Stainless steel flanges connecting to ductile iron valves - Corrosion?

[dsg1985]

We're using stainless steel fittings on a pipeline containing emulsion (HFA fluid for coal mining longwall). The 5km pipeline(other than the fittings for isolation points, drain points, etc.) is HDPE. This is all due to a client request.

The line requires pressure reducing stations. These have several components - drain points, isolation valves, pressure gauges, etc. - all joined by stainless steel spools. We've received advice in writing from valve manufacturers that their coated ductile iron valves will be suitable for use with the emulsion fluid.

The problem I have now is how to join the ductile iron flanges on the valves to the stainless steel flanges on the metal spools in the prv stations (this is the only place these metals will interface). My question is, how best to do it? I've looked at galvanic tables and cast iron and s-steel seem sufficiently 'far apart' to warrant my corrosion concerns.

Thanks.

 

[racookpe1978]

What gasket material?

RF flanges on both type material flanges?

What bolt material - if the gasket is "perfectly" inserted between two RF flanges of different materials, then only the bolts can carry current from one to the next.

 

[dsg1985]

Not sure on gasket material. I had a look at our pipe spec and it basically just stated 'suitable for system pressures, temperatures, contents, and coal mining conditions'.

The pipe spec. was written by another person, so I'm not sure what bolts either beyond 'Stainless steel bolts and nuts to AS 1111 and AS1112'.

This can all be changed if they're unsuitable. The problem as I saw it was that although, yes, gaskets would separate the flanges, the bolts would still join disimilar metals whether you used a stainless bolt or another bolt, and hence some form of insulation would be required. I'm just not sure what that is as I've never come across it (doubt anyone who's working here has either).

I'd like to point out I'm a grad engineer at this stage

 

[SNORGY]

When in doubt, use a flange electrical insulation / isolation kit.

Regards,

SNORGY.

 

[BigInch]

You could try flange protectors filled with grease, bolt sleeves and backing washers, essentially a hand made isolation kit. Then a double wrap of Tapecoat. Minimize bending moments at those locations to attempt to preserve the gasket.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[stanier]

Ductile iron bodied valves are normally Rilsan coated across the flanges so , with the gasket, provide the electrical insulation required.

Gasket needs to be selcted on design pressure.

I have used many ductile iron gate valves in pump stations with stainless steel piping. I have not seen evidence of corrosion. Please check out photo on my website.

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