In the manufacture of coated pipes, a typically metal pipe is heated to a high temperature and polymeric material is applied as a powder or extruded onto the external surface of the pipe. The polymeric 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 rollers on a conveying line.
There are numerous known processes for cooling the molten coating. In most processes, the cooling is carried out by flooding the outside surface with cold water using pipes having many open or spray nozzle, with the process lasting until the polymeric material has reached the predetermined temperature. These processes are laden with problems, specifically in relation to obtaining a defect free coating, in particular in pipes that have raised weld profiles. This is likely at least in part due to shrinkage when the coating solidifies, as well as the order in which the different regions solidify.
Solidification of the polymeric material on the outer surface first produces a skin layer which is highly stressed in tension while not yet bonded to the pipe surface. If the skin 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. In the meantime, the molten polymeric material at the pipe surface is 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 solidified skin can be squeezed away to produce a coating thinner than specified when the coating solidifies entirely.
To overcome some of the disadvantages of solidification of the outer surface first, alternative processes have been developed for cooling pipes being coated with hot plastic, comprising applying a liquid cooling medium to the interior surface of the pipe. Internal cooling has numerous advantages. For example, the material at the pipe surface is solidified first, which promotes better adhesion to the surface, and minimizes frozen-in stresses at the interface. Internal cooling also reduces damage at defects, and decreases or eliminates tenting at the weld areas. Having a solidification front move from the pipe-coating interface towards the outer surface of the coating exposed to the air provides an unhindered process resulting in low coating stress. Interior cooling provides improvements in uniformity of coating thickness and increased efficiency of heat transfer. Methods and apparatus for internal cooling of pipes are described, for example, in the Applicant's U.S. Pat. No. 6,270,847, incorporated herein by reference. These include procedures and apparatus using a fixed lance, and a self propelled cart.
In some prior art, a fixed lance may be connected from the outside of the pipe by means of a travelling lance connected to 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, resulting in a coating solidification front which is stationary relative to the position where the coating is applied. However, use of a travelling lance has several disadvantages, including the need for added factory floor space for the removal of the travel lance at the time each coated pipe length is removed from the assembly line.