Corrosion in glycol heating system

[SlAze216]

Hi everyone,
I'm looking after a propylene glycol heating system.
There has been corrosion issues with the system since installation, particularly for the aluminium boilers. I had taken samples of the system and measured a ph of 8.5 and chlorides as high as 150ppm. No other contaminants i.e. calcium, magnesium, sulphates were out of recommended range

Can someone suggest what I can do to remove the chlorides? I had flushed the system out last year and put in a new batch which I also had measured and found little to no chlorides or contaminants, I'm not sure how it developed in the system. Also what reactions does the presence of chloride induce? especially ones that would cause the pH to rise. The glycol in the system has also turned yellow colour from initially being clear, with a strong chemical smell to it.
Thank you.

 

[IRstuff]

Did you put in corrosion inhibitors, which is the typical solution?

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[romke]

is the propylene glycol used in pure, undiluted form? what temperatures are achieved within the system? is there a possibility for heated propylene glycol to have contact with air?

can you give some more info on system design and its operating conditions?

the questions you ask will be hard to answer without sufficient information.

 

[BronYrAur]

You mentioned a pH of 8.5. That is the maximum pH allowed for some Aluminum boilers. Be careful there. I put several aluminum boilers on old piping systems and it took a trial and error process by the chemical treatment company to get it right. Constant pH battle as solids dissolved.

 

[SlAze216]

Here are some details.
Corrosion inhibitors were added by the supplier, i know molybdenum is one of them.
The temperature of the system can be 170 deg. F.
The flow is approximately 80gpm.
I believe it is 40% propylene glycol.
There is a mechanical de-aerator in the system, but to ensure it was working i did test for dissolved oxygen and found minimal amounts and based on that air is not dissolving in it.

Hope this is helpful, please let me know if more is needed.

 

[georgeverghese]

The corrosion management folks in your company may have decided to use some corr inhibitors which are in chloride form to counter excessive corrosion in some parts of this PPEG circuit - corrosion always gets worse when there is dissolved O2 in the PPEG-water mix. Dissolved O2 is often difficult to measure at the levels when corrosion sets in. Is your expansion tank / drum nitrogen blanketed ? Is the makeup water doped with an O2 scavenger to mop up dissolved O2?

 

[SlAze216]

The system was flushed out last year and a new batch added. The fresh glycol, although has inhibitors in them, does not contain appreciable chlorides. And no corrosion inhibitors were added separately after the new glycol was put into the system. The inhibitors may be a source, but I don't believe it is the source in this case.
Could someone also suggest techniques of removing it without too much effort and costs.

There is no nitrogen blanket but there are air traps that expel any air to ensure there is no air-glycol interface. And no oxygen scavengers were added. Is the oxygen the cause of the rising pH? is it creating hydroxide ions? if so what reaction is taking place?

Your help is much appreciated, as BronYrAur said at a pH 8.5 the boilers are at high risk.

 

[georgeverghese]

If your makeup water is very low in chlorides, then you can eliminate this source. And if the Cl- in the corr inhibitor is low, then maybe you've got a leak in one of your heat exchangers ?

In the oil gas business I've worked in, all heat transfer media loop expansion drums I've come across were N2 blanketed.

 

[SlAze216]

George, perhaps there is a leak. I will look into. What would you suggest for the getting the system to a less corrosive state? Remove the chlorides? if so, how? add oxygen scavengers? The nitrogen blanket is likely too expensive to consider

 

[georgeverghese]

Yes, the O2 scavenger should help. If there is a leak, you may have to recharge the entire PPEG inventory once you've flushed the whole circuit clean. There may be more than one source for this leak. Shell and tube heat exchangers with roller expanded tubesheets often leak at the tube to tubesheet joint after some years in service. Alternatives are to go for all welded plate heat exchangers made of CRA materials or get CRA material construct heat exchangers with seal welded tube to tubesheet joints.

 

[SlAze216]

I will look into O2 scavengers, if you can provide some options to help streamline my search, that would be helpful.
In the case of the chlorides, would the use of an ion exchange resin specific to chlorides be applicable? Both from an ability to rid the system of chlorides and do it at a reasonable cost? This solution of course will be in addition to locating the source of chloride to prevent it from being present again and inducing corrosion.

 

[georgeverghese]

Directionally, the lower the dissolved O2, the higher the tolerable level of chlorides before onset of chloride SCC.
Have not heard of anyone using ion exchange resins in this recirc loop for chloride control, though it sounds sensible enough - the only solution is to find the source.
As you already know, aluminum leaches out into solution at both ends of the pH spectrum. Here also, there may be some link between how readily aluminum leaches into solution and dissolved O2 content - a materials selection engineer should help to advise.
A materials selection engineer may be able to advise on a suitable reference book - I'm only speaking from past experience from what I've learned from materials selection / corrosion management engineers.