When I first started in pipe sales they sat me down with a big thick book about pipe and told me to read for three days. On my fourth day they told me to pick up the phone, start dialing numbers, and sell pipe. I got my first inquiry on the fifth day, took it to my assigned sales mentor, and asked him what the customer meant by DSAW. I remember seeing it in the book, but what did that REALLY mean? Vince said that stood for Double Submerged Arc Weld; a big weld for big pipe, he further explained. “Submerged in what”, I asked of course. “Do they weld it underwater…twice?”. To which Vince artfully replied “they’re going to underwater weld you to that chair if you don’t stop asking me questions…quote what I told you to quote and go get another inquiry”. Okay so not quite as technical a response as I had hoped for, but at least I understood that more than I understood what the engineers had written in the literature I had. I spent the next few years simply explaining to customers that the DSAW weld was the generally preferred and available weld seam on the market for pipe thirty inches in diameter and up. What it was submerged in…I hadn’t a clue (they told me it was submerged in flux, the thing in Marty’s Delorean I guess)…let alone why or how they did it twice. Long story short, I figured it out so here I am today posting this to hopefully save someone from years of silly questions.
Pipe comes in two basic flavors: seamless and welded. Seamless pipe is more or less a solid bar with a hole punched through the middle. Welded pipe is basically a piece of flat plate rolled by big machines into a long tube which is then welded shut. Welded pipe itself generally comes in three types: Electric Resistance Weld (ERW), Double Submerged Arc Weld (DSAW), and Spiralweld (which is a form of DSAW).
For the most part, pipe manufactures use the ERW process for pipe 30” in diameter and smaller, while DSAW is the more common process for pipe 30” on up. The main reason for this is basic economics…it’s more efficient to use the ERW process for smaller pipe, and its more efficient to use the DSAW process for larger pipe. Turning the base metal in such a manner that a DSAW process yields a helical weld seam is typically more efficient manufacturing method than straight seam DSAW, but some applications require a straight seam (epically where internal flow is a consideration).
How The DSAW Process Works
As the focus of this essay is on the DSAW process, we’ll assume the reader has a basic understanding of plate steel and/or skelp, and will therefore break-down the process into two key functions; the forming of the cylinder (its not pipe until its welded) and the weld itself.
Pipe manufactured with the DSAW process starts off as flat steel in the form of either plate or a continues ribbon of flat plate-like steel called skelp. The flat steel is first processed and prepared to be formed (edges are prepped, further leveling may be required, etcetera). Once the flat steel is properly prepared it is then formed to the proper size and cylindrical shape via one of two typical methods: pyramid rolls or U-O-E. Pyramid rolls are typically used for shorter lengths of steel as plate is rolled to form by three rollers. The rollers simply pinch the plate as the plate rocks back and forth through the rolls gradually shaping the plate into a cylinder. Most DSAW pipe mills use the U-O-E process to form their flat steel. First the edges of the flat steel are pressed to form a ‘U’ shape. Next the now ‘U’ shaped steel is pressed into an ‘O’ shape forming a cylinder ready to be welded into a pipe.
The weld is created when welding wire is made molten by an electric arc (a rapid flow of electrons from one point to another creating friction which creates heat) and that molten welding wire is lain at the point where the edges of the formed cylinder meet. This is the ‘A’ and ‘W’ portion of DSAW. When an arc weld is made, the integrity of the weld may be compromised by unwanted elements surrounding the point of the weld. In the DSAW process, impurities are kept out by keeping the weld covered by a material called flux. Flux is a fine granular material specifically comprised to benefit the weld, and is typically poured over the weld seam as the welder travels down the pipe. This is the ‘S’ portion of DSAW. The ‘D’ in DSAW simply means that this same process takes place on both the inside and the outside of the pipe, forming a uniform weld seam on both sides.
After the welding processes the pipe is further treated (shaped, heat-treated, cut, etcetera) depending on the specifications.
There are both advantages and disadvantages to DSAW pipe, most of which are determined by the specific job. Structural engineers may like the sturdy weld, but find the seams to be astatically unpleasing. The cost of DSAW may appeal to estimators, but the weld seam’s effect on media traveling through the inside may trump any savings.