Tube Rolling

[DetroiTug]

On fixing tubes to the sheets: I spoke with the local boiler shop and they said that on a boiler with 150 psi WP (high pressure), the A 178 steel tubes/flues need to be straight rolled prior to welding. I forgot to ask, but would it be better to flare/taper roll? Or if the holes are reamed and chamfered and fit the tube, is it really necessary to roll them in?

She also said the sheets should be 5/16" A70 plate? I had planned on using 3/8" A36.

-Ron

[S. Weaver]

Ron:

Sheet size is a function of calculated design. Thickness depends on the number of tubes, stays, diameter, etc. Steel specifications, ditto. I don't have chapter and verse in my head. Fred?

This sounds counter-intuitive, but under ASME rolled in tubes are a stronger and required practice at certain pressures than welding. Our shop practice is to only weld where tube-to-sheet heat transfer is required to keep the tube ends from deteriorating and cracking in the combustion heat due to exposure. So, my new boiler will have the tubes rolled, flared and beaded at both ends, but only "seal-welded" (a misnomer since the seal is in rolling - tubes that have been "seal-welded" and still leak are rolled further to correct the leak), with a small, flat, concise bead of xx10 wire. If I could get away with it, I wouldn't bring any weld to bear on the fire-side tube sheet.

[DetroiTug]

It is for a 16" diameter shell. 16"OD X .375" w/t ASTM A106 seamless.

-Ron

[fredrosse]

I think all tube expanders flare the end of the tube a bit, but the tube should be expanded over the entire length of the reamed tubesheet hole. Some boiler tube rollers also bead the tube end, that is OK, but with welding the tubes in (“seal welding”), an ordinary expander works fine. It is important to have the tube ends stand proud of the tubesheet by 1/8 inch here, no more, no less. As stated previously, the rolling operation is what seals the tube to tubesheet joint from leakage.

There are many ways to make the tubesheet, CNC machining, old drill press, punching, laser cut or water jet, etc., but the holes do need to be reamed in the end.

The tubesheet holes have to be reamed about 0.004 to 0.008 over tube size for expanded tubes, when I had my tubesheets made the holes were perfect, but I specified 1.250 diameter, and that was too tight for the 1-1/4 inch boiler tubes. I had to open up the holes to 1.255, then everything went together easily.

Boiler tube rolling is not too difficult, the main technical points to remember, keep everything clean, lubricate and clean the roller after each rolling operation. Check alignment of the tube before the roll, and don’t over expand the tube. An underexpanded tube can be touched up with some more rolling, an overexpanded tube will have too much tube wall thinning, and may distort the tubesheet, which is very bad.

Just putting the tubes in place and welding them can also seal the joints, but that can lead to trouble after some service hours. If the tubes are rolled in, there is intimate contact between the tube OD and the tubesheet hole, no space for a gap. If the tube is a loose fit (a few thousandths clearance between the tube OD and the tubesheet hole, and welded in without rolling, then the gap will eventually fill up with boiler scale deposits, and there is no way to get rid of these deposits. This area can then become more susceptible to corrosion issues. Also differential thermal expansion (the tube runs hotter during hard firing, expands more than the tubesheet, and presses the hard deposit against the tubesheet hole. Then with moderate firing or shutdown, the tube contracts, is smaller diameter, opening the gap for more deposit accumulation. This is repeated while underway, maybe hundreds or thousands of times. This is why the ASME Code will not allow just welded tubes, unless the welds are true "full penetration welds", tube expansion is mandatory.

As to tubesheet thickness for a simple small VFT launch boiler, ASME Code specifies a minimum thickness of 5/16 inch, but I prefer 3/8, especially if the shell is 3/8 wall thickness. This makes for easier welding of tubesheet to boiler shell, , with both materials the same thickness.

[DetroiTug]

Steve, Fred,

Thanks for weighing in on this. I guess my puzzlement is coming from metal fab experience where any time material (the ones I'm accustomed to) is deflected it will return as it has some memory - have to bend it beyond the point to get it to stay at another. Boiler flue material is annealed (as I just learned) and has no memory and will retain it's shape once displaced.

I plan on cutting the sheets with plasma-arc and a template. Then throw them in the CNC machine and circle mill the holes to the 1.255 dim.

Apparently there are two types of rollers, the guy at Clifford Associates asked if I wanted a straight or tapered roller. I responded straight and he said that one was 313 dollars.

With rolling the tubes in the sheets, is the chamfer still necessary? We plan on seal-welding.

Thanks, Ron

[fredrosse]

If you would like, I will lend my roller (1-1/4 inch x 0.095 wall tubes) for your use. Send PM

[fredrosse]

Assuming you are using tubes with a wall thickness of 1/8 inch or less, the rules for firetube to tubesheet attachment allows a chamfered hole or a tube hole without any chamfer, with seal welded tubes. These are the rules:

For a non chamfered tubesheet hole, tube ends must stand proud on the tubesheet by at least 1/8 inch, and no more than 2 x the tube wall thickness. For example, 0.095 tube wall, maximum distance above the tubesheet is 2 x 0.095 - 0.19 inch, say 3/16 maximum. With the tube end excess heating issue, associated with a fierce firing, I would stay with 1/8 inch rather than 3/16 inch.

For a chamfered tubesheet hole, tube ends must stand proud on the tubesheet by no more than the tube wall thickness. For example, 0.095 tube wall, maximum distance above the tubesheet is 0.095 inch, say 3/32 maximum. The chamfer must be at least 1/8 inch deep, and must be no more than the tubesheet thickness x 1/3. This works out OK for a 3/8 tube sheet, as the 1/8 debth is exactly equal to one third of the tubesheet thickness. I like the chamfered arrangement, because it is easier to get a good seal weld by filling the cavity of the chamfer, rather than trying to run a weld bead around the outside of the tube. I guess good welders can do it either way, but I always try to setup welds as easy as possible, since I am not such a good welder.

Note that with a 5/16 tubesheet, the rules cannot be met with a chamfered tube hole, the rules require at least 1/8 inch chamfer debth, and that is more than one third of the tubesheet thickness, so that arrangement is not allowed.

[S. Weaver]

Ron, the only thing to Fred's counsel that I would add is for the smokebox end, where I'm not welding but instead flaring and beading, the only chamfer is a more a radius - 0.031 or so, inside the tube sheet and out - to eliminate the sharp corner and prevent the sheet from cutting the tube in the rolling process. This can be done while deburring the holes.

[DetroiTug]

Thanks for the offer of the roller, we will just purchase one as we plan on doing more than one boiler.

Looks like chamferring is the way to go and 3/8" sheets as well.

-Ron

[DetroiTug]

Machining a set of heads today:




Will Chamfer with a countersink. Burned out the outer diameter with the plasma arc then milled to 15.2" diameter . Then drilled and circle milled the tube holes to 1.252" diameter.



-Ron