Cavitation in Steam Header 1

[Jmoore1]

I have a steam header that is failing every 1-2 years consistently. The steam is part of a cleaning system where 625# steam is blown into atmospheric pressure environemnt where the steam rapidly expands and cleans the carbon build up on the walls.

When not in use the system is closed off a dead leg exists at a highpoint. What I beleive is happening is liquid is condensing and falling back down the line, then flashing when it hits the superheated steam and causing cavitation erosion in the pipe. Is a stainless steel going to remove my problem (currently carbon steel), or will this constant caviation effect eat through stainless steel at a relatively similar rate? Is there any other alternatives?

 

[metengr]

Are you sure this is really cavitation damage or FAC (flow accelerated corrosion)? What does the damage look like, exactly? Can you describe it?

 

[Jmoore1]

The line is holing through, I was considering flow accelerated corrosion recently,but this corrosion is so localized and none of the rest of the pipe is seeing corrosion.

 

[metengr]

FAC in carbon steel can have one or more of the following visual features and it can be highly localized ot widespread;

1. Tiger stripes widespread feature
2. orange peel surface texture where the surface looks black in color (local or general damage)

Cavitation damage -

1. Very rough texture, like the surface of a sponge with sharp edges.


Erosion
1. Can be smooth and higly localized or general damage


For FAC - go to Cr-Mo material (P11 or P22)

For erosion - increase Chrome content to P22

For cavitation - stop the source of water from forming droplets during contact with the surface.

 

[EdStainless]

Moving to a 2%Cr steel would be my suggestion.
The issue with SS is that while you would get fantastic erosion and cavitation resistance the thermal expansions are much higher. I see thermal fatigue failures in SS regularly.
Sounds like you need to re-route a line to minimize the risk of water returning.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[rmw]

Can you hear any noise associated with this activity? I have seen desuperheaters with greatly reduced flows where when the liquid comes into contact with the hot metal parts of the DSH exposed to SH Steam temp, banged like someone had left a worker in the pipe with a hammer and he was trying to beat his way out. Sorry, but no other way to describe it.

rmw

PS: the DSH had to be redesigned to take a much higher pressure drop on the water side which kept the water as liquid until it exited the nozzle. But I'll never forget the noise it made.

 

[Jmoore1]

Hey All, I found someone who had seen the corrosion when this part of the line was replaced. He saw some of the cavitation sponge like corrosion as well as pitting. I beleive along with the cavitation, (this is somewhat dirty steam), the CO2 is condensing out of the water and dissolving into the condensate then forming carbonic acid so I am going to replace the problem error with a CR-Mo steel and put little vents at the top of the line to vent about 1 #/hr of steam along with the non condensables.

 

[rmw]

That will be an interesting undertaking - or was there a typo in your flow rate?

rmw

 

[ione]

If the culprit is indeed cavitation the use of an alloyed steel won’t fix the problem. The issue is related to a mechanical action carried out by collapsing bubbles which act as bullets. You could even increase your pipe schedule (you’ve not mentioned it) to allow a longer life, but to definitely sort it out you have to eliminate the source of the problem as noted above.

 

[MJCronin]

Jmoore...

How about some pictures of the failed/cavitated metal area ? Can you get ahold of a camera and use it ? Help us out here.... we are trying to help you

Perhaps a sketch of the system showing how condensate repeatedly eroses a section would be helpful.

This type of failure sounds almost exactly like the failures at dripping desuperheaters in fossil power plants. The "fix" for these is usually a change to 2.25 Cr/1 Mo materials

 

[Jmoore1]

Cronin...unfortunately this stuff has long since disappeared, all I have to go off of is what was seen by the inspector in the past.

All I have is paint so ive drawn a very crappy brief drawing. General idea is that steam comes off the header into the dead leg area when steam is injected, then is sat idle otherwise the corrosion is right off the header.