Corrosion allowance is used to determine pipe wall thickness. Corrosion allowance should not be applied to valves. Valves wall thickness is determined to avoid material creep. If the valve wall thickness is reduced due to corrosion, or other critical dimensions that ensure operation and sealing are corroded, the valve may not function or worse, could fail and leak. This is why corrosion allowance is not mentioned for valves in B16.34 and valve standards. The standards expect the valve materials of construction to be resistant to the media being contained.

Despite this, it is common for some piping engineers to transfer the corrosion allowance they use to determine the pipe wall thickness to the valves. So some valve manufacturers have simply complied, which is why you can get some valves with a "corrosion allowance".

I had always wondered that a little, but bcd has it correct.

Valves are expensive and vital to operation. Spending a small amount to internally coat a valve with some sort of overlay or simply make it out of a non corrosive material is cost effective compared to doing the same for pipe.

But 6.4mm!! That's one of the highest I've ever seen and I suspect someone has converted this from 1/4"... A nominal value in one set of units shouldn't be directly transferred to another.

I can't think of a valve where the body has corroded to the extent that the valve either failed or had a hole in it, usually because the amount of material a valve needs anyway is well in excess of what a pipe needs. Hence why no one does it this way or that the codes don't include for it.

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

It is common for corrosion in valves to impair the ability to seal and shut off.
Most people spend the money required to assure that the valves will be one of the most durable parts of the system.
This usually means a CA of zero.

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P.E. Metallurgy, consulting work welcomed

Quote (bcd)

Despite this, it is common for some piping engineers to transfer the corrosion allowance they use to determine the pipe wall thickness to the valves. So some valve manufacturers have simply complied, which is why you can get some valves with a "corrosion allowance".

I work with agitators and occasionally we have customers who think corrosion allowance can be applied to a rotating agitator parts operating under pressure with a sealing device on the shaft. I agree, corrosion allowance only makes sense and has clear definition for piping, vessel walls, and flanges.

What I do is explain to the customer why it has no documented definition in this case and is therefore not required by the code. If they're stubborn, I listen to how they believe corrosion allowance can be applied to the situation and supply exactly that as long as it will still function as well as before. It becomes a cost adder. I tell them once about where these assumptions will likely not come true, and document thoroughly. I disclaim any ownership of how this "corrosion allowance" will improve the reliability of the system. I usually add a note clarifying how this was done "corrosion allowance of 1.0mm has been incorporated as additional impeller blade thicknesses and shaft diameter."

This seems endemic to engineering companies who take customer specifications for things that are mostly piping or vessels and apply those specifications without any nuance or intelligence to every single item in the package. It's usually because the EPCM didn't review the specifications or understand them, and they're trying to pass 100% of that nonsense onto the suppliers. So now they're trying to check a box and pass off responsibility. And they will, if you allow it.