Contrasting Welding Techniques Used On Pipelines And Refinery Piping: Uphill Versus Downhill
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.
- Coatings, pipe joint
- Compressor components
- Contractor, pipeline
- Contractor, river crossing/ directional drilling
- Directional drilling rigs, large
- Fittings, valves: plastic
- Meters, flow
- Pigs, cleaning
- Pigs, intelligent
- Pigs, scraper/ sphere launchers/ traps
- Scada systems
- Ultrasonic inspection
- Vacuum excavators/ potholing
- Valves, ball
- Welding systems, automatic