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PeterStarck

Potable Expansion Vessel Questions

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PeterStarck    108

Never having had an unvented DHW system before I've a couple of questions for the wise and knowledgeable.

1. In practice is there an advantage in having a replaceable membrane or 2. having a nitrogen filled vessel?

In the WRAS guidance on EV installation it states the EV should be installed bottom fed and upright so does that mean air going into the vessel with the water isn't a problem or is there a way of bleeding it out?

 

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JSHarris    871

We have three vertical pressure vessel/accumulators on out borehole supply, so they are carrying potable water.  The advice I've had is that vertically mounted ones last longer than horizontal ones, due to less friction from the bladder on the side of the vessel, and that the bladders fail from friction and folding fatigue, rather than oxidation, so filling with dry nitrogen, rather than air, may not make any difference.

 

Spare bladders are fairly expensive, and I wasn't sure of the wisdom of keeping some, as it is probably quicker to just buy a replacement accumulator and swap it out, then look at replacing the bladder in the old one.  I've also heard (from one of the US well web forums) that there's no point in replacing the bladder, as the chances are that the liner will be worn and have rough spots, so may cause early failure of any replacement.  Apparently, most of these things have a polythene sleeve inside that is there to reduce the friction between the bladder and the vessel inner surface, and this can wear and cannot be easily replaced.

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PeterStarck    108

@JSHarris Thanks for that info. The advertising blurb states that with a nitrogen filled expansion vessel there is less corrosion and the nitrogen permeates through the membrane more slowly than oxygen resulting in the charge pressure staying up. Don't know how true that is. As your vessels are vertical and bottom fed did you find it necessary to bleed air from the water side of the vessel or does it just get absorbed into the water over time?

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jack    417
8 minutes ago, PeterStarck said:

 nitrogen permeates through the membrane more slowly than oxygen resulting in the charge pressure staying up. 

 

I'm sure I saw a tyre place near me advertising nitrogen fill as part of their fitting deal. According to this at least, the advantages in that situation aren't necessarily all they're sold to be.

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JSHarris    871

I can understand the advantages of nitrogen, aircraft tyres are filled with nitrogen for similar reasons, plus others related to reduced rubber degradation from oxidation.  I'm not convinced it's worth the hassle for an accumulator, though.

 

There seems to be very little air in an empty, but pre-charged, vessel, as the bladder seems to completely compress in on itself.  On ours there is what looks like a small grill behind the base fitting that I think is there to help stop part of the bladder blowing down the connection pipe when the vessel is empty.  Air does seem to dissolve very easily in water under pressure, too, so I doubt there is any free air space in the bladder when it's filled with water.  I've never needed to bleed ours, and when I fitted a pressure gauge to the top port on one of them, no air came out when I undid the screw, so I'm guessing any air just gets dissolved into the water.

 

Before bladder-type accumulators came along, borehole supplies used to use a sealed water tank with an air space at the top.  These are still in common use in the USA.  They work by having a non-return valve on the inlet to the pressure tank, with a snifter valve in the inlet pipe, that opens when the system is static, draining down the inlet pipe and letting it fill with air.  When the pump comes on, the snifter valve closes and a slug of air is pumped into the pressure tank, to top up the air space.  In the air space there is a float valve that operates an air release, to maintain the air space at a set height.  Some still prefer these systems over a bladder type accumulator, as they tend to oxygenate the water and remove dissolved iron and hydrogen sulphide.  There are many tales on the US well forum of people who have changed from an old air space pressure tank to a new bladder type accumulator and then found their water quality has degraded.

 

We did have a problem with dissolved water in our system, with the water coming out of the taps looking a bit milky.  This upset the ultrasonic flow measurement sensor inside the Sunamp PV, so I fitted a special vent to remove the microbubbles, a Spirovent.  This works very well.

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PeterStarck    108

Okay, cross posted with Jeremy. So forget about replaceable membranes and nitrogen fill and any air in the vessel will dissolve back into the water. Thanks for your help.

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JSHarris    871

It was a useful exercise, though, @PeterStarck

 

One difference that may be relevant is that the rubber used in aircraft tyres is very different to the EPDM used in tank bladders and membranes.  It seems that nitrogen does delay the oxidation of aircraft tyre rubber compounds a fair bit, and that often oxidation of tyres is a significant cause of the need for replacement, rather than wear, especially on aircraft that are operated on low duty cycles.  From what I can gather, EPDM doesn't oxidise very readily at all, which may be why it's not common to fill accumulators with nitrogen.

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