PIPING SYSTEM FABRICATION AND SURVEY: A SHIPBUILDING PERSPECTIVE

INTRODUCTION
1. If one recounts the development of machines from steam engine to modern day gas turbine engine, one would observe, that all machines have their weakness in the unannounced bleeding of their veins and arteries i.e. it’s piping network. However piping industry has kept pace with changes in all other shop floor production methods. The new industry tool in the hands of a system manufacturer or repairer/surveyor, to reduce downtime and over-maintenance of marine equipment, by means of a robust and reliable piping system is elucidated in the succeeding paragraphs.

TRENDS IN PIPE MANUFACTURING
2. The manufacturing of a piping system consists of four major sub processes namely; bending, flanging, branching and welding/brazing. The automation of these sub-processes that has occurred in shipyards, to help in achieving consistency in quality of production, enhances speed and reduces the inventory of ferrous and non ferrous castings is amplified below:

3. Pipe Bending. The manual intensive process used for pipe bending, by filling the entire pipe with sand tightly and bending it to the required curvature by cold bending, used to take about two man-days and in case of hot bending about four man-days. But with the help of new 100 CNC 3 X pipe bending machine, it is now a matter of few hours for a pair of skilled pipe fitters. This machine can do cold bending of steel and Cu-Ni pipes of ODs, as large as 114.3 mm along with pipe thickness of 4.85 mm. The approximate cost of such machines is about Rs. 84.3 lakh. One such commercial machine manufactured by M/s Electro-pneumatic and hydraulic India Pvt. Ltd. Mumbai, is used in the construction for new ships. The advantage of this bending machine is that, the problems of residual stress, formation of folds on completion and pin holes at pipe bends, faced with manual bending method has been reduced. Further, pipes of large diameter and thickness do not require an additional inventory of pre-casted elbow pieces to be kept standby for use at bends.

4. Pipe Flanging. The making of flanges for any pipe is a time consuming process entailing, flange-casting, machining and erection of template/ jig followed by welding of flange to the pipe. A minimum of three man-days were required for making a flanged pipe. But technology infusion by means of the new T drill F-200 flanging machine, has shrunk the process time to less than an hour by deploying two skilled machinists only. This machine can make flanges up to 219.1 mm OD, the maximum thickness allowable for cold flanging is 5 mm and 8 mm for hot flanging. The purchase price of the equipment is about Rs.1.5 crore. The aid of a flanging machine manufactured by M/s T Drill OY company of Finland, has hastened the pace of production at shipyards. The use of this machine removes the intermediate step of flange welding, flange neck was a perennial source of leakage due to weld erosion.

5. Pipe Branching. The branching of a piping system, earlier entailed procurement of pre-casted T-pieces. But with help of new TEC-150 branching machine, the entire inventory for casted pipe T pieces can be done away with. The latest machine can make branching holes by extrusion and welding for pipes up to 420 mm OD for ferrous as well as non ferrous application. The machine manufactured by M/s T Drill OY Company of Finland costing about Rs. 2.0 Crore, is the latest addition to production floor for the new construction activity at shipyards. The use of this machine removes the use of a casting prone to pin holes due to porosity in casts and bimetallic corrosion due to difference in system pipe and T piece metallurgy.

TRENDS IN PIPE SURVEY
6. The manufacturing of pipes is just a beginning in the long journey of fault free machinery initiatives. The main part of the entire endeavor for enhanced reliability is early fault detection in post manufacturing stage onboard ships. In actual terms the process of fault detection is much more difficult since it is akin to ‘finding a dead person’s disease, without actually digging his grave’. The cutting edge technology for marine application in ILI (In Line Inspection) use non-intrusive methods of NDE (Non-destructive examination). The site practices for survey are influenced by benchmarked standards of API(American Petroleum Institute), ASTM (American Society for Testing Methods), ABS (American Bureau of shipping) and ASME (American Society for Mechanical Engineer). The three major NDE methods, practically usable in our own Indian Naval operating domain are discussed below:

7. Acoustic Emission Test. Acoustic Emission Test (AET) is a process whereby an elastic wave, in the range of ultrasound usually between 20 KHz and 1 MHz, is generated by the rapid release of energy from the source within a material. The elastic wave propagates through the solid to the surface, where it can be recorded by one or more sensors. The sensor is a transducer that converts the mechanical wave into an electrical signal. In this way information about the existence and location of possible sources is obtained. AE analysis is a useful method for the investigation of local damage in materials. One of the advantages compared to other ND techniques is its ability to observe damage processes during the entire load history without any disturbance to the specimen. The only disadvantage of AE is that it can only estimate the damage in the material and approximate life of the components qualitatively, not quantitatively. Moreover, service environments are generally very noisy, and the AE signals are usually very weak. Thus, signal discrimination and noise reduction are very difficult, which is extremely important for successful AE applications. This application is aptly suited for critical hydraulic and pneumatic systems (High pressure) used in main propulsion, steering and weapon controls, where pre-emptive change of possible cause of pipe failure is of paramount significance for operational readiness. A majority of HP pipes in pneumatic and hydraulic systems are prone to failure due to stress in pipes.

8. Magnetic Flux Leakage. Magnetic flux leakage (MFL) is a magnetic method of non-destructive testing that is used to detect corrosion and pitting in steel structures, most commonly in pipelines and storage tanks. The basic principle is that a powerful magnet is used to magnetize the steel. At places where there is corrosion or missing metal, the magnetic field "leaks" from the steel. In an MFL tool, a magnetic detector is placed between the poles of the magnet to detect the leakage field. Analysts interpret the chart recording of the leakage field to identify damaged areas and to estimate the depth of metal loss. Figure below illustrates the basic principle of the MFL method. A magnet mounted on a carriage induces a strong magnetic field in the plate or pipe wall. In the presence of a corrosion pit, a magnetic flux leakage field forms outside the plate or pipe wall. An array of sensors is positioned between the magnet poles to detect this flux leakage. The sensors are usually Hall Effect devices or coils; there are advantages and limitations with either type of sensor. The use of MFL technique is a better tool over visual examination, for gauging health of main fuel supply trunking and the fire-main system/ ring-main in alleyways during NR/MR of a ship. The main trunking of fuel and fire-main lines are critical pipes most prone to internal corrosion/ metal erosion.

9. Ultrasonic testing. Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements and material characterization. A typical UT inspection system consists of several functional units, such as the receiver, transducer, and display devices. A receiver is an electronic device that can produce high voltage electrical pulses. Driven by the receiver, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen.. Figure below illustrates a simple UT set-up using the pulse-echo principle and a twin crystal probe. In this configuration one crystal acts as transmitter and the other as the receiver. The transmitter is isolated from the receiving circuits so that the A-scan display is freed from the presence of a transmission signal. As a result the transmission pulse does not obscure the first back wall echo when testing relatively thin areas of plate or pipe. Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited are that, It is sensitive to both surface and subsurface discontinuities, the depth of penetration for flaw detection or measurement is superior to other NDT methods.

CONCLUSION
10। The plumbing industry outside shipyards is changing and so are the shipyard’s, engaged in construction of new ships for Indian Navy. Our Naval Dockyards, who are the custodians of maintenance for the 21st generation ship/s with new methods of piping fabrication, have to generate domain awareness towards this change in method of piping erection/ survey. An audit of our ability to augment the skill-set of the apprentice/ artificers, to keep pace with new methods of pipe manufacture/ inspection, in tandem with efforts to establish shore installations with new generation machines/ tools for piping departments of ship repair yard, is the need of the hour.
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Acknowledgement :
1. Comments on AET by Cdr S Mujumdar Ex Senior Manager at Systems Department Naval Dockyard Mumbai.
2. Comments on piping manufacture by Mr. M RoyChowhary; Consultant to M/s GRSE.
3.Comments on piping trends in marine industry by Cdr (Retd) A Bandhopadhyay at M/s ABG Shipyard.

References:
1.Detection of Mechanical Damage using the Magnetic Flux Leakage Technique by Mr. L. Clapham, Mr. V. Babbar and Mr. James Byrne. Queen’s University, Kingston, Ontario, Canada.
2.The Pipeline Defect Assessment Manual by Mr. Andrew Cosham & Mr. Phil.Hopkins at Proceedings Of IPC-2002: International Pipeline Conference 29 September - 3 October, 2002; Calgary, Alberta, Canada.
3. A Comparison of the Magnetic Flux Leakage and Ultrasonic Methods in the Detection and Measurement of Corrosion Pitting in Ferrous Plate and Pipe by Mr. J. C. Drury I.Eng. M.Inst. NDT.
4. American Petroleum Institute; Standards 570, 574, 578and 579.
5. American Society for Testing Methods; NDT Standards volume 03.03.
6. American Bureau of Shipping HSNC rules from section 4-6-2 for metallic pipes.
7. American Society for Mechanical Engineer; Spec no B31.3 for process piping and Section V for NDE.
8. QAP of Plumbing systems for New construction ships at WOT(Kol).