Services Piping in Seawater Tunnel at 55m depth

[stanier]

A seawater tunnel for the feed to a desalination plant is 2.2m diamter and 2.2km long. There are services that run the length of the tunnel 80mm up to 450mm diamter. They may be exposed to 55m external head.

The requirement is therefore a pipe material that will not fail due to buckling instability and be resistant to corrosion from seawater.

The pipes will be laid in the obvert of the tunnel after the tunnel is constructed.

What materials are recommended? I am after projects where materials have been successfully used.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[jonelv]

What fluids are you transferring throught the pipes and what are the internal pressures?

For general materials, I have used duplex stainless steels for seawater applications as you get good combination of strength and pitting and crevice corrosion resistance. These are expensive though and ultimately your decision will need to factor in the criticality of the particular service, whether it can be repaired / replaced if it fails etc,

Copper / nickel alloys may also be appropriate although line velocities may need to be limited. These alloys do have the advantage of better resistance to marine biofouling which may also be important.

Regards

Jon

 

[Ashereng]

I am just guessing here, but won't a lined CS pipe do the trick?

Other exotic materials like duplex stainles steel (for a 2.2m diameter and 2.2 km pipe) will be much more expensive.

I have used CS with Impreglon coating (though not for sea water) in other applications. Maybe this solution is worth a look see?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[MikeHalloran]

Lined pipe won't help when the seawater is on the outside.

The larger pipes may need extra wall to deal with the external pressure. E.g. a 450mm OD duplex tube that's no more than 6.3mm out of round will require a wall around 6mm to withstand 55m of external head with a FS=1.0. That's a lot of duplex. Multiple smaller thinner tubes in parallel might be more economical. The collapse equations (e.g. ref2) are nonlinear, so you can have a lot of fun balancing the economics.


ref 1: "Tubing Limits for Burst and Collapse", Tech Note, CTES, L.C., Conroe TX

http://www.ctes.com

ref 2: "Effect of Initial Eccentricity on Collapse Pressure of Circular Beam Tubes", S. Yadav
Fermi National Accelerator Laboratory, Batavia IL

http://www.flab.gov

Mike Halloran
Pembroke Pines, FL, USA

 

[Ashereng]

I guess I misunderstood. I took the problem to be sea water inside a pipe/tunnel, leading to the desal plant.

If the sea water is on the outside, why such a long pipe?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[MikeHalloran]

I'm guessing the tunnel is real long so it will inhale clean seawater from offshore, as opposed to what's near the beach, and so the eyesore of a plant can be located well away from the eyes of beachgoers.

I'm also guessing that one of the service pipes carries an outflow of hypersalinated effluent, and you'd also want to keep that away from the beach.

I'm curious about what other services would be required.



Mike Halloran
Pembroke Pines, FL, USA

 

[MJCronin]

stanier..

The classical method of evaluating the strength and stability of cylinders with external pressure is contained in ASME Section VIII. The rules work for piping as well as vessels.

You may want to evaluate the case where there is zero interna; pressure and ~55 meters of external water pressure (~78 psid?). You jave not given us enough details about the diameter of the pipe, flow rates or maximum pressure differential..

If you are using a premium cost material (like duplex SS or better)you may want to consider exterior stiffening rings on the piping.

My opinion only...

-MJC

 

[dcasto]

I don't see how you could have less than 1 atmoshpere difference between the inside and ouside of the line unless you had a submerged pump. With that, there should be no pressure related problems.

 

[RossABQ]

I think the question was misunderstood; he's talking about other piping that is inside the tunnel that is full of seawater at 55m head, right? So these other pipes are subject to seawater corrosion, and have an external head of 55m on them, and some other pressure internally (probably a few meters higher than the external head).

The mention of buckling puzzles me, I don't see that being a factor? Unless it is due to the support spacing?