Tube-Tubesheet Joint on Heat Exchanger

[JMAN2001]

Could someone please explain the difference between a rolled and seal welded tube to tubesheet joint versus groove/expanded?

What are the advantages and disadvantages and when would you use one over the other?

 

[speco]

JMAN2001,

The two joints are basically the same thing, except that one of them has a seal weld, usually around the OD of the tube at the tubesheet. Sometimes the manufacturer might skip the groove(s) in the tubesheet, but they are standard in most process heat exchangers.

The advantage with the seal weld is that there is less chance of leakage, especially in cases which are more prone to leaking such as exchangers using hot thermal transfer oils.

Regards,

Speco

 

[rmw]

Just remember that the rolling is for joint strength and the seal weld is for sealing. Don't confuse the two different functions.

rmw

 

[doct9960]

JMAN2001,

Try to get a hold of AWS Welding Journal 2000, Vol. 79. There is a three-page article in there titled Improving the Reliability of Tube-to-Tubesheet Joints written by Harry W. Ebert.

Ebert discussed four types of tube-to-tubesheet joints:
1. Roll or expand only (without grooves)
- strength is poor
- leak resistance is poor
- tube replacement is very easy
- application is limited (i.e. low pressure water or air)
2. Roll or expand only (with grooves)
- strength is limited (can be improved by explosive expansion)
- leak resistance is limited (can be improved by explosive expansion)
- tube replacement is easy
- applicable for low stresses and low consequences of leakage
3. Roll or Expand and Seal Welded (with grooves)
- commonly used by the oil industry
- strength is limited (can be improved by explosive expansion)
- leak resistance is good (can be improved by explosive expansion)
- fabrication problems are frequent
- tube replacement is more difficult
- applicable where stresses are not too high and where risks of leakage is low
4. Strength Weld (no grooves required)
- full strength
- leak resistance is good (maximum leak resistance can be achieved if two pass welds are used)
- some fabrication problems, esp. if there are grooves
- tube replacement is more difficult, esp. if there are grooves
- applicable where mechanical and thermal stresses are high and leakage is unacceptable

 

[Montemayor]

No one mentioned it, but it is very important to note the specific procedure if a rolled joint is compared with a seal-welded joint or if the two are combined:

A seal-welded joint can also be combined with a rolled joint; however, the joint must first be seal-welded and then, followed by a rolling of the welded tube. doct9960 doesn't identify this procedure as a "strength weld"; I presume that this is what he describes. Note that an expanded tube is not welded after being expanded. I was always warned by mechanical engineers that the stresses built up in the rolled joint make welding it into a weaker physical joint.

I have used the seal-welded + rolled joint many times with success, so I would have to give credance to my ME mentors.

 

[doct9960]

Montemayor,

Note that an expanded tube is not welded after being expanded. I was always warned by mechanical engineers that the stresses built up in the rolled joint make welding it into a weaker physical joint.

Some Users (or his designated agent) and manufacturers prefer to expand first and then seal weld. By doing this sequence, you eliminate the risk of cracking the seal weld during the expansion process. The decision on which sequence (seal weld then expand; or expand then seal weld) to use is still up to the User or his agent.

A seal weld is not a strength weld. A tube-to-tubesheet seal weld is just a supplement to an expanded joint to ensure leak tightness. It is not designed for strength. For an ASME VIII-1 exchanger, refer to UW-20 for the definition of different types of tube-to-tubesheet welds and for the requirements of strength welds. ASME classifies tube-to-tubesheet strength welds as either full or partial. From ASME VIII-1 UW-20...
"A full strength tube-to-tubesheet weld is one in which the design strength is equal to or greater than the axial tube strength...A partial strength weld is one in which the design strength is based on the mechanical and thermal axial tube loads (in either direction) that are determined from the actual design conditions."

 

[rmw]

Sorry, but I am with Montemeyer on this one (and I am still intend to telephone you Art-forgive the personal note).

The reason that seal welding is done first is to allow the welding gases behind the weld to escape along the tube. To roll first means that the gas is trapped behind the weld at the point of closure and it will blow out at the end of the pass destroying the seal.

The only exception is titanium tubes to titanium tube sheets where the proper procedure is to roll first and then weld.

If a seal welded joint is properly expanded, whether by rolling or explosive or hydraulic expansion, the expansion needs to begin about 1/2" behind the seal weld so that there is no danger of mechanical damage to the seal weld. Anyone who would roll or otherwise expand too close to the seal weld doesn't know what they are doing. Some explosive expansion charges have a duplex charge where the leader is a lower charge cardite and the cardite that does the expansion is a higher charge weight. Others don't-the cardite is uniform from the leader to inside the tube. Watch those guys.

All that said, a high pressure Feedwater heater (5000 PSI working pressure) on a supercritical unit in a state up north of where I am now was found some years ago to have a row of tubes where the entire row had failed to be expanded and it wasn't detected at initial manufacture. The error wasn't caught until the heater was field re-tubed some 15-20 years after it went into service. So much for seal welds not being strength welds. But, then again, the exception doesn't make the rule.

Unclesyd and I had this discussion some years ago in this same forum I think, and he has a different take in that he had a lot of success with the reverse of the process I advocate, so how can you argue with success.

A search can produce that discussion.

Most of my experience is in high pressure stuff, so I can't address low pressure heat exchangers.

rmw

 

[unclesyd]

I have to confirm the rmw and I had that discussion.

We have always rolled the tubes then welded on all tube bundles built on site. This is the majority of tube bundles on this site.
The site now only builds/rebuilds the smaller bundles up to 1000 tubes. With a few exceptions the highest pressure is 450 psig.
On all Austenitic SS bundles we do roll further past the tubesheet than most shops.

As corrosion is a problem on most of our tube bundles we go with a thicker than require tubesheet. This allows us to use 3 grooves and facilitates doing a light cut on the face to clean up the gasket surface.

 

[acrylonitrile]

I am sizing a new heat exchanger for 850F to 450F for lethal service.The tube sheet and tubes are 304L material.This heat exchanger sees the thermal cycles 24 times a year.
One of the vendor recommends tubeless and strength welded tube to tube sheet. Is this a good advise?

 

[rmw]

There is a word wrong somewhere in that previous post.

rmw

 

[Montemayor]

What is a tubeless tube?

 

[acrylonitrile]

I am sizing a new heat exchanger for 850F to 450F for lethal service.The tube sheet and tubes are 304L material.This heat exchanger sees the thermal cycles 24 times a year.
One of the vendor recommends seamless tube and strength welded tube to tube sheet. Is this a good advise?

 

[rmw]

Seamless, now that makes all the difference.

I think it is good advice. I hope you take it if I ever have to be in your plant.

Was the vendor going to do any kind of tube expansion?

rmw

 

[unclesyd]

Is the lethal substance your are referring to HCN?

As far as I know we still use this approach on tube bundles in this service.

 

[EdStainless]

For true lethal service it doesn't matter what type of tubes you use (welded, welded and drawn, seamless) what matters is your NDT. High quality Ultrasonic Testing is a must. I have seen He leak testing as well. It is more sensitive than any other NDT method.
Weather you seal weld or full strength weld make sure that you leak test after welding and then roll the tubes. Stay back from the weld and don't get too near the back edge of the tube sheet. You don't want to leave the open space.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm