Background of the Invention
1. Field of the Invention
This invention relates to delayed coking, and more particularly to
a method of reducing the metal stresses in delayed coking drums during the cooling
and quenching part of the coking cycle.
In a typical delayed coker unit, a pair of coke drums are alternately
filled and emptied, with coker feed being pumped into one of the drums while the
other drum is being emptied of coke and prepared for the next filling cycle.
2. Background Art
A conventional coking operation includes, in the process of emptying
the filled drum, the steps of steaming out the filled drum to remove residual volatile
material from the drum, quenching the steamed out coke bed with water, draining
quench water from the drum, opening the top and bottom of the coke drum (unheading
the drum), drilling a pilot hole in the coke bed from the top, drilling out the
remaining coke with a radially directed water jet drill, removing the drilled out
coke from the bottom of the drum, closing the top and bottom openings of the coke
drum, and preheating the empty coke drum by passing hot vapors from the other drum
being filled with hot coker feed. The preheating step is necessary to bring the
empty coke drum temperature up prior to switching the hot coker feed to the recently
emptied drum, as otherwise the thermal stresses from feeding hot feed into a relatively
cool drum would cause serious damage. In my U.S. patent No. 5,891,310, filed on
June 20, 1997, a method of reducing the time required for the preheating step is
described. That method includes the application of external heat to a critical area
of the coke drum during the preheat step of the coking cycle.
A typical coke drum is supported by a skirt which is welded to the
drum near the junction of the drum shell and the lower cone of the drum. As described
in my aforementioned U.S. Patent, the maximum thermal stresses occur at the time
the hot oil feed, at about 482°C (900°F), is switched to the preheated drum. These
thermal stresses are partly due to the fact that the interior surface of the preheated
drum is hotter than the exterior of the drum, including the area where the supporting
skirt is welded to the drum shell. The expansion rate of the interior of the shell,
upon being contacted with hot oil feed, is initially greater than the expansion
rate of the cooler exterior portion. If sufficient time is available, the preheat
step can be carried out over a time period sufficient to heat the drum exterior
to a temperature near that of the drum interior. However, this is a problem if preheat
time is to be minimized in order to reduce the overall cycle time.
There is another point in the coking cycle during which high metal
stresses develop in the area of the junction between the coke drum and its supporting
skirt. This occurs when quench water is introduced into the drum to quench the steamed
out coke. At the time the quench water is introduced, the drum exterior is much
hotter than the quench water, and the temperature differential between the drum
interior and the drum exterior sets up large thermal gradients which result in high
metal stresses. This is particularly critical in the area of the drum where the
supporting skirt is attached. The top portion of the support skirt remains at a
higher temperature than the cooling cone and shell. The resulting temperature differences
in the components results in the cone and shell contracting at a faster rate than
the skirt. The differential of expansion rates creates high metal stresses when
the contracting cone and shell pull away from the hotter skirt.
US patent 4,634,500 discloses a method of quenching heated coke in
a coke drum, whereby the rate of feeding the quench water into the coke drum is
regulated to prevent the stress in the coke drum wall, which is monitored by measuring
either the longitudinal thermal gradient or the rates of change in the drum wall
temperature over time, from exceeding a predetermined limit.
US patent 3,167,486 discloses a method of retarding skirt weld cracking
in coking vessels by reducing temperature gradients in and about the skirt weld
by creating a coke depositing and retention zone in a limited region within the
vessel near the skirt weld and utilizing the deposited coke itself as an internal
layer of insulation.
Summary of the Invention
According to the present invention, the metal stresses in a coke drum
during the quenching step of the coking cycle are reduced by applying a cooling
fluid to the external part of the coke drum adjacent the area where the drum and
its supporting skirt are connected. This external cooling fluid reduces the temperature
differential between the drum interior and the supporting skirt connection, thereby
reducing the metal stresses during the quenching step.
Description of the Drawings
Description of the Preferred Embodiments
- Figure 1 is a schematic view of a delayed coker unit showing a pair of coke
drums and associated equipment.
- Figure 2 is a chart showing the coke drum schedule for a coking cycle.
- Figure 3 is a side elevation, partly in cross section, showing details of a
coke drum and its supporting structure.
- Figure 4 is a side elevation, partially cut away, showing details of the junction
of a coke drum and its supporting skirt.
- Figure 5 is a cross section showing a coke drum supported by a skirt welded
to the knuckle section on the cone of the drum.
- Figure 6 is a cross section showing a coke drum supported by a skirt welded
to the shell of the drum.
The primary object of the present invention is to decrease the metal
stresses in a coke drum during the quenching step of the coke cycle.
Figure 1 shows a typical coker unit comprised of a pair of coke drums
10 and 12. Coker feed from feed line 14 enters coker fractionator 16 and is pumped
to furnace 54 and then fed to one of the coke drums. Overhead vapors from the drum
being filled return to fractionator 16 where they are separated into product streams.
Referring to Figure 2, a typical cycle schedule is shown. The example
illustrated is for an eighteen hour cycle, but longer and shorter cycles are common.
The means for applying external cooling fluid to the drum are best
shown in Figure 3. A cooling fluid jacket 48 encircles drum 10 around the area of
the skirt-to-drum junction. A cooling fluid inlet 50 and outlet 52 are provided
for passing cooling fluid, preferably water or low pressure steam, through the cooling
As seen in Figure 3, a coke drum 10 includes a bottom cone section
34 and a removable lower plate 36. Between the drum shell and the bottom cone section
34 there is a transition or knuckle section 44. As shown in Figures 3 and 6, near
the junction of the drum shell and knuckle section 44, a supporting skirt 38 is
welded to the drum, in what is sometimes referred to as a tangent line connection.
As shown in Figure 5, a knuckle section 44 is welded between the drum
shell and lower cone section 34. A supporting skirt 38 is welded to the knuckle
section 44 at weld 22, in what is sometimes referred to as a knuckle connection.
In one popular variation as shown in Figure 4, the skirt includes
a series of fingers 40 formed by slots extending from the top of the skirt, and
each finger has a curved top 46 to present a scalloped shape, and the curved finger
tops are welded to the drum shell. It is common to include rounded lower ends in
slots in the skirt to prevent stress risers from forming at the slot ends. In cases
where the cooling jacket 48 extends over part of the slots extending from the top
of the skirt as shown in Figure 4, it may be desirable to apply a packing material
in the slots to prevent leakage of cooling fluid.
Whichever type of skirt-to-drum system is used, the junction between
the drum shell and skirt is very hot when the quench step is initiated. The exterior
drum surface, and especially the welded junction of the drum shell and the supporting
skirt, does not cool down at the same rate as the interior of the drum. High metal
stresses then develop because of the thermal shock that occurs when quench water
is introduced into the bottom of the drum. This thermal shock can potentially damage
the skirt-to-drum connection.
To illustrate the process of the invention, the coking cycle including
the use of external drum cooling will now be described with reference to Figures
1 and 3.
Hot coker feed from furnace 54 is fed to the bottom of coke drum 10.
At the time feed to drum 10 is initiated, coke drum 12, which is full of coke, is
steamed with low pressure steam to strip residual volatile hydrocarbons from the
coke bed in the drum. The steam also removes some heat from the coke. After the
steamout step, the coke is quenched by filling the drum with quench water. Before
the thermal gradient caused by the quench water reaches the level of the drum-to-skirt
connection, a cooling fluid such as water, air or other gas, or low pressure steam,
is injected into cooling jacket 48 from inlet 50. The cooling fluid exits outlet
50, providing external cooling to the drum at the area of the drum-to-skirt junction,
and reducing the metal stresses in the drum. Once the coke bed is covered with water,
the drum drain is opened and water is drained out. The top and bottom drum head
covers are then removed. A pilot hole is drilled through the coke bed from the top,
and then a rotating high pressure water jet drill passing down through the pilot
hole directs a cutting stream horizontally against the coke bed. The drilled out
coke falls downwardly out of the drum. After the coke cutting is completed and the
coke has been removed from the drum, the head covers are reinstalled and the drum
is purged with steam and tested for leaks. Part of the hot vapor from the top of
the on-line drum is diverted into che cleaned drum to warm the drum to a predetermined
temperature. Hot feed from furnace 54 is then switched into the cleaned drum.
The essence of the invention is in externally applying cooling fluid
to the junction of the coke drum and its supporting skirt during and/or prior to
introducing quench water into the drum. The application of external cooling fluid
allows the area of the drum-to-skirt junction to more nearly approach the temperature
of the drum interior during the quench step, and allows the introduction of quench
water without the damaging metal stresses that would result if the exterior of the
drum, particularly around the drum-to-skirt welds, is at a much higher temperature
than the quench water.
The foregoing description of the preferred embodiments of the invention
is intended to be illustrative rather than limiting of the scope of the invention,
which is to be defined by the appended claims.