Rapid (erosion?) of stainless pipe in steam/oxygen service 2

[frostrobn]

There is a process in my facility that uses live steam (160 psig and desuperheated) mixed with oxygen which is piped to the process in 316L stainless steel piping. The piping just after the point of oxygen injection in the steam line is eroding rapidly. The line has to be patched every 3 months or less. Does anyone have any experience with this problem. The piping upstream of the oxygen injection point is fine, it's just the pipe after the introduction of oxygen that is troublesome. Other information: The line is under high frequency vibration caused by injecting live steam into the process a few feet after the oxygen injection point. I have pics if anyone is interested. Question: Would 2205 (duplex) stainless steel be a better metallurgy in this situation? Any response would be greatly appreciated.

 

[JLSeagull]

Look at the requirements for the oxygen service. Velocity is a big issue. Monel is used for impingement areas. I don't know that oxygen is the culprit but the O2 design documents are very different than for other stuff.

 

[metengr]

In this situation the Mo used as an alloy element in 316L might be working against you because for normal steam service, 316L will tend to oxide and spall (exfoliate) over time. The reason is believed to be the rapid oxidation characteristics of molybdenum.

Using a 304 stainless steel might provide better performance in this application. The duplex stainless steels are limited to lower service temperature (600 deg F or lower) but still contain Mo.

 

[drywall]

Frostrobn,

In my experience gaseous oxygen is a material to be handled with care. I rather suspect that JLSeagull is right, that the problem is one of velocity and impingement. Usually monel or inconel are appropriate materials for oxygen impingement areas. I think it would be worth your while to get a copy of CGA G-4.4 Edition 4 Oxygen Pipeline Systems.

 

[frostrobn]

Thanks for your responses. I checked out CGA G-4 and found that it is applicable to oxygen systems up to 292 Degf. I'm not sure if a steam/oxygen mixture at 370 Degf can be applied to this code.

 

[hacksaw]

are you using a sparger, are you preheating the O2, how far downstream from the desuperheater is the O2 injection point, what sort of damage (pitting, thinning...) is occuring?

 

[frostrobn]

The device (that the combined oxygen and steam enters the process through) is a sort of variable sparger that is controlled based on outlet temperature of the sparger. The desuperheating nozzle is approximately 30 ft upstream of the oxygen injection point in the steam line. There is approximately 5 ft of pipe after the oxygen injection point before it enters the sparger. The damage looks a lot like heavy corrosion that one would typically see in carbon steel water piping. There seems to be more of a generalized thinning of all surfaces with some washing and accelerated thinning in weld heat affected areas.

Thanks for your response.

 

[hacksaw]

how close to saturation do you control? you may be getting wetsteam erosion-corrosion . along this line 30 ft may not be enough distance between the desuperheater and the oxygen injection, typically 40-60D is required just for the temperature measurement

you can always add metal and the duplex stainless will likely stand up better than 316L, if you can keep it in place,

I am not sure that I buy the moly argument, especially with oxygen present, since moly improves resistance at least in wet steam services.

 

[frostrobn]

I have always understood that moly helps with only erosion with wet steam, but maybe there is something to the oxygen at 370-400 Degf that is causing corrosion in 316L. Now that I have seen the damage, it looks more like corrosion (like carbon steel water pipe with heavy scale) than erosion. I have not seen areas of "washing" from wet steam. Ideas?

 

[frostrobn]

If the desuperheater was not working the steam temp can go as high as 500 Degf. Is this temp high enough to cause rapid oxidation especially with oxygen also being injected into the line?

 

[hacksaw]

I am more concerned about water impingement rather than elevated temperature

my experience is that stainless is good to 500F in O2, given caveats of corrosion in any service.

160 # steam has a sat. temp of 370F, most desup installations can get to 5-10F superheat at best, and depending on the amount of run to the process.

what sort of desuperheater are you using?

 

[frostrobn]

I would think that if impingement was the issue the elbows in the line would be the first to show it. The damage occurs directly after the injection of oxygen and appears to be more like corrosion than erosion. There is an elbow in the steam line upstream of the oxygen injection point that is clean with no signs of erosion.

I will have to get back to you on the type of desuperheater (not sure).

Are you saying that 316L stainless steel should not corrode at 370-500 Degf. with a steam and oxygen mixture?

 

[btrueblood]

Is there a swirler/mixer associated with the oxygen injection? Is the oxygen preheated prior to injection?

If anwsers are yes and no, the oxygen may be locally condensing pockets of steam as it heats up in the flow. If there is swirl in the flow, the droplets will move to the pipe wall due to centrifugal effects, and erosion can/may/will occur.

 

[frostrobn]

Getting back to you all. I was on vacation. All the response has been very educational. I have moved the discussion to "metal and metallury engineering" forum. Thanks to all for your help.