1-1/4Cr for steam service? 2

[vesselguy]

Hi all,

I am filling in (temporary) for a piping engineer who quit recently. Crappy job but the garbage always falls in the lap of the remaining personal. This person really didn't know anything about piping nor material selection. Hence I'm rechecking his work. In doing so, I am reviewing the piping spec he put together and which he have "supposedly" reviewed it for accuracy. In it I find 2 new piping class for 2 phase steam that specify the use of 1-1/4Cr steel (SA335-P11) with a max temp for the class set at only 340°C (645°F). I have found no history on why this is specified.

I'm normally a vessel/equipment engineer and I don't do much piping, but I know just enough to be dangerous. I have used 1-1/4Cr only for high temperature service when the temperature is above 750°F to 900°F for its better strength at these temp, when C.S. won't work due to carburization and dramatically lowered strength. I see no good reason why 1-1/4 Cr should be used for steam at only 645°F. Am I missing something on the details? Can someone fill in the gap for me regarding steam service that demand 1-1/4Cr?


thanks.

 

[stanweld]

He may have specified it for piping in high velocity fluid service to prevent FAC (flow accelerated corrosion).

 

[vesselguy]

Stanweld,

Mmmmm, interesting thought. I am not aware of any literature that details the erosional properties of C.S. vs 1-1/4 Cr or other material for that matter. Is there such a difference between c.s. and 1-1/4Cr? Can anyone direct me to a paper so I read on it? Thanks.

 

[EdStainless]

Even a small amount of Cr (0.25%) will greatly boost resistance to FAC. If these lines could see high velocity conditions like blow down or such then it would be a good bet.

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Plymouth Tube

 

[stanweld]

Essentially FAC can be mitigated by a "Chrome Equivalency" of 0.15% - a function of %Cr + %Mo + %Cu. Because purchasing of carbon steel material having the chrome equivalency is somewhat difficult, a number of boiler designer/manufacturers have specified P11 piping instead - overkill, maybe, but it will do the job.

 

[vesselguy]

stanweld and EdStainless,

Thanks very much Gents for your tips. I search on FAC using Goggle and found 2 paper that talks about FAC on steam piping and HRSG. Vogt Power claims their use of 1-1/4Cr yields 4 times longer life than C.S. in resisting erosion. I do remember now that TIW once talked about possibly using 1-1/4Cr for their OTSG.

Looks like the EPC that inititally developed the piping spec had the foresight to propose 1-1/4Cr for this purpose. I think it would be used for elbows and places just down stream of flow changes where it would affect erosion/corrosion property.

Thanks again gents.