Seawater application 6

[FIF99]

Hi,

One of our contractors stated that for a Fire Water (seawater) system Stainless Steel may be specified (316L).
So far, I have never seen Fire Water systems specified in Stainless Steel.
Although the system temperature will not be that high (aprrox. 30 degr.C max), we believe that st.st. is not suitable. The system is filled but normally no flow.

Thanks in advance for any reply.

Regards.

 

[sathanasiou]

Try using PVC or PP. If you do use stainless steel be sure to use plenty of sacrificial anodes. I find the following links very useful:

http://www.corrosionsource.com/discuss2/ubb/Forum43/HTML/000006.html

http://www.corrosion-doctors.org/MatSelect/rouging.htm

http://www.corrosionsource.com/handbook/CPS/

 

[unclesyd]

I don't know your jurisdiction, but I know plastic piping for fire protection wasn't allowed at our site (US) and was told sevral times that it was not permitted by both insurance and government regulations.

When I left they were trying to get a ruling on using a Polyliner to rehabilitate some cement lined DI pipe.

 

[stevenal]

I don't know about the anodes. Inside the pipe? I suspect the corrosion products would plug the sprinkler heads just when they are needed. Why not recirculate the sea water to keep it oxygenated? I understand that oxygen depleted water will corrode the SS.

 

[EdStainless]

No, don't use SS for seawater piping. A Cu alloy is the standard choice. You could do it is brass, but a bronze (NAB) would be better. Aren't Naval systems mostly in Cu/Ni?

Stagnant seawater water isn't good on low alloy SS. The O2 depletion and sediemnt combine to kill you. There are stainless grades that would work (AL-6XN), but at a price.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[stevenal]

What about using a dry system? Compressed air keeps the valve closed and the water out until a sprinkler operates.

 

[SJones]

GRP is a strong candidate material.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[mshimko]

Need more specifics on the application:

Dry system or wet?
Standby pressure (if a wet system)?
Operating pressure?
If dry, how often is the system wetted for testing, and with what?
Land based, ocean structure, or ocean vessel?

As a general recommendation, use a copper alloy such as 90-10 or 70-30 CuNi (these are the standard alloys for US Navy ships).

 

[mthakur]

Hi,
GRP or GRE is a good solution, depending upon the media temperature ofcourse, if its buried line GRP?GRE is even better as it doesnt need protection from outside. Normal carbon steel piping with come internal coating such as glass flake etc. is a very good, proven and economical solution.

 

[FIF99]

Hi,
Thank you all for your input sofar! The original spec is indeed based on GRE/GRP. But now 316(L) stainless steel seems to be the selected material despite the fact that we already advised not to do so.
The system is wet and operating on a pressure of 16 barg at temperature of approx. 25 degr.C, and is circulating to avoid algal growth. But the system do have death ends.
All recommendations based on industrial practice or common sence makes sence, CuNi 90/10, GRE etc..
I'm just a piping engineer,so do anybody having ideas how to convince client based on corrosion (scientific) explanations?

Thanks again in advance.

Regards.

 

[MaxNeg63]

I have a table (unfortunately only on paper, I can't find the electronic format) where it is clearly shown that 316L has corrosion rates in quiet seawater up to 5 mils/y (0,125 mm/y) depending on other factors like temp, type of seawater, steel surface conditions and so on. My recent experience talks about the use of 90/10 CuNi for seawater fire system. Consider that even if small amount of corrosion products is formed inside the pipe system, this can easily block the spray nozzles and the system would not work even if no part is actually damaged by corrosion.

 

[mshimko]

One significant difference between CuNi alloys and stainless steel is the likely form of corrosion.

CuNi tends to corrode uniformly at a very low rate, forming a weak passive film. Crevice corrosion, should it occur, will manifest itself on the OUTSIDE of the crevice as opposed to inside the crevice.

Stainless steel for a stronger passive film, but are prone to pitting and crevice corrosion. Pitting and crevice corrosion or localized corrosion, and will lead to failure much, much quicker than the corrosion you'll likely see on CuNi alloys.

 

[joeswoes]

and you may also use GRP valves for the same reasons as GRP piping.

 

[EdStainless]

You can go to the Nickle Institute (nidi.org) and get publications on the use of alloys in seawater. Combine this with the similar info from copper.org and you can lay out your story.
As much as I love SS (it pays my bills) 90/10 is a much better option in stagnant seawater than 316.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[mthakur]

Hi FKE,
Cu Ni 90/10 is good option for low velocities as the protective layer it forms is not so strong as in case of SS, in addition fabrication and maintenance is not so easy due to its poor weldeability, Cu-Ni fittings might be expensive for that working pressure.
On the other hand, GRP/GRE piping system has higher initial cost but very low operating cost due to minimum maintenance and longer life compared all other options.

 

[FIF99]

Hi,

Again thank you all for your responses.

I have checked the site of Outokumpu,

http://www.outokumpu.com,

and found a corrosion handbook free of charge. It's an interesting site anyhow. I have received a copy and it provides a lot of usefull info, also related to this thread.

Thanks!

Regards.