RE: [PipingDesign] Field Weld(FW) and Fit Field Weld (FFW)

Hi Murthi,

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 450 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 1½ 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.

Hope this helps,
Richard B.
Calgary, Canada

  -----Original Message-----
From: PipingDesign@yahoogroups.com [mailto:PipingDesign@yahoogroups.com] On Behalf Of Paul Bowers
Sent: July 19, 2008 10:23 AM
To: PipingDesign@yahoogroups.com
Subject: Re: [PipingDesign] Field Weld(FW) and Fit Field Weld (FFW)

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).

Paul

chellappan easwaramurthi wrote:
> Hi guys,
>
> I am working as a Piping designer. In Isometric drawings, some sheets
FW is given in flanges and some other sheets FFW is given..
>
> i some clarifications
>
> 1. what is the difference between FW and FFW?
>
> 2. How to decide whether this joint is by FW or FFW?
>
> thanks n advance

[Non-text portions of this message have been removed] Received on Sat Jul 19 15:19:00 2008