INSTALLATION PROCESS ON PIPING AND FABRICATION

INSTALLATION ON PIPING AND FABRICATION

I think everyone who work and involved in the Piping, Maintenance, Fabrication and Mechanical on Piping and Fabrication must know about the whole process from the beginning what the piping system is and here we will talk start from drawing, erection, cold spring, joint alignment and etc.

Drawings

Drawings used for piping system installation may vary greatly. Often orthographic projections of the building showing several systems or single systems, depending on complexity, are used. In many cases single or multiple isometric drawings of a single system are used. These of course are not to scale but are convenient for planning, progress recording, or record keeping when required by quality programs. In all cases where prefabricated subassemblies are being erected, these drawings will have been marked up to show the locations and mark numbers of the individual subassemblies, the location and designations of field welds, and the locations and markings of hangers.

Erection Planning

Planning is vitally important in installing a piping system. Many factors must be considered, among them accessibility to the building location, coordination with other work, availability and accessibility of suitable welding and heat treatment equipment, availability and qualification of welders and welding procedures, rigging, scaffolding, and availability of terminal equipment. Each of the system components should also be carefully checked to assure correctness. Valves and other specialty items in particular should be checked to assure they are marked with flow arrows, that the handwheels or motor operators are properly oriented, and that the material to be welded is compatible with the material of the piping. Special valves for use in carbon steel systems are sometimes furnished as 5 percent chrome material, and thermowells are often not of the same chemical composition as the pipe. This may not be apparent from the drawings Such a preliminary check will indicate the need for alternate welding procedures and preclude problems later.

The location of the work and accessibility to it should be viewed. It may not be possible to install an overly long subassembly after other equipment or building structure is in place. A common practice in the power field is to have large, heavy assemblies often found in the main steam and reheat lines of large central stations erected with the structure. In other cases, a preliminary review may show interferences from an existing structure, cable trays, ducts, or other piping which are not apparent from the drawings. The locations of the terminal points on equipment should be checked to assure that they are correct. The type, size, rating, or weld preparation of the connection should be checked to assure that it will match the piping. Solutions to any problems can be devised with the designer before work starts.

The ideal way to begin erection is to start at some major piece of equipment or at a header with multiple outlets. Install the permanent hangers if possible. If these are to be welded to the structure, some prudence should be exercised, since the final location of the line may warrant some small relocation to assure that the hanger is properly oriented relative to the piping in its final position. Obviously a certain number of temporary supports will be needed. Welding of temporary supports to the building structure or to the piping itself should be avoided or used only with the approval of the responsible engineers. Variable spring and constantsupport type hangers should normally be installed with locking pins in place, assuring that they function as a rigid support during the erection cycle. Where welded attachments to the pipe are involved, it is preferred that they be installed in the shop as part of the subassembly. If possible, the major components of the system should be erected in their approximate final position prior to the start of any welding. This will reveal any unusually large discrepancies which may result from equipment mislocation, fabrication error, or tolerance accumulations. Adjustments or corrections can then be decided upon. Long, multiplane systems can absorb considerable tolerance accumulation without the need to modify any part. Short, rigid systems may not be able to accommodate any tolerance accumulation, and it may be necessary to rework one or more parts.

Cold Spring

Both the B31.1 and B31.3 Codes address cold springing in detail. Cold spring is the intentional stressing and elastic deformation of the piping system during the erection cycle to permit the system to attain more favorable reactions and stresses in the operating condition. The usual procedure is to fabricate the system dimensions short by an amount equal to some percentage of the calculated expansion value in each direction. The system is then erected with a gap at some final closure weld, equal to the ‘‘cut shorts’’ in each direction. Forces and moments are then applied to both ends as necessary to bring the final joint into alignment. Once this is done, it is usually necessary to provide anchors on both sides of the joint to preserve alignment during welding, postweld heat treatment, and final examination. When the weld is completed and the restraining forces are removed, the resulting reactions are absorbed by the terminal points, and the line is in a state of stress. During start-up the line expands as the temperature increases, and the levels of stress and terminal reactions resulting from the initial cold spring will decrease. For the 100 percent cold sprung condition, the reactions and stress will be maximum in the cold condition and theoretically zero in the hot condition. It should be borne in mind that it is very difficult to assure that a perfect cold spring has been attained and for this reason the codes do not permit full credit in the flexibility calculations. Also remember that lines operating in the creep range will ultimately attain the fully relaxed condition. Cold spring merely helps it get there faster. Cold spring was historically applied to high-temperature systems such as main steam and hot reheat lines in central power stations, but this practice is not as prevalent anymore.

For those involved with the repair of lines which have been cold sprung, or which have achieved some degree of creep, caution should be exercised when cutting into such lines since the line will be in a state of stress when cold. The line should be anchored on either side of the proposed cut to prevent a possible accident.

Joint Alignment

In aligning weld joints for field welding it may be necessary to compromise between a perfect weld fit-up and the location of the opposite (downstream) end of the assembly. The weld bevel may not be perfectly square with the longitudinal axis of the assembly. Even a 1/32-in (0.8 mm) deviation across the face of the weld bevel can result in an unacceptable deviation from the required downstream location if the joint is aligned as perfectly as possible. Often such a small gap can be tolerated in the welding. If, in order to maintain the downstream location, the gap at the joint is excessive, the joint should be disassembled, and the land filed or ground as needed to attain the required alignment of the weld joint while still maintaining the required downstream position. Flanged connections should be made up handtight so that advantage can be taken of the bolt-hole clearances to translate or rotate the assembly for better alignment of downstream connections.

Weld shrinkage of field welds may or may not be important in field assembly. In long flexible systems, they may be ignored. For more closely coupled systems, particularly those using GTAW root-pass welding, this factor should be considered. The degree of longitudinal shrinkage across a weld varies with welding process, heat input, thickness, and weld joint detail. See the section ‘‘Layout, Assembly, and Preparation for Welding.’’ In extreme cases closure pieces may be used. Here, the system is completed except for the final piece. A dummy assembly is then fabricated in place and the closure assembly is fabricated to match the dimensions of the dummy assembly with weld shrinkage of the final welds taken into account.

Just to remembering, that what we explain here is according to the ASME Standard.