Corrosion Allowance 1

[Valving]

This is the first time I ever saw below expression in a spec,
could you pls. advice how could I handle it?

Manufacturers shall conform the Corrosion
allowance of 3mm within the thickness
achievable as per the Design Standard of the
valve.

Type is high performance butterfly valves.

Thanks

 

[Ehzin]

Valving,

My only interpretation of what you've stated is that they intend to have corrosion allowance applied on top of your specified thickness/schedule taking into account mill undertolerance. What is the particular item you are specifying? Pipe/Valves/etc... More elaboration is needed to provide a decent answer.

I appreciate your effort in accurately wording your issue although it does add a slight confusion. Can anyone else provide their opinion?

Thanks,
Ehzin

 

[al1979]

Valving,
Looks like someone has used 'Google Translator' to build up this statement.
Ehzin is right. Min Wall thk = Design standard wall thk + 3mm corrosion allowance

Cheers !!!

 

[Valving]

As I mentioned it is stated on high performance butterfly valve RFQ data sheets.
These are fire safe valves with CAS bosy and we have proper coating etc. however
I could opine what I should do with this request.

 

[KKeong]

A way to "comply" is to show that your valve pressure envelope(min. body thickness) calculation takes into account an additional 3mm of corrosion allowance.

This corrosion allowance requirement is typically directly transferred from their Piping specification.

As a vendor, it will also give you an indication on the process fluid. Example: If an end user states 6mm corrosion allowance required then the valve material definitely should not be carbon steel.

 

[JohnGP]

Limited information available, but my interpretation would be that the specification is seeking confirmation that the standard wall thickness of the valve can include a 3mm corrosion allowance for the nominated design pressure, or maybe without derating from pressure class rating.
Cheers,
John

 

[bljnv]

Minimum wall thickness of valve casting = Calculated wall thickness required for handling the pressure and at temperature + Corrosion allowance of 3 mm.

 

[bcd]

Just to confirm what others are saying. The 3 mm corrosion allowance is taken from the piping specification where 3 mm extra is added to the required pipe wall thickness needed to contain the pressure. Transferring this requirement to the valves is not a smart thing to do. Valves will not shut-off and seal very well if their internal surfaces corrode by 3 mm. The wall thickness of valves is determined by other specifications and rules than the pipe, so the valves tend to be much stronger than the mating pipe. The valve manufacturer's tooling is established and they do not have the ability to add various extra mm to account for various corrosion allowances. What you will most likely get is a valve manufacturer's report which creatively shows that their standard valve wall thicknesses already includes the extra corrosion allowance you are requesting. What you should be asking for is a valve with internal materials that are compatible with the fluid inside the pipe, and proof or references to successful use of their product in your application.

 

[XL83NL]

I think bcd has the proper interpretation, or way to go.
Other replies on what the minimum tw should be are also correct in terms of piping specs for, say, elbows, pipe and tee's. For valves i think its not like that.
Convince your customer that what they need you to do is source valves that have internals, for the moving and sealing parts at least, that are corrosion resistant for the intended fluid. Other parts of the valve may have some corrosion allowance, as long as theyre parts that dont influence the sealing and shutoff/-on capacity of the valve.