Written by: Richard Beale and Bruce Raymond at Piping Design Central
FW = field weld
FFW = field-fit weld
A field-fit weld indicates that the designer has foreseen that the piping may need adjustment in the field so has added an extra length of pipe at a buttweld fitting or flange. The pipe end will be bevelled and welded on site after the installer determines the correct dimensions.
Field-fit welds are sometimes provided in all three planes and the extra pipe provided can be anywhere from 3" to a foot long (usually).
Just to add a little to what Paul has said and as you say, there are two types of field welds: A field weld (FW), and a field fit-up weld (FFW). The difference between a FW and a FFW weld is that an additional length of pipe is added to the calculated dimension at a FFW for field trim allowance . No such allowance is provided at a FW. As Paul has said, this extra length can vary. I usually add 150mm. Check with your piping lead as to what has been decided for your project.
Field welds are there because the piping system has to be broken into pieces for transportation, and has to be in pieces of manageable size for handling into place by construction. These pieces are referred to as spools. FFW welds have the additional reason of providing field trim allowance. The size of a spool is limited by the size of the "shipping box" limit. The shipping box limit is decided by following the transportation requirements of the jurisdiction(s) you'll be transporting through, i.e. width, height and length. An average spool limit is approximately 2.5m x 2.5m x 12m. All spools must fit into this "imaginary box". Again, check with your piping lead.
Natural breaks occur at all flanges, whereas it is necessary to select the field weld "breaks" in the piping system. However, more is required of the designer than doing the math and placing a field weld each time the shipping box limit is reached. Placing field weld locations also requires an effort to understand and consider the needs of construction. Therefore, judgement calls are required by the designer. The considerations are:
1. Avoid field weld locations that will be difficult or impossible for a welder to get to.
2. Avoid horizontal welding (field weld in a vertical pipe). Horizontal welding poses the most problems for the welders. Molten metal tends to run downward and requires extra care to overcome this difficulty. Avoid FW and FFW welds in 45 degree vertical piping also. It is preferable to place FW and FFW welds in horizontal piping.
3. Consider the threading of spools through floor, wall, and platform penetrations, and threading of spools into existing areas, particularly existing pipe racks. Extra field welds may be required to achieve this.
4. Strategically place FFW welds, e.g. in the horizontal piping between equipment in parallel, such as exchangers and pumps. Exact distances possible in the models between vessels, pumps, steel supports, foundations, etc., are unlikely to be so precise in the field. Therefore a FFW is introduced for a final hook-up, whereby the field erector takes a field measurement and trims the extra pipe to the required length.
5. Make FW and FFW placement consistent for similarly run piping configurations.
One last point; where I work, commonly spools are fabricated for piping systems NPS 2 and above, whereas NPS 1.5 and below (small bore) piping is field run. We had an interesting discussion about this recently on this forum, and you may wish to review this topic. Another thing though to check out with your lead.
Bruce Raymond adds:
Field fit up can be a cost saving or cost addition to a project.
Field Fit Up implies that a construction crew actually presents the piping spool pieces, marks the Field Fit Up section - removes it , cuts and bevels the section, re-presents the Field Fit Up section, and then proceeds with the actual field welding. This can be a very man-hour intensive procedure. The construction crew must perform the following steps - primary rigging / presentation / marking / derigging / cutting / beveling/ and rerigging prior to actual field welding.
Example #1: Replacement in kind of a 6" Sch. 40 CS piping section due to corrosion. No Unit Shutdown.
Designer has original drawings, isos, field verifies dimensions, and adds FFWs because he didn't want to go up in the piperack and physically verify the existing piping route.
Number of FFWs - 2.
Designer time not spent to verify piperack dimensions: 20 manhours.
Additional construction crew time to install piping system with 2 FFW's: 80 hours.
Example #2 - Replacement of a 12" Sch.160 Incoloy piping system due to metallurgical upgrade: Unit Shutdown
Designer has original drawings, isos, complete laser scanning has been performed.
Designer did no field verification of dimensions due to laser scanning - Number of FFWs - 0
Due to the confidence of laser scanning point clouds all spools were "pre-fabbed", with only FWs called out, but during the subsequent unit shutdown and installation there were numerous installation problems which required modifications to the original spool pieces.
Designer time to field verify dimensions - 0 manhours.
Additional construction crew and engineering time to install system - 240 hours.
Actual cost impact for lack of FFWs - 2 additional days of unit shutdown @ $1,250,000 per day.
Important Comments on Laser Scanning
If you are working on a laser scanning driven project don't forget the following:
1) Laser scanning only "sees" the outside surfaces of piping, if you have an insulated line it will not be able to pinpoint actual pipeline center lines.
2) Laser Scanning does not take into account piping "Shrinkage" due to operating vs. shutdown temperature differentials, normally a laser scan is performed while a unit is operating, but when the actual modification is done the affected piping systems, and or equipment will be at ambient temperature.
If in doubt as to the utilization of FFWs consult with your supervisor, and also with your construction group - good planning and coordination / engagement of all involved groups normally will assist in making the proper decisions.