Expansion sequence for Heat Exchangers 2

[Merzi]

We are in the process of fabricating heat exchanger where the shell side is to be tested at 770PSIG. The tube sheet has 2 grooves and the tubes will be welded with 2 passes.

My question is as follows.

1) should expansion be performed before or after welding
2) Is Hydraulic expansion a better option than coventional roller expansion.
3) Is helium test required prior to Hydrostatic test to ensure no leakage .

Thanks

 

[SnTMan]

Merzi, this is pretty commmon:

1) If adding filler metal, light contact roll, weld, full roll - this lets welding gasses escape
2) Hydraulic expansion is usually used with materials that work harden rapidly and heavy wall tubes. It will require a wider groove than a rolled joint.
3) Not required, depends on the consequences of a leak.

Regards,

Mike

 

[Christine74]

1) If adding filler metal, light contact roll, weld, full roll - this lets welding gasses escape

I agree that you need to make provisions for the welding gasses to escape, but if you're adding filler metal, why would you still need to do a full roll?

A light roll might be advisable to reduce the likelihood of crevice corrosion, but a full roll is usually considered unnecessary with a full strength weld.

-Christine

 

[rmw]

Belt and suspenders.

rmw

 

[SnTMan]

Done all the time

 

[Christine74]

Here's a pretty good summary of tube-to-tubesheet joints:

http://www.wahchanglabs.com/pdf/2k1ConfProc/2001011.pdf

"When a strength weld is employed and properly tested it is not necessary to perform the tube end expansion step. However if only seal welding is done then expansion of the tube end is needed in order to achieve the required mechanical strength at the joint."

"The preferred method of making the tube-to-tubesheet joint is first to expand and then seal weld. When this sequence is followed a higher quality expanded joint is possible and the risk of cracking the seal weld during the expansion step is eliminated. In addition a higher quality seal weld can be made."


-Christine

 

[rmw]

Consultants can write what they please, and it is a well written piece, but the companies that fabricate large HP Hx's as their day job rarely roll first then weld. Then, only with autogenous tube welds. They just can't afford the rework when the weld puddle blows out due to the build up of gasses behind the weld. They've tried, they know the difference. Production is the key to their success.

I have been closely associated with three of them and speak from their experience and practices. Others have stated success with the roll then weld method, but I hold out for the position of those that do tens, no maybe hundreds of thousands of tube welds each and every year.

Expansion can be done so that the welded end isn't disturbed. And the question was raised about hydraulic expansion. I'll mention explosive as well. Both lend themselves to protecting the welded end.

rmw

 

[gr2vessels]

My two cents;-
Two passes of weld are calling for full strength weld, not seal. Hence no roll is required (it also depends on the tube wall thickness).
However, to prevent the crevice corrosion, after weld a light roll might be justified. Also, the uneven weld heat and progressive heating of the tubes may loosen up the expansion if performed before welding.
Amazingly, another fabricator needs to expand first the tubes to prevent them moving in the tubesheet during weld!
That only proves that the fabricator experience and his equipment determine the priority of weld and expansion, not theoretical documentation.
On the other hand, I don't think is correct to leave this issue to the welder discretion, probably is time to get some proper code or similar ruling.
Cheers,
gr2vessels

 

[SnTMan]

Well, not much to add except if the customer specifies welded and rolled, that's what the customer gets.

Regards,

Mike

 

[MikeMet]

It needs to be stated that a hydrotest at 1.3 design is a strength test. It is a very poor leak test. If you require the weld to be truly leak free you need to perform a senstive leak test of the weld, such as helium leak test. This can only be accomplished if done after welding and before expansion. If done after expansion you run the risk of having a tight leak in the weld that is not found if the tube expansion produces a leak tight seal (the helium never makes it to the weld to find a leak). I agree that application of a helium leak test is dependent on the consequence of failure or if the equipment is considered PSM critical. As a compromise, you can have the welds tested by air soap at much lower cost and less time than a helium leak test. Not quite as sensitive as helium, but still magnitudes better than hydrotest.

 

[gr2vessels]

The helium test is a test required in specific services only. The helium test will show leak in an application when water won't leak at normal operating and hydrotest pressure. So horses for courses. If you can afford it and the Client demands the helium test, then do it, but careful with compressed gas in the shell! But do inhale a bit of the gas and start talking to the subordinates...hilarious.
Cheers,
gr2vessels

 

[rmw]

Helium leak tests are very common in Hx's for lethal service.

And I suppose in the old days when electric utilities in the USA made guaranteed mark up on the cost of building plants, it made the power companies money by requiring and paying for Helium leak tests. Much more common way back then as were Hx's that were grossly oversized.

Then deregulation happened.

rmw