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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a transfer line exchanger inlet cone for
connecting the process gas outlet of a hydrocarbon cracking heater with
the inlet of a transfer line heat exchanger. In particular, it relates to
an improved transfer line exchanger inlet cone and to methods of making
the same.
The pyrolysis of hydrocarbons to produce desired olefins and diolefins is
highly endothermic. Process temperatures are usually high, ranging from
1300.degree. to 1600.degree. F., with the pressure normally close to
atmospheric and residence time preferably short, so as to inhibit the
undesirable formation of coke particles. Quench and heat recovery
equipment is provided downstream from the heater in the pyrolysis reactor
system to effect cooling of the reaction products from the cracking
temperature, and to thereby inhibit additional secondary reactions which
have an adverse affect on yield and lead to coke formation. The
hydrocarbon cracking heater may be a conventional furnace comprising a row
of vertical tubes with process gas therein which is heated externally to
between 1300.degree. and 1600.degree. F. All the gas from the tubes is
collected and passes through the transfer line exchanger inlet cone to the
transfer line heat exchanger. The transfer line heat exchanger, which is
normally installed in the vertical position above the inlet cone,
comprises many small diameter tubes through which the process gas flows.
Shrouding these small tubes is a jacket which contains water. The hot
process gas is cooled by conduction through the tube walls to the water.
The water is elevated thereby to boiling temperature, and the resulting
steam can be used for power requirements in the plant.
Occasionally, tube failure will occur in the transfer line heat exchanger,
resulting in water leakage into the tubes due to the water being at a much
higher pressure than the gas. The water will then flow down until it
impinges upon the interior of the inlet cone. Complete vaporization does
not occur due to the amount of water flowing down and in some cases, due
to the relatively short distance the water must travel to reach the inlet
cone. In general, inlet cones are made of metal casting or fabricated
metal. Cones of this type will fail by cracking when suddenly quenched by
a water leak of the aforementioned type. The crack may allow the process
gas to escape to the atmosphere where it will burn. This presents a
potential safety hazard as personnel may be burned by the hot gas leakage.
Under these circumstances, the furnace must be shut down, which will
adversely affect plant production. If the inlet cone cannot be repaired,
for instance by welding, then it must be replaced resulting in equipment
loss. These problems may be alleviated somewhat by internally lining the
inlet cones with insulation, for example, refractory. However, it is still
possible for the inlet cone to crack due to water seepage if there is a
flaw in the insulation or refractory lining.
It is therefore desirable to define a repair procedure for cracked inlet
cones which are incapable of repair by welding and thus, normally
discarded. It is also desirable to redesign the inlet cone as a
preventative measure so as to inhibit interruption of furnace runs by an
inlet cone failure.
Attention is drawn to the following, related U.S. Pat. Nos. 3,374,832;
3,409,074; 3,416,598; 3,443,631; 3,449,212; and 3,525,389. None of these
patents teaches the repair method or the improved transfer line exchanger
inlet cone of the present invention.
SUMMARY OF THE INVENTION
The present invention provides a method for repairing a cracked metal inlet
cone which connects the process gas outlet of a hydrocarbon cracking
heater with the inlet of a transfer line heat exchanger. The method
comprises the steps of: cutting away a portion of the inlet cone's wall
which has the crack therein to create an aperture in the wall; providing a
pressure resistant exterior wall of metal which is connected to the inlet
cone to thereby create a gap therebetween; and filling this gap with a
castable refractory and extending the refractory inwardly through the
aperture until the refractory is approximately flush with the interior of
the wall of the inlet cone. Stress cracking and high temperature failures
are eliminated without affecting the flow pattern of the process gas when
using a transfer line exchanger inlet cone repaired according to this
method. Surprisingly, inlet cones produced by this repair method also have
a longer life span than conventional inlet cones and thus, a procedure for
modification of conventional transfer line exchanger inlet cones, prior to
any such cracking failure, has evolved. The method for preventing the
cracking of a transfer line exchanger inlet cone of the aforementioned
type comprises the steps of: cutting away a portion of the wall of said
inlet cone to create an aperture in said wall; providing a pressure
resistant exterior wall of metal which is connected to the inlet cone to
thereby create a gap therebetween; and filling this gap with a castable
refractory and extending the refractory inwardly through the aperture
until the refractory is approximately flush with the interior of the wall
of the inlet cone. It is preferred that the number of apertures created be
four, and that each be equispaced in its respective quadrant of the wall
of the inlet cone.
The apparatus of the present invention provides an inlet cone for passing
gases from the outlet side of a hydrocarbon cracking heater to the tube
side of a heat exchanger. The essential elements are a generally conical
wall, a pressure resistant exterior wall of metal, and a castable
refractory fill. The generally conical wall is connectable around the
periphery of its larger end to the heat exchanger and around the periphery
of its smaller end to the cracking heater. Between its larger and smaller
ends, the generally conical wall has a filled aperture. The pressure
resistant exterior wall of metal is spaced from the generally conical wall
and its ends are connected to the generally conical wall at points above
and below, respectively, the smaller and larger ends of the generally
conical wall. The castable refractory fill occupies the gap between the
pressure resistant exterior wall and the generally conical wall. The
refractory fill extends inwardly to form the filled aperture and is
approximately flush with the interior of the generally conical wall. Even
if, in operation, the generally conical wall of the inlet cone is quenched
and cracks due to tube failure in the heat exchanger, the flow pattern of
the process gas will remain unaffected as the castable refractory fill
protects the pressure resistant exterior wall. It is preferred that the
number of filled apertures be four and each equispaced in its respective
quadrant of the generally conical wall.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical cross-section of a conventional heater outlet, inlet
cone, and transfer line exchanger;
FIGS. 2A and 2B are perspective views of a conventional inlet cone and a
modified inlet cone; and
FIGS. 3A, 3B, 3C and 3D are vertical cross-sections showing the preferred
procedure of repairing a cracked conventional inlet cone to obtain the
modified inlet cone of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, like numbers indicate like apparatus. With
reference to FIGS. 1, 2A and 3A, a conventional inlet cone 11, which is a
continuous metal casting, comprises a generally conical wall 25, flange 14
at its smaller end, and flange 21 at its larger end. Flange 14 is attached
to flange 13 of heater outlet 10 by suitable means such as a plurality of
bolts (not shown) and sealing gasket 16. A second gasket 22 effects a seal
between flange 20 of transfer line exchanger 12 and flange 21 of inlet
cone 11. Transfer line exchanger 12 comprises a shell 17, a tube sheet 18,
a plurality of tubes 19, and flange 20. Only a small number of tubes 19
are shown for ease of illustration, although a larger number are
preferred. Shrouding tubes 19 is a jacket containing water 24.
The hydrocarbon cracking heater may be a conventional furnace comprising a
row of vertical tubes (not shown) with process gas therein which is
externally heated to approximately 1560.degree. F. Referring again to FIG.
1, all the gas from said tubes is collected and passes from heater outlet
10, through conventional inlet cone 11 to transfer line exchanger 12 where
it passes through a plurality of small diameter tubes 19. Process gas
pressure is approximately 14 to 20 p.s.i.g. Tubes 19 are shrouded by a
jacket containing water 24. The hot process gas is cooled by conduction
through the tube walls to the water 24. Water pressure is approximately
600 p.s.i.g. Occasionally, tube failure will occur in one or more of the
tubes 19 of transfer line exchanger 12, resulting in water leakage into
the failed tubes. The water 24 will flow down until it impinges upon wall
25 of conventional inlet cone 11, resulting in inlet cone cracking
failure. To avoid the unnecessary expense of replacing the cracked inlet
cone 11, and more especially to prevent future inlet cone failures, the
following repair procedure with resultant modified inlet cone 11' has been
devised.
With particular reference to FIGS. 3A, 3B, 3C, and 3D, the repair procedure
of the present invention is as follows. FIGS. 3A shows conventional inlet
cone 11 with crack 26 therein. The portion of wall 25 with crack 26
therein is cut away to create aperture 27 through wall 25. It is preferred
that a plurality of apertures 27 be formed, more especially four with each
equispaced in its respective quadrant of wall 25. The number and spacing
of apertures 27, however, may be dictated by the number and spacing of
cracks. After forming apertures 27, pressure resistant exterior wall 28
(see FIG. 3C) is connected to wall 25 immediately beneath and above,
respectively, flanges 21 and 14 by conventional means such as welding. It
will be appreciated that apertures 27 are between these connecting points.
The shape of pressure resistant exterior wall 28 is such that a gap is
left between it and wall 25 except at their points of connection. This gap
is filled with a castable refractory 29 (see FIG. 3D), which is extended
inwardly through apertures 27 until approximately flush with the contours
of wall 25. By this method, modified inlet cone 11' is formed. This is the
preferred sequence of modification, as apertures 27 provide windows by
which it can be ascertained that castable refractory 29 has completely
filled the gap between pressure resistant exterior wall 28 and wall 25 of
modified inlet cone 11'. However, pressure resistant exterior wall 28 can
be affixed to wall 25 before apertures 27 are formed. Surprisingly,
modified inlet cones 11' have a longer life span than conventional inlet
cones 11. When the interior of modified inlet cone 11' is quenched, the
uncut portions of wall 25 are still subject to cracking, but the flow
pattern of the process gas is not adversely affected. As a direct
consequence, the repair method of above has been transformed into a
preventative procedure whereby transfer line exchanger inlet cones are
modified prior to any such cracking failure.
The method for preventing the cracking of a conventional transfer line
exchanger inlet cone 11 comprises essentially the same steps as for
repairing a cracked one. The primary difference is that there are no
cracks 26 to dictate the number and spacing of apertures 27, and
therefore, the number and spacing of apertures 27 can be independently
chosen. As flanges 14 and 21 are generally unaffected by quenching of the
interior of wall 25, it is preferred that the height of apertures 27 be
substantially therebetween. Also, it is preferable that apertures 27 be
four in number, each equispaced in its respective quadrant of generally
conical wall 25. Since implementation of this preventative procedure, the
life of transfer line exchanger inlet cones has been greatly increased,
and the probability of an inlet cone failure by cracking substantially
reduced.
The modified transfer line exchanger inlet cone 11' of the present
invention passes gases from the outlet side 10 of a hydrocarbon cracking
heater to the tube side of a heat exchanger 12. Referring to FIGS. 2B and
3D, the essential elements are a generally conical wall 25, a pressure
resistant exterior wall 28 of metal, and a castable refractory fill 29.
Generally, conical wall 25 may be fabricated metal, but is preferably a
continuous metal casting. Generally, conical wall 25 is connectable around
the periphery of its larger end by flange 21 to flange 20 of the heat
exchanger 12 and around the periphery of its smaller end by flange 14 to
flange 13 of the cracking heater outlet 10. Between its flanges 14 and 21,
generally conical wall 25 has a filled aperture 27. The pressure resistant
exterior wall 28 of metal is spaced from generally conical wall 25 to
create a gap therebetween, and its ends are connected to, for instance by
welding, generally conical wall 25 at points above and below,
respectively, flanges 14 and 21. Castable refractory fill 29 occupies the
gap between pressure resistant exterior wall 28 and generally conical wall
25. The refractory fill 29 extends inwardly to form filled apertures 27
and is approximately flush with the interior of generally conical wall 25.
Even if, in operation, generally conical wall 25 of the connector duct is
quenched and cracks, due to tube failure in heat exchanger 12, the flow
pattern of the process gas will remain unaffected as the castable
refractory fill 29 protects the pressure resistant exterior wall 28.
The preferred materials of construction are as follows: for the wall of the
inlet cone, fabricated steel or a metal casting, more preferably the
latter; and for the pressure resistant exterior wall, a metal such as
stainless steel.
Various modifications and other advantages will be apparent to those
skilled in the art, and it is intended that this invention be limited only
as set forth in the appended claims. In particular, while the apparatus is
generally referred to herein as a "cone," it will be understood that other
configurations are possible so long as they present a small opening to the
outlet of a hydrocarbon cracking heater and a large opening to a transfer
line heat exchanger inlet.
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