FW & FFW

From the pipingdesigners.com Glossary:

FW = Field Weld – Refers to a weld (normally in piping) that is to be made in the field (Point of installation) as opposed to being made in a pipe fabrication shop. This weld can be pipe-to-pipe, pipe-to-fitting or fitting-to-fitting. The pipe and fitting components on both sides of this weld are dimensionally correct and no trimming is to be done without the approval of the Design Office.

FFW = Final Fit Weld - Refers to a piping weld that is to be made in the field (Point of installation) as opposed to being made in a pipe fabrication shop. This weld can only be pipe-to-pipe or pipe-to-fitting. The pipe component is to be fabricated long with (6” to 9”) of extra length for final field fit-up. This weld (for this line) is to be made only after all other Field Welds” have been completed.

From my own collection of Q&A:

Field Weld and Final Fit Weld
The question:
“Hi, I have been working for almost 12 years in design office of three companies. During my design experience I have not gotten any chance to have construction experience. Because of this I have a lack of knowledge regarding field welds and fit field welds.
Generally we mark field weld and fit field weld on the IFC (Issued For Construction) isometric while considering container size 40'x8'x8' for size 2" and above and for below 1 .5" we follow 20'x8'x8'.
Still I am not confident with what we marked.
Can somebody focus on this more like a rule of thumb?

My answer:
Based on what you described, you are doing it (field welds and fit field welds) just right.
I am not sure what your question is.
Are you asking about the "box" sizes you listed? Or Are you asking why are there field welds and fit field welds?

The box sizes are determined by a number of reasons:
1. shipping size. The 8' x 8' x 40' is the largest size normally allowed for over the highway truck that would carry prefabricated piping.
2. The 8' x 8' x 40' results in maximizing the number of less expensive shop welds and reduces the number of more expensive field welds
3. The 8' x 8' x 40' for the larger size piping is because larger piping is more stable for lifting without bending when handled during a crane lift for installation.
4. The 8' x 8' x 20' for the smaller size piping is because the small piping is less stable for lifting and prone to bending when handled during a crane lift for installation.


Why are there Field welds (FW) and why are there fit field welds (FFW)
FW = Piping is normally prefabricated in a pipe fabrication shop that is remote from the job site. Remote is a relative term, the shop may be located 10 miles or 10,000 miles away. The FW is a standard weld to be made in the field for the purpose of installation no extra pipe is included by the shop and both sides of the FW are to be shop beveled. The FW's in a given line may be made in any order. The piping designer who creates the isometric is the person who is best able to decide the best location or place to have a field weld.
FFW = The Final Fit Weld is intended to be the last weld made in the installation of a long or complex line where there is the possibility for a dimensional misfit. All normal Field Welds (FW) are to be made first and the Final Fit Weld (FFW) is to be the last weld made for this line. For this (FFW) purpose, you must instruct the shop to leave 6'' to 9" extra pipe (no bevel end required) on one side of the FFW.