Considerations for location of Field Welds and Shop Welds.

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Dec 10, 2012, 6:49:40 AM12/10/12
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There are 4 categories of weld names which are usually used in general ,in EPC project.

             Field Welds

             Field Fit Up Welds

             Shop Welds

             Tack Welds

In this Piping Design Tip we will address Field Welds (FW) & Field Fit Up Welds (FFW).  Shop Welds and Tack Welds will be touched on briefly.

DEFINITIONS:

Field Weld (FW)

A weld performed somewhere other than the pipe fabrication shop.  This is the welded point of connection between two Spool Pieces.

The end preparation of the pipe is a beveled end for butt welding.

The PREFERRED location is in a horizontal run.

Field Fit Up Weld (FFW)

 A weld that is performed somewhere other than the pipe fabrication shop.  This is the welded point of connection between two Spool Pieces but it differs from the Field weld in that this type of weld is used when Piping Design does not have information that enables them to establish a definite dimension for a Spool Piece.  Examples of this situation include

having only preliminary vendor information (equipment, instruments,etc.), or, not knowing the exact location of a point to which the Spool Piece is to connect.

THE FIELD FIT UP WELD IS USED ONLY WHEN NECESSARY - IT IS YOUR LAST CONSIDERATION.

The end preparation of the pipe is either a plain (square cut) end or is a torch (square cut) cut end both with 6 additional inches of pipe added to the end of the Spool Piece, the additional 6 inches is NOT REFLECTED in the dimension shown on the isometric. The purpose of the additional 6" length is for any adjustment in pipe length required at the location of installation.(6” additional depends on company standard)

The PREFERRED location is in a horizontal run.

Shop Weld

 A weld that is performed in the pipe fabrication shop.

Tack Weld

A weld performed either in the pipe fabrication shop or somewhere else that is only designed to temporarily "keep" piping components together during their transportation to the installation location.  At the location of installation the tack weld is "broken" to separate the two components and reposition them for final welding installation.

 
Additional Definitions:

Spool Piece

An assembly fabricated in a pipe fabrication shop that COULD consist of; pipe, fittings, flanges, attachments, and on some Projects - valves and instruments.

Spool Number

 A unique number assigned to a Spool Piece - used for identifying and tracking the Spool Piece - from design to installation..

Shipping Box

- Sometimes called a Shipping Container, or just Box.  This has a fixed set of 3 dimensions that into which any Spool Piece can fit in a square, not skewed (diagonal) position.  Even though no box or container may really exist, all Spool Pieces are designed to fit within it's limits. Handling of the Spool Piece is easier when the Spool Pieces are similar in size. The 3 dimensions are 11'-0" x 8'-0" x 38'-0"in general as per factory standard. Your Project will establish it's own dimensions.

 The following must be considered when locating Field Welds and Field Fit Up Welds:

 1 - The preferred FW/FFW location is in a horizontal run.

2 - The FW/FFW should be located, whenever possible, where the welder can perform the weld without scaffolding, ladders or other temporary means.  The ideal location for a welder is above a platform, floor, or something on which the welder can stand safely and comfortably - this is accounting for the "ease of installation."  Do consider the "ease of installation" when locating FW & FFW - BE AWARE of the constraints, obstacles, etc. in the area which the Spool Piece is to be installed.

3 - While number 2 above is 100% true, it's theory and application is changed somewhat when you have piping that is to be installed on a vertical vessel BEFORE the vessel is erected.  In this situation, the FW/FFW can be made from a more "controlled" and convenient position by the welder than what would be normally done - the welder could make a weld from a height of 5 feet in this instance when the "usual" distance could be from a height of 75 feet.

 4 - The FW/FFW should be located, whenever possible, so Spool Pieces are (self) supported without the need for temporary support or "blocking up" to keep the Spool Piece in place during installation.

5 - The FW/FFW must be located so that the Spool Piece will fit within the three dimensions of the Shipping Box without positioning the Spool Piece in a skewed (diagonal) shipping position. The result of having a skewed Spool Piece is that it may require too much of the Shipping Box volume and possibly crowding out other Spool Pieces in the box.

6 - The FW/FFW should be located so that the Spool Piece will squarely fit in the Shipping Box as in number 4 above, but also not be a "Press Fit" (a tight/squeezed-in fit). This means that the dimensions of the Spool Piece should not be the same dimension as any of the dimensions of the Shipping Box - make the Spool Piece a little shorter - it's good insurance.  There is no rule as to how much shorter, 2 or 3 inches will be sufficient.

7 - When you are checking to be sure your Spool Piece fits in the Shipping Box, be sure you do not fall into the trap of only looking at the dimensions on the isometric.  You must always remember there are UNDIMENSIONED ITEMS that will add to the size of your Spool Piece, some of

these UNDIMENSIONED ITEMS are:

a) 1/2 the outside diameter of the pipe

b) 1/2 the outside diameter of a flange

c) A valve handwheel or operator -- ONLY IF THE VALVE IS BEING INSTALLED IN THE SHOP

d) A pipe stub or structural shape that is welded to the Spool Piece. It could be part of an  attachment to a support, anchor, guide, a dummy support, a support trunnion, a shoe, a  directional anchor,etc. (often seen on heat-treated or alloy piping).

8 - It is considered good practice for a FW/FFW to be placed on a run of piping that connects to two or more nozzles on a piece of equipment - or - connects two pieces of equipment.  In either case it is desirable to have the option of adjustment in the field during installation.  The reason for this is that NOTHING IN THIS WORLD IS INSTALLED WHERE IT IS SUPPOSED TO BE INSTALLED - THERE ARE NO EXCEPTIONS TO THIS FACT.  Tolerances in fabrication, manufacture, and installation are the reason for this.  We can say something is correctly installed if it is installed within an acceptable variance or error.  You will see this expressed as plus or minus in some situations.  So, at a point where you need a weld, consider the above and make it a FW/FFW.

9 - If your Project has Piperack Loading Drawings, pay special attention to the required locations specified for FW.

 10 - Be absolutely certain you have interface with the Designers in Partitions that are adjacent to yours, you (or your neighbor) may have placed a FW/FFW in your Partition per all rules but a lack  of interface with the adjacent Partition may give us 2 FW/FFW within a few feet of each other - we look pretty stupid when this happens - this is something we let our competitor companies do - not us.

11 - There are definitely places and situations that are considered "bad" choices/locations for FW/FFW. They can be done but they do not reflect the most intelligent or efficient options due to their location making them difficult to weld, or, the configuration of the weld being expensive when done in the field as opposed to being done in the shop - some of these are:

"BAD CHOICES" of FW and the reasons they are "bad."

 a) Locating a FW at the "fish mouth" connection to the header at a stub-on connection. This presents a longer and more expensive weld than what you would have with a circumferential FW.

An alternative design would be to have a short pipe "stub" coming out of the header with a circumferential FW.

This is unavoidable when a stub-on connection occurs when the branch line is shop fabricated and the header is rack-loaded.

b) Locating a FW in such a manner that one of the spool pieces is a flange or fitting that is shipped loose. It is possible for the fitting or flange to be lost in transit somewhere from the shop to the installation location. Lost can also mean the fitting or flange is used elsewhere at the job site.



--
Regards,
 
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