Pressure transmitter with diaphragm seal for very high salinity application

[Steve010]

I have to use a pressure transmitter with a flanged (Raised Face rating 600#) diaphragm seal on a the on the outlet of a well. The material engineer specified that the wetted parts shall be Hastelloy-C, so i chose the diaphragm material to be hastelloy-C. What should the diaphragm seal flange material be? should it be Hastelloy-C as well ? or is it ok to use a carbon steel flange, and rely on the gaskets to make the process fluid does not contact the flange ?

 

[Steve010]

Also note that i can not use 316 SS flange because this will lead me to increasing the rating to 900# instead of 600#, as piping is working at the edge of rating 600# for carbon steel.

With regards to carbon steel, i am worrying about galvanic corrosion between the seal carbon steel flange and the hastelloy-C flush ring.
What do you think ?

 

[danw2]

These are screen shots from Honeywell's remote seal guide. (Your mileage may vary by vendor)

The 600# CS flange, Hastelloy C diaphragm and SS wetted body is Fig C construction, where the diaphragm extends over the wetted gasket surface separating the body from process exposure. As long as the diaphragm material holds out, the process does not contact the SS 'body'.

I suspect the diaphragm failure resulting in loss of pressure measurement/indication would be a higher risk than serious corrosion at the gasket that could weaken the SS 'body'.

Flanged flush seal materials Table, Smartline remote seals

Flanged flush seals figure C

 

[Steve010]

Thank you very much for your reply.

In your statement "I suspect the diaphragm failure resulting in loss of pressure measurement/indication would be a higher risk than serious corrosion at the gasket that could weaken the SS 'body'." In which case and why do you suspect that ?

 

[danw2]

My goof. I swear that when I first read that table I saw only Hastelloy as wetted, not SS.

I was thinking the extended diaphragm covered the SS area, but if SS is listed as wetted, then it is.

I think you're stuck with Hastelloy/Hastelloy unless another vendor does it slightly different.

Dan

 

[Steve010]

You mean a carbon steel flange + a hastelloy-c body + a hastelloy-c diaphragm, right ? Note that 316 SS flange is not an option for me because 316 SS flange in my case (unlike carbon steel)cannot withstand the design pressure/temperature.

 

[danw2]

Yes, given that Honeywell only provides flanges in CS or SS, not Hastelloy (ding ding goes the cash register). I assume CS is suitable material for the flange?

 

[Steve010]

I assume so too. it is non-wetted.
But the question remains. why would some body purchase a SS body if his diaphragm should be Hastelloy-C ? The body is considered wetted part as well and it's corrosion (corrosion at the contact circle between the body and the diaphragm) can result in diaphragm failure and loss of containment. Any idea ?

 

[roydm]

The body in Figure C is not a wetted part, it clearly says so on the drawing. I suspect the manufactures standard is to weld the diaphragm to a 316 SS body.
The flange can be CS but stainless will look better 10 years down the road.
What are the pipe flanges?

 

[Steve010]

Piping flanges are CS

 

[danw2]

The body is listed as 'wetted' in the table.
The body is declared 'non-wetted' in figure C.

Go Figure.

 

[Steve010]

The body is wetted, however in figure c it is non-wetted because of the special design of extending the diaphragm over the flange raised face which covers the body.