Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TUBE COATING METHOD AND APPARATUS
BACKGROUND OF THE INV~NTION
This invention relates to the internal coating of a heated
tube and in particular to a coating process wherein fusible particles
are transported through the tube in a flow of air.
It is frequently advantageous to coat the inside of steel
tubing with nonmetallic linings. Properly selected linings can
protect the tube against erosion or corrosion. A coating surface
which is smoother than that of the tubing itself can result in a
decreased pressure drop of fluid flowing through the tube. For such
a purpose materials have been used such as polypropylene, chlorinated
polyether, various polyester, certain epoxy and vinyl polymers,
polyethylene~ polytetrafluoroethylene, and nylon. These materials
may be either thermoplastic of thermoset~ing, and are normally
applied to the tube after it has been heated.
It is desirable that the internal coating be free from
voids or pin holes. It also should have a uniform thickness and a
smooth surface. The method used to coat the tube should be both
reproducible and simple in operation.
It is known to heat and rotate the tube to be coated so
that fusible particles conveyed in an air stream and injected into
the tube will stick to the surface and form a coating thereover.
There have been many methods, along with apparatus therefor, sug-
gested involving this general concept of coating a tube.
Fluidized beds have been suggested as a supply source for
the fusible particles ~rom which location they are carried through
the tube in a stream of air. This is generally a continous flowing
stream with excess particles collected at the other end. In some
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cases the apparatus provides a means for reversing flow through the
tube from a second fluidized bed to promote uniformity of coating
of the tubing.
It has been suggested to use a timed pulse of flow carry-
ing the fluidized particles followed with a flow of particle-free
air scavenging the tube.
These various methods in general require a significant
excess of particle flow and/or timing of the cycle. Effectiveness
of this operation requires that the timer be consistent and readily
adjustable and that there be no leaks in the system whereby the
timing would not represent an appropriate particle flow. A uniform,
preferably thin and smooth coating, remains difficult to obtain in a
consistent and reproducible manner.
SUMMARY OF THE INVENTION
1~ The tube to be coated is heated, horizontally supported,
and rotated in accordance with common practice. The first end of
the tube to be coated is connected to a charging container with a
pipe including a quick-opening valve which is closed at this time.
A measured charge of the coating material is placed in the container.
The second end of the tube is connected with a pipe to a
vacuum tank. A vacuum pump places the tank and the tube under
vacuum. The valve is suddenly opened so that flow suddenly occurs
from the container into the tube carrying the fusible particles
with it. The vacuum in the tank and the tube causes a high initial
flow rate which decreases with time as the vacuum is diss~pated by
the inflowing air. The vacuum pump may or may not be operated
during this time.
An additional valve may be added between the tube and the
vacuum tank so that vacuum may be drawn within the tank while the
tube is being placed into position. The valve is then opened to
place the tube under vacuum as well as the tank, and when the
vacuum has been established at the proper level, the quick-opening
valve initiates the process as described above.
The apparatus for carrying out this method includes a
measuring means for depositing only a predetermined quantity of
fusible particles within the container. These particles are placed
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within the container in an unpacked condition either by using a
fluidized bed or by pouring them into the chamber over a fall
breaking apparatus which will permit the particles to remain in a
fluffed condition for sufficient time to achieve operation of the
apparatus.
The vacuum level in the tank is sensed, and means are
provided for opening the quick-opening valve in response to a pre-
selected vacuum level.
A plurality of vacuum tanks may be connected to the
initial tank to provide a controllable variation in the volume
which is evacuated. This provides ability to easily manipulate the
time rate of flow variation when opening the quick-opening valve.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 illustrates a schematic arrangement of the system
including elementary controls;
Figure Z illustrates the relationship between vacuum and
time, and
Figure 3 represents the relationship between flow through
the tube and time.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The tube to be coated lO 1s first cleaned and preferably
internally sand blasted to white metal. A primer may or may not be
app1ied to the clean surface. This tube is then preheated to a
temperature level above the melting point of the powder which will
be deposited on the internal surface. The actual temperature level
depends on the coating material to be used.
This tube is then placed on rolls l2 while still hot and
rotated at a predetermined speed such as 100 rpm. This rotation aids
circumferential distribution of the coating.
Air ti~ht rotary connection l4 connects the first end of
the tube to be coated by pipe 16 to a char~ing location within
container 18. The pipe l6 includes a quick-opening val`ve 20 whereby
the container may be selectively placed in ~luid communication with
the tube interior.
A hopper 22 contains a supply of powdered material which
is to be used for coating the pipe. A predetermined weight of the
fusible particles 24 are to be used to coat the interior of the tube.
The desired amount of these particles for the tube to be coated are
weighed in scale 26, and the selected charge is then placed within
the container 18. They are located within the container at a loca-
tion generally near the outlet through pipe 16 and are preferably
maintained in a flu~fed or unpacked condition. As illustrated~ a
fluidized bed is used wherein a supply of fluidizing air passes in
through pipe 28 and valve 30 through bed support 32 in accordance
with well-known fluidizing principles. Alternately the material
could be placed in a container gently so that it retains a fluffed
condition which may be maintained so long as the container is not
unduly jarred or vibrated.
An imperforate vacuum ~ank 34 is connected to the second
end of the tube 10 with an air tight rotary coupling 38. The
connecting pipe 40 between the inlet connection 42 on the imper-
forate tank 34 and the rotary coupling 38 may include valve 44 if
certain operation as described below is required. Otherwise this
~0 valve is not required.
A vacuum pump 46 such as an air ejector is in fluid
communication with the tank 34 through pipe 48 and operates to place
the tank under vacuum. With valve 20 closed and valve 44 either
omitted or open, the tank and the interior of pipe 10 are both
placed under vacuum. Because of the low ~low rate through the pipe
existing while establishing a vacuum, the vacuum level in the tank
and the pipe are essentially the same and may be measured by pressure
sensor 50.
After the tube to be coated is placed on the rolls 10,
rotatable couplings 14 and 38 are made up; and with valve 20 closed,
the weighed charge is placed in tank 18. The vacuum pump 46 is
operated by an air line 52; and a vacuum is drawn in the tank 3~ and
pipe 10. The vacuum level during this period is illustrated in
Figure 2, wherein the vacuum generally increases (51), and as
illustrated, it is held at a preselected level (53).
When the vacuum reaches the desired level which is a
function of the tank size, the pipe diameter and length, and the
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coating material usedg quick-opening valve 20 is opened at time 55.
This may be done as soon as the vacuum reaches the desired level,
and the flat portion illustrated in Figure 2 is not necessary
With the opening of valve 20 there is a sudden flow of
air into the tube 10 as shown in Figure 3 by curve 57. The vacuum
in the tank decays as illustrated by 59 in Figure 2. Accordingly,
there is a sudden surge of air flow which then uniformly decreases.
The sudden change in the particle environment from atmospheric
pressure to the low pressure causes an explosion-like action which
creates a cloud of powder which is conveyed through the tube. The
rate of air flow peaks and decays in a manner related to the vacuum.
Curve 59 shows the continuing air flow when the vacuum pump continues
running.
The air flow pattern is a function of the tank volume and
the tube size as well as the vacuum level. Some experimentation is
at present required in order to determine the appropriate parameters
for a particular tube and tube coating. For a particular coating
too low,a tube temperature results in a considerable amount of the
particles passing completely through the tube, poor adhesion, and a
tendency for a powdery deposit. Any deposit also tends to be thin
on the upstream end. On the other hand~ too high a tube temperature
not only involves handling tubes at unnecessarily high temperature
levels and a waste of energy but also results in excessive coating
at the upstream end of the tube.
A roughened internal surface seems to occur when the tube
is too hot or when the powdcr is insufficiently fluffed in container
18.
Insufficient coating at the upstream end seems to be caused
by too high a vacuum.
Insufficient coating at the downstream end tends to be
caused by insufficient powder quantity or by too low a vacuum.
Inordinate wastage of material carried through the tube
seems to be caused by too cool a tube.
Speculation on the reasons for the good results obtained
in laboratory tests suggests that the result may be related to heat
transfer requirements between the particles and the tube itself~
High velocity is initially used when the incoming particles are
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radiantly exposed to the hot tubing whereby some stick at the
upstream end and others are heated as they pass down the tube.
The decreased flow with time retards those particles which have
already passed to locations near the outlet, and the flow is reduced
at a time when the particles within the dust cloud radiate not to
the wall directly but to other particles which have already formed
a portion of the surface. These particles, accordingly, radiate
to a lower temperature level; and more time is required for heat
transfer. The decreased air flow accommodates this need, and a
uniform coating is obtained with little wastage.
The pipe 16 should be of a smaller diameter than the tube
to be coated. This increases the velocity thru pipe 16 decreasing
or avoiding the possibility of powder remaining in this pipe.
Material left behind would cause erratic quantity delivery to the
successive pipes being coated. The high velocity in pipe 16 as
compared to the decreased velocity in the tube 10 results in good
dispersal of the material with the tube.
Any particles which are carried through the system are
collected in centrifugal vacuum tank 34 or in bag filter 54.
These particles may be intermittently removed through valves 56
and 58, respectively. While these particles may be recycled for
use, it still is not desirable to have a substantial amount o~
particles passing through the pipe. To the extent that there is
large wastage at the pipe discharge, this provides an increasingly
sizable percentage of the injected material which is passing
completely through the pipe and not being deposited on the surface.
The larger this amount the more variation there ~s in the actual
pipe coating with variations in system operation.
Auxiliary vacuum tanks 60 and 62 are isolatably connected
to the vacuum tank 34 through valves 64 and 66. These tanks may be
cut in or out as desired thereby varying the volume-tric capacity
of the ~acuum system. Accordingly, the time ~low characteristic
of the system may be adjusted.
The system may be automatically operated with pressure
sensor 50 sending a control signal through control line 68 to a set
point 70 where the signal is compared to a desired pressure leYel
signal or set point 72. This may operate to maintain a vacuum
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level in the tank by varying the air flow through line 52 by
modulating control valve 74 with controller 76. In the event that
the preselected vacuum level is reached within the vacuum tank
before the remainder of the system is prepared, this control will
limit the vacuum and hold it at a preselected level.
The control signal indicative of pressure level from
pressure sensor 50 also passes through control line 78 to set point
location 80 where it is compared with a set point 82 which represents
the desired pressure level at which the quick-opening valve 20 is to
open. When the vacuum reaches the preselected level, a signal is
sent through control line 84 to controller 86 which opens the quick
opening valve 20.
Depending on the characteristics of the particular vacuum
pump 46 being used, it may be advantageous at times to include valve
lS 44 which may be closed while pipe 10 is being changed. The vacuum
pump may operate at this time to establish a vacuum in tank 34 while
the pipe ls being changed. When the new pipe is in place, valve 44
is opened with pipe 10 being evacuated at which time it would be
ready to operate in accordance with the above-described procedure.
Experiments were carried out to internally coat pipe to a
thickness of 20 to 23 micrometers. The pipe was a range 2 drill
pipe which is a pipe approximately 9.5 m~ters long. The material
used to coat the pipe was an epoxy melt rnix of approximately the
following compos;tion: The particular material used was an epoxy
powder prepared by Whittaker Coatings and Chemicals Co. of Colton,
California, using the trade name, "Interior Oil Pipe Coating--E. S.
Powder". The manufacturer code identification was 8502-B235.
Four different pipe sizes were tried, and the preferred
quantities of powder, vacuum, and pipe temperature are set forth in
Table 1. The vacuum setting given in the table is the trip point
at which the quick-opening valve is opened. The vacuum tank had a
volume of 0.125 cubic meters. Considering the connecting hoses as
well as the pipe to be coated, the total volume varied from 0.221
cubic meters for 12.7 centimeter pipe down to 0.163 cubic meters
for 8.9 centimeter pipe.
TABLE 1
Pipe S_ze Powder VacuumPipe Temp.
cm kg mm Hg C
12.7 1.8 12 7 160
511.4 1.5 11 4 160
10.2 1.2 10 2 160
8.9 1.0 10 2 160
The flexible hose connecting the charging location to the
pipe was approximately 3.8 centimeters inside diameter. It was
estimated that the velocity in this line was maintained at approxi-
mately 20 meters per second with the venturi vacuum system being
kept in operation.
Satisfactory coatings were obtained over a range of about
28 C from 140 C to 170 C. It is, however, required that the
15 end-to-end pipe temperature not vary more than 5 to 8 C. Some
experimentation is obviously required to achieve the optimum
conditions for other coating materials and pipe sizes.
