Shell and Tube Assembly Methods? 1

[Outlaw500]

I'm looking at a new design for a shell and tube heat exchanger.

Can anyone tell me how the assembly of the tubes into the tube sheets is normally done?

I know we are going to roll the tube ends to attach them to the tube sheet at each end. In this case the tube sheet is welded to the shell on both ends on the finished product.

Is the best way to assemble to insert all the baffles into the shell and thread the tubes through the first tube sheet, baffles, and into the final tube sheet? Or is it better to assemble the whole tube section outside then slip it into the shell and complete the welds on the tube sheets?

Thanks for the help.

 

[pastyl]

There are two methods for attaching the tubes into tubesheet
First one is to expand the tubes into their holes in the tubesheet and a few mm beyond, and then to make a fillet weld at the circumferential of each tube (sealing weld acc. ASME VII UW20).
The other one is to make a groove at each lip of tubesheet hole and to make a groove weld (named strength weld acc. ASME VII UW20) and also a fillet circumferential weld (the sealing weld).
For the rest of your questions I will answer you in a few days, when I finish my heat exchanger construction. But I am thinking to attach the baffles with the tie rods at the tubesheet first and then to pass each tube for the baffles hole and weld it to the tubesheet.

 

[vpl]

I would think it would be very difficult to take a lot of very long, slender tubes and try to thread them through already installed tube sheets, baffles and out another tube sheet at the far end.

I'd sure love to see a video of someone trying it though.

Patricia Lougheed

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[SnTMan]

Outlaw500, some companies might consider this knowledge a competitive advantage and be unwilling to share it.

Regards,

Mike

 

[rmw]

Outlaw,

Either go to Babcock Power's TEi Division website and look through all their (especially condenser) pictures or find their local rep that handles plants on your domain and ask them for some of their promotional material.

The condenser tube bundles that VPL is familiar with in her world are tubed ALMOST as she describes; two tubesheets set well apart (some in the range of 50-60 ft) with a lot of support plates in between (some as close as on 18" centers) and the tubes are stabbed individually through the whole assembly, entering at one tubesheet and ending at the other. But, the key to that process is that the tubesheets are not fixed and are able to be moved as required. Tubesheets already welded in place as VPL notes wouldn't allow that.

Field units are retubed in the exact same manner once the old tubes are removed and all the supprort plates and tubesheets are carefully aligned. In both cases, a 'bullet' is placed into the tube end so that it overcomes the tube's tendency to nosedive due to gravity between spans and its pointy nose catches the next hole in the process (be it support plate or tubesheet on the other end.) But again, the key to this process is that the tubesheets are not welded to the shell and are able to be shifted around a minor amount for alignment purposes.

A real good condenser tubing crew can 'push' several tubes at a time but having the components well aligned is the key. I have seem as many as 5-6 tubes at a time pushed simultaneously. I have also seen a single tube have to be coaxed, cajoled, fought, and cussed in order to get it pushed through the support plates.

Sometimes in existing condensers where the existing support plate spans can be up to 39" in older units, a person will have to be stationed between support plates inside the unit to help lift the tube end so that it can enter the hole in the next support plate.

I also know of one condenser manufacturer of the smaller shorter round type condensers who tubes one tubesheet and all the support plates and then with bullets in all tube ends, coaxes the other tube sheet up and into place. I would not have believed it possible if I hadn't seen it for myself.

Now I want to address something that was said previously in this thread. Unless you are fusion welding Titanium tubes to Titanium (or Titanium overlaid) tubesheets, if you are welding with a consumable tip, do not hard roll the tubes into the tubesheet before welding. You can do a light contact roll to center the tube in the tube hole, but if the tube is hard rolled, there is no ability for the gases generated by the welding to escape and it will result in a blow out in the weld bead which can become a leak.

I know that there are those that read this forum who disagree and who have had success with rolling and then welding on specific situations and metallurgies, but I can tell you that the major equipment manufacturers that do this type of tube welding on a daily basis and do it for profit do not do it and recommend strongly against it when they receive specifications that call for it. When forced to do so, they will not guarantee a leakproof design.

To answer the origial question, I would only attempt to tube the bundle if it the ability to shift and adjust the tubesheets and support plates independently. If they are welded in place or fixed, tubing will become a nightmare.

In addition to TEi's website, another manufacturer that has some good photos of this type of work is Yuba. Yuba has a link that will take you to their condenser counterpoint as well.

rmw

 

[MikeMet]

rmw makes some good points but it should be noted that re-tubing of fixed tube sheet bundles is done without removing the tube sheets. Tubes are removed a few at a time and new tube inserted. This keeps the tube sheets and baffle skeleton aligned.

I agree that one should not hard roll prior to welding, the main reason being you have no way to perform a sensitive leak test on the completed weld if you hard roll. I have seen many times when hard rolled tube can be welded with no problems of entrapped gas causing leaks in the weld.

As for new fixed tube sheet bundles, a common general order of operation for fabrication is:

Roll shell and tack weld to one tube sheet
Build skeleton with tubes, baffles and tie rods
Insert skeleton and poke tubes through the fixed tube sheet
Tack weld second tube sheet to other end of shell
Poke some tubes through for alignment
Weld tube sheets
Poke through remainder of the tubes
Weld or hard roll.

It can vary a bit from shop to shop.

 

[boberator]

The preferred method of building a fixed tubesheet exchnager is to build the tube bundle outside the shell and attached to the tubesheet which the tie rods are bolted to. I don't think I am giving away a trade secret since everyone else I know builds them this way too.

The tubes are all in place in the bundle but protruding through the face of this "stationary" tubesheet as the entire bundle is slid into the shell. You then "hang" the other tubesheet on the end of the shell and push each tube through the proper tube hole. The tubesheets are welded to the shell and then the tubes are rolled into the tubesheets. As mentioned above if tube end welding is involved it is best not to roll the tubes first.

 

[SnTMan]

boberator, that's pretty much it.

Regards,

Mike