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Ovality Correction Methods for Pipes
M. Balachandran
U.G student, Mechanical Engineering Saranathan College of Engineering Trichy-12, India Email: [email protected]
Abstract—Ovality nothing but deviation from roundness is a main defect in all pipe fabrication industries where heat treatment is involved. Bending in pipes alter the mechanical properties of the material. The main alteration is formation of stress concentration. Stress concentration is a condition in which high localized stress is induced due to change in shape of the element and this will be higher than yield stress of that element. It leads to fracture, crack and failure of pipe. So a pipe should undergo a heat treatment process before further process. Heat treatment refines grain size, improves mechanical properties and machinability. Our main objective is to reduce ovality of pipes during stress relieving in the furnace. A new concept is suggested to correct the ovality.
Index Terms— ovality, stress concentration, stress relieving, roundness deviation, heat treatment.
I. INTRODUCTION
Pipe bends are generally most economical means of changing directions while providing flexibility and end reactions to piping systems within the allowable limits[l].The bend section may be a source of damage during working due to internal pressure and other loads particularly where ovality and wall thickness variation exists which are introduced during manufacturing process[2-4]. The acceptability of pipe bends depends on magnitude of shape functions [5]. The cross section of the pipe bend is often assumed to be perfectively oval for numerical investigation [6-9]. Distortion in cold tubes is usually limited to the outer half of the bend where flattening occurs and described as a semi-oval section [10]. Both the shapes do not represent true cross section but are approximated. In industries the contour of pipe bend cross section are captured in their first off trail test(FOT) to study about ovality and reveals that the cross section is neither elliptical nor semi oval. Analysis in pipe bends rely on the assumptions of constant wall thickness along the contour of pipe’s cross section and no initial ovality and normally have variable wall thickness along the contour of the pipe’s cross section[11]. Bending of a curved pipe is accompanied by flattening forces and transforms initial circular cross sections into oval cross sections [12]. Pipe bending is mainly caused by the ovality [13]. Our objective is to correct the ovality in pipes by using refractory bricks and structural member.
Refractory bricks is used to make the pipe to withstand high temperature during heat treatment and structural member is used reduce the deformation as compared to the using of refractory bricks.
II. EXISTING OVALITY CORRECTION METHODS
1. HYDRAULIC JACK METHOD
The ovality was corrected using a hydraulic jack by placing the jack inside the pipe with suitable wedge fixes.
When the jack is operated the hydraulic energy is converted to mechanical force which pulls out the pipe in vertical direction. This method was commonly used in plant because of its simplicity. The whole operation was done manually including all the set up and wedge fixing.
Despite of its hazards such as oil seal break up, slipping of wedge etc this method gave good results.
ADVANTAGES:
This was simpler method of ovality correction.
There were no need of fixtures and holding devices.
DISADVANTAGES:
The maximum capacity of hydraulic used was 200 tons, but load needed to correct the ovality was 300 tons.
Since the working pressure exceeds the permissible pressure of the jack the seal failure occurs. This leads to total jack failure and thus increases the cost.
Fig.1 Hydraulic jack method
2. EXTERNAL PRESSING METHOD
In this method two stationery actuators and two movable actuators were used. The stationery actuators were used to hold the pipe, the pressure was given using two movable actuators operated by hydraulics. This method was not widely used because of formation of wrinkles, buckling and poor work holding techniques. This method of correcting ovality was not successful as much as jack method.
ADVANTAGES:
This method was simple and convenient.
The cost and working hours were less comparatively.
DISADVANTAGES:
It required fixtures and complex work holding devices.
Setting of job was complex and setting time was high.
Fig.2 External Pressing Method
ΙΙΙ. OTHER SUGGESTED METHOD
1. REFRACTORY METHOD
The refractory bricks which were used in the furnace were stacked and built inside the pipe like a wall using refractory material. This holds the pipe from bending in any direction and the heat conduction was not disturbed as well. The human working accuracy was less and not reliable. Refractory disposal and handling were time consuming. Wall may collapse due to prolonged usage of same refractory brick.
2. EDGE FILLET
The pipe’s edge is cut to suitable angle so that the ovality becomes negligible. The welding can be done in between groves of the joint. The strength of joint reduces, not all the pipes can be edge filleted due to variation.
3. EXTERNAL HYDRAULIC LIFTING
This mechanism is the combination of fork lift and universal testing machine, two forks acts as jaws and expands the pipe from inside. The power is given by hydraulic which uses the same mechanism as the hydraulic jack. Job setting is difficult, need of platform.
4. DOUBLE ACTING HYDRAULIC JACK The hydraulic jack failure occurs due to over loading. If the jack is made as double acting, the load divides as two.
And seal becomes still stronger. Manufacturing of such jack is costly.
5. GAS TRAP METHOD
The Co2 gas is filled as trap in enclosed pipe. This when heated the gas expands and treats the ovality. There is a possibility of puncturing of pipes.
REFRACTORY METHOD
The ovality correction in pipes using refractory was experimentally done. The refractory bricks which were used in the furnace were stacked and built inside the pipe like a wall. This holds the pipe from bending in any direction and the heat conduction was not disturbed as well.
Fig.3 Refractory method OBSERVATION:
Std. diameter of pipe =760 mm; Thickness =30 mm BEFORE HEAT
TREATMENT PROCESS
AFTER HEAT
TREATMENT PROCESS Diameter at both ends
762 mm
Diameter at one end 762 mm Diameter at another end 776 mm
Table.1 Dimensions of pipe before and after heat treatment
This proved beyond doubt that the Ovality can be prevented using refractory materials without affecting its heat treatment process since it was built inside the pipe.
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ADVANTAGES:
Ability to withstand high temperatures.
Ability to withstand sudden changes of temperatures.
Ability to withstand action of molten metal slag, glass, hot gases, etc.
Ability to withstand load at service conditions.
Low coefficient of thermal expansion and cost.
DISADVANTAGES:
The human working accuracy was less and not reliable.
Refractory disposal and handling were time consuming.
Wall may collapse due to prolonged usage of same refractory brick.
During crane handling, the wall may fall down.
CONCLUSION:
Due to the issue such as handling, wall collapse etc, made this method difficult to process further hence another convenient method was used.
STRUCTURAL MEMBER
The structural member was a plus section made of cast steel which was also the material of furnace pedal cast. It could withstand high temperature more than 1773 K. This material was used because it does not undergo deformation at the working temperature of the job. So this plus section can act as a support and holds the job preventing the deformation of the pipe. As the temperature needed to deform the cast steel was far higher than job (P91 pipe material) the self weight which causes the deformation was prevented by plus section. If there is no structural member section deformation occurs.
Fig.4 Structural member
Fig.5 Structural member inside the pipe S.
No.
PART NAME
MATERIAL COUNT
1 CENTRE
PIECE
CAST STEEL
GRADE 11
1
2 THREADED
PIPE
CASTE STEEL 11 4
3 NUT CAST STEEL
GRADE 11
8
4 WEDGE CAST STEEL
GRADE 11
4 Table.2 Components of Structural member
ΙV. ANALYSIS
COUPLED FIELD ANALYSIS OF P91 PIPE
The couple field analysis combines two kinds of analysis.
In this case we combine the structural and thermal analysis.
Our aim is to analyze the deformation of pipe P91 using ANSYS software.
MATERIAL:
P91 (ASME SA335 91)
Density =7850 Kg/m3
Young’s modulus =2.08*e11 N/m2
Poisson ratio =0.3 Thermal expansion coefficient =1.1e-5 / K Thermal conductivity =42.7 W/mK Specific heat =461 KJ/Kg ELEMENTS:
COUPLED FIELD, TET 10 NODE 227. This is the higher element in coupled field analysis and this element gives high accuracy.
LOADING:
Colors (blue, aqua and pink) depict the supports Uy = 0
Gravity: y = - 9. 81m/s 2
Temperature: 1323 K (whole volume) (max. temp of furnace)
Temperature film coefficient: 2864.19 W/m2 K (at outer surface)
Fig.6 Modelling of P91 pipe
Fig.7 Meshing into successive finite element
Fig.8 Deformation of pipe along X-axis
Fig.9 Deformation of pipe along Y-axis
Fig.10 Deformation along Z-axis
Fig.11 Deformation along all co-ordinate axes
Fig.12 Von Messis total mechanical strain+ thermal strain
Fig.13 Von Messis total mechanical strain RESULT:
The deformation of the pipe is analyzed using couple field analysis the results are
Minimum deformation = 0
Maximum deformation =0.007623 m = 7.62 mm.
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COUPLED FIELD ANALYSIS OF PIPE WITH STRUCTURAL MEMBER
To analyze the deformation of structural member that undergoes the temperature and self-weight.
MATERIAL:
CAST STEEL IS 4522 GRADE 11
Density =7700 Kg/m3 Young’s modulus =1.9*e11 N/m2
Poisson ratio =0.26 Thermal expansion coefficient =1.51e-5 / K Thermal conductivity =37 W/mK Specific heat =530 KJ/Kg LOADING:
Displacement at
Uy=0 (area 21,44and their opposite sides) Gravity: y = - 9. 81m/s 2
Temperature: 1323 K
Temperature film coefficient: 2864.19 W/m2 K (at outer surface)
Fig.14 Modelling of structural member inside a pipe
Fig.15 Meshing of structural member and pipe
Fig.16 Deformation along X-axis
Fig.17 Deformation along Y-axis
Fig.18 Deformation along Z-axis
Fig.19 Deformation along all co-ordinate axes
Fig.20 Von Messis total mechanical strain+ thermal strain
Fig.21 Von Messis total mechanical strain RESULT:
The deformation of the pipe is analyzed using couple field analysis the results are
Minimum deformation = 0 Maximum deformation =0.00361m = 3.61 mm.
V. CONCLUSION
Our aim is to prevent the ovality of pipes during heat treatment processes. Using structural member made of cast steel grade 11 instead of external pressing method in correcting ovality can reduce the time consumption, human effort etc. This structural member can acts as a support and holds the job preventing the deformation of the pipe. Due to its high thermal properties the cast steel can be best suited material. We have proved practically by using refractory bricks (IS 8) that can withstand high temperature of about 2000 ◦C and also we have proved analytically using couple field analysis. From this analysis we have reduced the deformation of 7.623mm to3.61mm by providing structural member.
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