how to prevent rust from stack to atmosphere

[Bothkanin]

I have a big problem about the rust in flue gas blow out to atmosphere (only when start up HRSG) and impact to the environment.
now,i have an idea to spray water to mix with deposits in flue gas. Is it suitable or not ?
Does anybody have problem like this or anybody can suggest me how to execute it (may be how to prevent rust in the power boiler tube )
thank you veru much

bothkanin

 

[imok2]

If this is a power boiler and it's allowed to cool down in an atmosphire with a hi wet bulb/ humidity, condensation will rust fire tubes and develop iron oxides then when boiler starts forced air will blow this into atmosphere (solution) Keep boiler from getting too cool.

 

[Sharik]

I am assuming your HRSG is a gas turbine/watertube boiler recovery unit. We have several of these and had the same rusting/corrosion problem on the shutdown units. When the units cooled down, condensation would form inside the stack and outlet ductwork.

I wouldn't imagine spraying water into the flue gases would be a good idea during startup.

We got around the problem in two ways. For the standby units we flood the boiler and have a separate low pressure feedwater heater and maintain a feedwater flow through the boiler. We keep the natural draft flue gases above the dew point and no condensation forms.

For the units which are down more often and longer, we painted the inside of the stack and the outlet ductwork with a high temperature paint. As long as the condensation can't reach bare metal, no rust or corrosion occurs.

 

[Bothkanin]

thanks imok2 and Sharik for your reply.
First, the reason i spray water to flue gas to increase weight of sludge(rust+water) and thought that it would drop in my plant.
Sharik can you tell me how you prevent rust at boiler tube when unit shutdown.

thank you very much

bothkanin

 

[Sharik]

A lot depends on your location, the design/installation of the HRSG units and the resources at your disposal. The main thing is to prevent the condensation from forming in the first place.

Does your unit have a damper in the exhaust ducting or in the stack? One some of our units we close the inlet and outlet dampers, remove the manway from the ductwork between the gas turbine and the boiler, and stick a Herman Nelson in the manway. The Herman Nelson is a portable, gas fired heater/blower. It does not take much energy to keep the boiler section warm enough to prevent condensation. Enough air leaks by the outlet damper so there is always fresh air movement through the furnace but not so much that we can't control the tmeperature/humidity.

On some of our other units, we've piped in small high pressure feedwater supply and return connections. When the unit comes down for an extended period, we flood the superheaters and continuously circulate this small amount of feedwater through the boiler and superheater and returning to the deaerator. Again the intent is to keep the furnace above the dew point.

Sometimes, all that is necessary is to ensure the boiler is completely drained. It is more difficult to have external condensation on empty boiler tubes than on tubes filled with cooler water. This would depend on your supply of boiler feedwater and the run/standby times of the units.

There are more options but it depends on the boiler design the unit location. Keep the boiler warm and dry and you won't have any problems. Try to avoid spraying water into the flue gas.

 

[Bothkanin]

Sharik
I would like to explain you about my plant . When unit shutdowns for a month to inspect. I must inspect boiler tube and duct for a period so I could not install heater or tap hot water from another unit to keep the tube warm.
Anyway, I would like to thank again for good advice.

bothkanin

 

[Sharik]

Are your units inside buildings or outside? For inspection/maintenance shutdowns of our units inside buildings, we put plastic over the air inlet to the turbine to stop any natural draft from drawing cool, humid air from the outside. This usually makes the millwrights working on the turbine happier as well. We open the duct manways and draw warm air from inside the building.

We still use the Herman Nelson heaters, if required. These heaters are quite small (< 0.5m in diameter, 1m in length and come with wheels or a skid) and are easily shutdown during inspections. It takes about 2 minutes to install the heater inside the ducting between the gas turbine and the superheater.

For the outside HRSG units, we still put plastic or wood over the gas turbine inlet and use the heaters.