When
one looks on a huge oil and gas installation like a refinery spread across many
acres and representing millions of dollars on equipment and infrastructure
investment, one does not see the raw crude oil injected into the refinery for
the refining process. This is because the crude oil is transported through an
underground pipeline. The pump station which pumps oil into the pipeline is
also located far away from the refinery.
But the underground pipeline and the aboveground refinery are
there for one purpose. That is to provide refined oil and other products to the
public. The pipeline is for transporting the crude oil and the refinery is for
refining this crude oil.
It is interesting to know that the welding techniques for
these complementary structures are entirely opposite to each other. Downhill
welding techniques are used for welding pipeline whereas uphill welding is used
for welding refinery piping systems. Even the welding codes and inspection
methods are different. The pipeline welding is controlled by API 1104 whereas
refinery piping work is controlled by ASME Sec IX.
In this
article we are going to discuss one by one how and why the two welding methods
differ from each other. Following are the main areas where we mostly find the
difference: 1.) weld joint, 2.) use of clamps, 3.) welding technique, 4.) codes
and standards, 5.) electrode coating, and 6.) welding speed.
Weld Joint
The pipe thickness used on pipeline
is usually less than that used in refinery piping and the pipe ends of a
pipeline are machine beveled whereas pipe ends of a refinery piping joint are
manually cut and beveled. These two factors play a major role in determining
the opposite welding techniques.
Since the pipe end of a pipeline pipe is factory machined and
smooth, it is easy to use an internal clamp to adjust both ends of a pipeline
joint keeping uniform root gap without tacks, thus downhill welding technique
(Figure 1) is a better choice for speedy welding. In contrast, in the case of
refinery piping, not only is the pipe thickness greater but also the handmade
bevels are not so smooth. Tack welds are also used instead of clamps and the
root gap is not as uniform as in the case of the pipeline joint. Therefore the
uphill welding technique (Figure 2) is a better choice.
One more reason is the size of root gap between pipeline and
piping weld joints. Root gap for pipeline joint is 1.6 mm (Figure 3) as
compared to 3 mm in piping weld joint (Figure 4). A joint with a smaller root
gap can be easily welded with downhill technique, fusing both the root faces,
whereas in bigger root gaps you need a weaving motion of the electrode to fuse
both root faces.
Use Of Clamps
Cross-country pipelines which are
spread for miles are welded on the right-of-way. In contrast, plant piping
joints are prepared and welded in a workshop. Weld joint preparation are done
keeping this factor.
An internal clamp (Figure 5) is used inside the pipeline
joint for speedy alignment and can be removed from the second end of the pipe
once the root and hot passes are complete. Whereas, due to short and bent
lengths of piping joints having fittings, the weld joints are prepared with or
without using external clamps.
Another
difference is the use of tacks. On pipeline joints, no tacks are used as the
root and hot passes are completed immediately when the internal clamp is in
place whereas in the case of plant piping, weld tacks are used to prepare weld
joints for weld at a later stage.
Welding Technique
As explained earlier as to why
pipelines are welded by using the downhill technique and piping with an uphill
technique, in the downhill technique two welders weld one joint simultaneously
from the top to bottom of the pipe on opposite sides whereas in a piping joint
job, one welder completes the whole joint welding from bottom to top of a
piping joint.
Codes And Standards
Codes And Standards
As stated earlier, the pipeline
welding is performed with respect to API 1104 code and plant welding by ASME
Sec IX.
For a pipeline, the welder test piece is held in a horizontal
position (Figure 7) whereas the test piece of a plant piping welder is held in
a 45-degree position (Figure 8). This is because pipeline welds are made in a
horizontal position and piping welds are carried out in horizontal, vertical
and at 45-degree angle positions.
Electrode Coating
For downhill welding, all electrodes
used are of cellulose coating whereas for uphill welding the electrode used for
the root pass is of cellulose coating and the rest are accomplished with low
hydrogen-coated electrodes. The reasons for this are 1) The pipeline wall
thickness, which is usually less than 12.5 mm; 2) easy removal of slag; 3)
welding speed, and 4) a thin bead of a cellulose electrode. All of these are the
requirements of pipeline welding whereas in plant piping the pipe thickness is
greater, therefore a weaving motion of an electrode is required to weld heavy
thickness piping joints. For this purpose low-hydrogen electrodes are used.
Welding Speed
Welding Speed
The last – but not least – big
difference between pipeline and plant piping welding is the welding production
speed. Here are some of the reasons for this difference in welding speed:
1. Piping joints are adjusted and tacked in a workshop and
usually one welder completes the whole joint, welding root, filling and cap
passes. Whereas on a cross-country pipeline, the joint is adjusted with an
internal clamp on the site and welding is performed by a team of mostly two
root pass welders, two hot pass welders, two filling pass welders and two
capping pass welders.
Both welders perform welding on opposite side of a pipeline
joint and the welding crew moves in a caravan in open air. As a result, welding
production speed is much more than piping joints welded in a workshop.
2. Downhill welding technique gives good welding production
on a pipeline where pipe thickness is mostly 12 mm or less, whereas piping
joints are of greater thickness and the uphill welding technique requires more
time; thus, the welding production is less as compared to the pipeline.
3. Another reason for faster welding speeds on pipelines is
the electrode movement from top to bottom and with no weaving motion. Whereas
the electrode moves from bottom to top on a piping joint and the weaving of the
electrode slows the welding speed.
Author
Engr. M. Younas Malik
Engr. M. Younas Malik
Source :
Tidak ada komentar:
Posting Komentar