Expanded tube clearance

[BJaffa]

ASME Sections I, IV and VIII-1 all have brief specifications for tube expanding. Common is the 1/32" clearance between tube OD and tube-hole ID (see HG 360.2). This is the common .031" clearance we see in new construction. Rarely, during a repair do we find this hole size on firetube boilers. TEMA has slightly a different take on the clearance, but has a standard as well. NBIC is silent on what size tolerance is acceptable. We have some company policies on what we have found will work based on service life after tube replacement regarding overall size and out of roundness of the tubesheet hole. We are being called to qualify our procedures. What written practice is available for guidance?

 

[metengr]

We have some company policies on what we have found will work based on service life after tube replacement regarding overall size and out of roundness of the tubesheet hole. We are being called to qualify our procedures. What written practice is available for guidance?

Basically, the reason that most Codes and Standards are silent is that a procedure should be qualified using a mock-up with pull out tests. One can simulate existing hole conditions and tolerance and write a tube rolling procedure for a mock-up with pull out tests to qualify this procedure. This is how it should be done for repairs or new construction.

 

[BJaffa]

I am sure I read a requirement for qualified expanding procedures recently. I cannot relocate what I saw. I have studied expanding principals written by the USN from prior to WWII. Unlike Welding Procedures (WPS) the Codes have not required "Expandinng procedures. Is this about to change?

 

[metengr]

BJaffa;
Most of the HX work in the Power Generation sector requires tube rolling procedure and qualification. Just some advice.

 

[rmw]

And to reinforce metengr's point, most of those procedures are verified with mock-up's and pull out tests.

rmw

 

[saplanti]

"AS4458 Pressure Equipment Manufacture Sect19 - Tube hole drilling and tube expansion" has some guidance on tube hole diameter, tolerance and clearance for expanded tubes.

Tolerance on outside diameter of tube or swaged tube is very important in addition to others to control the expansion process. When purchasing the tube or swaged tube this part is mainly forgetten. Additionally you need keep the tube or swaged tube hardness under certain value to be able to achieve the full expansion. I suggest you contact your expansion contractor to get this hardness value.

I attached a spreadsheet to give you an idea about expectation from the expansion and tolerances. It might be helping.

Regards,

Ibrahim Demir

 

[saplanti]

Additionally, "AS 3857-Heat Exchangers_Tube Plates" Appx A is called "Tube-to-tubeplate joint-Determination of axial breaking load and joint efficiency".

It give the procedure for the breaking axial load determination of the expanded joints.

Harvey's book called ' Theory and design of Pressure Vessels" Clause 3.11 gives very good information about tube-to-tubesheet joints. Figures 3-24 and 3-25 are very valuable to get an idea about typical design allowable holding force on several type connections.

Some companies established their own standards on tube expansion by depending on the drum/tubesheet thiskness and tube diameters. Most drum connection have grooves for stresngth and leakage. I guess they are supported by tests.

Ibrahim Demir

 

[saplanti]

After a search on the internet I have found a spreasheet for the wall thinning at following link:

http://www.mgt-inc.com/docs/2006 January21-PercentWallReductionCalculationRevised.xls

Ibrahim Demir

 

[BJaffa]

Thank saplanti for your research. We have used these formuli and our own version on 10's of thousands of tube joints from 3/8" copper thru 4" x .220" WT at operating pressure in the thousands of PSI. The tube hole serations in HEX and drums resist axial expansion that tends to rip, the tubes rolled into thick tubesheets, loose as the expaning progresses. Our problem has always been with excessive tube holes and out of round tube holes. We have replaced tubesheets, welded and remachined tube holes and even used sleeves. We have proven success with holes larger than the 1/32" specifically called out in the Code. I believe the the input above is that a proof test (qualification) bypasses the Code requirement. NBIC in Alaska is the repair doctrine with provisons of the construction code incorporated. I don't know if We could prove that a tube hole .090 over size would work or not. It would cerainly stress the tube and shorten the life.

 

[saplanti]

I did not work with copper tubes, mainly used CS tubes in the high pressure water tube boiler applications. We basically go up to a wall reduction of 10% to achieve the strength and leak proof connection. Naturally, this helps to fill the clearance given by the code I mentioned above.

The clearance of 2.28 mm(0.09 in) seems very large even for 4" of tube diameter. I guess again everything is depending on the hardness of copper (guessing you use copper for this application ) tube.

If the HEXCH code is asking for certain maximum clearance for tube-to-tubesheet connections I would not use grater than that in applications in case there may be a problem with the authority or you may fructure the tube. I believe these clearance numbers come from years of experience with materials, wall thinning, bulging and the maintenance requirements. Therefore, if you do not comply with those you do not comply with the code rules and the result will not be economical either.

Ordering tubes with the right tolerance on OD and wall thickness for the expansion application is vital, otherwise you can not achieve the required max clearance. By machining the tubesheet hole with very fine tolerance is almost impossible and not economical. You need to controll both side tolerances before purchasing. I guess you are the expansion contractor and probably not involved in the purchasing of the tubes either unfortunately.

You did not mention the diameter in your application. If you can provide it I might provide you the required max clearance in acc. with AS1228 rules I mentioned above.

Regards,

Ibrahim Demir

 

[saplanti]

I attached the extraction of the section19 I mentioned above to give you the acceptable tolerances in accordance with diameters.

Hope it helps,

Ibrahim Demir

 

[BJaffa]

Thank you again saplanti. I errored in omitting that the range of tubes we work with included SS and CS in many sizes. We do often purchase the material from the mill and apply actual dimensions to our calculations. Your reference is new to me. Could you send the section 19.2.2 boilers. I am trying to find a compromise using some tube hole weld buildup and extreme limits of tube expansion. We will start today with full size mock up per metengr and rmw to have qualified procedures with establish tolerances. I am still not sure how this can comly to ASME or NBIC but we will have the demonstration as support.

 

[saplanti]

Here is the section you asked for. In case you need more information you need to purchase the standards I mentioned above.

For information to everyone; I suggest you to ask for suitable tolerance on OD of the piping in case you need to use them for expansion process on boilers or heat exchangers by considering the clearance permitted and reasonable machining tolerance on the hole. British piping standards have the option for different OD tolerances. I am not sure about the new European standards now.

Regards,

Ibrahim Demir