Lap Flange Materials

[aussiengineer]

Hi guys. Firstly apologies if I have failed to find a post where this has been discussed previously.
I am trying to resolve a dispute at work
My question is regarding the use of dissimilar materials in lap joint flanges. As i understand, as long as the pressure-temperature requirements are met for the material group selected, for the class rating applicable, then there is nothing that precludes me using dissimilar metals in a lap joint flange connection. In fact, I thought it was fundamental since one of the main reasons for the use of lap flanges is that where expensive exotic wetted parts are required, the backing flange need not be.
My question is- is this stated explicitly anywhere in the codes?

Cheers
-Dean

 

[pennpiper]

In my opinion you will not find this kind of answer in the Piping Codes.
This is a logic related issue based on economics.

 

[aussiengineer]

thanks pennpiper, i did have a look myself but came up short. its one of those cases where it would be great to be able to grab someone by the ear, point and say 'look here'.

-Dean

 

[whammett]

Another issue is safety which isnt readily addressed. Disimmilar matterials will expand at different rates. For example, a few years back at Exxon's Light ends plant at the Baton Rouge Refinery, the firefighters who sprayed water on the fire just made it much much worse. That's because the materials contracted at different rates, leaking all the light CH(2,3,4,5) into the fire.

"Scientists dream about doing great things. Engineers do them." -James Michener

 

[StevenHPerry]

Can anyone weigh in on the galvanic reaction between, say, a CS backing flange and an exotic stainless stub end?

Let's assume that there's no leakage at the flange and the insulation cladding is sealed up tight, so after erection there's no medium (water or otherwise) but we still have direction contact between dissimilar metals.

In my opinion, painting the backing flange isn't a permanent solution because the paint may wear off during construction.

Typically exotic stainless lines are going to be bought as thin as possible. If it was an XS wall that only needed half the wall to hold pressure that's one thing. But when t_min is 1/16" and 0.083" Sched 5S is used, it seems to me there's potential for trouble.

Anyone have practical experience with this?

- Steve Perry

 

[moltenmetal]

If there is galvanic corrosion in the case you're talking about, you're going to lose the carbon steel backing flange and protect the stainless lap joint stub end. Fortunately the carbon steel has LOTS of effective thickness. Depending on the service, there are plenty of choices for suitable coatings to reduce this loss, and other ways to mitigate the corrosion of the backing flanges as well.

Titanium and numerous other exotic materials are NOT ANSI flange materials, for good reason. People still construct pipnig systems out of these materials. EVERY flange in piping systems made from these materials is either a lap joint or a designed flange which is part of a valve or vessel.

Yes, lap joint flanges are different than weld necks or other solid construction flanges. They have their own unique strengths and weaknesses. But properly selected and designed, they're perfectly adequate and safe. Solid flanges are an easy choice when materials are cheap- you'll see much more use of lap joint construction if and when alloy prices climb to the heights they saw during the last economic boom.

 

[rneill]

Direct contact between dissimilar metals will not result in any galvanic corrosion unless their is a conductive fluid present (e.g., an electrolyte). If the parts remain dry, there will be no consequence.

As mentioned by StevenHPerry, if the parts are wet, the corrosion will be on the carbon steel flange which can likely tolerate a lot of corrosion anyway.