3 replies to this topic

[kybele39]

Dear All,

This was a discussion between my coworkers couple days ago. The question arise like when should we use an inert gas blanket in the expansion tank of the heater. Is it over 150 degree F or is it close to boiling temperature? or really close to max. film temperature?

The question came up because of oil degradation via oxidizing and occurring of the acidic environment due to degradation. So when should we worry for the oxidizing step in the tank? IS the corrosion will occur more at the 150deg F or in the elevated temperature and what should be the corrosion mechanism that will occur. Is glycol acid corrosive as much as H2SO4,HCl or like HNO3?

thank you.

Kybele

Edited by kybele39, 16 July 2010 - 03:10 PM.

[Art Montemayor]

Kybele:

If you want to deter the oxidation of glycol into degraded, acidic compounds, then you should be inerting the bath heater circuit with nitogen – whether at 150 oF or higher. Here, I am assuming you are referring to the bath heater design you mentioned in a previous thread. You fail to tell us that this is a hot water-glycol solution heater with a natural gas (or crude oil) stream in the internal coils. I have to assume that is what you mean and that you don’t want to vent it to atmospheric pressure.

Note that I use the term “deter”. You can’t, in my opinion, arrest the decomposition of a glycol solution 100%. It will decompose eventually – especially when heated and cooled cyclically.

Now you write about “oil degradation via oxidizing” as well as corrosion. I have to assume you are concerned that the glycol in the water solution will degrade and form corrosive acids that will attack the shell of the bath heater as well as the internal coil. Am I correct? If so, then let me confirm that you will have some corrosion if you don’t inert and take preventive steps such as inspections and possible chemical additions. When this will take place or when it should be done is a matter of how hot you get the solution and what type of materials and glycol you have. I would not expect abnormal corrosion – a few mils per year perhaps – if you monitor closely. The corrosion exhibited can never get as bad as mineral acid corrosion on carbon steel. If you keep your solution temperature at 150 ^oF you should only exhibit mild – if any – corrosion that can be tolerated and lived with. Add a corrosion allowance to all wetted parts (approximately 1/16” and you should be fine for the 15-year life of the equipment.

[kybele39]

Dear Art,

Yes all your assumptions are correct related to my post. My bath temperature is around 180 degree F. We are going to use 30% Ethylene glycol in the bath. I thought that if I do not use inert gas blanket system, air will be in contact with ethylene glycol and oxygen in the air will start to dissolve in the glycol solution. By time, because of the acidity the tank will start to corrode.However I do not have industry experience for this mechanism if occurs rapidly or not. Looking at your response, I should not worry too much about it.

Wwhat if I am over the 150 degree F, I should use inert gas blanket system with a relief valve, right?

Corrosion rate that you assumed is with the Inert Gas Blanket system, right?

If I run the heater continuously and maybe 1 or 2 times shutdown in a year corrosion may decrease its rate, right?

How about the thermal fluid heaters? Heaters have expansion tank and the temperature is in a heater may go up till 750 degree F. But in the expansion tank the temperature may not be this high. According required temperature of users I can never go under the 150F for a closed loop system. In this case, I should definitely use IGBlanket, right?

Thank you very much for your help.

Kybele

[mvp]

Kybele
I suggest you need an inhibitor to the solution. Most of the commercial heat transfer glycol solutions do contain them.
Two things that cause degradation are temperature and the residence time inside the furnace.
You are aware that The skin temperatures will be higher than the bulk.
MVP