Support of SS pipe 2

[B.L.Smith]

Hi again,

Can I place a bare stainless steel directly on a CS structure such as T-post or Piperack? If not what is the reason? Which type of corrosion is the matter?

 

[curtis2004]

Hi BradSmith,

What is the reason for replacing CS with SST? Can you place CS instead?

Curtis

 

[B.L.Smith]

Excuse me if the question was not clear.
The pipe is Stainless steel and structural support is Carbon steel. Can I put this S.S pipe on the CS structure?

 

[rconner]

While I suspect ther may be ways this can be done, you may wish to google "sensitization".

 

[SNORGY]

I suspect that the primary corrosion concern is galvanic corrosion.

If the line doesn't move around too much (i.e., it isn't a "hot" line, I would just use a couple of yellow jacket heat shrink sleeves around the pipe and be done with it. Otherwise, bond a plate of stainless steel or teflon or something to the structure to keep the two metals separate. (I assume shoes are not involved...).

A contractor once told me he had a great idea for exactly this situation in the field; it was for a stainless steel breathing air system. He suggested that we put the stainless steel piping onto a bunch of galvanized aluminum conduit cantresses. I went home and looked at the aluminum siding on my house that had been fastened in certain locations by stainless steel nails (the heads of which were now rather brown) and looked up the relative galvanic potentials. I came back the next day and suggested that my heat shrink sleeve idea ($1.90 per support) was probably more economical and less problematic.

Being a true contractor, they built the cantresses anyway and did whatever the heck they wanted. To this day I don't know which one of us was "right". I do know that my idea would have worked and it cost a lot less money.

 

[bimr]

I don't think that corrosion will be a significant issue. It is common to use carbon steel legs on stainless steel vessels and there is no great corrosion issue with that fabrication technique.

However, the stainless steel will pick up the iron and will show surface staining and rust which will be unsightly.

Your best solution is to install stainless steel slide plates.

http://www.pipingtech.com/faq/article/what-is-a-pipe-guide-131.html

 

[curtis2004]

BradSmith,

Please provide more details: What size piping, temperature-pressure of fluid.

Curtis

 

[B.L.Smith]

size= 1"-12"
temp=85 C dec
150#

But I think this data is not matter.

 

[SNORGY]

At that temperature, it matters. Slide plate is best option. I am sure the pipe is on shoes though, right? At 85 C? If so, I suspect your issue becomes a moot point.

 

[B.L.Smith]

Dear SNORKY
The pipe in not on the shoe. I'm trying to provide best support for my case.
If the pipe be placed on the rack or T-post directly, then SS pipe won't corrode because in galvanic table CS is above SS.
According to above reason, I don't understand why a pad or clamp shoe or bearing plate should provide between CS Structure and SS pipe for avoiding corrosion. Please guide me.

 

[rconner]

While not a whole lot of detail like grades of SS, environment and service etc. involved were provided, some well-meaning folks have attempted to provide guidance. While “your case” (it appears that bare SS pipe can be simply spaced on bare carbon steel supports) may well be right, you should nevertheless at least be aware of many references regarding perhaps some non-obvious concerns of contact of SS with other materials. While you may be right, you should probably see e.g.

http://www.millerwelds.com/resource...rosion-resistance-and-increasing-productivity

e.g., “Cleanliness is critical. Even trace elements of foreign materials incorporated into the weld joint can cause flaws and lead to reduced corrosion resistance and strength…” (and may be hard to keep/police weld seams off all supports) and even from some notable
promoters of stainless steel on page 19 at

http://www.euro-inox.org/pdf/map/Contact_with_Other_EN.pdf

etc., “Here, too it is essential to avoid crevices between stainless steel components and less noble materials, in which corrosive attacks can occur due to dirt and humidity.” (this warning does not appear limited to welded areas). While I don’t know whether or not you have an entirely dehumidified environment, I would think something resembling
“crevices” would kind of hard to avoid if you put bare stainless steel pipe on a carbon steel plate or support that is not welded to same.
For whatever it is also worth, I think I have heard over the years of some problems with stainless steel pipes at least blamed on carbon steel peck hammers or brushes, or even stainless steel brushes that were alleged previously used to brush some regular steel etc. or were otherwise contaminated etc.(again such maybe kind of hard as a practical matter to police nowadays). Good luck.

 

[Kiwi2671]

Brad,
These are issues that should be addressed by knowledgable engineers - not on an internet forum.
I have spent 35 years as a pipewelder or Welding Inspector in oil refineries, LNG, petro-chemical and nuclear and in every one of these there has been a project requirement to "isolate" the S/S pipe from the C/S structure.
These vary from teflon pads to welded shoes but in all cases there was no contact between the S/S pipe and the C/S structure.
Regards,
Kiwi

 

[SNORGY]

BradSmith,

What you are saying has merit. CS is anodic relative to austenitic SS. I am not a metallurgist or specialist in this area, but from what I understand, in the presence of a moist environment the sliding contact between SS pipe and CS supports can result in the transfer of metallic material and oxides from the CS onto the surface of the SS, and the surface "rust" that forms is actually the oxidation of that transferred material. By itself, this damage is cosmetic, but prolonged contact in such an environment can increase the susceptibility of the SS to intergranular attack.

 

[jtseng123]

Bimr:
you are partially correct about "it is common to use CS leg on SS vessel". You will never weld a CS leg or skirt to a SS vessel. No one dare to do that.
What you missed is that there is a SS "poison pad" to isolate the CS leg to SS vessel. If it is a skirt, top 2'-3' of skirt will be SS to serve as a isolation.

BradSmith, by the same practice used in pressure vessel, you shall weld a SS poison pad to pipe to isolate the SS pipe to CS support, that will take care galvanic corrosion or any other problems. Vessel is much critical than your pipe and that is the way the vessel is designed and can last for many decades, why you want to do something else ?

 

[B.L.Smith]

jtseng, as I told above, galvanic corrosion is not matter for SS pipe. Also using welding pad for small bore pipe is not best option because of difficulties in welding.

 

[SNORGY]

I'm still trying to get over why a SS line at 85 C wouldn't be insulated and, therefore, on shoes - unless one is deliberately trying to shed the heat to the environment up in a location in a rack or in a utilidor where an Operator cannot come into contact with it. If one is looking for the best support in this case - and hasn't actually picked one yet - then, in my judgement, a pipe shoe is the best support and all of the bi-metal contact issues disappear.