8 replies to this topic

[johneyissac98]

During the shutdown of fired heater why the tube skin temperature should not fall below 150degC and in case of cooling why should we coat it with soda ash solution?

[Pilesar]

I have never heard that requirement. My guess is to avoid conditions that might cause the tubes to rust. At 150C there will be no condensate formed on the tubes. Soda ash solution is alkaline which also inhibits rust. Let us know if you get a definitive answer elsewhere.

[johneyissac98]

yes ! @pilesar After shut downing the fired heater the tubes are coated with soda ash solution so as to prevent the oxygen contacting the tubes which causes corrosion. 

I didn't get the answer for why maintaining tube temperature at 150 DegC and for your kind information the tube side hydrocarbon not water therefore condensate is not an issue here.

[Pilesar]

The fire side of the tube has water in the air (humidity) if the furnace is cold. Water is a product of combustion, but as long as the firebox is hot, there will be no water condensing on the tube surface. The 150 C may just be a convenient target temperature for where action is taken. Tubes may also need to be hot to evaporate the water from the soda ash solution. My experience is with fired tube reactors such as for ammonia and ethylene and I never before heard of a soda ash bath. Does the original question arise from an academic situation or a real equipment experience? If real, ask your questions of those more directly involved with the procedure.

[johneyissac98]

The fire side of the tube has water in the air (humidity) if the furnace is cold. Water is a product of combustion, but as long as the firebox is hot, there will be no water condensing on the tube surface. The 150 C may just be a convenient target temperature for where action is taken. Tubes may also need to be hot to evaporate the water from the soda ash solution. My experience is with fired tube reactors such as for ammonia and ethylene and I never before heard of a soda ash bath. Does the original question arise from an academic situation or a real equipment experience? If real, ask your questions of those more directly involved with the procedure.

[johneyissac98]

Thank you pilesar

[breizh]

Hi,

My experience with soda ash plant and boiler. During shut down we were spraying on the bank of tubes, a solution of ammoniacal condensate which is a mixture of sodium carbonate, ammonia and water (available from the process) to protect the tubes form corrosion.

 

https://www.watertec...rrosion-control

 

My 2 cents

Breizh 

[Pilesar]

The handbook reference is informative and I learned some stuff. At the end of the chapter linked, it suggests that high pressure alkaline water is used for cleaning the fireside area. Is that the same thing as what you describe? I was envisioning a low pressure spray coating of some sort -- similar to a car wash -- with the sodium carbonate left on the tubes as a precipitate. I think my imagination was way off. Is the real procedure just hydroblasting the tubes with high pH water or is this something else? I would think it would cause problems to get the firebrick wet.

Edited by Pilesar, 13 January 2023 - 12:01 AM.

[breizh]

Hi Pilesar,

Yes but it was 30 years ago. I don't remember the details of the cleaning (spray gun!)

To be frank this handbook is very nice and covers a lot of topics.

Breizh