type flash arrester as claimed in claim 1, said granular heat resistant material being refractory of the group consisting of alumina and silicon carbide, and the particle size diflerence being at least two grit sizes of standard commercial specifications as defined in U.S. Department of Commerce publication entitled Simplified Practice Recommendation No. 11850.

3. In a dry type flash arrester as claimed in claim 1,

6 said granular heat resistant material being metal shot of the group consisting of nickel, steel and stainless steel, and the particle size difference between SAE shot numbers being at least 120.

4. 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.

References Cited in the file of this patent UNITED STATES PATENTS 1,170,055 Ellis Feb. 1, 1916 1,629,085 Robertson May 17, 1927 1,787,698 Montgomery Jan. 6, 1931 1,907,976 Jones May 9, 1933 2,409,278 Hedges Oct. 15, 1946 2,435,781 Heydorn Feb. 10, 1948 2,808,319 Hufl Oct. 1, 1957 2,810,631 Kanenbley Oct. 22, 1957

Claims (1)

1. IN A DRY TYPE FLASH ARRESTER FOR COMBUSTIBLE GAS SERVICE COMPRISING A CONTAINER HAVING AN INLET AND AN OUTLET FOR COMBUSTIBLE GAS, AND A PACKING OF GRANULAR HEAT RESISTANT MATERIAL ALL OF WHICH PASSES THROUGH A NO. 4 SIZE SCREEN OF U.S. STANDARD FINE SIEVE SERIES, SAID PACKING COMPRISES AT LEAST THREE DISCRETE CONTACTING LAYERS OF RESPECTIVE PARTICLE SIZES, SAID LAYERS BEING DISPOSED TRANSVERSELY TO THE PASSAGE OF SAID GAS THERETHROUGH, THE PARTICLES IN EACH LAYER BEING APPROXIMATELY 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, THE LAYERS BEING ARRANGED IN DECREASING ORDER OF PARTICLE SIZE FROM THE END OF THE ARRESTER NEAREST THE SOURCE OF THE FLASH.

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