Monday, 31 December 2018

What is meant by pipe fabrication?

Pipelines are used to transport fluids, including vapors, gases, slurries and powders which flow through pipes at various pressures and temperatures. The transmission of these materials leads to intense stresses and strains and this demands the highest possible performance from the pipe. All pipework must be designed and fabricated in a way that ensures the safety of plant operators, the plant, the public and the environment. The fabricator who is responsible for the quality of the pipework fabrication must carry out work in accordance with all relevant standards and specifications. One mistake may lead to damage of millions of dollars, personal injury and in serious cases even death. Excellence in materials and quality of work is what is expected from welded pipe systems. Because high standards are required, the cost of pipework is extremely high. There must be a high degree of quality control when high-pressure pipe systems are being fabricated to ensure that they meet the specifications of the relevant codes. 
The inspecting authority may be the machinery inspection branch of the state or territory government, the insurance company underwriting the job or the representatives of the client for whom the pipework is being fabricated. The steps involved in the quality control program are: 1. Approval of the design of the pipe system. 2. Approval of the fabricator who may have to prove that there is adequate equipment and expert personnel to successfully meet the requirements of the relevant code(s). 3. Frequent checks to ensure that the specified material is being used. 4. Approval of welding procedures and regular inspections to ensure that these procedures are being followed. 5. Qualification of welders who are tested to ensure that they are capable of carrying out the approved procedures successfully. 6. Non-destructive testing as required by the code(s). 7. Post-weld heat treatment as required by the code(s). 8. Final hydrostatic testing or alternative testing method. Fabricators are responsible for the quality of the work they perform and for the coordination of all inspection and testing procedures as required by the relevant code(s). Pipe is specified by stating its nominal size (NS), which is only approximate. Nominal size is neither the inside nor the outside diameter and it is necessary to use standard tables or manufacturers’ tables to ascertain these two dimensions exactly. Pipe thicknesses are often described as ‘standard’ (STD), ‘extra strong’ (XS) and ‘double extra strong’ (XXS). However, most manufacturers have adopted the American piping code classification which classifies pipes into a schedule system. The mass of the pipe is referred to as the schedule of pipe, and may vary from Schedule 10 to Schedule 160. As the schedule number increases so does the wall thickness, and as a consequence the inside diameter reduces. The outside diameters remain constant enabling standardization of pipe brackets or threading. Pipes are sometimes referred to as tubes. The primary difference is in how they are measured, Boiler tubes and tubing are known by their outside diameter and their wall thickness. Socket-welded fittings allow for easy fabrication and fit-up of piping. Square end preparation of the pipe is all that is required and fillet welds are used, preferably made in two passes. Lower levels of operator skill are required to assemble and weld socket joints than are required for butt joints. The socket-welding system is preferable to the screwed system as it is simpler to assemble and avoids the possibility of leakage which sometimes arises in screwed joints.

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