FIELD OF THE INVENTION

The present invention generally relates to measurement of weld-induced distortion of large cylindrical shells with high thicknesses which are used in pressure vessel fabrication. During the welded fabrication, the shells are experiencing axis shift distortion in different segments of cylindrical shells during welding. More particularly, the invention relates to an axis shift distortion measurement system to measure the distortion induced during circumferential welding on a large thick-walled circular shell component of pressure vessel.
BACKGROUND OF THE INVENTION
Large size thick walled cylindrical shells are welded in the pressure vessel fabrication. The welding is carried out using semi-mechanized welding processes. The shells are circumferentially butt-welded to construct the long shells. Due to the welding induced shrinkage and associated time lag in the welding process, the welded shells experience axis shift distortion. The axis shift experienced between two shells along the length of welding needs to be measured for the quality control purposes. The proper measurement of distortion leads to proper distortion correction and control techniques.
There are known distortion measurement systems, which are not applicable readily to the large size thick walled shells. The reasons are attributed to the difficulty involved in the measurement such as, time consuming, more number of persons required, data cannot be automatically stored, online measurement during welding is not possible and certain techniques are not suitable for shop floor applications, which are only suitable to laboratory conditions.

PRIOR ART SEARCH
US 6,594,555 B2- teaches a method for predicting an amount of distortion experienced by turbine components due to the replacement of an upper turbine portion, such as an upper casing, is disclosed. This method requires the definition of reference for the distortion measurement, which is practically difficult in case of large shells. Moreover, distortion is predicted manually and distortion is measured only in left and right direction alignments. This method is not suitable as large shells are experiencing axis shift in all the directions around the shell circumference.
US patent 3,475,825- describes an apparatus for checking distortion of a compound reactor-fuel element by probing the space between inner and outer sections of the fuel element and the exterior of the outer section. This is not applicable to the large diameter and longer shells.
US 6,253,697 B1 - discloses a process for distortion measurement of ship structure using laser-based sensors. This method is applicable to the circular pipe sections and recording of the distortion data during welding (online) is not possible.
US Patent 3448607- describes a method for evaluating quantities relative to the distortion of a nuclear fuel assembly, in particular the bow and the twist of the assembly. As per the method, the assembly is immersed in the water and based on the water level, distortion is evaluated using the camera at least for one surface. This is not possible to measure distortion during welding also; the material handling is very difficult for the large structures such as boiler shells.

US 8,345,333 B2 - teaches displacement/ distortion measuring method and apparatus using line scanner alternative to the CCD camera. The apparatus is costly in terms of number of scanners required around the circumference of the large shell component. In addition, the distortion induce during the welding process is transient and analysing different segment of the large circumference using ttie line scanner is not the better option.
All of these prior art inventions are related to the distortion measurement methodology and distortion measurement apparatus, which are not generally suitable for the large thick walled shells, which are circumferentially welded. In most of the prior art, the measured distortion data can not be stored for the future analysis. Similarly, the known systems could not be employed during the welding process, as it requires high frequency measurement systems since the welding induced distortions are transient in nature.
Hence, the measurement system for the thick walled large shells during circumferential welding is not readily available.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an axis shift distortion measurement system to measure the distortion induced during circumferential welding on a large thick-walled circular shell component of pressure vessel.
Another object of the invention is to propose, an axis shift distortion measurement system to measure the distortion induced during circumferential welding on a large thick-walled circular shell component of pressure vessel, which is enabled to measure the distortion data during welding using displacement sensors positioned at eight directions around the circumference of the shell.

A further object of the invention is to propose an axis shift distortion measurement system to measure the distortion induced during circumferential welding on a large thick-walled circular shell component of pressure vessel, which is capable to process the distortion data in real time during the measurement and recording the processed data for future reference.
SUMMARY OF THE INVENTION
Accordingly, there is provided an axis shift distortion measurement system to measure the distortion induced during Circumferential welding on a large thick-walled circular shell component of pressure vessel. It is known that axis shift distortion is experienced in the large thick walled shells. The measurement of distortion at the one end of the shells during drcumferentJal welding is to be measured. Large thick walled shells are positioned on the positioner rolls for welding. The drcumferentJal butt-welding of shells are carried out either by completely mechanized or semi mechanized welding. The axis shift distortion measurement system of the invention is positioned at the one end of the shell where the distortion is to be measured. The measurement system consists of eight displacement sensors in eight directions around the circumference of the shell connected to the frame endrding the shell. The frame is supported by the supporting members, which are resting on the movable table.
The table houses the data logger set up including the on board computer and sensor busters, connector pins etc. The radial dearance between the frame and the shell to be welded can be adjusted. Similarly, the distance between the sensors and the shell can be adjusted. In this way, different thick and diameter shells can be accommodated in the measurement system. The whole circumference of the shell is divided into eight numbers of segmental welding and each sensor is located on each segments of welding. The displacement sensors are positioned in such a way that it can read both the positive and

negative movements of the shell during welding. The displacement sensors are connected to the data logging system and data logger is connected to the onboard computer for the data viewing requirements. The measurements can be captured simultaneously in all the eight sensors and 'the displacements are transiently observed.
Huge quantum of welding is involved, as the thickness of the shell is very large. During each layer of welding, the axis shift is increasing incrementally. The incremental distortion is having its effect on displacement in all the segments, which are captured using the data logger. The whole assembly including frame, support members, and fixed table is movable for which rolls are provided on the bottom of the table. The table arrangement can be made fixed at particular position using the locating pins provided in-line with rolls so that during distortion measurement table acts as a fixed reference.
According to the invention, a computer controlled distortion measurement system is used for measurement and recording including course correction after fabrication. The axis shift distortion experienced due to the circumferential welding of large shells are captured using displacement sensors at eight locations around the shell at one end of the shell. The distortions at different segments are displayed digitally using the computer during welding (online) and can be preserved for future analysis. This new system requires only one operator during sensor setting and for process monitoring.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - A schematic view of the axis shift distortion experienced in the large shells due to welding.
Figure 2 - A schematic view of positioning of the inventive axis shift distortion measurement system during welding of large shells.

Figure 3 - A schematic view of the axis shift distortion measurement system according to the invention
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
According to the invention, there is provided a measurement method and measurement system by which a computer controlled measurement of axis shift distortion is carried out for the large cylindrical shells during welding. The distortion measurement system consists of a plurality of displacement sensors, a frame with support members supporting the sensors around the circumference of the shell to be welded, and a movable table a data acquisition system with an onboard computer for the data display and analysis.
The cylindrical shells to be welded is positioned on the supporting rollers of welding arrangement with tack-welded condition. The axis shift distortion measurement system is positioned at a first end of the cylindrical shells. The positioning is done in such a way that the sensors are having contact with the cylindrical shell. The table containing the sensors and supporting frame is fixed steadily without any movement or vibration using a multiple locating pins. The sensor and data acquisition controls are put on before the welding is started. During welding and associated shrinkage, the axis shift distortion experienced in outer circumference of the shell causing displacement disturbance, which is captured by the respective sensors in different locations around the shell. The sensor readings are stored by the computer for further analysis. The constructional details pertaining to the axis shift distortion measurement system is explained with reference to the accompanying drawings.
The following are the different parts that comprise the axis shift distortion measurement system.

1) Cylindrical shell 1
2) Cylindrical shell 2
3) Load support
4) Fixed rollers
5) Weld metal
6) Encircling frame
7) Displacement sensors
8) Support members
9) Movable Table
10) Data acquisition system
11) Connecting cable
The typical axis shift distortion experienced in the large shells is shown in Figure.1. The large diameter shells (1&2) are welded using mechanized or semi-mechanized processes. The shells are positioned on Load Supports (3). The shells are tack welded aligning shell axes of both shells co-axial and collinear. The shells can be rotated during welding using the fixed rollers (4). The weld metal (5) deposited during the circumferential welding, causes both weld metal and base metal shrinkage leading to axis shift distortion.
To measure this axis shift distortion, a measurement system is herein proposed. The positioning of the system is shown in Figure.2. The displacement measurement system is shown in Figure.3.The system is positioned in such a way that the enriching frame (6) has free annular space around the cylindrical shells (1&2) a plurality of displacement sensors (7) positioned to establish a full

contact. The support members (8) are provide support to the frame (6) to ensure a rigid positioning of the displacement sensors (7). The whole assembly rests on a movable table (9). The table (9) has a housing for a data acquisition system (10). The height of the table (9) can be adjusted according to the support level. The data acquisition system (10) contains a data logger and computer. The displacement sensors (7) are connected to the data acquisition system (10) using an optical cable (11). The axis shift distortion is experienced at the end of the large shell (2) and is sensed by the displacement sensors (7). Finally, the computer available in the data acquisition system (10) displays the distortion reading pertaining to different segments around the shell (2).

WE CLAIM :
1. An axis shift distortion measurement system to measure the distortion
induced during circumferential welding on a large thick-walled circular
shell component of pressure vessel, comprising :
a frame member (6) enriching a first and a second cylindrical shell (1 & 2) to be welded in an welding arrangement, the frame member (6) forming an annular free space around the shells;
a plurality of displacement sensors (7) disposed around the shells (1 & 2);
multiple support members (8) supporting the frame member (6) and rigidly locating the sensors (7) to establish a full contact around the shells (1 & 2);
a movable table (9) accommodating the assembly of sensors and frame (6 & 7), the movable table (9) housing a data acquisition device (10) having a data logger and computer apparatus;
an optical cable (11) connecting the data acquisition device (10) and the sensors (7), wherein the sensors (7) are enabled to sense an axis shift distortion experienced at the end of the large shell (2) during welding, and wherein the distortion data sensed at different segment around the shell (2) are processed and displayed in the data acquisition device (10).
2. The system as claimed in claim 1, wherein the cylindrical shells (1 & 2)
are positioned over a plurality of rollers (4) of the welding arrangement,
and wherein the measurement system is located at a first end of the
shells (1 & 2).

3. The system as claimed in claim 1, wherein the cylindrical shells (1 & 2) are tack-welded aligning shell axes of both the shells (1 & 2) co-axial and collinear.
4. The system as claimed in any of the proceeding claims, wherein the weld metal (5) deposited during the circumferential welding causes both the weld metal (5) and the base metal shrinkage leading to axis shift distortion.