We supply both components and entire systems into critical high purity applications that have stringent requirements for both cleanliness as well as integrity. In the day to day course of business we read a lot of customer specifications for sanitary stainless steel process piping and fittings. Most of the specifications today are written for welded tubing versus seamless, but not all. We normally try to take exception to systems specified with seamless tubing as it is expensive and can be difficult to source. There is a lot of seamless pipe and tubing made, but very little seems to be available in tube sizes meeting sanitary specifications. We thought we would use this post to do a comparison of sanitary seamless tubing versus sanitary welded tubing.
Because there is no longitudinal weld seam, seamless tubing is often considered to be a higher integrity product than welded sanitary tube. Case in point, there is no weld that could possibly leak. Also, with welded tubing, the heat affected zone of the weld seam is considered to be less corrosion resistant than the base metal of the tubing. This could be an issue with highly corrosive fluids.
That being said, let’s take a closer look at both and help you decide which is suitable for your application.
Seamless Sanitary Tubing
Most seamless tubing is made using the rotary piercing and rolling method. A conditioned steel round bar is heated to a suitable forging temperature and pierced on a mill which works the bar, causing it to flow around a piercer point forming a hollow billet. This billet is then roller elongated in a succession of mills with a mandrel or plug to support the interior. Finally, the steel tube is sized by further rolling on the OD.
Welded Sanitary Tubing
The sanitary stainless steel welded tubing manufacturing process starts with rolling a steel coil at the steel mill to the proper gauge thickness. This yields a coil of flat strip. The strip is then slit to the proper width dependent upon the size tube to be made. The slit coil is then loaded onto a tube mill. From there the tube is formed by going through a series or rollers then the seam is welded in an inert atmosphere. The weld seam is then cold worked (forged) by going through a rolling operation. Finally the finished tubing is heat treated, normally through an induction annealing process.
What are the Differences and Which is Better?
There was a time when welded tubing was thought to have poorer corrosion resistance than seamless because of the weld seam. Recent improvements in the welding and forging process of welded tube have negated that argument in all but the most extreme conditions. There are some areas, like the micro electronic and semiconductor markets where that can become an issue. But in the food, beverage, and biopharmaceutical market this argument is really a non-starter.
Continually, seamless tube used to have better surface finishes, especially in smaller sizes. You can buy ¾” and ½”OD seamless tube that right off the mill has a 15RA ID surface finish. That used to be impossible with welded tube because of surface roughness around the weld bead. Improved manufacturing techniques in welded tubes have also mitigated this concern.
It’s also important to note that welding seamless tubing with a computerized orbital welder can be challenging because of inconsistencies in the weld thickness. Welded tube tends to have more uniformity in this area and can be easier to orbital weld.
Then, there is the commercial area. 20Ra ID welded tubing in both 304 and 316L is readily available in sizes through 6” from a variety of manufacturers. In fact, Holland has one of the Midwest’s largest inventory of sanitary tubing. Seamless tubing is almost non-existent above 2” and is hard to find in sizes above 1”. It is also considerably more expensive.
Finally, if you are looking at using sanitary stainless steel tubing, you are also probably going to need sanitary fittings. Above ½” size, the availability of sanitary fittings made from seamless tubing is very limited. Most of the off shelf ASME BPE fittings used in the biopharmaceutical industry are made from welded tubing. So if your fittings are made that way, why go to the expense of buying seamless tube to connect to it?
Hopefully this serves as a good primer on seamless vs. welded tube. Stay tuned for new posts on all aspects of sanitary process piping and ASME BPE fittings and weld techniques in 2014 or contact Holland today for any of your sanitary process needs.