7 replies to this topic

[Shree317]

we apply 10 13th rule for S&T exchanger and multitube exchanger. but why dont we apply for double pipe exchangers. what is reason for it?

[fallah]

Hi,

 

It was better you submitted the query in "Relief Devices Forum"...

 

However, as per API 521, in fact only those double-pipe heat exchangers that included "schedule pipe" as the inner tube, wouldn't require provision for overpressure relieving. It is due to the assumption that a complete rupture for such inner tube (actually pipe) cannot be a credible scenario.

 

Then, those use "gauge tubes" should be investigated for overpressure relieving requirement.

Edited by fallah, 05 November 2013 - 02:46 AM.

[Bobby Strain]

And, there is no such thing as 10/13 rule. If you look at the API RP you will clearly note that when the test pressure of the low pressure side is => the maximum operating pressure of the high pressure side, then no relief is required for tube rupture for the low pressure side.

 

Bobby

[Shree317]

@fallah,

 

for scheduled pipes why we dont take provision of rupture. is it because it has more thickness than tubes or something? for example 1" NPS sch 40 having the thickness of 0.133" whereas 1" Tube BWG 10 has about 0.134". so it has more or less same thickness.. so why dont we consider. 

Edited by Shree317, 05 November 2013 - 11:09 PM.

[fallah]

Hi,

 

Pipes are designed to more conservative rules than tubes. A double-pipe heat exchanger, including "schedule pipe" as the inner tube side, can rightly be supposed to be a jacketed vessel. Then, if the shell rupture of a jacketed vessel can be a credible scenario, inner pipe rupture of a double-pipe HX can be as well...!

[Shree317]

hi fallah,

 

what do u mean by "conservative rules", AFAIK even thought pipe or tube it has to validate "Stress=(pressure*dia)/(2*Thickness)"(hoop's law). so where do we get conservative rules. even i dint find any such hard and fast rule in ASME B31.3.

 

and another query that, you said pipes have some rules so we can avoid rupture case. but if u see in fired heater we do consider tube rupture case.  

[fallah]

_Hi,

 

By...designed to "conservative rules"... i meant: e.g. the pipe is normally used as is and rarely bent while the tube most often is bent if needed; or the pipes are joined by weld, flange,... while tubes are joined by solder, weak fittings,...Indeed ASME B.31.3 covers schedule pipes while don't cover gauge tubes...See attached...

 

There is no special code for design and fabricating the double pipe heat exchangers as a pipe in pipe HX and the piping codes are normally used in this regard...Then it can be evaluated as a jacketed pipe within an applicable piping code...

 

The tubes inside the fired heaters is another story and not only they are subject to high thermal radiation but also aren't covered by applicable piping codes such as ASME B.31.3 (see attached)...

Attached Files

•  Pages from ASME B31.31.pdf   82.72KB   68 downloads

[Shree317]

thanq fallah. for fast rplies.

 

but which codes we do apply to design tubes. i dont understand that much difference between the design of the both.. basically both are comes under in Thick or Thin cylinders. so what does it make difference. you said that we solder the tubes to join whereas pipes can be weld. for example lets say KCS of 6"NPS sch40 and tube KCS 168.3OD BWG 12 have almost of same thickness.. and moreover same material. so the mechanical properties should be same right. why they change? if they have same properties why cant we weld the tubes?

 

yes. i know that ASME B31.3 does cover heater tube design. it actually design acc. API 530.