Heat exchanger Evaluation 2

[babak528]

I have a shell&tube heat exchanger with 15% of tubes plugged!! How Can enusre that exchanger is still safe to operate? is there any sort of calculation that I can perform?

 

[framxo]

Babak528,

You need to check whether the remaining area of your heat exchanger can still meet the duty of the services (be it out let temperature etc.). You can use any of the available exchanger simulation tools in rating mode to estimate the exchanger performance. What might also be worthwhile to check, is your new tube side velocity after plugging the tubes. Check if the new velocity is still within the allowable operating range and no excessive pressure drop is experienced. I hope this helps.

 

[LittleInch]

If the tubes were plugged because they leaked, then I doubt there is any calculation you can do on the tubes to ensure it is "safe". All the other parts should be able to be measured and a design checked. Sounds to me like you needed to pressure test this and then apply a working pressure at a suitable percent below the test pressure. Regular retesting sounds likely.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gr2vessels]

It sounds like an old exchanger with heaps of design margin, otherwise your process would have shown long ago significant change in the outlet temperature. As LittleInch said, your main problem is most likely the sealing of those plugs. If the operating pressure and temperature have not changed, the process appears to cope quite well with 15% of plugged tubes, so you don't need a new heat transfer calculation. The safety has nothing to do with the plugged tubes, as long as you don't crank-up the pressure or temperature to force the desired output. I am also assuming the plugging has been done correctly and you don't have reasons to suspect leakage or further degradation of the exchanger.
Cheers,
gr2vessels

 

[babak528]

Thanks for the reply.

The tubes were initially plugged as a result of a recent shut down and due to fouling of produced water, and now they are completely sealed with tapper plugs. The expectation is that some form of engineering calculation be documented on record.

This is how I am planning to do my calculations:
Q= UA (ΔTlog mean)

I assume (ΔTlog mean ) changes will be negligible. I will calculate “U(new with 15% tubes plugged)” and “A (new with 15% tubes)”.
Knowing “Q (old with no tube plugging)”, New heat duty can be calculated by writing a ratio between heat duty old and heat duty new:
Q (old with no tube plugging) /Q (new with 15% tubes plugged) = U (old).A (old)/ U(new).A(new)

And then the outlet temperature (t2) can be calculated from equation:
Q(new) = m.c.(t2- t1)

Using equation for pressure drop (found in the literature) and velocity equation I can calculate the new velocity as well as the pressure drop.

But then I have to compare these values with the acceptable design values!! However, I don’t know what’s the acceptable design values !!
Can you guys please give me a feed back for my calculation and tell me in you know the acceptable values for pressure drop, velocity, heat duty and temperature.

 

[TD2K]

< The expectation is that some form of engineering calculation be documented on record >

LOL. Is the unit operating now? You are doing to see two things with 15% of your tubes plugged, outlet temperatures will be higher than they were previously unless it's a constant temperature process on one side such as steam condensing and tube side pressure drops will be higher (the dP should be about 30% greater than it was before the tubes were plugged).

I'm going to assume you have access to operating data while the exchanger was in service prior to this work. Calculate the Q and the dTlm. You know the original area so you can calculate the overall heat transfer coefficient. If it's not been returned to service, I'd likely assume UA is constant and solve for the new dT log mean and see what that means for outlet temperatures. Decide if the higher temperatures (see the above caveat) is acceptable. If the unit is already back in service, compare your numbers to the actual field data as a check and make your evaluation if the operation is acceptable.

< and tell me in you know the acceptable values for pressure drop, velocity, heat duty and temperature >

Depends on your system. There's no way anyone can give you specific limits when you've provided no real information.