7 replies to this topic

[lafondejs]

Dear All,

Can anybody tell me, what are the limitations of using of welded tubes (CS or SS) for heat exchanges (Shell & Tubes Or Air Cooled). I wanna know Upto what temperature, pressure and size (Diameter and Length) limit, I can use welded tubes for heat exchanges. Actually I can find Temperature, pressure rating for welded tubes, But i wanna know the limitations from a experianced heat exchangers desiner's point of view.

Thank you.

[Art Montemayor]

lafondejs:

Why would you even consider welded tubes for any exchanger? You realize, of course, that it is a risk on the quality of the tubes that you are taking. Are you, or your employer, that hard up for capital or maintenance cash?

I simply have never employed any welded tubes because I didn't consider the risk worth the money I was presumably saving. When you have to calculate and prepare the mitigation for a ruptured tube taking place inside your exchanger, you can appreciate what I am stating.

[kkala]

Following notes are from someone inexperienced; that is why I made a search to learn a bit, comments would be welcomed. Local Process is not responsible for the matter, so text below represents views from other Depts of an Engineering Company, or from literature. References in parenthesis concern Perry's Chemical Engineers' handbook, 7th edition.
1. In local refineries there is a tendency for seamless tubes. Exchangers, whose data sheets were prepared, had all seemless tubes (specified by others); same for boiler superheaters; and for boiler economisers (finned tubes).
2. Nevertheless Perry indicates that exchanger tubes can be either seamless or welded (Heat-transfer equipment / Tube-side construction). So this is valid for TEMA exchangers (all classes?) (Perry, TEMA-style shell-and-tube heat exchangers / General design considerations).
3. TEMA exchanger tube data is given in "section 9, Table D-7 of TEMA"; it must comply with ASME, Sec VIII, Div 1, at least for class C, http://inquiries.tema.org/inq9/SelectedInquiryList8.htm - 9th question from top.
4. Apart from a bit better manufacturing thickness tolerance, welded tubes do not seem to have a technical advantage; they have some risk of premature leaks due to corrosion; besides expansion or bending seems to need extra consideration (Perry, Process-Plant piping / Metallic pipe systems: carbon steel and stainless steel).
5. Design pressure (for given design temperature) of welded tubes is 80% of seamless tubes of same nominal diameter, material and thickness (excluding corrosion allowance), in the case of attached "76b9666ec1764971a877b924f2e17cdd3.pdf" (not referring to exchangers). Perry indicates a factor 60% - 100%, depending on welding.
Allowable stresses for several temperatures are reported for carbon / stainless steels; no distinction is understood between pipes and tubes. Upper temperature limits can be concluded from this (Perry, Process-Plant piping / Piping-system design).
For more precise info, welded pipe standards for exchangers have to be looked into (TEMA?).
6. Cost advantage of using welded tubes (instead of seamless ones) in exchangers is not known (to me), but could be doubtful.
7. Stainless steel welded tubes for exchangers have been found in the market, http://www.dynamiceximcorp.com/stainless-steel-welded-tube.htm, http://www.valtimet.com/index.php?option=com_content&view=article&id=4&Itemid=7.

Attached Files

•  76b9666ec1764971a877b924f2e17cdd3.pdf   166.3KB   1936 downloads

Edited by kkala, 18 November 2012 - 05:12 PM.

[T.S.MURALI]

welded tubes recognized by codes are used.But seamless tubes are without doubt , safe for use in the long run.

welded tubes are susceptible for cracks and propagation there of-may be in corrosive and critical applications.

To avoid downtime due to weld cracks,HFS,CDS tubes are preferred.

[lafondejs]

Dear Mr. Art,

Actually I do agree with you...there is a risk. But sometimes customers/clients request that they need welded tubes. Such a case if that heat exchanger is a low pressure & low temperature heat exchanger, we are proceeding with welded tubes (Normally pressure < 3bar & Temp < 150C), if it is a high temperature & high pressure heat exchanger, then normally we discuss with their engineering team. Normally they are selecting welded tubes because of the less price. According to my experience welded tube price is 20 to 30% cheaper then seamless.(in Asia Pacific region, sometimes even 40%...!!! depend on the material origin). I asked above question because the above limits(150C & 3Bar) which my team uses are not form books, they are just values which we are thinking that welded tubes will be ok below that limit.

Thanks.

[lafondejs]

Dear Mr.kkala,

Thanks for your post. According to my experience welded tubes are 20 to 30% cheaper than seamless tubes (In Asia Pacific region). Cost of a heat exchanger with large number of tubes is majorly depend on the tube price, using of welded tubes instead of seamless reduces the cost of heat exchanger around 20% or even more (with a risk..!!!).
Please read post #5 if you can.

Thanks.

[kkala]

Thanks, lafondejs, for the information; actually I had not detected post No 5 when posting No 3.
Welded pipes (and most probably tubes) seem easier to make than seamless ones; so a country could avoid imports, if welded tubes are locally produced.
Large diameter welded pipes for natural gas (made here) have design pressure of about 80 Barg http://www.cpw.gr/Resource.aspx?id=25. Well, welding methods can differ for small diameter tubes, but look at https://www.swagelok.com/search/find_products_home.aspx?show_results=Y&item=a6f62494-4dec-443b-a7e8-0de9f61ea57c (*). Probably considered limits (3 barg, 150 oC) could get higher (especially for non corrosive fluids), after market investigation or an expert's advice.

(*) in the site go to
tubing and tube accessories / tubing / 304-304L welded tubing, Features: size 1/8" to 1", pressure ratings up to 8160 psig.

Edited by kkala, 19 November 2012 - 07:36 AM.

[Jack Lion]

Stainless steel welded tubes have been approved to be applicable in heat exchanger, but shall be in compliance with ASTM A249 standard.

 

Tubes shall be heat treated and inner weld seam removed. The most important is hydro-static test and eddy current test shall be 100% performed.

 

Compared with seamless tubes, the advantage of welded tubes are mainly as follows:

 

1. cost effective

 

2. small tolerance on wall thickness