We have recently experienced what our corrosion engineer has labeled as Vapour Space SCC of 316SS condenser tubes in a vertical heat exchanger unit. I'm looking for online and would appretiate a nod in the right direction for any information on this fairly new phenomena in our plant
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Condenser Vapour Space SSC 1
- Thread starter pvcguy
- Start date
I'll be looking for someone's kmowledgable answer to this one. I have never found much of value on SCC on the web. That said, SCC of 316 is preceded by pitting which initiates at chromium depleted zones around inclusions. The cracking follows these zones.
In vapor space this process must occur through condensation within pits and then within the cracks. No reason for it not to.
In vapor space this process must occur through condensation within pits and then within the cracks. No reason for it not to.
Some general resources for information on SCC are MIL-HDBK-729 and DEF STAN 02-738:
Some more specific data that may be useful can be obtained from the following links:
Some more specific data that may be useful can be obtained from the following links:
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1
- #4
Just for starters.
Are they any tubesheet vents on this exchanger?
What are the shell side conditions?
What are the flow directions, which end is the hottest?
What type exchanger, fixed, floating head, etc.
SCC of tubes in vertical S/S heat exchnagers is quite common in the chemical industry. This covers all the Austinetic S/S. The mechanism is usually Chloride induced SCC. This occurs due fact that most vertical exchangers have
an air space at the top tube sheet. If you have pitting there is strong possibility that you may also have MIC.
These are not the only causes, but they are the most common in my experience. If there is water on the shell, in my opinion tubesheet vents are an absolute must.
Are they any tubesheet vents on this exchanger?
What are the shell side conditions?
What are the flow directions, which end is the hottest?
What type exchanger, fixed, floating head, etc.
SCC of tubes in vertical S/S heat exchnagers is quite common in the chemical industry. This covers all the Austinetic S/S. The mechanism is usually Chloride induced SCC. This occurs due fact that most vertical exchangers have
an air space at the top tube sheet. If you have pitting there is strong possibility that you may also have MIC.
These are not the only causes, but they are the most common in my experience. If there is water on the shell, in my opinion tubesheet vents are an absolute must.
- Thread starter
- #5
unclesyd
Sorry about the long reply time but issues, issues, issues. Tube sheet venting appears to be the most appropriate design change for our vertical heat exchangers with cooling water on the shell side. Our process design department is questioning the effectivness of say a 1/2" or 1/4" vent hole. We have 4 vertical exchangers with 2 - 1/2" vent holes already drilled but not in use. Will this be enough to vent this space? The exchanger in question has a 60" OD. We will be directing the vent into the cooling water outlet as this cannot go to the sewer. What is the normal piping / tubing design for this type of installation?
Sorry about the long reply time but issues, issues, issues. Tube sheet venting appears to be the most appropriate design change for our vertical heat exchangers with cooling water on the shell side. Our process design department is questioning the effectivness of say a 1/2" or 1/4" vent hole. We have 4 vertical exchangers with 2 - 1/2" vent holes already drilled but not in use. Will this be enough to vent this space? The exchanger in question has a 60" OD. We will be directing the vent into the cooling water outlet as this cannot go to the sewer. What is the normal piping / tubing design for this type of installation?
We normally have a vent, 3/8" or 1/2", in all four quadrants. We use stainless drain tubes with brass valves due to the local enviroment. We have the same problem with the cooling water. It goes to the process sewer in most cases, though we do have some that go to the storm drains and some to the CTW return sumps.
Your case is not the first. We spent millions with this problem over the years.
Every vertical Hx get vents whether they are needed or not. This due to changing uses due to process changes.
Where the process permitted we safe-ended, normally 18" with one of the Inconels, process dependent.
We have had cracking in all the austenitic stainless steels, 304/304L, 316, 317, 321.
We also had a problem with MIC attacking the S/S tube below the safe-ends. When we increased the safe-end to 36" to 48" we found we had a bug that like some of the Inconels.
These were 1" tubes 30' ft long. We did all our safe-ending.
The biggest problem we had was keeping the vents open and the water regulated. A big operator problem.
Until you can modify your condensers you need to get a supply of Pop-A-Plugs and tube inserts. These work very well if the process permits.
Your case is not the first. We spent millions with this problem over the years.
Every vertical Hx get vents whether they are needed or not. This due to changing uses due to process changes.
Where the process permitted we safe-ended, normally 18" with one of the Inconels, process dependent.
We have had cracking in all the austenitic stainless steels, 304/304L, 316, 317, 321.
We also had a problem with MIC attacking the S/S tube below the safe-ends. When we increased the safe-end to 36" to 48" we found we had a bug that like some of the Inconels.
These were 1" tubes 30' ft long. We did all our safe-ending.
The biggest problem we had was keeping the vents open and the water regulated. A big operator problem.
Until you can modify your condensers you need to get a supply of Pop-A-Plugs and tube inserts. These work very well if the process permits.
- Thread starter
- #7
This is a method to get the Austenitic Stainless Steel out of the danger zone in a Hx.
We mainly used safe ending on S/S vertical condensers to prevent SCC in the vapor space.
Safe ending is accomplished by selecting a material that is compatible with the process and immune/very resistant to SCC. We usually use 18" of a tubing material that meets this criteria and use an autogenous weld to join the two tubes.
This way you ended up with the resistant material in the top 18" of the Hx.
This method produces a tube only slightly more expensive than a full length of S/S but far less than full length tube of the resistant material. When you are talking 30' tubes this is a lot of money.
We have actually safe ended tubes of Inconel and Hastalloy C where the processed changed and caused corrosion.
We mainly used safe ending on S/S vertical condensers to prevent SCC in the vapor space.
Safe ending is accomplished by selecting a material that is compatible with the process and immune/very resistant to SCC. We usually use 18" of a tubing material that meets this criteria and use an autogenous weld to join the two tubes.
This way you ended up with the resistant material in the top 18" of the Hx.
This method produces a tube only slightly more expensive than a full length of S/S but far less than full length tube of the resistant material. When you are talking 30' tubes this is a lot of money.
We have actually safe ended tubes of Inconel and Hastalloy C where the processed changed and caused corrosion.
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