GB2049491A - Internally coating metal pipes with thermoplastic materials - Google Patents

Abstract

A thermoplastic internal coating is formed on a pre-heated metal pipe (1) by drawing an air stream through the pipe from the atmosphere using a pump (10) and periodically supplying a charge of thermoplastic particles entrained in gas through conduit (17) from a fluidised bed (20) of the particles and maintaining the velocity of the air stream through the pipe close to but not above the air transport velocity of the particles. The particles thus fall out of the air stream onto the internal pipe surface and form a thermoplastic coating thereon. The fluidised bed is pressurized to feed the powder through conduit (17) which leads into the end of the pipe (1) which is open to the atmosphere. <IMAGE>

Description

SPECIFICATION Method and apparatus for coating metal surfaces with thermoplastic materials The present invention relates to a method and apparatus for coating a preheated metal surface such as a length of pre-heated pipe, with thermoplastic material derived from a fluidised bed source of thermoplastic particles.

There are many industrial applications wherein it is desirable to provide a relatively thin layer of protective plastic on the interior of metal pipe in order to protect the pipe from the corrosive action of fluids passing therethrough. Known attempts to supply such coatings formed from thermoplastic materials have often involved spray application of the thermoplastic material and great difficulty has been encountered in achieving the desired uniformity of the depth of the coating.

It has previously been proposed to supply the thermoplastic coating in the form of particles which are introduced into the interior of the pipe by connecting one end of the pipe to a fluidised bed source of such particles and the other end of the pipe to a vacuum source. When the pre-heated length of pipe is rotated those particles contacting the surface of the pre-heated pipe are melted, adhere thereto and gradually build up a protective film.

This last mentioned method has hitherto only been practical for relativeiy small diameter pipe.

When coating are desired for pipes of, for example, 10cm or greater diameters, the cost of generating the required vacuum applied to one end of the pipe often has been excessive. Moreover the uniformity of coating achieved through the vacuum deposition method often left much to be desired.

A method according to the invention of forming a thermoplastic coating on a pre-heated metal surface comprises forming a fluidised bed of thermoplastic particles, passing an air stream along and above the surface, and periodically supplying a charge of the thermoplastic particles entrained in gas from the fluidised bed to the air stream by passage through a conduit leading from the fluidised bed and discharging in the air stream, and the velocity of the air stream along the surface is close to but not above the air transport velocity of the particles, as a result of which particles fall out of the air stream onto the surface and form a thermoplastic coating on the surface.

The surface to be coated is generally within a pipe in which event the air stream is drawn through the pipe by an air pump such as a fan.

The invention also includes apparatus for carrying out the method where the surface is within a pipe.

The apparatus comprises a pressure vessel, means for increasing the pressure within the vessel, means for forming a fluidised bed in the vessel, a conduit leading from the pressure vessel for removing fluidised material from the vessel, means for mounting a pipe having one end open to the atmosphere and with the conduit discharging into that end or into a duct leading into that end, an air pump connected to the other end of the pipe, or to a duct leading from that other end, for sucking air stream through the pipe and means for regulating velocity of air through the pipe.

The apparatus may include the pipe and the surfaces may be positioned within it but generally the surfaces to be coated are the interior of the pipe, in which event the pipe is fitted in the apparatus only when it is to be coated.

The end of the pipe distant from the pump is open to the atmosphere either directly or through the conduit, so that air can be sucked into that end by the pump connected to the other end of the pipe.

The air flow velocity of the stream is adjusted to be close to, and generally just under, the transport velocity of the thermoplastic particles. Generally the pipe is connected to the pump inlet by a duct and there may be an inlet for introduction of air into this duct so as to cause a higher air velocity through the duct and pump inlet than through the pipe. This reduces the risk of blockage in the duct and pump inlet.

Generally the air stream that has passed over the surfaces being coated is passed through a filter to collect any particles entrained in the air stream.

Often there is a bag type filter connected to one side of the air pump. For instance there may be a bag house between the pump and the pipe.

When the surface to be coated is the inner surface of the pipe the apparatus will generally include means for rotating the pipe.

Thermoplastic particles entrained in a gas stream may be supplied periodically when desired by providing a pressure above the fluidised bed greater than atmospheric pressure. There is generally a valve in the conduit to control the duration of the passage of entrained particles, and thus the size of the charge supplied to the air stream. Preferably there are means for intermittently connecting the conduit to either the fluidised bed or to a pressurised portion of the vessel above the fluidised bed. This renders it possible to periodically purge the conduit, for instance between each charge, by passing pressurised air through the conduit so asto eliminate the possibility of build up of particles in the conduit.

The invention is illustrated in the acompanying drawing which is a schematic elevation, partly in section, of apparatus suitable for applying a uniform coating of thermoplastic particles to a pre-heated length of pipe.

Referring to the drawing, a discrete length of pre-heated pipe 1 is supported on a pair of laterally spaced rails 2 and rotated in a fixed position on the rails by a pair of friction rollers 3. The rollers are rotatably journaled at the top of a support structure 4 which includes a motor 5 and driving connections to one roller 3 including intermediate drive pulleys 6 and belts 7. Other drives such as a chain may be used instead of the belt. The pipe 1 may constitute any type of pipe for which an internal coating of thermoplastic material is desired, but the pipe illustrated is a length of oil well drill pipe having an enlarged female coupling end la and a reduced diameter male coupling end 1b. The rate of rotation is from 100 to 300 r.p.m.

Pipe 1 can be pre-heated to the desired tempera ture by any conventional form of heating apparatus, such as an oil or electric or gas fired oven, or it can be rapidly pre-heated through the application of induction heating. Generally it is pre-heated before being located on the rails 2 in a position such that it can be rotated by roller 3. The pre-heating temperature of the pipe is selected in accordance with the melting characteristics of the thermoplastic material to be applied. Obviously, the internal surface of the pipe must be heated to a temperature in excess of the melting point of the thermoplastic material, and yet not be so hot as to cause degradation of the applied thermoplastic material.When a commercially available epoxy type powder material such as Polymer 1432 Gray is employed, the temperature of the interior metal surface of the pipe is generally approximately 300 to 330"F. Other thermoplastic materials are available with melt points from 225"F to 750"F.

The pipe to be coated can have a wide range of diameters and lengths. Diameters as small as about 2.5cm can be readily coated with a thermoplastic material by use of this invention but much larger pipes can also be coated.

There is an air pumping unit 10 adjacent to one end of the rotating pipe. The pump may be any conventional form of low or high pressure air moving apparatus such as a centrifugal fan, and preferably includes a filter at the input end of the pump that is capable of removing and collecting any thermoplastic particles from the air stream. A conventional bag house filtering arrangement can be used and is not illustrated.

A conduit 12 leads from one end of the pipe to the intake side of the pump 10 and a flow regulating valve 13 is connected to conduit 12 by a conduit 13a.

Valve 13 permits adjustment of the amount of atmospheric air entering the conduit 12 so as to raise the flow velocity from the velocity within pipe 1 (slightly below the air transport velocity of the particles) to a value above the flow velocity of the particles. As a result the particles will not fall out and clog the conduit 12. Also the air admitted through valve l3coolstheparticlesto preventcoating ofthe conduit 12. The air transport velocity of the previously mentioned epoxy material is from about 3200 to about 3900 feet per minute.

Conduit 12 is connected by a conventional form of rotary joint 14 in sealing reltionship to the adjacent rotating male end 1b of the pipe 1. On the other end la of pipe 1, a rotary joint 15 is provided which connects a stationary conduit 16 in sealing relationship with the rotating female end 1a of the pipe length 1. The outer end of conduit 16 is open to the atmosphere, thus providing the necessary air to maintain the air flow through the interior of the pipe.

Thus, an air stream flow through the interior of pipe 1 is created having a velocity slightly less than the transport velocity of the thermoplastic particles.

Intermittent charges of fluidised thermoplastic particles are injected into the inlet end of conduit 16 by a smaller diameter conduit 17 which connects through an intermittently operating valve 18 to the bottom portions of a fluidised bed 20 of thermoplastic particles. The bed 20 is created within the interior of a pressure vessel 30 and has a conventional screen or membrane 21 at the bottom portion thereof which is sufficiently porous to permit a fluidising flow of air upwardly through the screen from a supply conduit 22. Supply conduit 22 incorporates a pressure regulator 22a and a flow rate indicator 22b which the operator can refer to to ensure that sufficient air or other fluidising medium is being supplied to the fluidised bed 20 to maintain the particles in a fluidised condition.The top portion of the pressure vessel 30 defines an air chamber 30a and a flow regulating exit valve 31 is provided on an exhaust conduit 31a connecting with the top of the vessel 30. A filling neck 32 with an appropriate sealing closure is also provided in the top portion of the pressure vessel 30.

By adjusting the rate of air flow through the pressure regulating valve 31, the interior of the pressure vessel 30, and hence the fluidised bed 20 can be maintained at a pressure significantly above that of atmosphere. As mentioned, the conduit 17 connects directly with the lower regions of the fluidised bed 20 and whenever the intermittently operating valve 18 is open, a charge of fluidised particles will be forced by the pressure within the vessel 30 to flow through conduit 17 and then through conduit 16 into the interior of the pre-heated rotating length of pipe 1.Since the velocity of the air flow through the pipe 1 is slightly lower than the transport velocity af the thermoplastic particles, a great majority of such particles drop out of the air flow and are deposited on the surface portion of the pipe 1 that is immediately beneath the particle at the time it falls out of the stream. The rotation of the pipe thus ensures that a uniform coating of such particles will be applied. The temperature of the internal surface of pipe 1 assures the sticking and melting of the particles and the flowing out thereof to form a uniform coating.

Any particles that are still entrained within the air stream after passing entirely through the pipe 1 are drawn into the intake of the air pumping unit 10 and entrapped by the bag house filter contained in such unit. The particles can thus be recovered for subset quent use in the process.

To purge the plastic particles from the supply conduit 17 after each open cycle of valve 18, a purging conduit 35 is provided which connects between the supply conduit 17 at a point beyond the intermittently operating valve 18 and the top portion of the air chamber 30a defined by the pressure vessel 30. A purging valve 36 is incorporated in conduit 35 and is intermittently operated, and is sequenced to open for a period of time up to, for example, 60 seconds after valve 18 closes. The movement of purge valve 36 to an open position permits a blast of pressurised air to flow into the supply conduit 17 and literally blow out any entrapped particles within conduit 16 or the pipe-feeding conduit 17.

Atypical cycle of operation for 2-inch internal diameter pipe would involve a pressure in the vessel 30 of 10-15 p.s.i., a powder supply time of two seconds and a purge time often seconds. Hence a predetermined charge of plastic particles is supplied which results in a particular coating thickness.

Larger pipe requires a longer charging time or a higher pressure in vessel 30.

The illustrated apparatus can easily be modified to make it suitable for coating metal surfaces other than the inner surfaces of pipe.

Claims (15)

1. A method of forming a thermoplastic coating on a pre-heated metal surface comprising forming a fluidised bed of thermoplastic particles, passing an airstream along and above the surface, and periodically supplying a charge of the thermoplastic particles entrained in gas from the fluidised bed to the air stream by passage though a conduit leading from the fluidised bed and discharging in the air stream, and in which the velocity of the air stream along the surface is close to but not above the air transport velocity of the particles whereby particles fall out of the air stream onto the surface and form a thermoplastic coating on the surface.

2. A method according to claim 1 in which the conduit is periodically purged of particles by pressurised air.

3. A method according to claim 1 or claim 2 in which the passage of particles through the conduit is caused by providing a pressure above atmospheric in the vessel containing the fluidised bed.

4. A method according to any preceding claim in which the air stream that is passed over the particles is passed through a filter to collect particles entrained in the stream.

5. A method according to any preceding claim in which the surface to be coated is within a pipe and the air stream is drawn through the pipe by an air pump.

6. A method according to claim 5 which the surface to be coated is the interior of the pipe and the pipe is rotated during the supply of the charge of particles.

7. A method according to claim 5 or claim 6 in which the pipe is connected to the pump by a duct having an air inlet and air is sucked into the duct through the inlet in an amount such that the air velocity in the duct is above the transport velocity of the particles.

8. A method according to claim 1 substantially as herein described with reference to the accompanying drawing.

9. Apparatus suitable for use in the method of claim 5 and comprising a pressure vessel, means for increasing the pressure within the vessel, means for forming a fluidised bed in the vessel, a conduit leading from the vessel for removing fluidised material from the vessel, means for mounting a pipe having one end open to the atmosphere and with the conduit discharging into that end or into a duct leading to that end, an air pump connected to the other end of the pipe or to a duct leading from that other end for sucking air through the pipe and means for regulating the velocity of air through the pipe.

10. Apparatus according to claim 9 including means for rotating the pipe.

11. Apparatus according to claim 9 or claim 10 including the pipe.

12. Apparatus according to any of claims 9 to 11 including a duct for connecting the said other end of the pipe to the pump and in which there is an inlet in the duct for introduction of air into the duct so as to cause a higher air velocity through the duct than through the pipe.

13. Apparatus according to any of claims 9 to 12 including means for intermittently connecting the conduit to either the fluidised bed or to a pressurised portion of the vessel above the fluidised bed.

14. Apparatus according to any of claims 9 to 12 including a bag type filter connected to one side of the air pump to recover thermoplastic particles.

15. Apparatus according to claim 9 substantially as herein described with reference to the accompanying drawing.