TO POTECT AN AUSTENITIC ST ST. DURI 2

[38621]

TO POTECT AN AUSTENITIC ST ST. DURING SHUTDOWN WE INTEND TO WASH IT WITH A 2WT% OF SODA ASH SOLUTION. WE ARE GOIUNG TO HAVE A WASHING SYS. CONSISTS OF A STORAGE AND MIXING TANK (5 M^3) AND A PUMP (30 M^3/H). DOES THIS DESIGN SOUND OK? COULD WE USE AN AIR OPERATED DIAPHRAGM PUMP? COULD WE USE SUCH PUMP AS A SELF PRIMING AND INSTAL IT ABOVE THE TANK? IS IT BETTER TO USE DRY SODA ASH OR TO USE SLLURY?
THANK IN ADVANCE TO YOUR ANSWERS.

 

[kbander]

I don't think it matters too much what you use to make the blend, as long as the final mix is correct. Hopefully someone can comment on the pumping rates...I always thought was the main thing was that everything was completely soaked with the solution, so I'm not certain that a high rate is really required.

Do you have any H2S in your fuel gas system? If so, you may want to wash down the outside of the tubes also to prevent cracking.


Regards,

Bob

 

[kenvlach]

What are you protecting the SS from?
Is the washing also intended to clean off something corrosive?
Is this solution intended to leave a neutralizing residue on the surface?
What is the SS alloy?

Subject to my not knowing your environment, I would wash the SS with a warm citric acid solution, rinse with water, then final rinse with DI water. Any carbonate residue would attract moisture and dirt from the air, possibly leading to corrosion.

 

[owg]

He is protecting the SS for stress corrosion cracking. Keep the steel wet and alkaline while it is open to air. HAZOP at

http://www.curryhydrocarbons.ca

 

[kbander]

The stress corrosion cracking is caused by polythionic acid attack which occurs when the austenitic stainless steel is exposed to air and moisture.

Even if there isn't much sulfur present, we usually assume that there is a layer of sulfide scale that will form polythionic acid when there is O2 and moisture. So, you either have to keep the steel from being in contact with air during a turnaround...or you have to neutralize the weak acid.

By the way, I believe that it is usually recommended to do a hot H2 strip to get as much sulfide as possible off of the steel prior to neutralization. Other things to consider are:

1. The mix tank water should be at least 38 C to help dissolve the soda ash.

2. The mix water has to be low in chloride content (<50 ppm), and the soda ash shouldn't be >500 ppm Cl. Add 0.5 wt% sodium nitrate (do not exceed this) for added protection against chloride attack.

3. Add a wetting agent for better penetration of coke, scale and films.

4. Soak everything for at least 2 hours, and have a good circulation rate if you suspect you may have some air pockets.


Also, I remember taking to someone about cracking of ss fins on heater tubes due to the amount of sulfur used for odorizing natural gas...which stresses that it really doesn't take a lot of sulfur to cause a problem.


Regards,

Bob

 

[kenvlach]

kbander (Bob),
Thanks, I gave you a star for being both helpful to 38621 and informative to me.
Ken

 

[38621]

THANKS kbander, kenvlach and owg ( I COULDNT FIND ANY RELEVAT INFORMATION ON THE

http://www.curryhydrocarbons.ca/

WEB SITE). AS FAR AS I KNOW THE EQUIPMENT (HE TUBES-INSIDE) SHOULD BE SOAKED FOR A MINIMUM OF TWO HOURS. HOW COULD I KNOW THAT THERE IS NO AIR POKECT IN THE HE?
ALSO WE INTEND (TO BE ECONOMICAL AND HAVE 5 M^3 TANK AND NOT A 15 M^3 TANK) TO FILL THE TUBES VOLUME WITH 3 BATCHES OF SODA ASH SOLUTION 2 -3 WT% - IS THIS WISE? COULD WE THAN CIRCULATE THE SOLUTION INTO THE HE (14 M^3 TOTAL INC PIPES)BY THE DIAPHRAGM PUMP? HOW DO WE KNOW THAT THE SODA ASH SOLUTION LEAVE A THIN ALKALINE FILM ON THE ALL SURFACE ?

 

[22082002]

YOu can also use NH3, if it's available, since the ammonia cylinder handling would be easy, and less cumbersum. check with the licensor of heater, for his approval of NH3 filling in fire box(actually we did it in the servise you mentioned, in diesel hydrotreater charge heater).
It was quite simple, ammonia cylinder + regulator + instrument tubing. close the vent stack(keep some opening for avoiding pressurisation of fire box).
This was quite simple, in 2 or 3 cylinders we could get the result.

Also how will you ensure that whole tubes are coated with soda ash solution, spraying in firebox with available openings is difficult.

Also you can keep instrument air positive pressure inside the firebox, to prevent moisture entering, once you have sprayed ammonia.

Hope you find this useful...

Regards,
SAA

 

[owg]

Just keep in mind that ammonia is a flammable and toxic gas. HAZOP at

http://www.curryhydrocarbons.ca

 

[FRChem]

Further to the above, is soda ash nutrilazation done to ALL 300 series S.S. Waht about vessels made of C.S. but LINED or CLADDED internally with any of the 300 series S.S.
Regards ,,

 

[FRChem]

Guys, still waiting your response. Thnx.

 

[PAN]

FRChem,
This may be too late for the reply. However, I would like to confirm my understanding with all members.

The reference should be NACE RP 0170.

The clad material of 300 series S.S. should be nutralized. However, the service temperature of the vessel should be concerned. In case of service temperature less than 400 C, sensitization will not occur.