No Tube In Window (NTIW) Tubesheet Design

[roca]

Both TEMA and UHX do not cover the design of a NTIW tubesheet.
How are these tubesheets normally designed?
Is FEA the only solution?

 

[SnTMan]

roca, formerly TEMA, now both TEMA and Part UHX. I don't know that anybody ever paid much attention to that particular limitation. I'll bet less than a tenth of a percent, if that, of the tubesheets in service were FEA.

Regards,

Mike

 

[roca]

Thanks SnTMan
I too agree with what you say

 

[SnTMan]

roca, I have found Part UHX tubesheet rules to be so limited in applicability as to be nearly useless, strictly speaking. I use it anyway. As I said in the other thread, I use TEMA also. U-2(g).

As for FEA, it is not necessarily more accurate, and you can bet the customers don't want to pay for it.

Regards,

Mike

 

[Bobfromoh]

What is an NTIW tubesheet?
Were is it used?....Thanks

 

[SnTMan]

Bobfromoh, NITW design is a tube layout such that there are no tubes beyond the cut portion of the baffle (the "window"). Mostly used when tubes are required to be supported between baffles, such as wide baffle spacing. The support is just a baffle with cuts on both sides of the tubefield, so that it does not turn the flow.

Regards,

Mike

 

[SnTMan]

Like this. Plus this one has a long baffle. The hidden line is the opposite baffle cut.



Regards,

Mike

 

[Shmulik]

roca,
Both TEMA and UHX do not let you credit plate thickness for the area that is not been drilled.

On the other hand,
as a significant area of your tubesheet is unsupported, you should be aware about the possibility of falling into UG-93 (d)(3) examination requirement:
"When a pressure part is to be welded to a flat plate thicker than ½" to form a corner joint under the provision of UW-13 (e), the weld joint preparation in the flat plate shall be examined before welding as specified in (d)(4) below…. The requirements of this paragraph shall not be apply to those joints when 80% or more of the pressure load is carried by tubes, stays, or braces".

 

[SnTMan]

As a practical matter, I do not attempt to determine whether "80% or more of the pressure load is carried by tubes, stays, or braces", and just do the NDE. Its cheap.

Regards,

Mike

 

[roca]

he NTIW design is sometimes required to eliminate tube vibration. The NTIW design has no tubes in the baffle cut out. By using intermediate supports between the baffles the natural frequency of the tubes can be increased to resist tube vibration.

 

[Mandeep1]

From what I have heard, the TEMA committee is working on developing factors that could be used when there are large untubed areas. These factors would plug in UHX design rules. So, you can essentially use UHX for this design.

Hopefully this would be available in the next TEMA update.

 

[eliebl]

FYI,

UHX does in fact take into account untubed areas. It is considered in the effective tube pitch but limited to 4 times the product of equivalent diameter and tube pitch. I have not seen it make a big difference but it does have an impact.

EJL

 

[SnTMan]

eliebl, UHX accounts for untubed LANES such as pass lanes. According to an interpretation I've seen elsewhere on this board, it specifically does not apply to NTIW design. UHX-10(a) says tubed area shall be nominally circular.

Regards,

Mike

 

[eliebl]

Mike,

I was not trying to imply that UHX was applicable. In fact, I could probably make a convincing argument that UHX is not applicable in all but a few cases. The advantage that UHX has is that the background is widely available.

An assumption that I have seen several times on the NTIW design is to consider that the area is in fact tubed. In the case of UHX this area is considered to have a lesser strength because of the tube holes. These holes do not exist in the as-built exchanger. I cannot say if this assumption is more or less conservative, just that it was used more than once.

EJL

 

[SnTMan]

eliebl, yeah that is pretty much what you have to do, just consider it fully tubed, or in other words, ignore that limitation of Part UHX. I'm with you, I don't know whether it is more or less conservative either.

I do know the work still has to get done

Mike