definition of 'unsupported tube length' in TEMA

[ksquare]

Hi ya, can anybody define the term 'unsupported tube length' in TEMA code? according to TEMA, its mean the span tube length between central baffles, but if one have the segmentel baffles, eg baffle#1, baffle#2. can we define it as the span length between baffle#1 and baffle #2 instead of baffle #1 to baffle#1 in order to reduce the baffle plate thickness. mean while is there any method to calculate the baffle plate thikness instead of table RCB4.41 shown in TEMA. any answer for this will be nice.

 

[ijzer]

Unsupported means in your case from baffle 1 to baffle 1.
There is no method to calculate , the load is very low. If you are allowed to deviate from the TEMA , it is adviced to make the baffle at least two times the shell-side corrosion allowance.

 

[ksquare]

Thanks ijzer, can you please further your explanation about 'two times the shell side corrosion allowance'. may i know where this practise come from? i wish not to follow the baffle thickness given by TEMA as we would like to adopt the thinning one provided to ensure the baffle thickness is adequate.

 

[vpl]

ksqurare

I think the key words are "if you are allowed to deviate from TEMA". Just because you don't want to follow a code or regulation doesn't mean you're allowed to deviate from it.

Is there some key factor that is driving you to need a thinner baffle plate?

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[SnTMan]

FYI, I believe API 660 REQUIRES baffles to be at least 2X shell side corrosion allowance, if you are working to that.

ksqurare, I can't prove it, but the practice of making the baffles 2X the shell CA would seem to insure that the baffles do not disappear before the shell has reached the end of its useful life.

You want to make the baffles thinner, use high alloy or non-ferrous.

Regards,

Mike

 

[ijzer]

My advice of 2 times the shell side corrosion is indeed coming from API 660. I do not think it is wise to save some money on baffle plate thickness. I have seen exchangers that had baffles as thin as a rasor blade after some years of operation . The shell and the tubes were still acceptable , but due to the thin baffles , the bundle had to be replaced. Penny wise pound foulish.

 

[Bobfromoh]

Also, the baffles are there for thermal design purposes. The baffles direct the shell side fluid over the tubes.
If the baffles are corroded away, the thermal efficiency will be affected.