FIELD OF THE INVENTION;
The proposed invention relates to a method of producing tubular T joints or bifurcate used in connecting the pressure parts in a fossil fuel boiler power plant. Specifying the proposed invention relates to produce the bifurcates by hydroforming in a die.
BACKGROUND OF THE INVENTION:
In the power station boiler plants, tubular welded joints are made which are interconnected between the boiler wall tubes.
The Y connection or bifurcate between tube to tube is done in existing practice by drilling in the center of the U tube at one end and pulling out the drilled end and connecting the second tube and welding together the U shaped tube manually. This manual method of welding to prepare Y connection involve high proportion of handling and is time consuming.
Casted bifurcate also does not end with good properties as desired.
Japanese Patent JP1100756 by FURUKAWA HIROFUMI et al describes a process of preparing bifurcate tube having high dimensional precision without developing a cavity defect by hot hydrostatic pressing after casting a cast steel material by a lost wax precision casting (investment casting) method.
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The said process involve high cost with separate manufacturing units for precision casting and hydrostatic pressing.
The industrial production of bifurcate therefore was in search for alternative of lessening fabrication time by welding and cost effective method since long.
The present invention in this field is aimed to solve the above difficulties of prior art by developing a simple cost effective and time saving method of producing bifurcate boiler tubes.
DESCRIPTION OF THE INVENTION:
According to main objective of the invention it is proposed to produce bifurcates tubes of boiler by preparing the U tubes in a conventional way and hydroforming the U tubes by pressing the tubes in a die to produce bifurcates/ Y connections.
Another objective of the invention is to propose for alternative process for welding of bifurcates on the boiler wall tubes without any difficulty of distortion or any leakage, on joining the produced bifurcate/Y connection between the boiler tubes.
According to the invention it is proposed to produce bifurcate which enables the U shaped bend tube to be converted into Y shaped tube by hydroforming i.e producing the Y shaped tube by forcing the material by means of a punch against a die partly filled with hydraulic fluid.
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Firstly a bend tube is prepared in U shape by conventional tube forming method and the tube blank is pressurized internally using high pressure water to form the bifurcate in a single hydroforming step. The said pressing operation subject the bend tube to axial compressive stresses from the action of axial punches. The hydroformed Y shape connections are connected with wall tubes of boiler by butt joints.
Thus in the proposed development the multifold operations i.e drilling in the center U shape tube and then pulling out the drilled end are replaced, resulting shortened time of manufacture with cost saving technical features, as the cost of hydroforming is less than conventional process of steel casting or precision lost wax method casting.
SUMMARY OF THE INVEVNTION:
Distinguishing features of the present hydroforming method of producing bifurcates are:
• Die is closed before introducing fluid
• Fluid is the primary forming medium
• Cross section of the U tube expands due to material flow.
• Relatively high pressure (usually grater than 20,000 psi) is maintained
during hydroforming.
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According to the invention there is provided a method of producing boiler bifurcate tubes comprising the steps of bending a straight tube in a conventional way to form a U tube blank, characterized in that the U tube blank is placed in a modified die on coating the blank with a lubricant and sealing the ends of the die by axial counter punch means and hydroforming the blank by pressuring it internally by means of a punch on the die partly filled with water to form a bifurcate/Y connection with protrusion.
According to the invention there is also provided a boiler bifurcate tube to be connected in boiler tube wall comprising of Y shape tube connection consisting of multiple open ended U tubes with a protrusion being hydroformed in a die by application of internal pressure through a punch on the die partly filled with water and through a counter punch sealing the end of the die.
The present invention will be better understood from the following description with reference to the accompanying drawings in which
Figure 1 shows the existing method of producing Y connection by drilling hole in U shaped formed tube and welding a straight tube after fixing in the pulled out hole.
Figure 2 shows the bifurcate produced by investment casting according to Japanese patent no. JP 11000756.
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Figure 3 shows the hydroformed bifurcate tube of Y shape according to the present invention.
Figure 4 shows the hydroforming die through which a bifurcate is formed according to the present invention.
Figure 5 shows the hydroforming die through which two bifurcates are formed simultaneously according to the present invention.
In Figure 1 a hole (H) is drilled on the preformed tube (3) and a Joint (4) is welded on the tube (3) to form Y connection.
Figure 2 shows the bifurcate (B) produced through investment casting according to Japanese Patent No. 11000756.
Figure 3 represents the developed bifurcate according to the invention in which the bend U tube is formed by conventional method of bending followed by hydroforming to form the bifurcate/Y connection in one operation thus eliminating operation of drilling and pulling out hole and welding a joint. The bifurcate protrusion is drilled out after hydroforming and finally set with straight wall tube through butt joint.
Figures 4 and 5 show the modified dies for single and double bifurcates. Through these dies the bifurcates are produced by hydroforming the formed U tube (3) by forcing the U tube by means of a punch against a partly filled with hydraulic fluid
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on the die (1). During hydroforming a bulge of the tube material (3) occurs due to plastic flow and a considerable amount of U tube material is pushed into the die cavity, to form the bifurcate/Y connection with protrusion (5) as a single piece resulted from the flow of tube material during hydroforming operation. During hydroforming higher expansion ratio near curved ends of long hydroformed members as illustrated in Figures 4 and 5 to draw out branch in Y joints, are maintained to maintain uniform cross-sectional shape continuously along its curved end. During tube end feeding for hydroforming the magnitude of tube bulging due to plastic flow is maintained equal to the amount of material of produced protrusion in the Y shaped bifurcate.The bifurcate tube hydroforming operation involves a controlled application of internal pressure and axial feed of tube material from the tube ends into the die (1). A means of counter punch to support the protrusion (branch) during hydroforming Y shapes is also maintained. This counterpunch means allow the forming of protrusion with a greater length and with uniform thickness distribution during hydroforming operation.
During axial feeding of tube, the feed pressure (Pa) for round sections is maintained according to the following equation:
Pa = (2 x 0.85 UTS x 1)1 (D-2T) Where UTS is the ultimate tensile stress (Mpa), D is tube diameter which is hydroformed (mm), T thickness (mm).
Axial pressure maintained by counter punches at the end of the die is same as that of feed pressure Pa.
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On forming the U shaped tube blank the blank is coated with a solid film lubricant, placing the said coated blank into the modified die accordingly made to accommodate the blank U tubes and sealing the ends of die by axial punches and pressurising it internally using high-pressure water while subjecting it to axial compressive stresses from the action of the axial punches. A counterpunch usually provides compressive stresses against the growing protrusion to delay bursting at the protrusion tip, thus increasing the protrusion heights.
The internal pressure (Pi) maintained by punch is given according to the following relationship:
Pi= 2/V3 flow stress (In rb/(rb-t))
Where rb is the corner radius, t thickness of tube
The hydroforming is maintained in automatic mode through controlled maintenance of pressure on the die and amount of tube end feeding dependent on wall thickness.
The process parameters of the hydroforming operation is maintained such that Protrusion height (max) = Diameter of the tube= Feeding length of U tube.
On completion of forming Y connection by hydro-forming the open ends of U tubes are coupled with boiler wall tubes and the bifurcate (5) is drilled and joined with boiled wall tube by butt joints.
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ADVANTAGES OF THE INVENTION:
• The transformation of blank U shape tube to Y shape bifurcate tube is
done by a single step hydroforming operation, instead of drilling in the
center of the U tube and then pulling out in two step operation as was
considered out in the existing practice.
• An enormous time saving operation is carried by hydroforming operation
in a modified die in the order of 10% than that of the existing art.
• The cost of hydroforming is less than that of existing art of drilling and
welding wherein the rejection rate is more than 40%.
• The cost of the present invention through hydroforming is much less
compared to the Japanese patent as herein described, in which
investment casting and hydrostatic pressure maintaining equipments are
costly and time consuming than the present invention.
Though the invention has been illustrated with a particular embodiment, it should not be read and construed in a restrictive manner as various adaptation, alterations and modifications are possible within the scope and limit of the invention as defined in the encompassed appended claims.

WE CLAIM
1. A method of producing boiler bifurcate tubes comprising the steps of :
bending a straight tube in a conventional way yo form a U tube blank (3);
coating the blank (3) with a solid film lubricant;
placing the said coated blank into the modified die made to accomodate
the blank U tubes;
sealing the ends of die by axial punches;
characterised in that,
bifurcates/y connection with protrusion (5) are produced through modified
dies of single and double bifurcates by hydroforming the formed U tube (3)
by forcing the U tube by means of a punch against a partly filled with hydraulic
fluid on the die (1) when the bifurcate tube hydroforming operation involves a
controlled application of internal pressure and axial feed of tube material
from the tube ends into the die (1) wherein the feed pressure ( Pa ) for round
sections is maintained according to following relationship
Pa = (2 x 0.85 UTS xT)/ (D-2T)
Where UTS is the ultimate tensile stress (Mpa)
D is tube diameter which is hydroformed (mm)
T thickness (mm), and internal pressure (Pi)
maintained by the punch according to the relationship
Pi = 2/V3 flow stress (In rb/(rb-t))
Where rb is the corner radius, t thickness of tube
2. A method of producing bifurcate as claimed in claims 1 and 2 wherein an axial counter punch means is provided on the die to seal the ends of the die in which axial pressure of the counter punch is maintained equal to the feed pressure Pa.

3. A boiler bifurcate tube to be connected in boiler tube wall comprising of Y shape tube connection consisting of a multiple open ended U tubes (3) with a protrusion (5) being hydroformed in a die (1) by application of internal pressure through a punch on the die partly filled with water and through a counter punch sealing the ends of the die.
4. A boiler bifurcate tube as claimed in claim 5 wherein the process parameters during hydroforming to produce the bifurcate tube are maintained such that Protrusion height (max) = Diameter of the tube = Feeding length of the U tube.
5. A boiler bifurcate tube as claimed in claim 5 and 6 wherein the protrusion (5) is drilled and butt jointed with the straight boiler wall tube.
6. A modified hydroforming die to produce a bifurcate tube to be connected in boiler tube wall consisting of performed groove in the die to accommodate the U tube blank and hydroforming the blank according to method of claim 1.