Gasket Centering and Inner Ring Material for Carbon Steel Flange 1

[patchlam]

Dear all engineers,

Good day to you all. I have a question on the spiral wound gasket material. Normally what I have found for the centering or inner ring, the material is always in SS 304 or SS 316.

I am using A 350 LF2 flanges for my services, if I am going to get the spiral wound gasket for that, can I still use those with stainless steel ring? Or I have to use carbon steel just to avoid galvanic corrosion between stainless steel and carbon steel?

Looking forward to your comments.

Thanks!

 

[moltenmetal]

LF2 material indicates extreme cold service, to -50 F, correct?

Then I wouldn't be worried much about galvanic corrosion on the interior...You either have water, and hence the risk of galvanic corrosion but ALSO the need to heat trace and insulate the lines against cold conditions (i.e. probably no need for LF2 material), or you have uncontrolled cold and don't have water, in which case there's no realistic risk of galvanic corrosion.

The spiral windings themselves are also stainless steel, typically.

The outer ring is usually carbon steel.

 

[patchlam]

Hi moltenmetal,

Thanks for your reply. Just wanted to further explain on my process, I am using LF2 material due to the extreme low ambient temperature at nearly -25 degC. However, during operation, the metal temperature will reach around 350 degC with the thermal oil inside. In this case, should I still be worried about the galvanic corrosion?

Thank you.

 

[moltenmetal]

No. Thermal oil won't contain any water. No electrolyte = no galvanic corrosion.

Still puzzled- why would your piping system be under significant pressure with thermal oil while ALSO being at -25 C temperature? Sounds like yet another case where low temperature was needlessly specified just because of cold ambient conditions.

 

[patchlam]

Hi moltenmetal,

The low temperature was set simply to cater for the start-up of the equipment during winter.

Thanks!

 

[moltenmetal]

OK, then I'm likely correct, and the LF2 material was probably needlessly specified, as regrettably it usually is.

You need low temperature materials when you have a pressure case which occurs simultaneously with low temperature. Low temperature gas services, or services where liquefied gases can cause autorefrigeration etc., are typical examples.

I struggle to imagine a credible winter start-up case for a hot oil system which would require materials rated for pressure service at -50 C, but I don't know your system- maybe you have such a case. But more likely, you don't, and someone was covering their rear end by matching minimum ambient temperature with minimum design metal temperature for the whole system without thinking about the implications.

 

[patchlam]

I totally agree with your point. However would you please advise what are the implication you meant above?

 

[moltenmetal]

The implications are that you're needlessly using a material which has been specially processed for low temperature service, instead of a more widely available and cheaper material. That has nothing to do with your original question, which we've answered- it's merely an observation.