Diesel Engine Exhaust Corrosion

[mamoore]

Proposed System: 1825 kW standby diesel power plant with very long horizontal exhaust pipe and vertical stack.

Specifications: Would like pipe to be std wt. carbon steel to keep costs down. Pipe is insulated with 6" mineral wool. Engines will be execrised at 50% load once a week for approximately 2 hours. Engines reach 777F at full load.

Issues: Two hours may not enough time to completely warm-up the piping system; therefore condensation is expected to occur and remain in the pipe.

Questions:
1. What is estimated life expectancy of this piping system?
2. How will the horizontal pipe corrode? Entire inner surface area? Just on the bottom? Will this increase the roughness significantly? What roughness value should be used? 0.001 ft?
3. Does it make sense to use Carbon Steel at all?
4. If diesel engines were to run long enough to completely heat the piping system above condensation temperature (approximately 250F) would pipe last longer?
5. If a purge fan were installed to "dry-out" the piping system after running the diesel generators would this increase pipe life?

 

[Metalguy]

Briefly, if you can dry the pipe and keep it dry, you shouldn't have a problem.

 

[hacksaw]

Heat tracing has been used as have properly sloped and drained stacks.

 

[martinwhelan]

This problem is similar to the problem of HCl corrosion.
Our company uses this sort of insulation system a great deal, mainly in turbine exhaust systems but occasionally for diesels. A lot of operators see safety problems with a combination of oil firing and ceramic fibre internal insulation, but I am sure you have considered that!

On gas-fired exhausts where we suspect acid corrosion may occur due to low temperature/dewpoint we either use Corten, available from U.S. steel or other suppliers in Europe or we apply a proprietry acid resistant bitumastic coating to the carbon steel duct before installing the ceramic fibre.
The main problem is that it is very messy in the workshop if you have other activities going on at the same time.

 

[mamoore]

Thanks for the input Metalguy, Hacksaw, and Martinwhelan.
I wanted to clarify the primary concern with this exhaust system is condensation of diesel exhaust gases on carbon steel due to the extreme length of piping. Insulation is placed on the outside of the pipe only and no coatings are planned inside the pipe.

Our current plan, in addition to drying out the pipe after operation, is to place sumps in the bottom of the pipe with small reservoirs (length of pipe w/valve) placed before each EJ to collect the condensate (like a steam system consensate return if you will). This condensate will be manually collected after operation of the engines.

Thanks Again,

MM