Expansion Tank Losing Pressure On Compressed Air Side

matt_sunwaysolar

Curious if anyone has any info or experience about bladder-type (non-replaceable) expansion tanks and the loss of pressure on the compressed air side?

A few times now, with no identifiable leak on either the bladder or schrader valve, I have had expansion tanks lose compressed air. I've used leak detecting bubbles on the valve and also confirmed no leak coming through to the water side of the tank. No evidence of PRV opening or any "extreme pressure events".

Is this just expected to happen once in a while? Like bike tires?

In the most recent case, I tightened the schrader valve with a valve-stem screwdriver and re-pressurized and commissioned. So far, so good.

Thanks in advance for any info or experience.

hot_rod

How much pressure and over how long of a period? Is it a bladder or diaphragm tank?

Is it a solar system? Is the tank a solar rated tank, if so?

There was a time when tanks were pre-charged with nitrogen to limit pressure loss, from what I am told. I think they use dried air now.

SWEI

I've been told that Amtrol pre-charges theirs with helium.

matt_sunwaysolar

It is a bladder tank, not diaphragm.

It is installed on the solar loop of a larger, commercial solar water heating system. 400 square feet, flat plate collectors/ 600 gallons storage.

It is a Zilmet Solar Plus Tank and rated for 150psi. Charge pressure is 50psi on a three story building with 10' collectors. The model # is ZSP150, which is the 39.6 gallon model (huge).

Typically, I use Caleffi's expansion tank sizing formula for applications like this to get a base size, but then defer to Zilmet's reco's from their spec. sheets, which are even more conservative (larger) than Caleffi's.

Mark Eatherton

I'd heard they use dry nitrogen. Amtrol was the company that used dry ice when they first started. Wonder how they figured out how large a chunk they needed to use to drive the internal pressure up to 12 PSI. Seems to me if they used helium, the molecules are much smaller and would have a tendency to diffuse through the diaphragm easier.

They use helium to locate leaks in radiant floors with concrete because it can work it's way straight up from the leak.

ME

hot_rod

Does it have a large ring to replace the insides? It looks like they have a Solar Plus and TM series with a replaceable membrane.

Does the system go into stagnation often, that you know of?

I wonder how O2 permeable that "reduced permeability" membrane material is?

matt_sunwaysolar

Not replaceable.

System shouldn't be going into stagnation often, or at all. Collector to Storage Ratio is 1 : 1.5, plus it's a large multi unit building with lots of families in it (to say that plenty of water is being used). Would have to check on the number of units, but would guess at least 30 of various size.

SWEI

Nitrogen actually makes a lot more sense. Whatever it is, the MSDS turned them into truck freight only items at Ferguson -- major PITA.