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.
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.
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
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.
Destructive test coupons are also different for the techniques as shown in Figure 9 and Figure 10.
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.
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.
Engr. M. Younas Malik is a qualified mechanical engineer from The University of Engineering and Technology, Lahore, Pakistan, and holder of a B.S. degree in technical education from a polytechnic school established by Oklahoma State University in Rawalpindi, Pakistan. He has been a member of the American Welding Society (AWS) and The American Society of Mechanical Engineers USA. He served for a period of 30 years in oil and gas in the UAE, Iran, Pakistan, Oman and Kuwait with international companies such as Saipem, Snamprogetti, Techint, Bureau Veritas, Palmer & Tritton, OGDC, KNPC and OMV. The positions he held included mechanical engineer, project engineer, contract engineer, quality control engineer, site manager and welding and fabrication engineer. He can be reached at email@example.com.