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Design Condition Heat Exchanger

heat exchanger

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#1 rikakose

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Posted 21 February 2019 - 04:25 AM

We have a heat exchanger from 1980's. 

On the spec sheet, the design pressure of the shell size is 5 bara, tube side is 0bara. 

Test pressure shell side 7,9 barg, tube side is 3 barg. 

 

Design pressure of tube side is very strange. 

Since the shell size pressure is 5 barg, can we also say the tubes can handle 5 barg?

Test pressure, when the equipment is being tested, only the shell is tested or with the bundle in?

 

Thanks.

 



#2 Pilesar

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Posted 21 February 2019 - 10:16 AM

'Design Pressure' on a spec sheet is probably the expected maximum pressure as determined by the original process engineer. The exchanger was probably intended to be in service on the tube side where little internal pressure was anticipated. The 'Maximum Allowable Working Pressure' is a mechanical constraint and may be much greater than the 'Design Pressure'. Try to find the documentation from the vendor where they calculated MAWP. Where did you get the test pressure documentation? If this is a certified pressure vessel, then the documentation should be available and you should also have MAWP on the physical equipment tag attached to the vessel. If you have no documentation and no equipment tag, this vessel should only be used in very limited circumstances. While an authorized vessel engineer might can calculate and certify the exchanger for pressure, this is much easier to do at the time of manufacture and it may be more cost effective to replace the exchanger with a certified pressure vessel. If this is not a certified pressure vessel, why are you pressure testing it? Is this just for leak testing? Or is the exchanger being moved to a different service? As a practical matter, tubes generally can withstand very high external pressure due to their cylindrical shape and there is little chance of them bursting inwards when the shell side is pressured. If you pressure test, do it with the exchanger assembled.



#3 Art Montemayor

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Posted 21 February 2019 - 04:58 PM

In spite of meager data you have furnished, you have received some very valuable advice and comments from Pilesar, a very knowledgeable and experienced engineer.  His comments and questions should be seriously addressed by you.  I add some personal experienced comments here to reinforce what Pilesar has submitted:

  • Your heat exchanger is almost 40 years old and you fail to state what physical condition it is in.  Are the tubes new?  Or are they the originals?
  • Is the Specification Sheet one issued for design or purchase?  Or is it really a Data Sheet - a document stating the “as-built” condition of the exchanger?
  • The specifications are not what one would expect: the higher pressure fluid is almost always introduced in the tubes and not in the shellside.  This sounds like a very unique application.  But all exchanger tubes are sold on the basis of standard thickness - and therefore, the MAWP of the tubes is fixed and not a function of the application.
  • The inherent high pressure rating of all exchanger tubes is related to the basic equation for their design pressure.  The smaller the tube diameter, the higher the rated pressure allowance, just as Pilesar has stated.  Therefore, I highly suspect the test pressures given for the shell and the tubes.  The tubes should normally withstand a much higher pressure than the shellside.  The hydrotest should have been done taking the MAWP of both shell and tube in consideration.  Who did the hydro test and why is the set pressures not explained?
  • Who designed and fabricated this heat exchanger?  Isn’t there a name plate on it?
  • You can calculate an estimated MAWP for both the shell and tube side.  But you must have a detailed inspection report on the physical condition of both the shell and tube side.  I would suspect a lot corrosion and erosion has taken its toll in the last 40 years and this must be accounted for.
  • You haven’t even told us what TEMA type the unit is, so we can’t comment on the feasibility of re-tubing or putting in another shell - especially since we don’t also know the physical size of the unit.
  • You are probably better off buying a new unit if you can’t come up with credible, detailed basic data on its design and fabrication.


#4 rikakose

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Posted 22 February 2019 - 06:31 PM

Pilesar:


 

Thanks a lot for the reply.

 

I only have an simple mechanical drawing. Spec sheet is actually the spec table on the drawing. very limited information is listed in this table. And sadly the original manufacture has been bankrupt. Test pressure is mentioned in the spec table but no authority documentation. 

 

Currently this heat exchanger is connected with a vacuum system, intended to condense some vacuum vapor (that's probably the reason the tube side is 0 bara.)  Apparently, it doesn't work as it supposed to be. 

 

Now, I want to move it to another system, to condense some pressure vapor 3 barg. 

Without proper documentation/ certificate, I am worrying about the safety issue. 



#5 rikakose

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Posted 22 February 2019 - 06:50 PM

Thanks Art for the reply. 

 

This 38 years old heat exchanger has not been inspected for couple of years. The tubes are original. I want to move it to a 3 barg system instead of the vacuum system where it works now. it is too small to the vacuum system, hardly condense anything. Material is SS316, tube size 15 x 2mm.  The tubes are far more than enough to take 3 barg pressure. But if I move it to another system, I still need some certificate or solid information. People may question about the welding points or connections.......






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