Top Secrets for a Lasting Tube-to-Tubesheet Joint: Hole Finish and Groove Design
Updated: May 8, 2020
One of the most overlooked aspects of heat exchanger manufacturing is the tube-to-tubesheet joint. Sure, countless hours go into the design of the unit itself, the pressure, the temperature, the material selection, etc. When production starts the tubesheet joint certainly gets some attention; tube sheet hole drilling, reaming, and grooving take days to complete. Multiple shifts are dedicated to tube insertion and more days are set aside for joint welding and subsequent testing. All of this happens before tube-to-tubesheet expansion ever happens.
But how much thought actually goes into hole finish or groove design? Tube expansion method? How about whether to pre-set the tubes prior to welding? Or tube-end prep in general for that matter? Over the next few posts, we’re going to look at the importance of proper tube end prep, and hopefully help you ensure that one of the most important sealing points in your heat exchanger is designed for success.
Let’s get started with the tube hole itself. One of the most common questions we at HydroPro are asked is about the desired machine finish on the hole ID. The truth is, we like a semi-rough surface to expand into. What I mean by this is an as machined finish, with no polishing. We don't necessarily want a smooth or electropolished finish here.
The only thing we really don’t want to see on the hole ID is a longitudinal scratch that can become a leak path. When you hydraulically expand a tube joint, the tube takes on the shape of the hole, this includes imperfections such as machine marks. Every one of those imperfections acts to hold the tube more securely in place. So, keeping the hole a little “rough” can be a good thing when it comes to joint tightness.
When expanding thin tubes (usually 0.035” wall thickness or thinner depending on the tube OD), you can actually see the more obvious imperfections embedded on the tube ID after expansion. This is because hydraulic expansion forms the tube into the tube sheet hole using water, hydroforming it to the exact shape of the hole, including those imperfections.
Another major aspect of the tube hole is the groove design. Traditionally, the Tubular Exchanger Manufacturer Association (TEMA) has called for one or two 1/8” wide grooves depending on tubesheet thickness. When using hydraulic expansion they specify 1/4" wide grooves in place of the 1/8” width.
While the 1/4" width definitely helps with groove penetration when using hydraulic expansion, we prefer the formula W=1.56√Rt where W = the optimal groove width, R = tube radius and t = tube wall thickness. For example, the optimal groove width for a 3/4" x 0.083” AW tube would be W=1.56√(0.375x0.083) in which W = an optimal groove width of 0.275”, slightly wider than the TEMA recommended 0.250” width. Conversely the optimal width for a 3/4" x 0.049” tube is slightly thinner, at only 0.211”.
This doesn’t mean that every unit needs to have a different groove width in precise dimensions. It means that we would recommend running these calculations and using your best judgement when determining groove width, as opposed to simply rubber stamping things using a single groove dimension for every application. For the above mentioned 3/4" x 0.083” application, the 0.250” is likely sufficient depending the tube material. 300 series stainless or carbon steel would likely do well with the 0.250” wide grooves, while duplex or 254SMO may need something a little wider for adequate groove penetration. The 3/4” x 0.049” example above would do well with the 1/4" groove width and wouldn’t require any retooling to unnecessarily make the grooves thinner.
In addition to the groove width, consideration should be made as to the shape of the groove. Rather than square grooves, using tapered or radiused groove allows for more consistent contact between the tube and serrations, further helping with the desired strength gained from adequate groove penetration, and without creating a shear point like with a square groove.
Well there you have it. Using the above formula, tapered grooves and a machined hole finish is a great start to ensuring your tube-to-tubesheet joints are designed for success.
If you have any questions about your specific application, call HydroPro at (573) 732-3318 or email our engineering team at firstname.lastname@example.org.
For additional resources visit our website at www.hpro.com/engineering.