Baffle Inspection on Shell & Tube Heat Exchanger

[tommynugroz]

Hello !

We had an unused Sulphuric Acid Cooler, a horizontal fixed tubesheet Shell & Tube type. It was used to cooling down the circulated Sulphuric Acid liquid on its Shell Side. Sea water was the coolant on its Tube Side. This cooler operation time was relatively short since it was installed on 2016 (3 years old). However, it had an increasing trend of LMTD, even with the same Q load, Sea Water and Acid flow. It meant it heat transfer performance was decreasing. During twice periodical maintenance shutdown time, we had opened its Tube Side to confirm for any clogging materials, the condition was clean. We also conducted water jet cleaning on each tube and there were no clogging tubes. When operating, the acid circulation pump's pressured discharge and motor current were still normal, same as previous.However, its LMTD was still gradually rising up. And finally, we decided to replace it with the new one. And everything was back to normal.
If the tube side was normal, then there might be something wrong on its Shell side. We suspected that some baffles position might be changing even it did not clogged the Shell Side flow, or, some Baffle Plates had been corroded and perforated, so the acid were mostly flowing passing through the baffles instead of up and down following the baffle cuts. Hence, we would like to inspect inside condition of the Shell Side and its Baffles. But we would like to avoid any damaging test.
Is there any kind of Non Destructive Test to check the baffle condition inside of a fixed tubesheet Shell & Tube Exchanger ?

 

[georgeverghese]

More obvious suspect cause for increasing LMTD ( since you have found clean condition on tube side ID) would be corrosion on OD of tube -ie tube OD which is part of shellside. High deposition of corrosion product here would impede heat transfer and reduce shellside htc. To see this, you have to pull out the tube bundle. Presume you cannot do this, hence your request for some kind of non destructive means of inspection.

 

[tommynugroz]

@georgeverghese:
That is also one possibility that cause increasing of the LMTD.
If possible, we would like to know any kind of non destructive inspection method that did not require physically damaging the cooler parts.

 

[EdStainless]

Use a video borescope and look inside. You can rent these with long probes (I have seen 100') and some are semi-rigid and steerable.
Take a look.

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P.E. Metallurgy, Plymouth Tube

 

[MJCronin]

Seawater cooling Sulfuric acid, ehhhh???

And you feel that it's service life was too short... Right ?

All Carbon steel construction ???....

MJCronin
Sr. Process Engineer

 

[tommynugroz]

@EdStainless:
We'd had think about the video borescope too. But currently we only had a short probe, while our cooler was about 10 meters across the shell side length. We need to seek steerable borescope with at least 15~20 meters probe length.
Alternatively, we would like to know the possibility of other method for checking the baffles.

@MJCronin:
As on other Sulfuric Acid Plant in the world, the acid circulation cooler is usually made from a combination of ni-based alloy and SUS304/L material that are quite suitable for sulfuric acid concentration above 80% (also depend on the temperature). And, the corrosion rate was inhibited by cathodic-anodic protection system.

 

[georgeverghese]

Tube to tube sheet joint leaks are common in heat exchangers. If sea water pressure is higher than acid pressure, then sea water may have leaked into shellside. To check if there are leaks at these joints, do a hydrotest of tubeside. Are these tubes seal welded or just plain roller expanded into the tubesheet? Dissimilar materials of construction for tubes and tubesheet can result in localised galvanic corrosion and accelerate corrosion at these joints.

 

[tommynugroz]

@georgeverghese:
There was no tube leakage and no tube clogging. Our cooler problem was that the LMTD was increasing, the acid temp on the shell side outlet was increasing. That’s was why we would like to know what happened inside of the shell side.

 

[EdStainless]

Tommy, yes, you could either have lost baffles or you have fouled the OD of the tubes. Both of these would reduce the effectiveness of the HX. Take a look, it will tell you a lot.

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P.E. Metallurgy, Plymouth Tube