The present invention relates generally to the field of bending tubes for use in a furnace, among other things, and in particular to a new and useful apparatus and method for bending a boiler tube to permit tighter radius tube bends.
Tubes for steam generating equipment, such as fossil-fueled boilers, must be bent to form the various shaped components within them. Rotary draw bending machines are currently used to bends much of the 1 to 3 inch O.D. (outside diameter) tubing used in high pressure steam boilers for electric power generation. U.S. Pat. No. 5,315,852, issued to The Babcock & Wilcox Company, is an example of a current rotary draw bending apparatus and method, which is incorporated herein by reference as though fully set forth.
In rotary draw tube bending, a tube is located in the semi-circular groove of a bend die having an overall radius approximately equal to the desired tube bend radius. Next, a pressure die and clamp die are moved up against the opposite side of the tube with the clamp die clamping the front part of the tube to the bend die. The bend die and clamp die are then rotated about a bending axis while the pressure die moves forward in a line carrying the tube tangentially to the bend point. The pressure die holds back the reaction force to create the bend.
When hollow tubes are bent, there is a very decided tendency for the circular cross section to become oval in the curved or bent part of the tube. In particular, the inner radius of the bent portion of the tubing (the intrados) will wrinkle, or the outer radius (the extrados) will pull in short of a full circular arc and thin below the original tube wall thickness. These results are undesirable since they reduce cross sectional area of the tube through the bend, produce a tube which is not as strong as a tube with a circular cross section, and may result in thinner tube wall thickness along the extrados of the tube bend.
In some applications a wiper die, a die having a tangential groove with a knife edge that conforms to the bend die groove, is located adjacent the intrados opposite the pressure die to prevent wrinkling of the tube.
When thin walled tubes are bent to small bend radii, for example when tubes with a wall thickness of 0.095″–0.280″ are bent to a bend radius between 1X–2X times the tube outer diameter, internal mandrels must typically be used to support the tubes during bending to improve and maintain the tube shape. The ASME Pressure Vessel Code Sections I and VIII permit engineering design of certain elements using tubes with thinner walls, provided they can be bent successfully. However, the need for an internal mandrel limits the practical length of tube that can be bent, since the tube must be slipped over the mandrel bars. The use of an internal mandrel also increases the amount of wall thinning that occurs on the extrados of the tube. Bending without the use of a mandrel is generally reserved for bends that are less than 180 degrees, or with tubing that has relatively thick walls, e.g. tubes with wall thicknesses greater than about 10% of the tube outer diameter.
Wiper dies and internal mandrels also frequently wear out or break, and are expensive to replace, which are further disadvantages of current tube bending apparatus and methods.