Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations The Obturator on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Cold End Corrosion in HRSG

Status
Not open for further replies.

Metallurgist1991

Materials
Apr 14, 2016
3
There is a 100Ton industrial boiler which has to work with high sulfur content gas ( about 1000 ppm). Which kind of material you recommend us use for economizer tubes?

In HRSGs during Shut down period, gas side of the tubes may face with sulfuric acid corrosion (cold end corrosion) due to deposition of sulfur containing products and their reaction with condensate specially in humid plants. How can we calculate sulfuric acid concentration on tube surfaces? As you Know different materials have different corrosion resistant to different concentration of sulfuric acid. I think that the best way to overcome problem is to prevent condensation due to use of dehumidifiers. The use of dehumidifiers is not very common and I search for materials that could withstand this acidic condition. I think of using 304, 316, 430, 409 but sometimes A178 and A192 are used as a substitution. So could you please share your opinion about bellow questions :
-How much SO3 or So2 convert into sulfuric acid?
-How to calculate the sulfuric acid concentration?
- How to calculate the corrosion rate of tubes in this situation?
- Which material could withstand this acidic condition due to the corrosion rate and SCC?
- Is there any way to prevent acid formation apart from using dehumidifiers or increasing the water temperature of preheater?
 
Replies continue below

Recommended for you

If it is a long shutdown period, then the corrosion could attack all tubes, not just the preheater. For the long shutdown, it would likely be useful to prevent air circulation by use of a stack damper or tight shutoff ID fan damper. This will reduce the sizing of the dehumidifier and warm-up system.
There are correlations for sulphuric acid conversion to be found in older issues of Power magazine, Combined Cycle Journal, and Chemical engineering magazine- just spend a few hours on Google.
If the shutdowns are sort and there is a need for maximum stack gas heat recovery, there is a recent ASME code case 2795 that allows for use of ptfe tubes for corrosive service- but the downstream componetns and stack also will require equivalent protection.

"In this bright future, you can't forget your past..." Bob Marley
 
The acid dew point corrosion is a problem with HRSG's subjected to high sulfur exhaust gas. For this type of corrosion, the economizer tube material should be either a Ti-stabilized grade of ferritic stainless steel (Type 439), duplex stainless steel or even chromized carbon steel tube materials.

Regarding corrosion rates associated with acid dew point corrosion, they can vary considerably. If you have carbon steel economizer tube material, you can inspect and obtain a tube wastage rate.
 
Both 439 and 2205 have been used for cold end HRSG tubes. Since both are limited to an upper temperature of 600F (for metallurgical stability reasons) this tends to work out fine.
Determining the acid dew point is tricky, so most people use methods that consider the worst case (highest dew point possible) and work from there.
The actual corrosion rates can be very high, how fast does CS dissolve in medium strength Sulfuric?
Most plants have a method to keep the HRSG warm and dry for short periods, this usually involves recirculating hot water and dry air purge. Remember that when you shut down the entire HRSG becomes susceptible to acid attack.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor