ASME Sec VIII Div 1 for anode plug

[rmw]

Does ASME BPV deal with the use of a sacrificial anode plug in the pressure boundary of a stamped vessel?

It seems to me that with Corrosion Allowance, additional thickness is added over and above min required metal thickness with the expectation that over time corrosion could reduced the thickness - but hopefully not, but with the plug, it is there to virtually 'go away' instead of the cathodic surface it protects.

What does the code say about the minimum thicnkess of this plug as it "disappears"?

I am referring to a brass plug in a CS channel against a brass tubesheet and brass tubes. It is called out on the dwg as an anode.

When does the code say this plug is too thin?

rmw

 

[rmw]

Was this a hard question?

This is not a homework problem.

rmw

 

[TJOrlowski]

The code deals with corrosion allowances in a very limited way. TEMA and API have some specifications, but they allow for a great deal of flexibility: usually stating that if their respective "norms" are not used, design shall be by mutual agreement between the purchaser and the manufacturer.

Is the plug screwed into a coupling? And is the anode itself brass? Or is it a brass plug, with a zinc anode?

-TJ Orlowski

 

[vpl]

rmw

I hesitated to reply, because I'm not a Code expert. However, I'd expect that this would be something covered under the OMI (operations and maintenance inspections??) code, if it was anywhere. As the real code experts frequently mention, Sections I/III/VIII are design codes.

If this can wait for the weekend, when I might have time to look, I'll look through the company's reference documents and see if I can find anything.

Patricia Lougheed

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[trottiey]

They're good question; I'm not sure if I have any good answers. I don't know of any explicit rules.

Section VIII Div. 1 UG-25 allows different corrosion allowances on different parts of the same vessel to account for different rates of attack, so I would think an anode plug is just a part with a larger corrosion allowance.

The weak point would be the thread, so I think you would design to maintain the minimum thread engagement of UG-43 after corrosion. Those limits probably don't account for much corrosion of the thread itself, so I would make sure to use thread sealant on that.

 

[rmw]

TJOrlowski,

It is unclear as to what the plug is exactly. The BOM on the drawing merely states "Brass Plug". The bullet on the dwg states coupling with "Zinc Anode". So it is unclear to me if this is a brass plug with a zinc extension designed to act as the sacrificial anode or just a brass plug with the zinc content of the plug intended to act as the sacrificial metal. I expect the latter.

It is screwed into a steel coupling (or half coupling) in a CS bonnet. THe bonnet and end cover are all CS bolted to a brass TS with brass tubes.

Patricia, thanks. I can wait.

Trottiey,

So the basic question is; what is the corrosion allowance for this plug?

I understand corrosion allowance added to minimum required metal thickness, and I understand the code allowing the use of a brass plug at full size as part of the pressure boundary in a threaded coupling, but I don't know when that coupling becomes too thin as it sacrifices itself to suffice to meet the code requrement for min req metal thickness.

I know when the bonnet for example becomes too thin, and that is when the corrosion starts getting deeper than the corrosion allowance and into the minimum required metal thickness. At that point something has to be done.

BTW, I would think that if the thread is sealed, it defeats the purpose of the plug as an anode because it breaks the electrical path.

rmw

 

[trottiey]

I don't think thread sealant paste would break the electrical path. Even tape would develop some tears as you thread it in. You could check with a multimeter.

I don't know what more to suggest about the corrosion allowance. It's the length of thread engagement and how good shape the threads are in that should matter most to a small pipe plug.

 

[metengr]

Does ASME BPV deal with the use of a sacrificial anode plug in the pressure boundary of a stamped vessel?

No. Only the method of attachment to the pressure boundary.

 

[TJOrlowski]

rmw,

Most zinc anodes I've seen and used are a two-piece plug. The pressure retaining portion is usually an MPT hex-head brass plug, which would screw into your coupling on the head. There should be no corrosion allowance on this piece. The sacrificial portion is a zinc extension that screws to the inside of the plug, and the length can vary.

As to how to design the corrosion allowance, it should be 0 on the brass plug, and inconsequential on the anode. The anode just needs to be changed as required (get a longer anode than the original if possible).

If the whole head is corroding to the point where a portion thereof becomes too thin to meet Code, the whole head needs to be changed.

-TJ Orlowski

 

[vpl]

rmw

Sorry for the late reply. The best information I could find was that reference to two specs, neither of which I have access to:

1) ASTM F1182 "Standard Specification for Anodes, Sacrificial Zinc Alloy." (available for $45 from ASTM)

2) MIL-A-19521B "Military Specification: Anode Retaining Support Plugs and Anode Selection and Installation Design Criteria for Shipboard Condensers and Heat Exchangers (04 MAY 1992) (I did find a copy available for downloading (apparently for free) at

http://www.everyspec.com/MIL-SPECS/

. I haven't checked it out)

Patricia Lougheed

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[trottiey]

I have ASTM F1182. It includes specifications for "type ZDM" anode plugs. They consist of zinc cylinders press fit onto steel stems that thread into a blind hole on the inside face of a pipe plug. Sounds like the same thing TJOrlowski was talking about.

You can get official copies of all MIL specs from the assist database:

https://assist.daps.dla.mil/quicksearch

(Your taxes have already paid for them.) It looks like basically the same item as ASTM F1182 type ZDM.

 

[TJOrlowski]

They consist of zinc cylinders press fit onto steel stems that thread into a blind hole on the inside face of a pipe plug. Sounds like the same thing TJOrlowski was talking about.

Similar. The ones we use the anode and stem are a single piece. So for a 3/4" coupling on a bonnet, we'd have a 3/4" hex head plug with a 3/8"-16NC blind hole on the inside. The zinc would then have a 3/8"-16NC stem, and usually a slightly larger cylinder (can be specified up to the ID of Sch80 pipe), and length to the specification we need.

There is a huge selection of readily available plugs like this. There should be several distributors than can help.

-TJ Orlowski