Heat exchanger H2S CO2 corrosion 2

[Bain89]

G'day guys

We have a tube heat exchanger which cools process gas from the Claus sulfur recovery unit (inlet 340 deg C) to about 140 deg C (outlet). The process gas contains H2S, CO2. The waste heat is used to generate steam. The boiler feed water is 100 C, steam leaves at about 140 C.

We noticed the outlet gas temperature was 70 C for 1 month! Which didn't make sense, as the lowest temperature medium is 100 C. And alarm bells rang in my head for condensation in the exchanger!

Opened up the heat exchanger, sure enough bottom was full of water, leaks found in bottom tubes. As the boiler water (shell side) is at 3 bar and process gas is 0.1 bar (tube side), a leak would introduce water into the process gas. This no doubt caused sulfuric acid to form, and there was a sludge of FeS, pyrite, and similar on the bottom of the heat exchanger that was covered in water.

Tubes plugged and all is fine now, exit temp back up to 140 C. Turns out during start up they did not purge with N2, so H2SO4 was forming which probably caused the original leak. Then once water came in, it went from bad to worse.

Can anyone provide an idea how the gas is able to leave at 70 C, when all fluid flowing in are over 100 C? This has stumped us.

Many thanks,
Rob

Rob Muggleton
Corrosion, Oil & Gas

 

[btrueblood]

Corroded and damaged temperature instrumentation?

 

[Bain89]

btrueblood this was also our first guess, however, there was nothing wrong with the instrumentation...

We were considering that a small leak would have let water into the process stream, making the air saturated.
The gas outlet pipe diameter is 16", vertical, and not insulated.
We theorized there could be condensation on the pipe wall due to the saturation with water, which would run back down the pipe towards the exchanger gas outlet.

This condensate running back down the pipe wall may have cooled the temperature sensor. However, upon checking, the temperature sensor is right in the centre of the pipe, not at the pipe wall, so condensation would not be running over the temperature sensor making it read a lower value...

Rob Muggleton
Corrosion, Oil & Gas

 

Lower temperature is a strong indicator of a problem, in equipment that is exposed to nasty corrodents at high temperature. Why did operators permit operation for an entire month at such a significant deviation from the normal temperature!?
The HX needs a complete teardown for investigation of not just the physical condition, but the process conditions and history. Involve the process engineer (who in my neighbourhood are numerous but extremely elusive).
Don't want to sound harsh, but it suggests a wider issue, that your organization's safety culture needs upgrading.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

How complete was your exam of the HX? I would worry about lots of corrosion, in lots of places.
This has a lot of risks.

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P.E. Metallurgy, Plymouth Tube

 

[pradipgoswami]

Hi Rob89,
Sulfur recovery units are typically the downstream units in refinery. Often made of inexpensive materials and resulting in dew point and other low temperature corrosion mechanisms. A good resource would be to look at is API-RP-571-2011, Page 5-106. The total layout of SRU including the vulnerable areas with possible damage mechanisms are narrated.
The next part would be know the detailed mechanisms about the damages and how to mitigate them, by process/operational control, inspection techniques adopted, materials upgrading(when required) and so on...

In addition a google search would reveal many documents which you may use to your advantage.
Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[Dhurjati Sen]

Hi Rob,

Are you talking of the Sulphur Condenser? Then, the liquid Sulphur explains everything.

DHURJATI SEN

 

[NBrink]

Two things to consider, having dealt with substantial sulfur condenser leaks in the past:

1: What side was the corrosion actually on? Yes, the process can be rather nasty if it gets too cool, but unless inspection ruled it out, consider the possibility of the leak starting on the waterside. Have had that case before where pitting from poor BFW quality lead to water leaking into the process, and creating a lot of (suprisingly, green) acidic water.

2: Is there anything downstream that may have been damaged? A coalescer with a blocked drain? A inline burner-mixer in the tail gas plant that could have refractory damage from the extra moisture? Just things to consider...



As far as how it could be seeing a lower temperature, the only thing that springs to mind for me is if the thermowell is in a non-traced deadleg that filled with water and slowly cooled. Otherwise, it's possible for the actual thermocouple to become dislodged and read low. Would have to verify the reading with an instrument guy though...

Nathan Brink

 

[Bain89]

Thanks all for your input.

NBrink - was the pitting on the water side for SS tubes? This is all carbon. That been said, there are some chlorides in the boiler water, I am told.

I am diagnosing this remotely...

 

[Chumpes]

hello
can the vaporization of water induce the encountered low temperature ?

 

[Bain89]

Thanks Gentleman for the comments.

We will be opening it next week, as some water has again accumulated, so there are new leaks from the tubes...

They have a water sample form inside the tubes (process gas side), pH was 4.5. Does anyone know any specific species to test for that may give hints to the cause?

Thanks

 

[EdStainless]

4.5? You have some real problems. That will dissolve about anything.
Do you have the replacement HX on order?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Bain89]

Let me just clarify the pH. The water leaked in, and this water was measured at 4.5 to 6.45 with a chloride concentration of 0.7 mg/L.
The boiler water is about pH 10.

I will now be examining the tubes in the lab.

 

[Bain89]

Just an update for whoever is interested.

Seems the second leak was simply a leaking plug. Had a thorough look at the outlet pipe (from the inside), nice running condensation gouges, so yes the outlet gas was condensing.

Also looked though the man hole at the water side of the tubes, they seem to be in good condition. No scale, which is impressive for a 22 year old boiler. The pipes had a uniform, clean looking, darkish magnetite layer, no spalling.

So probably the formation of poor start up without purging with nitrogen, and the low temp, making H2SO4, ate the pipe somewhere from the fire side.

 

[Bain89]

One question:

Has anyone experienced on the water side, a black oily substance? It was not on the pipes, but it was all over the shell. I have seen this in other old heat exchangers as well.

 

[Dhurjati Sen]

A photograph would have been helpful! Iron sulphide is a possibility.

DHURJATI SEN

 

[Bain89]

It's on the water side, is black, oily, and you can smear it around...

 

[Chumpes]

what is the composition of the black deposit ?

 

[NBrink]

Talk to whoever does your water treatment.

Have seen hydrocarbon leak into condensate return, which ends up in the feedwater. Can make for some nasty deposits.

Nathan Brink

 

[Dhurjati Sen]

Bain89

You have to study the upstream processes for both shell and tube side to come up with an accurate failure analysis.
The one that you have done earlier is surely incomplete.
As a preliminary step, send the black deposits to the lab and test for sulphide, iron and even for hydro-carbons.

Regards.

DHURJATI SEN