Corrosion allowance:- Why diaphragm material selection is crucial

For Material selection our basis is Pipe material specs.

It is logical to assume that if our instrument sits on the pipe, we can blindly chose the material the pipe is made up of.

No!! This is not the cases always, but before we dive into a story from one of my projects lets understand basics of corrosion allowance.

Basics of Corrosion Allowance

let’s take a hypothetical Example:-

if a Pipe corrodes at a Rate of 0.02 inches per year you will provide corrosion allowance of maybe a 0.5 inch.

That means if in one year the Pipe gets corroded 0.02 inches then in 25 Years it will get corroded up to 0.5 inches.

So if the pipe required is of 1 inch thickness we will specify 1.5 inches for the above example.

A practical story

Now let’s explore a project scenario which I encountered which would make this simple to understand

Diaphragm seal material specified in Datasheet that I found was "Duplex 2205"

And the pipe material spec for that line stated Duplex 2205 but did NOT mention trim material.

So as the pipe material (Duplex) is compatible with the fluid so the Diaphragm seal material was also selected as Duplex 2205 by requisition engineer 

Here’s the catch !!!!

Piping guys have pipe with good thickness (also referred as schedule ) so that could be compatible for corrosion allowance for them but not for us 

Most metal diaphragms are in the range of 0.002˝ or 0.004” and mostly upto 0.006” (inches) thick for design reasons.

And the pipe class had a CORROSION & EROSION RATE of 0.03 inch/yr this value looks insignificant but compare this to diaphragm seal's membrane thickness.

i.e in 6 months 0.015 inches would be corroded & eroded and in 3 month 0.0075 inches, so your diaphragm would last for up till maybe a month or more ONLY !!!! (depending on its thickness)

And imagine this, In one of Hydrocracker unit I was working before, the corrosion allowance was 0.125”/year so much that ½ inch pipe were not recommended

Imagine our SMALL INNOCENT DIAPHRAGM EXPOSED TO SUCH HARSH FLUID !!!

SOLUTION :-

Usually the above being one of the governing factor why we use the TRIM MATERIAL mentioned in pipe spec for diaphragm seal application.

Also when the corrosion & erosion rate is very high like 0.125inches/year and above better to be safe and consult with materials that whether the trim material sufficient for diaphragm seal or some other higher/extra thick exotic material must be specified !

Which has happened in few cases!!

so choose wisely!!

PS:- Note that usually the pipe material is Carbon steel and we instrumentation engineer take stainless steel as diaphragm seal material which has high corrosion resistance but for exotic material it is of paramount importance to be sure of the corrosion and erosion rate. Like the one highlighted above.

Thanks for reading.

Hope this has been of value to you.

PS: This is as per best of my current understanding.