Process and apparatus for internally coating a tubing

A process for internally coating a tubing comprising the steps of:

• introducing a supply of a coating liquid into the tubing;
• inflating the tubing above the level of the coating liquid with a support gas;
• conveying the tubing vertically upwardly with the tubing carrying a relatively thick layer of coating liquid on its inside surface;
• flattening the inflated tubing;
• conveying the flattened tubing along a winding course through a metering device comprising a series of at least two wipers positioned in the direction of travel of the tubing on opposite sides of the tubing in such a way that the two outside surfaces of the flattened tubing are alternately drawn tightly against contact surfaces of successive wipers, said wiper surfaces being rounded in the longitudinal direction of the tubing, whereby the major quantity of the coating liquid carried on the inside surface of the flattened tubing is retained and only a thin layer of the coating liquid remains on the inside surface of the tubing exiting from the metering device;
• reinflating the tubing upstream of the metering device with support gas;
• and
• conveying the reinflated tubing through a drying station and removing volatile constituents of the coating liquid in said drying station to form a uniform, continuous coating on the inside surface of the tubing;
• the contact pressure between the tubing and the contact surfaces of the first and last wipers in said metering device being greater than the pressure of the support gas in the tubing upstream and downstream from the metering device whereby support gas is excluded from the tubing in the metering device.