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Author Topic: Corrosion in plate heat exchanger  (Read 7973 times)
wookey
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« on: July 17, 2011, 10:08:20 PM »

So, my solar system sprang a leak last week (or so, that's when I noticed), and had nearly filled a 2l ice cream tub in a week of dripping, so clearly (as I'm about to go away for three weeks), it needed to be fixed.

My setup (for those who haven't been reading here for years) is not the usual bottom-coil but a 'side-arm' plate heat exchanger fitted to a standard 114l coper cylinder between the cold feed and the vent pipe. So the PHE is hydraulically part of the cyclinder (mounted level with the bottom of the tank). When the solar heats the PHE, that sets up a thermosiphon in the PHE loop which puts hot water in at the top of the cylinder. The thermosiphon piping is plastic (JG speedfit), so the PHE is electrically isolated from the cyclinder (other than by the water).

So, on draining the cylinder and the solar loop and taking it to bits I found that the PHE threaded mounts on the DHW side are quite badly corroded, whilst the ones on the solar side remain shiny and new. The PHE has been fitted since approx May 2008 - i.e a little over 3 years. It's a standard combi PHE (intended for a vaillant boiler).

So, the question is - why has it gone all rusty? PHEs are normally used for CH water on one side and DHW on the other. DHW is agressive as it's full of oxygen and not full of inhibitor, but as this is the normal use case I don't understand why mine's rotting at a fair rate of knots. It's not the PHE body, which is stainless, but the threaded mount. Is it possible that the mounts on one side are made of different stuff to the other, because the manufacturers know that only one side needs to be agressive-water proof?  The rotted pair is 3/4"BSP, the OK ones are 1/2"BSP. Which size is normally the DHW? Pity they aren't all the same size - I could have just turned it round and got another 3 years out of it :-)

This is the PHE: http://www.ekmpowershop22.com/ekmps/shops/euroboilerparts/he110-vaillant-turbomax-06-4946-064946-289-p.asp

Below is what it looked like once unbolted, and after I cleaned it up - you can see it's got quite pitted. I had to take a good mm off to get a surface the O-ring would seal against. Both those spigots looked the same when it was originally bought and assembled - some kind of coating on the steel, presumably zinc?

Clues welcome. I can keep buying new PHEs every 3 years, but this shouldn't be happening, so I want to understand what's going on first. I also took the opportunity to service my solar loop by replacing the fluid. It was fairly brown (I'm not very surprised as I've had quite a few stagnation days, mostly due to 1-wire unreliability and my dodgy software (maybe 10 days in total?). Pic at the bottom. I need to get some pH paper to test it properly.


* PHEcorrosion1.jpeg (85.07 KB, 640x480 - viewed 1000 times.)

* PHEcorrosion2.jpeg (87.82 KB, 640x480 - viewed 858 times.)

* 3yrsolar fluid.jpeg (100.66 KB, 640x480 - viewed 827 times.)
« Last Edit: July 17, 2011, 10:32:46 PM by wookey » Logged

Wookey
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« Reply #1 on: July 17, 2011, 10:20:58 PM »

Hallo Wookey,

That's a fine spot of rust you have there, I have a feeling that a clue may be in the fact that normally there is mains water in that exchanger which hasn't sat around in a cold cistern exposed to the atmosphere giving it the chance to absorb oxygen. I wonder if there is a test you could do that would measure the difference in the oxygen content of fresh mains water and water drawn from your hot tap?

I have done a similar thing with my neighbours hot cylinder as my heat dump, but I made it out of 4 parallel lengths of 10mm inside a length of 28mm, it seems OK so far..........fingers crossed.

Desperate
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EccentricAnomaly
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« Reply #2 on: July 18, 2011, 09:56:00 AM »

I wonder if there is a test you could do that would measure the difference in the oxygen content of fresh mains water and water drawn from your hot tap?

Put a goldfish in it? Or, perhaps:

How to Measure the Amount of Oxygen in Water

The relative complexity of that does indicate there's not a really simple method. Maybe people who keep fish have a method.

Wookey, you mention that the PHE is electrically isolated from the cylinder (apart from through the water) but don't take the thought any further. Might it not be better to connect the two to short out any galvanic potential or would the resulting current just make things worse? That's something I'm a bit confused about.
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Brandon
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« Reply #3 on: July 18, 2011, 10:30:43 AM »

wookey, in a combi I would expect to find the DHW in the 1/2" tappings, and the system in the 3/4".

Had never thought about it before though
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djh
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« Reply #4 on: July 18, 2011, 11:10:14 AM »

Just speculation here ...

What material screws onto those terminals? Is it setting up a galvanic couple?

Presumably your solar piping is copper and is earthed somewhere and presumably your cylinder is earthed. If you measure the voltage between your cylinder and the PHE terminal (when installed), is it really zero?

I believe it is possible to fit anodes to potable water systems. That might be worth exploring if no other solution appears.
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dhaslam
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« Reply #5 on: July 18, 2011, 11:34:41 AM »

Although it doesn't look like it  part of the  build up could be limescale with  a little rust adding the colour.   

Connecting the DHW side to the 3/4 " connection seems logical since in the original use  the heat exchanger the other circuit is pumped  and very short.
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dimengineer
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« Reply #6 on: July 18, 2011, 01:47:12 PM »

Isn't it the chlorides in the DHW doing the corrosion? You will have warm/hot water, with chlorides in there. Very corrosive.
Its a thought.

TIm
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baker
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« Reply #7 on: July 18, 2011, 09:58:38 PM »

Hi
it looks like electrolyse from common brass bsp fittings to stainless steel  corrosion
this can be sorted by using a dzr  type brass fittings designed for your application
can be got from / BES
john
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Philip R
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« Reply #8 on: July 18, 2011, 10:52:52 PM »

High chlorides would destoy the copper cylinder, the water would run verdigris stains everywhere.

Heat exhanger possibly contains bosses off different grades of stainless, ferritic v austenitic.

Have you got earth currents running through your hot water system. May need to check earth bonding around the cylinder.

Philip R
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wookey
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« Reply #9 on: July 19, 2011, 12:20:57 AM »

Some useful thoughts here. The solar piping is not earthed explicitly, and indeed wrapped entirely in insulation, so only earthing is through expansion vessel attached to wall and panels attached to roof - both pretty shoddy earths.

The connections to the PHE at the corroded spigots are JG speedfit straight tap connectors (7088 on
http://www.bes.co.uk/products/154.asp ). (I had to cut down the flanges slightly to make them fit). So are those using DZR brass or not? As they are intended to connect to taps, I'd expect them to be DZR. So that's a plastic bodied fitting with brass, just for the nuts. On the non-corroded side it's 15mm angled tap fittings. Those are chrome-coated brass.

Yes there could well be some limescale in those deposits, as the PHE is often hot enough to scale up. However the inside of the PHE is very clean in fact, with no signs of limescale.

I'm not convinced by the 'water that's been via a roof tank is much more oxidised' theory. If that was really a problem then everyone who lived near a water tower would have terribly corroded combis wouldn't they?

I'll see if any voltages can be measured.
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Wookey
tony.
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« Reply #10 on: July 19, 2011, 08:15:28 AM »

Could it be bacterial growth?


Wookey,

Whats your current email as i have solat thermal installers guide which may be of use.
12MB email ok for you?

Tony
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JohnH
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« Reply #11 on: July 19, 2011, 12:49:40 PM »

Do you have any soldered joints anywhere?  The stuff around the inside of the PHE flange looks like rosin flux.
Flux can be corrosive and the flange itself could be acting as a cold spot.
JohnH
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ecogen
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« Reply #12 on: July 19, 2011, 01:22:27 PM »

Two forms of corrosion at work here. Firstly Crevice corrosion which is a type of pitting corrosion, the cause of which is often overlooked as nothing appears to be visible that could cause such an attack. In this case it is likely to have started around the rubber O ring where the fitting face and boss face come together, helped along by crud producing a stagnant crevice. The stainless steel becomes less noble due to low oxygen weakening the passive film. Chlorine in the tap water is only a few ppm at most, even in warm water this is not a problem. However, permeation through crud of such corrosive material into the stagnant crevice can lead to concentrations where pitting will occur.
Cleaning the boss face with a steel wire brush prior to assembly is certain to initiate pitting. Lower cost lower chrome nickel steels are more susceptible.
Secondly, a dissimilar metal nut on the outside of the joint in a dry environment will only start corrosion pitting once electrolyte seeps past the O ring. The three conditions needed for galvanic corrosion now present.
Partial solution is to remove all pitting from the boss face and reassemble using a new O ring and paste both mating parts, threads too with an anaerobic sealant eliminating all crevices. I find Loctite 572 enables many a good bodge. Screwfix 10. Corrosion will continue inside the pitted boss bore, not helped by low flow rates, periods of no flow, crud and biofilm.
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wookey
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« Reply #13 on: July 20, 2011, 02:02:20 AM »

That sound like you know what you are talking about :-) I didn't clean the spigots with a wire brush. The only surface dmaage could have come from the extra bit of plastic flange on the fitting that I had to chop off. But I can't imagine speedfit plastic doing much harm to steel.

Also we seem to have corrosion on both spigots. The pic is the top one - I couldn't get the bottom one off at all so just left it as it wasn't leaking. Why would both have the necessary scratches to induce corrosion. And I can imagine a bit of crud collecting at bottom fitting, but top as well?

I did wonder about something to re-seal the surface, but couldn't think what would last. I'll try the loctite - cheers.
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Wookey
biff
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« Reply #14 on: July 20, 2011, 09:06:21 AM »

hi wookey,
          i used to fit a salamder to the mains inlet in the house,it was supposed to polarise the iron ore contents in the water which meant that the crud passed straight through the system and would not stick to anything.
    that gunge in the 3/4 boss looks suspiciously like ore,horrible stuff, the salamder might be a good bet,it will not get rid of it but it will keep it in suspension, the febrox in the c/h does a similar job for the c/h but in a different way.
                                                                                                               biff
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