US2636507A - Liquid seal drum - Google Patents

Description

April 1953 G. s. HOUGHLAND LIQUID SEAL DRUM Filed Oct. 24, 1950 GASES SEAL DRU M LIGHT V FRACTION LIQUID FRACTI ONATOR CHARGE HEAVY FRACTION INVENTOR.

G LEN S. HOUGH LAND ATTORNEYS pheric pressure.

Patented Apr. 28, 1953 UNITED STATES PATENT OFFICE 2,636,507 LIQUID SEAL DRUM Glen S. Houghland, Larchmont, N. Y., assignor to The M. W. Kellogg Company, Jersey City, N. J a corporation of Delaware Application October 24, 1950, Serial No. 191,776

Claims.

This invention relates in general'to sealdrums from a lower pressure region by means of a ;manometric head of sealing liquid, and more particularly to a seal drum separated into a reservoir section and a sealing section which open into a other through an orifice opening into said reservoir section below the elevation at which gas enters said sealing section at break-seal pressure.

In a preferred species of the invention, gas enters said sealing section through a substantially vertical inlet pipe and said orifice, opening into said reservoir section, communicates with the intem to a region of predetermined low pressure,

whenever the pressure in said system exceeds a predetermined value, it is particularly valuable in petroleum refineries in which it is necessary to operate fractionation systems and the like at pressures which are a few pounds above atmos- For example, in a large catalytic cracking unit in which higher boiling hydrocarbons are subjected to catalytic cracking,it

is necessary to subject the vapor efiluent from the 0f the type wherein gas under pressure is sealed common vapor region above the level of said sealing liquid and which communicate with each cracking process to vapor fractionation in a massive'fractionation tower. From the upper end of this tower, vapor containing gasoline and the socalled low pressure or incondensable gases are withdrawn and subjected to cooling to condense v the gasoline.

The incondensable gases are then with-drawn and compressed preliminary'to catalytic polymerization. The pressure ofthe incondensable gases prior to compression may sometimes increase to unsafe maximums because ofcompressorfailure or unsatisfactory operation of the fractionation tower.

some provision must be made for releasing gases from thefractionation system to the atmosphere or to a fiare in which the gases can be burned at a safe distance from the refinery in case the maximum safe pressure is exceeded. The means best suited for this is some liquid at a sufficient distance below its surface to provide a hydrostatic head of. sealing liquid equivalent to themaximum safe pressure. How- .ple.

ever, the fractionation system in large refineries is capable of handling very large volumes of gas; and if pressure suddenly increases, the gas is capable of expelling most of the sealing liquid from the seal drum so that the device is not capable of rescaling until it is filled with new sealing liquid. The present device is designed to overcome this trouble by providing a drum which can be unsealed without disturbing the bulk of sealing liquid. 7

Moreover, it is sometimes desirable that the seal drum be capable of rescaling itself at some predetermined pressure lower than the maximum safe pressure or break-seal pressure. The present invention also provides a means for accomplishfractionation system is shown in Fig. 1 in which a charge (which may be a gaseous effiuent from a Houdry-type or fiuid catalytic cracker, for example) is introduced into fractionator 10 through line I l and separated by vapor fractionation into at least two fractions, a heavy fraction which is withdrawn as a liquid from the bottom of fractionator I!) through line I2, and a light fraction Which is withdrawn as a vapor through line l3 at the top of fractionator [0. A typical temperature and pressure for the light fraction at this point is 285 F., and 6 lbs. per square inch gage pressure. The light fraction is passed through a condenser l l in which the vapors are cooled to a temperature of F. and thus partially condensed. The gases and liquid enter a refiux drum IS in which the light fraction 1iquid separates as indicated by a liquid-level l6 and .is

the remainder is withdrawn through pump 20 as a product, which might be gasoline, for exam- The incondensable gases are withdrawn overhead from refiux drum l5 through'line 2| at a pressure of about 4 lbs. per square inch gage and compressed in compressor 22 to a high pressure, about lbs. per square inch gage, for

transfer through line 23 to a recovery unit of some type suitable for recovery of incondensable gases. For example, incondensable gases may be passed through a recovery process which separates desirable constituents and converts them into gasoline by means of catalytic polymerization. Compressor 22 may be of any type suitable for compressing large volumes of gas to a high pressure; in petroleum refineries such compressors are commonly of piston type, and driven by a gas engine or by a reciprocating steam engine; however, sometimes centrifugal compressors driven by steam turbines are used. In the case of reciprocating compressors there is, always. the danger that the compressor may stall if the intake pressure increases to say to .20 lbs. per square inch gage, for the compressor is then pipe opening 33.

loaded by too great a mass of gas for its power.

source to handle. In the case of a steam turbine,

stalling is not as critical a problem, but compressor bearings may fail and require immediate .Lshutdown. In either event, it is important that the cracking system ,not be upset and that the fractionation system not be subjected to danger of bursting by the ,build up of excessive pressure. An absolutely reliable method of venting large volumes of gases to a lower pressure zone must be available. In the case of an excessively high pressure in reflux drum 15 as a result of compressor failure, or for .any other reason, it is necessary that the incondensable gases be allowed to escape through line 24 either directly to the atmosphere or to a flare 25 in which the gases may be safely burned at some distance from the refinery. A valve 26 is available for operation by an operator if he detects the rising pressure in time but, if on account of some other distraction, such as a fire, he is delayed in venting the gas then an automatically opening by-pass is essen-. tial. This is provided by means of a seal drum indicated generally by a numeral 21 and located in the by-passline 2B in parallel with valve 26.

vessel '30 through inlet opening 33. .Asleeve as, H

shown in vertical .crosssection is supported in ,vessel 30 concentrically with inlet pipe 32 by means of supporting legs 35., which may be studs welded to the bottom of the drum and supporting sleeve 3 at a sufiicient height so that liquid or gas may readily flow between the lower end of :sleeve and the part of vessel 30 external to sleeve 34. An annular .disc 35, shown in vertical cross-section is horizontally positioned within sleeve 34 at a point substantially above its lower end so that there is :a cavity 3'? underneath it.

- Annular disc 35 has a center opening 38 which serves as an orifice to communicate between cavity 31 and inlet pipe 32. Preferably, this orifice communicates with the interior of inlet pipe 32 by means of a tubular nozzle .39 which extends upwardly into inlet pipe opening 33. The distance to which this nozzle extends upwardly is critical and may be used for determining the con- 'ditions under which seal drum 2! will reseal itself. Sleeve 34 and inlet pipe 32 may be braced with respect to one another by means of welded studs '40.- It will be seen that under ordinary operating conditions the gage pressure-should be zero in the upper region I! (the region-above liquid-level 3 carried entirely out of seal drum 2?.

since this communicates directly with the atmosphere through discharge line 29. However, the slight pressure, say 4 to 5 lbs., existing in reflux drum l5 will cause the liquid-level inlet within pipe 32 to be somewhat lower than liquid-level 3| .as indicated by the liquiddevel 42.. In case of excessive pressure in reflux drum 15, however,

liquid-level 42 will drop until it falls below inlet At the instant this happens a substantial slug of liquid occupying the annular space 33 between the outer surface inlet pipe 32 and the inner surface of pipe 3 5 will be violently ejected upwardly. Indeed, it may tend to form a foam with the passage of so much gas and be However, this slug of liquid is substantially smaller than the annular space 44 between the outer surface of sleeve 34 and the inner surface of vessel 38.

The flow of gas through inlet line 32 and back up through annular space &3 on its way to escape through line 29 causes the existence of a velocity head at orifice 38 or at the upper end of nozzle 39, if the latter is employed. The existence of this velocity head prevents the flow of liquid from the large reservoir space M into pipe 32 or annular space 43,, through which gases are flowing in great volume. Most of the liquid slug which is ejected from annular space 43 may be prevented from escaping by surmounting the upper end 45 of sleeve 3 with some kind of deflecting means. A simple annular disc as, shown in vertical cross-section surrounding inlet pipe 32, may be used. The eiiiciency of this disc maybe improved if it is further provided with radially positioned spiral deflecting means 41 on its under surface, seen in horizontal cross-section in Figure 3 as viewed along a cross-section taken substantially as indicated by the arrows 3--3. The area indicated in Figure 3. by line 4'8 is overhung by disc .8 as shown by the fragment of 5B in Figure 3.

Fractionation tower ill may be further protected from excessive pressure by means of one or more safety valves :such as safety valve to in line [3.

It must be understood that a seal drum of the nature disclosed herein is especially important because of the vastness of the gas systems in which it is employed. For example, fractionation tower Hi may be 20 feet in diameter, over feet high and capable of delivering 382,000 lbs. per hour of vapor. Line 13 must be very large, for example, 36 inches in diameter. Condensed gasoline is typically less than one-half, sometimes less than one-third, of the vapors withdrawn through line 13. In a typical case line 2| would be 28 inches in diameter and lines 24 and 28 only slightly less, say 24 inches in diameter. Seal drum 2'5 may be typically 35 or 40 feet high and about 6 feet in diameter. Inlet pipe 28 may be reduced to say 16 inches in diameter in vertical section 32 within seal drum vessel 30. The sleeve 34 ma be typically 24 inches or more in diameter and nozzle 39 would correspondingly be about 3 or 4 inches in diameter.

The first point to be brought home by these dimensions is that a satisfactory seal drum may be essential because safety valves may not be enough for such a gigantic gas system. It is customary to design tower l0 and its connecting piping not for any maximum pressure they may encounter, but for a. maximum pressure of about 23 lbs. per square inch gage. To protect the tower with safety valves would require 10 or more valves of the largest size. Although these valves are not excessively expensive, they are troublesome in operation because they do not reset themselves easily and a'ccurately,.but have a tendency, after pressure-has returned to normal, to remain partially unset and to permit the leakage of highly inflammable gas to the atmosphere. If leakage threatens to become serious, it is necessary to detach the 'valve or valves and take them to the machine shop for resetting. The seal drum of 'the' type herein disclosed is simpler and more re- *liable in a proper application. 9

' The second feature to be noticed is that the volume of gas that is to be passed by seal drum 21 is enormous. Seal drums which arent separated into a sealing section and reservoir section, as herein described, are in danger of having all of the sealing liquid carried oif each time the break seal pressure is reached. Of course, it would be theoretically possible to make seal drum '21 one hundred feet in diameter instead of six so that there would be an adequate reservoir of sealing liquid regardless of how violently excess gas- '-was vented; but, it is obvious from the size of the device, that this is not practical, and that it is desirable to save the sealing liquid without using .a drum which is larger than the minimum required, for that minimum is already a large and expensive piece of equipment costing between $15,000 and $50,000.

Under ordinary operating conditions, the liquid-level 42 within inlet pipe 32 will remain substantially constant at an elevation lower than liquid-level 3|, by a distance corresponding to the gage pressure within reflux drum E or pipe 2 I usually about 4 or 5 lbs, per square'inch gage. But if the pressure increases to break-seal pressure, say 15' lbs. per square inch gage, all the liquid is expelled from inlet pipe 32 and from the annular space 43 in the sealing section of the seal drum 21. The displacement of this liquid naturally raises liquid-level 3| a little, but this displacement should be relatively small since the volume of reservoir section 44 is large relative to the volume of the displaced liquid. It is to be understood that when the liquid-level in the reservoir section 44 is referred to in the specifica- 'tion or claimsthat this small variation is ignored and that the change in liquid-level 3! is not substantial; indeed, this may be referred to as'the upper surface region of the sealing liquid.

Gas escaping from the fractionation system "will enter the seal drum through line 28, pass downwardly through inlet pipe 32 and up through annular space 43 and out through line 29 to the atmosphere or to flare 25, and the velocityof gases moving against nozzle'39, which is pointed upstream, or in opposition to the fiow of gas through inlet pipe 32 so that a velocity head is exerted on the liquid within nozzle 39 or below it in cavity 31. On account of this velocity head there will be a tendency for the drum not to reseal even after pressure in the system returns to ordinary operating pressures in the system. If pipe 29 is connected to a flare, which would ordinarily be about 2,000 feet distant, then pressure drop in the flare line would cause the pressure within vapor space 4! to rise to 3 or 4 lbs; gage and the tendency of the seal drum not to reseal itself on account of velocity head would be overcome by the increased pressure in vapor space 4|. However, if seal drum 2'! discharges directly to the atmosphere so that the pressure within vapor space 4! is 0 lbs. per square inch gage at all times, then it may actually be necessary to reduce thgpressure across sealdrum 21to some fore it will reseal itself. This problem',.how've1-,

is ordinarily easy to meet by proper design of ori fice 38 and nozzle 39. One may select the degree to which velocity head will affect the liquid within nozzle 39 by pointing it directly into the flow of escaping gas or by pointing it in a direction with the flow of the escaping gas as by having it open into annular space 43, v

On the other hand, if it is desired that sealing drum 21 be incapable of rescaling itself, even after'ordinary pressure conditions are resumed in the fractionation system, nozzle 39 maybe extended upwardly into inlet pipe 32 a' substantial distance; this has no effect on the break-seal pressure, but it will prevent rescaling until the pressure within the fractionation system falls to a. value equivalent to (or perhaps even less than) the head of liquid between the upper end of 'nozzle 39 (within inlet pipe 32) and liquid-level 3| in the reservoir section 44 of seal drum vessel 3 1.v

The most convenient means for employing-the seal drum is in parallel with a valve 26 which may conveniently be a butterfly valve which is diaphragm controlled byair pressure controlled by the operator at some convenient point. The ordinary sequence of operation in case of a break-seal pressure would be that seal drum 21 would blow and sealing liquid would be ejected from sealing section 43 into reservoir section 44 (but not lost out the stack as in previously'known seal drums). In due course, an operator would open valve 26 and escaping gas would be provided with an easier passage directly to the atmosphere or to flare 25. At this point, seal drum 21 would automatically reseal itself. After the causes for excessive pressure had been corrected, then valve 26 would be closed and the fractionation system would resume ordinary operating conditions, seal drum 21 hav'- ing already resumed a reseal condition in readiness for any future excess pressure.

The sealing liquid is ordinarily water in moderate climates, but in place where the seal drum is exposed to freezing weather, oil should be used as sealing liquid.

It is to be understood that the design for the seal drum is not limited to the one shown'in the Thesealing section need'not be formed by means of a sleeve 34, but merely bymeans of a partition across one side of the sealing drum vessel 39; it is only necessary that the'volume of the sealing section be small relative to that of the reservoir section so that there may be ample liquid available for resealing even if some of the sealing section liquid is carried away. The size .proper depth below liquid-level 3i toinsure a break-seal pressure head of sealingliquid. It is not necessary that cavity 31 be provided; this cavity is merely a refinement which allows for a little fluctuation of gas volume (if there is sufficient velocity head to force the level of liquid down within cavity. 31) It will also be understood that orifice. 38 must communicate. withthereservoir section" 44 at some point below the level of outlet-31B, for otherwise liquid would be continually emptying into an escaping stream, whichis precisely what the present seal drum is designed to avoid.

I claim:

1.. A seal drum means. of the type wherein s under pressure is sealed 111131 a lower pressure region by means of a predetermined manometric head of sealing liquid, which means includes; a

vessel adapted to contain said sealing liquid to a dept at least as great as said predetermined manometric head and adapted to provide a vapor space above the surface of said scaling liquid, said vapor space being provided with an outlet to said region :of lower pressure; partition means separating the interior of said vessel below the surface Ofsaid scaling liquid into .a sealing section of relatively small volume and a relatively larger reservoir section, both of which sections open at their upper ends into said vapor space and which communicate with one anotherthrough an orifice, said orifice opening into said reservoir section at a point below the surface of said liquid a distance not substantially less than said predetermined .manometric head; and an inlet pipe communicating with said gas under pressure and opening into said sealing section at a depth below the surface of said sealing liquid 2. distance at least equivalent to said predetermined manometric head and in the vicinity of said orifice opening whereby the rescaling characteristics of the sealed drum means are affected by the velocity of the outgoing gas, said inlet pipe having an ap-- proach section leading to said opening from an elevation higher than the surface of said sealing liquid, and having a cross-section substantially greater than the cross-section of said orifice.

:2, A seal drum means as described in claim 1 in which said sealing section is surmounted by Joafile means above said surface of said sealing liquid and adapted to divert into said reservoir section sealing liquid ejected from said sealing section.

8. A seal drum means communicating with a system containing gas under pressure and adapted to release gas from said system to a region of lower pressure when the pressure in said system attains a predetermined break-seal pressure, and to reseal itself with sealing liquid when the total pressure in said seal drum means, including velocity head of escaping gases, falls to a predetermined pressure lower than said breakseal, pressure, which seal drum means includes: a vessel having a lower portion adapted to contain said sealing liquid to at least a depth equivalent to said break-seal pressure, and having an upper portion adapted to provide a vapor space above the surface of said sealing liquid, and an outlet for discharging gases from said vapor space to said region of lower pressure; an inlet pipe leading from said system containing gases under pressure and opening into said inlet vessel section than said inlet pipe, said nozzle opening :2 into said reservoir section at a depth below the surface of said sealing liquid equivalent to said break-seal pressure,- and into said sealing section at an elevation above said inlet pipe opening.

i, A seal drum means of the type wherein gas under pressure is sealed from a lower pressure region by means of a manometric head of sealing liquid equivalent to a break-seal pressure, which includes: a vessel vertically extended sufiiciently to contain said sealing liquid below a liquid surface region to a depth sufficient to provide said manometric head, and to provide a vapor space above said liquid surface region, said vapor space being provided with an outlet to said region of lower pressure; partition means separating the interior of said vessel below said liquid surface region into a sealing section of relatively small volume and a reservoir section of relatively large volume, both of said sections being open at their upper ends into said vapor space; an inlet pipe communicating with said gas under pressure and opening into said sealing section at a depth below said liquid surface region sufiicient to provide said predetermined mancmetric head, and having an approach section leading to said opening from an elevation higher than said liquid surface region; a nozzle means communicating between said reservoir section and said sealing section, from a point in said reservoir section lower than the opening of said inlet pipe and extending into said inlet pipe to an elevation between that of said inlet pipe opening and that of said liquid surface region.

5. A seal drum means, wherein gas under pressure is sealed from a lower pressure region by means of a predetermined manometric head of sealing liquid, which means includes: a vessel having a liquid-containing portion for containing sealing liquid to a depth at least as great as said predetermined manomctric head and having a vapor space above the surface of said scaling liquid, said vapor space being provided with an outlet to said region of lower pressure; a sealing well supported in said liquid-containing portion of said vessel, said sealing well opening at its upper surface into said vapor space and enclosing a volume which is small in comparison with volume of said liquid-containing portion external to said sealing well, and having a depth below said liquid surface at least as great as said predetermined manometric head; an inlet pipe entering said. vessel above said liquid surface region and descending into said sealing well to a depth below said. liquid surface at least as reat as said predetermined manometric head; a nozzle means of substantially smaller internal cross-section than said inlet pipe, communicating with the liquid portion of said vessel external to said sealing well at a depth below the opening of said inlet pipe and communicating with the interior of said inlet pipe, said nozzle being directed in an upstream position so as to be subjected to velocity head .of gases escaping into said sealing well from said inlet pipe; and horizontal bafilc means covering and overhanging the upper end or said sealing well and adapted to divert against the walls of said vessel sealing liquid upwardly ejected from said sealing well.

GLEN S. HOUGHLAND.

Name Date lioneywellv-amwfl Oct 16,1917

Number v 1,243,604

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