Abstract:
Pipe coated with a hot plastic coating is cooled by applying a liquid cooling medium to the interior of the pipe. The cooling medium may be applied from a lance or pressurised cart that is stationary relative to the surroundings and moves internally relative to sections of pipe that pass successively through coating and cooling stations.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT/CA97/00903 filed Nov. 18, 1997. 
    
    
     TECHNICAL FIELD 
     This invention related to the art of coating pipes. In particular, the invention relates to the art of cooling a coating after its application. 
     BACKGROUND 
     In the manufacture of pipe coatings, the pipe is heated to a high temperature and polymeric material applied as a powder or extruded on. The material is or becomes molten and conforms to the pipe surface. Usually, the pipe is spun or rotated about its axis. After sufficient time has elapsed for flow and/or curing to occur, the material is cooled to solidify it and to prevent damage during further handling. Damage can occur if the still molten coating comes into contact with equipment used to transport it such as supporting tires on a conveying line. In known processes cooling has been carried out by flooding the outside surface with cold water using many open or spray nozzle pipes. The process lasts until the material has reached the predetermined temperature. 
     With the known cooling procedures, it has always been a problem to obtain a defect free coating, especially with pipes that have raised weld profiles. It has been found that the difficulty arises in part due to shrinkage when the coating solidifies as well as in which order the different regions solidify. 
     Solidification of the outer surface first produces a skin layer which is highly stressed in tension and not yet bonded to the pipe surface. If the layer has a defect such as a pinhole or bubble, this becomes the weakest point and the coating can tear at this position. Where there is a concave curvature on the surface, such as at a neck area of a weld, the tension in the skin layer causes it to pull away from the pipe surface. The material at the pipe surface is still molten and yields, but at the same time creating pinholes and cavities to replace the displaced material. The cavities in the coating at the neck of the weld, which are referred to as tenting, can run for considerable distances along the weld length. 
     On a convex surface such as on the top of a weld, the still molten material under the frozen skin can be squeezed away to produce a lower than specified coating thickness when the coating becomes entirely frozen. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of cooling hot plastic coated pipe, comprising applying a liquid cooling medium to the interior surface of the pipe. 
     The transformation of the coating from a flowable or molten state to solid state using cooling of the pipe interior has numerous advantages compared to exterior cooling. For example, the material at the pipe surface is solidified first. This promotes better adhesion to the surface, and minimizes any frozen-in stresses at the interface which can affect the coating adhesion at a later time. 
     Interior cooling eliminates damage at defects and, in the case in which the pipe is welded pipe, that is to say metal, usually steel, pipe having a longitudinal externally raised weld profile, eliminates tenting at the weld areas. The solidification front moves from the pipe-coating interface towards the outer surface of the coating exposed to the air (the coating-air interface) which is the last area to become solidified. The coating material, which shrinks during solidification, can flow and shrink inwards at the air exposed surface. This process is not hindered and results in low coating stress. 
     The molten outer surface of the coating does not come into contact with the cooling medium which can deform and affect it to produce an irregular surface. With interior cooling, the outer surface solidifies without any physical interference, leaving a uniform and aesthetically satisfactory surface. 
     A uniform coating thickness can be achieved even on pronounced weld profiles. This means that less coating material need be used to maintain a minimum coating thickness. 
     Interior cooling is also considerably more efficient than exterior cooling. The overall heat transfer rate to the pipe surface is much higher. Further, the water or other cooling medium remains in the pipe and continues to remove heat whereas with exterior cooling, the water is gone after its initial contact. The invention therefore allows for less water usage as well as a shorter cooling time to get the pipe to the required temperature. 
     In the preferred procedure for carrying out the present process, a water dispensing device is positioned on the inside of the pipe at the location where the coating is to be cooled. Cooling is applied after the coating has had sufficient time to melt, flow and become smooth. 
     The water or other medium may be applied using multiple spray tip nozzles, each of which produces a 360° spray pattern. In this manner, the entire circumference of the pipe is cooled along a longitudinal zone covered by the multiple nozzles. This water dispensing device can be held stationary relative to a fixed outside position while the pipe moves forward relative to the nozzles. This results in the cooling of the entire pipe as it travels along. Alternatively the water dispensing device can be made to move in a controlled manner relative to the surroundings, so as to cool the entire pipe. 
     Many different ways of connecting and controlling the water dispensing device are possible. For example procedures using a fixed lance, or a self propelled cart may be employed. 
     A fixed lance may be connected from the outside of the pipe by means of a strong flexible hose which also supplies the water. Wheels support the lance on the inside of the pipe. At the end of the lance is a set of nozzles. The lance is held stationary while the pipe is rotating and moving forward. This results in a coating solidification front which is stationary relative to the position where the coating is applied. 
     A self propelled cart may be supported on the pipe by wheels and is unattached to the outside of the pipe. The cart may contain a pressured reservoir of water that is replenished after cooling each pipe. The alignment of the wheels may be controlled in such a manner as to maintain the relative position of the cart. While the pipe rotates and moves forward, the cart may remain stationary relative to a fixed outside point. The pipe is cooled as it moves forward. 
    
    
     BRIEF DESCIPTION OF THE DRAWINGS 
     Some cooling procedures are described in more detail, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 shows somewhat schematically a side view of a coating and cooling process. 
     FIG. 2 shows somewhat schematically on an enlarged scale a coupling and cooling medium feed unit used in the apparatus of FIG.  1 . 
     FIG. 3 shows somewhat schematically a plan view of a further form of coating and cooling process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a coated and cooled pipe length  11  and a subsequent pipe length  12  undergoing coating and cooling. The pipes are supported on respective spiral conveyors  13  and  14  comprising driven rubber tires or like rollers inclined to the pipe axis so that the pipe is spun about its axis while being conveyed forwarded in the direction of the arrow  16 . Pipe  12  is preheated before entering a coating application station  17 , for example a powder application booth wherein polymer powder is applied on the pipe and melts and fuses to the pipe surface. A cart  18  is supported within the pipe  12  on roller members such as wheels  19  journalled for free rotation on axes adjusted to an angle inclined relative to the pipe axis such that as the pipe rotates and travels forwardly, the cart  18  maintains a stationary position relative to the surroundings such as the powder booth  17  and the conveyor  14 . Extending rearwardly from the cart  18  is a rigid spray lance  21  supported at an intermediate position through connection to a frame  22  connected to wheels running freely on the interior of the pipe surface. In a zone  23 , the end of the spray lance  21  is provided with spray nozzles. 
     The cart  18  carries one or more pressurizable reservoirs  24 , provided, for example, with diaphragms, bladders or the like confining a compressed gas. Normally, the spray lance  21  and nozzles in the zone  23  are fed continuously with pressurized cooling medium, such as cold water supplied by the reservoirs  24 . 
     Normally, successive pipe lengths, such as lengths  11  and  12  travel through the station  17  with their ends in close proximity. FIG. 1 shows a stage at which a leading pipe length  11  has been accelerated to open a gap between its trailing end and the leading end of the following pipe  12 , allowing a recharging coupling device  26  seen in FIG. 2 to be actuated to raise its coupling portion  27  from a lower position shown in broken lines in FIG. 2 to an upper position shown in solid lines wherein it is aligned with a forwardly projecting recharging lance  28  connected to the cart  18 . The device  26  is then driven rearwardly so that its coupling device  27  receives a complementarily shaped fitting  29  on the forward end of the recharging lance  28 , allowing water or other cooling medium to be passed under pressure from a supply line  31  through the lance  28  to recharge the reservoirs  24 . During this operation, the fitting  29  may be retained by clamping devices  32 . The reservoirs  24  are fully charged by the time the leading end of the pipe  12  approaches the device  26 . At this point, the clamping devices  32  are unlocked, the device  26  displaced forwardly on its wheels  33  and the coupling portion  27  dropped downwardly to the dotted line position as seen in FIG. 2, so that the pipe length  12  can be passed forwardly to be received by the tires or like conveyor members  13   a  of the conveyor  13  which in the meantime has been vacated by the pipe length  11 . The pipe length next following the length  12  continues through the spray booth  17  and is cooled by application of the spray from the nozzles in the region  23 . After the wheels of the cart  18  have run into the interior of this next following pipe length, when it reaches approximately the position shown for the pipe  12  in FIG. 1, the pipe  12  is accelerated forwardly to the position shown for the pipe length  11  in FIG. 1, and the above described cycle of operation is repeated. 
     It may be noted that, in the procedure illustrated, the cooled zone  23  is located between the coating application station  17  and the point at which the cooled and coated pipe contacts the rear most of the tires  14   a  or other conveying devices constituting the spiral conveyor  14 . 
     FIG. 3 illustrates a further form of process wherein a pipe  41  is undergoing coating and cooling while pipes  42 ,  43  and  44  are detained on a lateral conveyor and rack  46 , while a further pipe  47  disposed at an entry station awaits loading onto the rack  46 . 
     Pipe  41  is conveyed on the tires or other roller members of spiral conveyors  48  through an acid and rinse booth  49  and through coils  51  that preheat the pipe for reception of powder in a powder booth  52  to form a flowable plastic coating. 
     Cooling is applied to the inside of pipe  41  at a region  53  from spray nozzles provided on an end of a rigid spray lance  54  running on angle tracking wheels within the pipe  41  so that the lance  54  maintains station with the surroundings and, in effect, moves rearwardly relative to the pipe  41  as it advances. The lance  54  is supplied with water or other cooling medium through a detachable coupling  56  at its rear end that connects to an auxiliary lance  57  that runs through the next succeeding pipe  42 . A rear end of the auxiliary lance  57  is connected to a main water supply through a detachable coupling  58 . The next length of pipe  43  contains a length of the auxiliary lance material  57   a  preinstalled through it. 
     In operation, the pipe length  42  is accelerated forwardly by spiral conveyors  59  disposed beneath it and forming part of the rack  46  so that its leading edge catches up with the trailing edge of the pipe length  41  and the wheels supporting the rigid lance  54  enter the pipe length  42 . Once the pipe  42  has cleared the rack  46  and has reached approximately the position shown for the pipe  41  in FIG. 3, a temporary water supply  61  indicated in broken lines in FIG. 3 is attached to the coupling  56  to supply water to the lance  54 , the auxiliary lance  57  is detached from the couplings  56  and  58  and is relocated within the pipe length  44  as shown by a broken line and reference numeral  57 . The pipe length  43  containing the auxiliary lance  57   a  is then moved forwardly to the position shown for pipe  42  in FIG.  3  and the lance  57   a  is coupled to the supply  58  and to coupling  56 , to re-establish supply of water to the lance  54  from the main water supply at  58 , and the temporary supply  61  is disconnected. A fresh length of pipe, such as length  47  is then rolled onto the rack, the pipe length  54  now containing the auxiliary lance  57  is rolled forwardly to the position shown for the pipe length  43  in FIG. 3, a fresh pipe length is delivered to the entry station to take the place of pipe length  47  and the above cycle of operation is repeated. 
     In the preferred form, the auxiliary lance  57  is a flexible pipe so that it can be fed in a part circular path  62  by a caterpillar drive  63  through a guide  64  to enter the pipe length  44 .