Seawater piping anode sizing

[vinhermes]

Good day gents,

I have been looking through various threads but can not find an answer to my query.

I am trying to determine the required zinc anodes on a galvanised steel seawater piping inside a ship. It does not need to be a very refined calculation as I am only looking at a ball park estimate. For this, I am using the DNV-RP-B401 methodology that is straightforward. My only concern is the mean current density assumed. It is averaged for the temperature of interest at 0.1 A/m². I am surprised that the flow velocity is not accounted for (in my case from 1.5 to 3 m/s). Only publications I can find for pipelines are for the external protection. I am interested in the internal cathodic protection.

Any feedback on this figure or alternative references would be appreciated. Thanks, Vincent

 

[TugboatEng]

Galvanized? That doesn't sound seamless... Welded pipe in sea water is going to be disposable, not really worth the effort to protect.

 

[EdStainless]

The problem with internal CP is the limited 'throw' or reach of the current. Unless you use a lot of anodes you will get a series of spots down the length of the pipe.
The other reason is that people don't like drilling all of holes through the pipe wall for isolated mounts and test points. And then there is the issue of placing anodes and replacing anodes in the future. Unless this large diameter that is highly problematic. And hence why a lot of this pipe is made from CuNi.
You need to hire a consultant who is certified to design CP systems.
High flow velocities can lead to erosion related issues.

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P.E. Metallurgy, consulting work welcomed

 

[vinhermes]

Thanks for your feedback.

About the material, my company has done a CAPEX/OPEX assessment and found that even if somehow disposable, the GS solution is optimal. Debatable but ok so far. We build dozens of ships like this every year.

We use seemless pipes and hot galvanise the pipes once flanges and fittings are welded.

We apply zinc anodes only at interfaces with equipment.

My question where I would like your help is the current density to be considered and the impact of velocity on it.

Thanks by advance

 

[TugboatEng]

Oh, from a builder's standpoint, galvanized makes sense. It last just long enough to get through the sea trial and warranty. Seamless galvanized actually hold does up pretty well in low use applications such as fire mains even without anodes. I have some systems that are 40 years old and only have a few pin holes. My grief is that it sheds rust and debris that clog everything attached to the system, especially check valves.

 

[EdStainless]

The material shed from the Zn as it does it job (mostly hydroxides) will clog control valves, regulators, and fire nozzles. I know of many sites where it would last long enough but causes so many other issues that they have banned it.
The Zn also makes these nearly non-repairable since the Zn has to be stripped for welding.
Using double heavy galv will help, but anodes don't work well in smaller diameter ID pipe applications.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[vinhermes]

Honestly, I agree on all cons of GS, don't try to convince me... I am just trying to size a few anodes at the equipment interfaces and try to estimate the current density (A/m2) in seawater with some flow. Any help would be appreciated...

 

[SJones]

If it's a rough and ready calculation don't worry about those sort of velocities. Currents of that velocity are not unknown offshore, and, on the whole, B401 is a reasonably conservative design standard. The thing will be attaching anodes and getting them to stay on, so you had better think of the consequences of an anode getting pushed downstream.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[LittleInch]

See this?

https://www.materialsperformance.co...on-retrofit-for-auxiliary-cooling-water-pipes

Still don't think it will work unless you have lined/coated pipe

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

There are pencil anodes that have a wire core. They do work well to keep the anode from breaking apart.

 

[vinhermes]

Thanks LitleInch. I read the article, very interesting. What I kept from it:

- 0.1 mA/m2 is also assumed in this study with the following reference: Metals Handbook. So thanks for that!
- It is not a praise against CS for seawater, it confirms that non galvanised pipes, badly coated and with welds defects, without CP are a bad solution. This is different...

Vincent

 

[LittleInch]

But as far as CP current is concerned it's like bare pipe no? The galvanising is metal therefore conducts electricity. How does the CP know where to concentrate the current?

As I understand galvanising, this is the addition of a thin layer of zinc which is corrosion resistant but has a very strong bond to the steel underneath.

It is similar in many ways to a protective layer of plastic or other film. The big difference is that it conducts electricity.

Am I barking up the wrong tree here?

The transport of seawater in pipes seems to come up here frequently, but I've never heard so far of anyone using CP on a non coated pipe and so far never on the inside.

Sometimes you do need to think that if it was such a good idea, why is not done by anyone??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SJones]

@LI - coughing up the wrong tree really. The zinc is not corrosion resistant, it too is a sacrificial anode. In acting as a sacrificial anode it will polarise the steel. The proximity of the potential to that of any additional sacrificial anode reduces the current requirement to maintain protection. Internal CP is not unknown in seawater systems but the RCP technology has been more for protecting stainless steels.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.