Coating processes – With post-treatment of coating or coating material – Cooling
Reexamination Certificate
2000-05-12
2001-08-07
Beck, Shrive (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Cooling
C427S230000, C427S233000, C427S236000, C118S306000
Reexamination Certificate
active
06270847
ABSTRACT:
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.
REFERENCES:
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patent: 3411933 (1968-11-01), Moore
patent: 3904346 (1975-09-01), Shaw et al.
patent: 3941087 (1976-03-01), Yazaki
patent: 3965551 (1976-06-01), Ostrowski
patent: 4490411 (1984-12-01), Feder
patent: 4510007 (1985-04-01), Stucke
patent: 4800104 (1989-01-01), Cruickshank
patent: 0137663 (1985-04-01), None
patent: 1052431 (1966-12-01), None
Patent Abstracts of Japan, vol. 007, No. 053 (C-154), Mar. 3, 1983 JP 57-201571A (Sumitomo Kiazoku Kogyo KK).
Patent Abstracts of Japan, vol. 010, No. 182 (C-356) Jun. 25, 1986 JP 61-028477A (Sumitomo Metal Ind. Ltd.).
Holub Jiri F.
Johnston Richard A.
Lemenn Louis
Wong Dennis T.
Beck Shrive
Cleveland Michael
Dresser-Shaw Company
Ridout & Maybee
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