Welding in Heat Exchanger Tubes

[ViconServices]

Hi All,

Some quick questions regarding high pressure steam heat exchanger tube installation.

My query is whether or not there is a requirement ISO, AS/NZS or generally OEM requirement to weld in tubes at the tubesheet for the following specifications of heater? In all the times that we've maintained heat exchangers rolling and expanding tubes has always been sufficient I think the welding may be overkill but I'm not sure just want to get some confirmation. If anyone can also point me to any exchanger / pressure vessel design engineers in Sydney who we can get to do up a report if welding is unnecessary?

Operating specs:
1. Pressure of exchanger is ~4800kpa / 700psi
2. Operating temp of exchanger is 264 degrees Celsius
3. Tube bundle no. 1356
4. Tubesheet is Carbon Manganese Steel and UNS 32304 duplex stainless weld overlay 6mm thick
5. Tubes are ATSM A789
6. NDT Standard to be BS EN473
7. ASME Standard ASME Sec VII Div 1
8. Test Certs on Stainless to EN10204
9. Depth of Flange Faced Tubesheets is approximatley 140mm
10. Multiple baffles internally
11. No grooves or cuts in tubesheet straight
12. Only one small inspection port on side of vessel flanged
13. Vessel heads are torqued to OEM specification not tensioned (as far as I know)

Any help or thoughts would be much appreciated. Please feel free to ask any clarifying questions.

 

[rmw]

To me this is not a terribly high pressure or temperature for a steam heater. You didn't state what was on the side opposite the steam. Is the steam on the shell side? The CS tubesheet indicates to me that the steam is most likely on the shell side.

Duplex overlay and some grade of SS tubes (A789 covers a wide range) indicate that you may have something particilarly corrosive on the tube side.

Welding of the tubes to the tubesheet(s) comes in two forms. Seal welding and strength welding. With seal welding, the joint strength is in the roll and the seal weld is added for additional protection against leakage. With strength welding the joint strength plus the seal is all in the weld. Rolling may be nothing more than contact rolling to center the tube in the hole.

There are many, many Hx's fabricated without welding and the roll takes care of both the joint strength and the sealing successfully.

So to me it depends on how much faith you want to put on a SS tube rolled into a CS tubesheet with no grooves (I don't like grooves except in soft metal joints, but that is just me.)

But if you do weld, the companies that make really HP Hx's contact roll to center the tube - but this is not a tight roll - weld the tube, then final (strength) roll the tube. Unless the joint is Titanium to Titanium, then the order is reversed (the weld is autogenous so there isn't the danger of gas build up behind the weld.)

As to recommending anyone in Sydney you have stepped out of my league. I can't picture Sydney being covered up with HP Heat Exchanger designers, but I can be taught new stuff.

rmw

 

[gr2vessels]

Sydney (in Australia) has a number of reputed pressure vessel fabricators. Look up on the Yellow Pages under 'pressure vessel fabricators'. You'll find L&A Pressure Welding, WE Smith, Skinner, then Baipini, ER Curtain, etc...They are qualified to tell you what you need for that tube to tubesheet joint.
There is no specific rulling, but only recommendations for using one or other type of tube-to-tubesheet joint in accordance with the code (TEMA, ASME VIII or even AS 1210). For 6 mm cladded tubesheet, you easily can groove into the stainless steel cladding and strength weld the tubes to the tubesheet (or only seal weld, depends on the HTRI design for tube and shell expansion diferential), given the fact that the temperature diferential of 264 dge C might losen up the joint after a number of start/stop cycles. Also, as rmv said, you must have some corrosive fluid on the tube side, hence the welding of the tubes prevent infiltration of corrosiv fluid between the tube and tubesheet hole, causing perhaps crevice corrosion.
Cheers,
gr2vessels

 

[ViconServices]

Thanks for the information rmw and gr2vessels.

By way of some further information please note:

Tubeside fluid is Caustic Solution of 168 g/l as NA2O and total soda concentration of 215 g/l as NA2O.Shellside fluid is steam/flash vapour.

What's the point of the stainless steel 6mm overlay, is this even worth utilising?

Also with regards to welding into the tubesheet is there any reason why when the steam is 380 degrees celsius and the pressure operating 5000kpa as to why you would really need to weld when the tubesheet is 235mm deep? surely that's enough contact surface area to have a strong join?

Also one last thing the Stainless Steel vessel has been having problems with flashing of the tubeside liquids in the actual heads and baffles themselves.

Duplex Stainless Steel tubes are pin-holing inside 4 years when their shelf lives are 12-16 years. Carbon steel tubes put in as a trial have been living for 2.5years so confused as to what's happening here?

 

[MJCronin]

With regard to your question of welded tubesheet attachment vs. rolled tubes.... consider power plant feedwater heaters.

The low pressure heaters are usually rolled tube and high pressure heaters are welded. There are guidelines available from HEI that may help. (HEI 2622, 2623)

http://www.heatexchange.org/pub/pdf/edu/Tech Sheet 127.pdf

http://www.heatexchange.org/

With regard to your pitting problem, I believe that you may have to upgrade materials. The only material I know of that will absolutely be acceptable for hot caustic solutions is Nickel 200

Let us know about you final solutions.....please

regards

 

[rmw]

Welding tubes to tubesheets is for two reasons. The more common is for seal welding - where the seal weld does the sealing and the roll does the strength work. The less common (in my experience) is the strength welding where, well, you know and the rolling merely centers the tube in the hole.

The purpose for the overlay (if it is only on the tubesheet and not the rest of the channel) is so that the Duplex tubes will have a surface to which they can be welded.

Tubes are often welded when the service on one side of the Hx is not desired to leak into the other side. In your case, it would seem to me that they were probably trying to avoid caustic leakage into the steam system.

You used a different temperature in your second post than you did in the original post. Why?

Where is the pin-holing? Can you determine which way the pin-holing progresses, inside to out, or ourside to in?

Sounds like your CS tubes still have 1.5 years to prove themselves. They could be real champions or due to go any day now.

rmw