RTJ Gasket Material 2

[DiegoMartinez]

Guys,

Can anyone of you know about this?

Which of the following RTG (2R) gaskets are suitable to fit between SS PSV flange and CS ball valve flange? Piping classes are 900 and 1500#.

1- RTG Gasket, 316SS Hardness below BH 160
2- RTG Gasket, 5Cr-0.5Mo (F5) Hardness below BH 130

 

[Cockroach]

Option 1 is better.

First, these are typical of commercial availability fo a ring donut type seal, second, you need to achieve deformation in the ring joint in order to get a proper seal, hence material softness. The second alternative is acceptable, but a little too soft.

Properly, these are referred to "ring joint" rather than gasket, which is more raised faced seal. You are talking about an RTJ seal as used in ANSI Class 900 and 1500 flanged end connections.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[11echo]

I have a question, wouldn't you have to worry about the galvanic action between the two dissimilar metals ..ie S.S. in both flg./gkt. and C.S. valve? ...Mark

 

[DiegoMartinez]

Mark, Dissimilar materials is not an issue in this case.

 

[Cockroach]

Galvanic corrosion only occurs where an exchange of electrons is set up in a working fluid or medium. This mechanism works not too dissimilar from a typical voltage cell or two poled battery.

In the case of stainless steel, a thin layer of oxidation develops shortly after the metal is poured at the foundry. This layer, the passification layer, shields stainless from the rest of the world, hence the inertness of the metal relative to others. The passification layer is extremely tough but can be breached by heavy wear, scratching or some chemical action.

Also, the relative closeness of stainless steel in the Galvanic Series with carbon steel make for a poor coupling. In our case, the carbon steel would be anodic (active) and the stainless cathodic (noble). In the case of 13% Cr stainless, there is neglegable potential with iron in a carbon steel composition. 18-8 stainless is slightly more noble, but again, I would not expect much in the way of strong electronic coupling between the dissimilar metals.

Therefore as London2002 correctly points out, this is not an issue in our particular case.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[grampi1]

If the flanges are CS why use a SS ring? Use soft iron.

If must use 316, then require a Vacuum Anneal and hardness of 130HB max.

 

[DiegoMartinez]

Grampi1,

It is not like that. Max hardness of SS flange is 200HB and of CS is 180HB. I can use both gaskets between these flanges. Remember the gasket shall be softer than the flange (Less hardness). If Hardness is going to be an issue, then I will use F5 gaskets.

 

[unclesyd]

A soft iron (ARMCO IRON) is in the range of Rb 50-60 and is normally the gasket of choice for CS flanges unless limited by other parameters.

 

[SJones]

On the subject of galvanic corrosion, may I suggest that the following be reviewed:

http://www.npl.co.uk/lmm/docs/bimetallic.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[spayette]

A bigger issue may be the operating temperature and the size of the flanges since the two flange materials have significantly different coefficients of expansion. Operating experience would limit the use of dissimilar flanges to approximately 800F, assuming the piping size was no larger than 10 inch NPS. For larger diameter lines the maximum recommended temperature may be as low as 500F.