EP2090382A2

EP2090382A2 - Method for cold-bending of pipes and the like

Info

Publication number: EP2090382A2
Application number: EP09001857A
Authority: EP
Inventors: Giorgio Mongiardino
Original assignee: Officine Metallurgiche Piemontesi SpA
Current assignee: Officine Metallurgiche Piemontesi SpA
Priority date: 2008-02-11
Filing date: 2009-02-10
Publication date: 2009-08-19
Also published as: ITTO20080107A1; EP2090382A3

Abstract

Method for cold-bending of pipes (1) and the like characterised by a step for preparing a pipe (1) to be bent, a step for marking the significant dimensions in the method, an actual bending step performed by a press (2) provided with a curvilinear saddle (3) and with two supports or cribs (4), a control of the bending parameters obtained and a step for calibrating the straight section of the bent pipe (1).

Description

An object of the present finding is a method for cold-bending of pipes and the like.
Generally known to the prior art are methods for hot-bending pipes having a large diameter. The technical problem linked with this technology regards the fact that during the bending process the straight section of the pipe is deformed acquiring an oval shape. Just to mention one of the most common methods in the current art, the metal pipe having large diameter is filled with sand or any other incompressible material, heated in a furnace and thus hot-bent. Such system, as clear to a man skilled in the art is definitely expensive and thus not convenient. Thus, it can be easily understood why, over the years technology has focused on developing cold-bending methods that are advantageous from an economical point of view . However, such methods, are not easily attainable and today they are commonly used when bending pipes with a thin wall that, evidently, creates less problems. For example, the European patent application N. EP 0901847 has the object of a device for cold-bending of pipes in a press, comprising a plurality of elastically deformable disks, almost cylindrical, which serve as means for supporting the internal wall of the pipe to be bent and at least one rigid separation element between the disks, capable of allowing the radial expansion of the disks. The Chinese patent application CN 1047231 has the object of a device for cold-bending of pipes with thin wall. Such device uses three press rollers for bending and ensures a good accuracy for internal and external diameters of the pipe and of the radius of curvature and also allows minimising the oval shaping of the transverse sections of the pipe. Lastly, the Japanese patent application N. JP60087929 still has the object of a device for bending pipes, where a metal pipe with a thin wall 1 is arranged inside a mould provided with a cavity and having a determined radius of curvature. A pair of arms that form a V-shape oscillates around a shaft of the mould to which a cylinder is connected by means of brackets. Rollers are arranged in a suitable position of the arms, above the metal pipe to be processed. By actuating a cylinder that serves as a pressure actuator, the arm rotates around the shaft of the mould, pressed and rotated in such a manner that the V-shaped angle "V" increases without varying the distance between the centre of the roller and the element. The metal pipe is bent according to the curvature of the mould.
As observable, all these solutions have the drawback of not being suitable for bending pipes having a large diameter and above all in presence of substantial thicknesses.
Object of the method, subject of the present invention, is that of overcoming such drawback.
The finding subject of the present invention attains the present objects according to the description outlined in claim 1.
These and other advantages shall be apparent over the detailed description of the invention which shall specifically refer to the drawing 1/1 represented in which is an absolutely non-limiting preferred embodiment of the present finding.
In particular:

Fig. 1, schematizes the operation of curving the pipe by means of a vertical press;
Fig. 2, shows the preparation of the pipe to be bent in the two cases of empty bending (Fig. 2a) and by filling with water (Fig. 2b);

Referring to the abovementioned figures, the method in question is applied to obtain the cold-bending of pipes 1, preferably having a medium and large diameter, by means of a vertical hydraulic press 2 and special equipment 3, 4. The method essentially consists in a step for preparing the pipe to be bent, the step for marking the significant dimensions of the method, an actual bending step with relative control of the parameters achieved and a step for calibrating the straight section of the curved pipe. As observable hereinafter, further accessory steps are possible. Over the description the following specific terminology shall be used:

pressing: this term is used to indicate the single operation which, when repeated, allows complete bending of the pipe under a movement in axial direction of the same;
pass: the entirety of the pressings and movements that produce the entire deformation of the pipe;
saddle 3: a member, integral to the press, which produces a bending deformation of the pipe, pressing the same between two supports;
supports 4: two crib-like constraints arranged at a given distance from each other and which, suitably hinged, react against the force imparted by the saddle against the pipe.

The method is articulated in a series of steps and expedients all contributing to obtaining a good quality product, fully complying with the bending and circularity angular tolerances of the straight section of the pipe.
Firstly, the bending preparation is performed after moving the pipe from the storage area to the cold-bending machinery area.
During such operation, after identifying the diameter "d" of the pipe and the final length of the bending "I", that is the piece of the pipe that shall acquire the designed bending, the pipe is prepared considering a greater length with respect to that of the bending. Different situations may arise: in case of pipes bendable in an easier manner or without particular requirements regarding the area and/or the shape of the straight section of the pipe, the bending shall be performed "empty" and the total length "l_t" of the pipe to be subjected to processing shall be given by the following formula: $l_{t} = 2 d + 1 + 2 d$
The two pipe sections having a length equivalent to 2d, arranged at the two sides of the portion to be bent (Fig. 2a), shall be arranged on the supports or cribs of the equipment. The support pieces are equally suitable should the length be comprised in the range of 1.5d ÷ 2.5d.
The empty bending inevitably implies the necking of the straight section of the pipe. In case it is required to eliminate or at least limit the reduction of the section of the pipe to the utmost, it is convenient to perform the bending after filling the pipe itself with water or, more generally, with an incompressible fluid at suitable pressure up to about 50 bars). In this case, the preparation of the pipe (see fig. 2b) also consists in providing for - alongside the pieces of pipe to be held on the constraints as in the previous case - a closed bottom for each end provided with access for filling with water and the subsequent pressurisation and welded against the pipe or integrated thereto by means of glass-coupling or the like. The overall length of the pipe shall be almost the same as the previous case, but the formula suitable for calculation thereof shall be as follows: $l_{t} = 0.5 d + 1.5 d + 1 + 1.5 d + 0, 5 d$

wherein the term 0.5 d represents the length of the bottoms, while the term 1.5d represents that of the support portions. Also in this case, the support portions may have their length variable in a given range, ad for example 1.5d ÷ 2.2d.
It is clear that both the bottoms and the support portions represent a trimming of the material. In case of pipes made of alloyed steel, typically chromium or stainless steel, these trimmings represent a substantial cost: it is therefore advantageous to weld, or integrally join to the useable section of the pipe to be bent both the support pieces and the bottoms to be made of common steel.
The subsequent operation regards marking. This operation graphically identifies both the length of the section to be bent, and the straight support portions, as resulted from the previously outlined calculation
After preparing and marking the pipe the following step regards the actual bending. It is appropriate to state that the cold-bending method, subject of the present invention, is particularly suitable for radius of curvature not smaller than 4d, with "d" still representing the diameter of the pipe to be bent. Furthermore, the number of passes, according to the previously outlined definition, shall depend both on the radius of curvature, and the s/d ratio of the pipe, "s" being the thickness of the pipe. More in particular, the number of passes increases proportionally to the reduction of the radius of curvature and to the reduction of the s/d ratio. For example, in case of the radius of curvature a good correlation is represented in the following table: Table A

Radius of curvature N. of passes

7d 1.5

5d 2

4d 3
Therefore, the bending operation is made up of a number of passes to be determined according to the correlation outlined above, where each pass is represented by a plurality of pressings and movement of the pipe depending on the length of the section to be bent. In practice, the pipe is positioned on the supports with the saddle of the press in proximity to the beginning of the section to be bent and the first pressing is performed. Thus, the pipe is moved forward for a suitable section and a second pressing is performed. These operations are performed in an alternating manner until the pressing is performed on the end portion of the length of the pipe to be bent. Given that the pies in question are usually of medium and large thickness, the movement and positioning thereof is performed by means of synchronised cranes, manually or automatically. At the end of the pass the pipe is once again positioned below the press at its initial section to be bent and the subsequent pass is started.
The control of the angle underlying the radius of curvature "α", easily obtainable from the ratio between the length to be bent and the radius of curvature, is performed at the end of the pass immediately preceding the final one or according to a constantly updated experimental database, when the operators perceive to be in proximity to the final value of the angle to be achieved. The control is performed by means of shaped templates or similar equipment. Regarding the information provided hereinafter, it is preferable that the achieved bending be slightly greater than the target value of the project: the Δα must be advantageously comprised in the range between 0.5° and 7°, using greater values in case of small radius of curvature and/or or high thicknesses of the pipe. The final control of the angle is then performed by means of suitable equipment such as inclinometers, compasses and the like. Advantageously, the final control of the angle may be automated and displayed on a monitor. For such purpose, it is sufficient to use suitable inclinometers, for example with double axis having measurement and control channels, suitably positioned for an accurate measurement, a data acquisition card, a suitably configured computer, a cabling dimensioned to limit the fragility of the system during use and the interferences and, lastly, a control software, such as for example a Labview platform.
After the bending operation, there follows the calibrating operation. This operation - performed on a different mould, horizontal, with respect to the vertical bending press - is used to obtain a recovery of the straight section of the pipe within the geometric and shape tolerances.
Further operations of the bending method regard cutting the end pieces (and the possible bottoms) and the bevelling. The cutting of the pieces occurs after having graphically marked the cutting surfaces and having subjected the bent pipe to possible chemical treatments, if required. Lastly, the bevelling operation consists in performing a bevelling at the end of the bent pipe in such a manner to prepare a possible welding joint.

Claims

Method for cold-bending of pipes of medium and large diameter characterised by a step of preparing a pipe to be bent, a step for marking the significant dimensions in the method, an actual bending step, performed by a press provided with a curvilinear saddle and with two supports or cribs, a control of the bending parameters obtained and a step for calibrating the straight section of the bent pipe.
Method according to claim 1, wherein said step of preparing the bending consists in defining a length of the pipe to be bent greater than the actual one, with the aim of providing for two portions for supporting the pipe on two cribs spaced from each other and underlying the press and the relative saddle.
Method according to claim 2, wherein the bending shall be performed "empty" and the total length of the pipe to be subjected to processing shall be increased with respect to the length of the actual bending of two side support portions having length comprised between 1.5 and 2.5 times the diameter of the pipe.
Method according to claim 2, where the bending shall be performed after filling the pipe with an incompressible fluid at a suitable pressure and the total length of the pipe to be subjected to processing shall be increased with respect to the actual bending length of two side support portions with length comprised between 1.5 and 2.2 times the diameter of the pipe and twice the closed bottoms, each length equivalent to about half the diameter of the pipe.
Method according to claim 4, where both the support pieces and the bottoms are integrally joined to the useable section of the pipe to be bent.
Method according to one of the preceding claims, characterised in that said bending operation comprises at least one pass, in turn made up of a plurality of pressings and movements of the pipe depending on the length of the section to be bent, where these two operations are performed in an alternating manner until the pressing is performed on the end portion of the length of the pipe to be bent.
Method according to claim 6, where the movement and positioning of the pipe is performed by means of two synchronised cranes, manually or automatically.
Method according to claim 6 or 7, where the intermediate control of the angle underlying the radius of curvature is performed at the end of the first pass or the pass immediately preceding the final one or, in proximity to the final value of the angle to be achieved, by means of shaped templates or similar equipment.
Method according to one of the preceding claims, where the final control of the angle is performed by means of suitable equipment such as inclinometers, compasses and the like.
Method according to claim 9, where said final control is automated and displayed on a monitor, using suitable inclinometers, for example of the double axis type with measuring and control channels, a data acquisition card, a computer, a cabling and control software.
Method according to one of the preceding claims, where said calibrating operation, performed on a different mould - horizontal with respect to the vertical bending press, allows obtaining a recovery of the straight section of the pipe within the geometric and shape tolerances.
Method according to one of the preceding claims, where further operations of the bending method are cutting the end pieces (and of possible bottoms) and bevelling.
Method according to claim 12, where the cutting of the pieces occurs after having graphically marked the cutting surfaces and after having subjected the bent pipe to possible heat treatment.
Method according to claim 12 or 13, where said bevelling operation consists in performing a bevelling at the end of the bent pipe in such a manner to prepare a possible welding joint.