ECE High Pitting Rate 1

[Hajiwaji]

Dears,

I have a question about possible mitigations and corrosion control for this steam I modeled on ECE. A 4" pipe is used to transport sour gas from HP compressor discharge KO drum (Dry Gas) to a water well to left water to a degassing vessel. Is it possible to use CS with 3.2 mm corrosion allowance for 20 year design for the pipe?

H2S Mole% 0.5
CO2 Mole% 9.8
Gas flow rate: 1.1 MMSCF
Crude Flow rate: 0
Water Flow rate: 0 but ECE can only take 0.1 Sm3/d as a minimum input.
Pipe size: 4"
Temp: 72 C
Pressure: 500 psig
Cl-: 50 ppm

The pitting rate is coming 15 MPY which is a very high value. I believe this is caused by the low water speed in a 4" pipe. But now water will condense in the pipe as the pipe is also electrically traced to prevent any condensations. Can we justify the use of bare CS in this case?

 

[EdStainless]

Sure, but if you ever lose steam trace you must have the compressor shut down and purge the line.
And we know that operation will never allow that.
Just know that a single failure may render the line unsafe.
I have seen this so many times with tracing, chem injection, impressed current, and others.
One failure will either force a system shutdown or risk consuming 100% (or more) of the CA.


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P.E. Metallurgy, consulting work welcomed

 

[SJones]

Of course it is possible to use “CS with 3.2 mm corrosion allowance” if the risk associated with a future loss of containment is tolerable. You probably won’t be able to change the consequence side of the equation if the equipment inventory and layout is fixed, so you will have to focus on the likelihood side, and then work out how much it will cost over the lifetime to maintain and verify the barriers, including replacement. Compare that cost to alternative materials and see which is the more attractive to management. Bear in mind that the modelling results could be substantially over pessimistic, so make sure that you have followed the ECE user manual, and that you have taken into account guidance from NACE TR21413.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.