U-type tube bundles

[Kazi Ghulam]

Hi All,

I am working in oil plant. My question is regarding heat excangers/reboilers. One of U-tube type reboilers has oil in shell and steam in tubes with 360 tubes overall, till now 70 tubes have been plugged. What is the margin in tubes is it 20% or 30%....what are TEMA standards. Apprecaite if any one help on it.

 

[iangineer]

TEMA is more for the design of new exchangers and allows the manufacturer to plug a maximum of 1% or 2 tubes without prior agreement.

For in the field, I think after 10% of the tubes are plugged, the unit would need to consider if enough heat is being transferred for their needs. If not, it's time to retube the bundle, or just purchase a new bundle if the shell and channel are in good condition.

 

[Kazi Ghulam]

Thanks Brother - That means thee is no TEMA recommnedation to replace or retube bundle if 10% or more tubes have been plugged. In our case, out of 360, sevety tubes (around 20%) have been plugged so far but still heat transfer is fine.

 

[iangineer]

It's surprising that 20% loss in exchanging capacity is not causing any heat transfer issues. Another thing to consider is the increased flow rate through the tube and increased tubeside pressure drop. This could cause vibration issues possibly or starve the downstream steam flow.

 

[Some Curious Guy]

Most Exchanger have 10 to 15 % over design on duty hence I think they can tolerate some loss of heat transfer area of tubes due to plugging. However it is a little surprising that even after loss of 20 % tubes, the heat transfer objectives are achieved. It is recommended to recheck all the design and operating cases of the original design. Maybe the exchanger is insufficient for some infrequent operating condition.

 

[Dhurjati Sen]

What is causing the leaks should be investigated.

Very soon the reboiler would have to be taken out of service leading to unplanned shutdown of the unit, unless, of course, there is a stand-by.

DHURJATI SEN

 

[MJCronin]

"SomeCuriousGuy" gives good advice, and I agree .. Most units are designed with a small oversurface margin, just for the record ...

Because each shell and tube unit is unique, only the HX owner can determine what percentage of plugged tubes is acceptable.

TEMA contains no guidance on the maximum percentage of plugged tubes, ..... However, TEMA (and other sources) do contain recommended maximum tubeside steam velocities based on the materials of construction. IMHO, this is kind of a back-door limit on plugging

It would be both simple and straightforward to calculate your original design tubeside velocity and compare it to your new plugged conditions.

Harder tube materials (e.g. stainless steel, titanium) are permitted to have much higher design velocities than, for example softer coppers and brass

https://jmpco.com/Files/Files/Technical Documents/B&G Steam Heat Exchanger Tech Manual.pdf

Of course, if you block too many tubes, the liquid velocity screams and your HX will suffer a quick and ignominious death

Higher velocities of steam may also induce destructive tube vibration into your HX

Give us more specific details about your HX and materials .... we cannot help if you resist sharing information !!!

What about your recordkeeping ? Have you made a tubesheet map to keep track of the blocked tubes and the pattern that they make ?

Can you post drawings of datasheets ?

MJCronin
Sr. Process Engineer