Poison pads - Who, what, when, where and why ?

[MJCronin]

To all..

Thanks to everyone who makes these forums an important and useful use of the internet..

My question: Over the years, I have delt with fabricators, customers, clients and geezers who all have thier own memories and opinions about "poison pads"

(These are the pads that are welded to a vessel where there is a change in material, supposedly to protect the vessel shell from galvanic corrosion) Yes, I know what galvanic corroision is....

Does anybody out there have anything more concrete than a bunch of recollections and memories ?. Does Shell, Penzoil or somebody have a guideline about the size and nature of tese pads ? Were they discussed in a paper or article somewhere at sometime. (Repeated GOOGLE searches yield nothing, but perhaps you are better than me...)

Anyone ?

I am looking for more than memories..


-MJC

 

[metengr]

I have seen poison pads used on heavy wall shells and drums on Section I components to allow for non-pressure part attachment welding after PWHT and hydrotesting. The pads are placed into the drum and or shell prior to PWHT so that any welding performed after hydrotesting can be done on the pad w/o having to re-hydrotest.

 

[moltenmetal]

A "poison pad" is intended to permit the welding of dissimilar materials without affecting the parent (corrosion resistant) material's metallurgy. A prime example is the welding of carbon steel legs to stainless steel vessels to save cost- doing so directly without a poison pad would potentially introduce carbon into the HAZ of the vessel shell.

 

[vesselguy]

I have never seen any paper on this subject in the past. This is just one of those rule of thumb knowledge passed on by senior guys. As for sizes, yes Shell DEP/ESTG, for example have some rules for the sizes but you can use what ever you want as long as it technically make sense.

I agree with the definition given by moltenmetal as that is exactly what the term was meant to discribe. To prevent carbon contamination of a s.s. shell by a carbon steel attachement.

Metengr's definition is not really correct for "poison pad". I know what he is describing but I don't think there is a term for a pad to by-pass the Code requirement for re-PWHT and rehydrotest of pressure membrane.

 

[doct9960]

My understanding of "poison pad" is the same as that of moltenmetal. But there were also instances where we used poison pads on similar metals. This is brought upon by Client requirements where they state that "material for nonpressure parts welded directly to the vessel shall be the same material as the vessel." When we clarified what the "same material" means, they literally meant the same material (not just the same P-number). We asked them for the reason why it should be the same material, and I also referred them to UG-4 of ASME VIII-1, but they were reluctant to tell me...probably a big secret (Maybe some of the metallurgical experts can answer this.)

As an example, we had a SA 516-70 vessel that needs to have a top head platform. The vessel does not need reinforcement pads for the clips, but because of the client requirement about the "same material" philosophy, we can either use fabricated channels made out of SA 516-70, or use a poison pad made of SA 516-70 with the A36 channel welded on it. We opted for the latter because it was easy to fabricate.

I have not seen any written guidelines on the size of the poison pad. For carbon steel vessels, I usually size the thickness depending on material availability (i.e. if I have used a nozzle repad on the vessel, I also use the same thickness on the poison pad) and just as long as the poison pad thickness is not greater than the vessel thickness. For the poison pad width, I usually use 1 1/2" wide all around the clip, preferably maintaining a rectangular profile. Sometimes I go down to 1" wide, but I have to ask our welding expert if there would be an overlapping of HAZ.

 

[AravindSujay]

Hi,

The points given by both Moltenmetal & metengr about Poison Pads are correct. PP's are used to Prevent the corrosion of the parent metal of two dissimilar metals. For instance in my current project for a Amine vessel, PP's are to be welded to a carbon shell & PWHT will be done after which, it is to be welded to SS316L cleats for ladder, so that Metallurgical properties of SS316L remains the same.

My doubt is that-What to do if the vessel is to be Weld overlayed with one layer of SS309L and another layer of SS317L. What is the procedure to be followed in this project. Need your expertise.

Regards,
Aravind Sujay

 

[gr2vessels]

Aravind,
First part is correct and agree with you.
The second part of your reply should be a new thread. I didn't quite get your doubt, however, do you intend to weld cleats to the overlay? It's OK as far as chemical composition concerns, since 317L is basically identical with 316L and as long as your cleat weldment is OK for bending and shear stresses. There is no code clause to prevent welding SS cleats on the SS overlay. There is however, a specific process for stripping the overlay, testing the base plate, welding the attachments to the base metal, testing and re-welding with SS overlay. Obviously, the ferrite content is an issue and sometimes the base metal and weldment hardness for compliance with corrosion resistance requirements. Please advise if you need specific answers.
gr2vessels

 

[AravindSujay]

Ok then I will start a new thread with the title,

"Procedure for Weld Overlay"