Contractors interested in starting a prefabrication facility should visit as many existing fabrication shops as possible during the planning process. A good method to gain facility access is through peer groups or contractors you meet while attending educational seminars or conventions across the country, not in your geographical market. Just ask, and chances are they would love to show off their facility. Additionally, material suppliers and equipment vendors should be a good source of information.
Other compelling reasons to prefabricate include quality, cost, and the schedule requirements the mechanical contractor faces in this ever-increasing challenge of profitability. Strict government and industry guidelines in the chemical, pharmaceutical, and food industries set the bar for quality. Critical tolerances can be maintained in a fabrication facility, ensuring quality is not compromised as could happen if fabricated in the field in a non-controlled environment. Inspection and testing can be performed more efficiently in the shop identifying any problems which can be corrected before installation in the field.
Work in a fabrication shop is performed more efficiently than work in the field, thereby reducing cost. This is due to automation in machinery and the assembly line process which produce layouts, cuts, and welds faster than is possible in the field. Once a subassembly or module is assembled and tested in the shop, it can be safely transported to the jobsite and installed as a single unit.
The construction schedule can be optimized by utilizing the fabrication shop, reducing the number of man hours expended in the field. The shop activities can begin long before mobilization to the jobsite. Reduction of field hours will relieve jobsite congestion and stacking of trades.
Planning the prefabrication is one of the most important steps. It allows the contractor to step back and consider the whole job and then decide on how to proceed in a way that will allow as much cost control as possible.
The planning process identifies what portion of work is to be done in the shop based on many factors, including pipe size, access to the site for finished product, trucking requirements, and how quickly the fabrication is needed. Once it has been determined exactly what to fabricate, the field measurements and fabrication drawings begin. Verifying field measurements may be the most critical step in the process. If measurements are incorrect, the fabrication won’t fit, requiring it to be cut and re-welded in the field at a very high cost to the project. The locations of field welds are of paramount importance. Make sure they are in a location that is easy for the welder to perform, or in other words, not out in no-man’s land, which would require access.
The field dimensions are used to create isometric drawings, from which spool drawings are produced. The spool drawings are typically printed on 11” x 17” paper and used by the craftsmen in the shop as fabrication instructions. Plan views are also produced with elevation views as needed for clarity.
Materials can be ordered and delivered to the fabrication shop “bagged and tagged” per spool sheet or isometric drawing. This method of material delivery will reduce the time required to find all the necessary parts to fabricate each spool. The craftsmen simply looks for the bag identified by the spool number he is starting instead of going through boxes or crates of fittings to gather the items.
The work typically starts at the pipe feed rack, or conveyor, where lengths of pipe from the storage yard are loaded for the pipe cutting station. The conveyor loads each length of pipe to the cutting torch head, where the operator monitors the automatic cutting and beveling of the pipe. The cut piece is labeled and removed from the cutting conveyor by overhead crane or jib and placed on pipe stands which can be rolled to the layout station.
The layout station is where assembly of the fabrication begins. The cut lengths of pipe and the “bagged and tagged” fittings are assembled and tack-welded together according to the spool drawing. Layout jigs and adjustable height work tables help the craftsmen assemble the part efficiently. Jigs can be labeled and used to assemble future projects as well.
The spool is then rolled or moved from the layout station to the welding station where final welding is performed. Depending on the welding procedure required by the customer, the correct method is used, which could be stick, MIG, Heliarc, STT, or submerged arc to name a few. Submerged arc welding is an automated process usually used on larger bore pipe welding. A turntable will facilitate the rolling of as many welds as possible, reducing the number of position welds.
The spool is then rolled or moved from the welding station to the testing station where final inspection and testing is performed. The test can be performed in a variety of methods including hydrotesting or nondestructive radiographic testing based on the requirements of the customer and service in which the pipe is to be used. Another advantage of radiographic testing in the shop is performing the testing in an area of the shop or outside away from the work area, which would not be possible if done at the worksite, as it would require “clearing the area”.
The finished spool is then loaded directly on a flat-bed trailer for scheduled delivery to the jobsite.
There are many other tasks that can be performed in the shop such as hydraulic punching or shearing of steel plate or angles and shapes. Punching of holes is much quicker than conventional drilling. A pulling tee station is used to pull a nozzle on the side of a pipe, thus eliminating a fitting and joint to attach a conventional branch connection. A grooving or threading station is used to place a groove or thread on the end of a pipe. A hanger station is used to fabricate hangers to be shipped to the jobsite as a completed piece. Be creative and think of other labor saving stations you could utilize to cut fabrication costs in your respective markets.
The decision to prefabricate is a major one, requiring much thought and commitment. Be sure to involve your craftsmen in the planning process for their good ideas and to mitigate any possible negative attitudes they may have had at the outset. The attitudes of management will also have a strong influence on the success of the prefabrication facility. Once labor and management ideas are aligned, the decision to fabricate can have a positive effect on the bottom line. Prefabrication can be considered at the estimating stage to gain a competitive edge and will become commonplace in your procedure to execute work.