Chilled Fluid Header Design

[StoneCold]

I am designing a new header for a secondary chilled fluid system. The process conditions are 350 gpm, -40F, 80psig. Fluid is 20%water 40%Methanol 40%Ethanol by volume.
The header runs on a pipe bridge above a high traffic area. Piping code applied is B31.3. Location is chemical plant in Colorado. Piping is about 400 ft total.
There are four possible piping options that I am looking at.
A333 carbon steel sch 40 Flanged and welded. We would have to add corrosion inhibitor to the solution but it would work.
Second option is 304ss Sch 10, Flanged and welded. This seems like the best choice.

Third option is 304ss, Sch 10, Groove lock fittings.

Fourth option is fiberglass piping (Red thread from Smith Fibercast) Bell and socket fittings, and Flanged.

My delema is that the A333 and 304ss welded versions are about $150,000 to install and inspect.
The Groove lock fitting option does not appeal to me but maybe I am missing something.
The fiberglass pipe is compatible with the fluid and there have been aplications at this temperature. My hesitation here is how thin the pipe is and the fact that one of the major failure modes of this piping is a fitting coming off the pipe. NDT for this type of piping is pretty much certifying the installers by doing test joints and then hydo on the final piping runs. The big attraction of this piping is that the install cost is about $100,000 less. Basiclly because the jointing method is so much faster and there is no x-raying involved.
Since the sytem will be insulated with 4 inches of insulation I don't think that the piping lends it's self to secondary containment in the method of double lined pipe. I have thought about making a pan under the piping on the bridge and putting a drain at a safe location.

What piping would you choose? Why?

Thanks
StoneCold

 

[Montemayor]

Stone:

For a system operating at a nominal 100 psig and -70 oF I have, in the past used 316SS, sch 10s with lap joint, carbon steel backup flanges. The system ran very well and I never identified any galvanic corrosion between the SS pipe and the carbon steel backup flanges. I welded as much of the pipe as I could except where I needed a flanged joint. The laps were obtained by using lapped stubs. For 350 gpm I believe you are dealing with 4" to 6" sizes and these should be rugged enough (with sch 10s) to lay on an external, elevated pipe rack.

I prefer the SS pipe because it is more rugged and mechanically sounder than the FRP and readily weldable and corrosion-free. The 316 grade is readily availale and I found it cheaper than the 304 - because it is the standard mill run. Manufacturers were only making 304 on demand and the volumes were less, ergo the higher price.

I believe the 316ss as described would be cheaper (on an installed, insulated basis) than the A333, sch 40. And don't forget that the carbon steel will corrode EXTERNALLY because at -40 oF, the insulation vapor barrier will always leak humidity and deposit water ice under the insulation. The 316ss will also, but it won't corrode.

I hope this experience helps.

 

[StoneCold]

Thanks Montemayor
I agree with your assement that stainless piping is probably the most durable.
However I am still not ready to give up on the fiberglass pipe yet. It is a very cost effective way to handle the fluid, and it does meet the B31.3 specifications for non metalic piping. Since my experience with the fiberglass is very small I wonder if I am over estimating the risks involved in using it.

Regards
StoneCold.

 

[MJCronin]

Stone....

Since joint seperation and leakage is a concern, and since we are dealing with modest sizes, have you considered a double-walled FRP system ?

Double walled systems are durable, have been used for many years in above ground installations and can be specified with FRP/CS/SS inner pipes and various FRP/PVC containment pipes.

Montemayer, it is my opinion that galvanic corrosion between SS and carbon steel is a myth.....they are not "far enough apart" on the gavanic series and I have never experienced this. However, I have also read many goofy piping specifications that require isolation kits installed between SS and CS flanges......I have yet to understand why.

-MJC

 

[StoneCold]

MJC
Because the piping is insulated, a secondary containment system is a problem. Unless I am able to pull really good vacuum on the secondary piping and use it as vacuum jacketing, I don't think it will work.


Regards
StoneCold

 

[StoneCold]

What did we do?
We are going with the welded stainless pipe. I like the fiberglass and if it was in a low personnel traffic area of the facility I might have used it. However the failure mode of the fiberglass piping is a fitting coming off and the concequences would be too great.

Regards
StoneCold