Here I enclosed the other responsibilities for the piping designers while the piping designer is also responsible for locating the field welds and the field fit up welds.
There are 4 categories of welds -
· 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 Mark 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 Mark 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 Mark 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 Mark 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 Mark 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.
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.
Mark Piece
- An assembly fabricated in a pipe fabrication shop that COULD consist of; pipe, fittings, flanges,
attachments, and on some Projects - valves and instruments.
Mark Number
- A unique number assigned to a Mark Piece - used for identifying and tracking the Mark 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 Mark Piece can fit in a square, not skewed (diagonal) position. Even though no
box or container may really exist, all Mark Pieces are designed to fit within it's limits. Handling of
the Mark Piece is easier when the Mark Pieces are similar in size. The 3 dimensions are 11'-0" x
8'-0" x 38'-0" per Fluor Daniel 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 Mark Piece is to be installed.
3 - While number 3 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 Mark Pieces are (self) supported
without the need for temporary support or "blocking up" to keep the Mark Piece in place during
installation.
5 - The FW/FFW must be located so that the Mark Piece will fit within the three dimensions of the
Shipping Box without positioning the Mark Piece in a skewed (diagonal) shipping position. The
result of having a skewed Mark Piece is that it may require too much of the Shipping Box volume
and possibly crowding out other Mark Pieces in the box.
6 - The FW/FFW should be located so that the Mark 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 Mark Piece should not be the same dimension as any of the dimensions of the
Shipping Box - make the Mark 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 Mark 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 Mark 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 Mark 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" 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 mark pieces is a flangeor 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. Be aware that PDS can cause a FW to be located to cause the situation.
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