Pressure vessel vales keep failing ?? at a loss.

paulmapp8306 · Jan 31, 2021

Im having problems with a pressurised hot water system. Keep getting leaks every 4-8 months as the pressure valve fails.

The first valve failed after approx. 4 years from installation. Since then (another 4 years) Ive replaced the vessel 3 times, its had had vales cores 5 times, and Ive put an "extender" on twice.

Keeps failing every 4-8 months, and not apparent reason why. Any help here?

ShaunCorbs · Jan 31, 2021

When was the last time the unvented cylinder was serviced ?

paulmapp8306 · Jan 31, 2021

ShaunCorbs said:
When was the last time the unvented cylinder was serviced ?

Not sure it has been as such to be honest. Its only 8 years old. Its had its temp valves changed, as well as its pressure ones. The only thing its not been is "pressure tested" itself.

Its been looked at by qualified persons every year though.

paulmapp8306 · Jan 31, 2021

OK - Just had a look at a guide for annual service of a pressurised hot water tank.

While not done as an annual inspection as such - all the aspects covered have been done regularly - more than once a year, as they are all checked every time a "leak" occurs (its always been the valve on the expansion vessel). All the over temp valves have been replaced at least once (to rule them out as culprits).

Scott_D · Jan 31, 2021

Undersized?
Got a secondary hot return?
How is it piped, from the cold, hot or combination valve?

paulmapp8306 · Jan 31, 2021

scott_d said:
Undersized?
Got a secondary hot return?
How is it piped, from the cold, hot or combination valve?

It did get replaced with a larger one at one point - but that made a horrible noise when the pressure was released (opening a hot tap), so the original size was put back on. That is what Im actually thinking though to be honest.

Piped from the cold side.

There is a pressure valve just before the expansion vessel on the cold feed (which leads to the tundish). The bottom of the tank (cold feed) is fed post pressure vessel. The top of the tank has a pressure/temp valve which also feeds the tundish.

Basically as per attached image.

John.g · Jan 31, 2021

paulmapp8306 said:
OK - Just had a look at a guide for annual service of a pressurised hot water tank.

While not done as an annual inspection as such - all the aspects covered have been done regularly - more than once a year, as they are all checked every time a "leak" occurs (its always been the valve on the expansion vessel). All the over temp valves have been replaced at least once (to rule them out as culprits).

Do you mean the pre charging air schrader valve on the expansion vessel itself and if so is it water?.

paulmapp8306 · Jan 31, 2021

John.g said:
Do you mean the pre charging air schrader valve on the expansion vessel itself and if so is it water?.

No - I mean we are getting water (slow drips) from the tundish (warm) after about 6 months - when the tank heats up. If the pressure is released (hot tap opened) after the heat cycle - drips stop. Checking the pressure of the expansion tank shows no pressure. Pressurising it - and it looses all pressure within a short time (can actually hear the slow seep of air) - though the time it takes varies with each failure, and starts very slow (a few hours) and gets quicker (to a few minutes) after a few days. Replacing the core of the schrader valve - and its all fine again - no leaks from tundish...for another few months. Rinse and repeat for the last 4 to 5 years.

As I say - Im asking as out engineer has no idea left. We've replaced the vessel 4 times (including one of a larger size) plus the actual valve several times. We've added the extension - same deal from that valve. We've replaced both pressure release valves (the one pre vessel on the cold side and the pressure/temp on on the top of the tank) in case it was those.

Ever time - the expansion vessel has had no air - and every time replacing the valve (or core - or indeed the vessel itself) fixes it for a few months. Then its back to square one.

There is no water leak from the vessel valve itself (so the internal diaphragm would seem to be in tact).

John.g · Jan 31, 2021

The reason the expansion valve etc is/was "leaking" was because it was just doing its job, when all the air had leaked from the EV you effectively had no EV as it was water logged, same as if diaphragm had failed so no place for expanded water to go.
Very strange that the valve keeps leaking though, especially as these vessels are pre charged with nitrogen when new but are then re charged with air which is quite normal procedure, what is this extension?, if the valve core is just leaking for whatever reason then you should be able to get a metal type dust cap to seal the end of the valve so doesn't matter if the core is leaking or not.

paulmapp8306 · Jan 31, 2021

John.g said:
The reason the expansion valve etc is/was "leaking" was because it was just doing its job, when all the air had leaked from the EV you effectively had no EV as it was water logged, same as if diaphragm had failed so no place for expanded water to go.
Very strange that the valve keeps leaking though, especially as these vessels are pre charged with nitrogen when new but are then re charged with air which is quite normal procedure, what is this extension?, if the valve core is just leaking for whatever reason then you should be able to get a metal type dust cap to seal the end of the valve so doesn't matter if the core is leaking or not.

Yeh - I get its doing its job - its the "why do they keep failing" thats the question. Its several valves, several vessles, several cores etc. Doesnt matter what we change it doesnt last that long.

The extension is one of the hose type extensions meant to be used it you have a faulty valve but cant get it out. Screw the extension on - and theres a new valve at the end of it. We tried one of those, and the valve on the end of the extension failed in the same way.

The vessel originally fitted (which did develop a leak in the diaphragm) was the tanks manufacturers recommended capacity, and its always had the same size replacement except the once - that was swapped because of the horrible noise it made when the diaphragm moved (when releasing pressure after a heat cycle). Maybe its worth trying a larger vessel again?

John.g · Jan 31, 2021

A sealing cap nut is what you need even though this problem is extraordinary.

What is the cylinder capacity and the present EV vessel volume, a larger EV should not cause any noise as the diaphragm moves even less, also what is the PRV (pressure reducing valve) set to and does the cylinder pressure stay the same when not being heated?.

paulmapp8306 · Feb 1, 2021

Ill have to see if I can dig that out.

Tank is definately 150L

Pressure seems fine - at lease there is no change in water pressure so I assume its good. Cant see the markings on the valve (eyes arnt great any more) but Ill have a look later with some help.

No markings on the vessel, so cant be sure of the size.

John.g · Feb 1, 2021

OK then have to assume a few EV sizes and will assume that the PRV is set to 3.0 bar, if then its advised that the air end pressure is set 0.2 bar lower but not essential as long as someone (G3) hasn't set it higher. also assume water heated to 70C.

Final pressures, from charging pressure of 2.8 bar
10 litre 4.85 bar
12 litre 4.43 bar
18 litre 3.85 bar
Final pressures, from (inadvertent) charging pressure to 4.0 bar
10 litre 6.14 bar and possible lifting of expansion valve, .
12 litre 5.67 bar Possible EV diaphragm noise until pressure falls to 3 bar when hot tap opened.
18 litre 5.0 bar. Ditto as above but unlikely IMO.

paulmapp8306 · Feb 1, 2021

Water is heated to 60 deg. I know that.

"12 litre 5.67 bar Possible EV diaphragm noise until pressure falls to 3 bar when hot tap opened." - that bit was the noise we got from the larger vessel (that looked like an 18 L) and why changed. Not from any of the others though.

Engineer that fitted, is saying the vessels are 18L though. Not convinced by pure look but as no markings I can see on the vessel, i have to take his word I guess.

Original charge/re charge (after valve failure) of vessel was 2 Bar I believe. Have checked with engineer - who is saying (once I put a new valve core in for now) charge to 1.3 Bar (when under no pressure - ie feed turned off, pressure released by opening hot water tap until flow stops).

John.g · Feb 1, 2021

What is the "cold" cylinder pressure or the hot pressure after opening a hot tap for a minute or so?.

John.g · Feb 1, 2021

assuming 3 bar cold pressure a 18 litre EV will give a final pressure of 4.16 bar at 60c from charging pressure of 1.3 bar and 3.88 bar from a charging pressure of 2.0 bar.

Ben-gee · Feb 1, 2021

We need to know your incoming mains pressure. And what the pressure reducing valve/combination valve pressure is set to , and this needs to verified after the valve with a gauge- can be temporarily fitted to measure and then removed.

paulmapp8306 · Feb 1, 2021

incoming mains is approx. 1.5 to 2 bar normally (its been measured by trent water - fluctuates between those 2). Flow is aprox 10-12 L/min (not as if that makes a difference though).

Gauge wise - that needs more help than I can do myself.

Heating engineer is flummoxed as well. Hes spoken to water cylinder manufacturers today to confirm correct piping and vessel size. They are head scratching as well.

Dont think its installation - it was fine for 3 years before the first cylinder lost its charge., Never been the same since.

At his point, its getting as many odd idea possibilities as we can to check out.

The temp/pressure release valve is set at 90 deg and 7 bar - thats rated mind and not checked with gauge. Cold side valve is 95 deg and 6 bar.

John.g · Feb 1, 2021

The above will only give a final pressure of ~ 2.64 bar. The valve problem should have nothing whatsoever to do with your installation, the valve should, theoretically, still leak if you left the EV sitting there unconnected to anything, the only other suggestion I can make is to buy a 0 to 10 bar pressure gauge that you can attach permanently to the schrader valve, that will kill two birds, it will stop any leakage through the valve and will measure the cylinder pressure as well as the pressure on both sides of the diaphragm will be equal but only use a initial air side pressure of 1.3 bar.

Edit: I would initially pump it up to ~ 3 bar and then reduce it to 1.3 bar to ensure that the diaphragm is pushed up against the water end.

paulmapp8306 · Feb 1, 2021

John.g said:
The above will only give a final pressure of ~ 2.64 bar. The valve problem should have nothing whatsoever to do with your installation, the valve should, theoretically, still leak if you left the EV sitting there unconnected to anything, the only other suggestion I can make is to buy a 0 to 10 bar pressure gauge that you can attach permanently to the schrader valve, that will kill two birds, it will stop any leakage through the valve and will measure the cylinder pressure as well as the pressure on both sides of the diaphragm will be equal but only use a initial air side pressure of 1.3 bar.

yeh - may well do that for peace of mind.

I tried to get the old core out today as replacements have arrived. I cant get the core out ..... it unscrews OK - but jams when trying to lift it out of the housing. Consequently (without completely changing the ….again, or the whole valve) adding an extension is my option. Using a pressure gage in its place makes sense for monitoring going forward.

Kelsall mcewen · Jul 23, 2022

Just read this thread. I have exactly the same problem with my Expansion Vessel. EXACTLY. Fine for 3 or 4yeears then the EV failing every 6 months. I've tried everything and spoken to everyone.

Did you manage to solve this problem.

Kelsall · Jul 23, 2022

Just read this thread. I have exactly the same problem with my Expansion Vessel. EXACTLY. Fine for 3 or 4yeears then the EV failing every 6 months. I've tried everything and spoken to everyone.

Did you manage to solve this problem.

John.g · Jul 23, 2022

Is it exactly the same though, ie losing pressure out of a leaking schrader valve?.

Chuck · Jul 23, 2022

If it's the valve failing, maybe a batch of valves with a manufacturing defect? Or far-eastern fakes using incorrect materials?

You'll need the manufacturer, or an independent 'failures engineer' (expensive), to investigate the failed valves and get a root cause to really know what's going on.

kelsall mcewen · Jul 23, 2022

John.g said:
Is it exactly the same though, ie losing pressure out of a leaking schrader valve?.

Exactly the same. Every 6 months I have to replace the expansion vessel. Put in a bigger and 10 bar tank and it made virtually no difference. Changed pressure reducing and relief valves and makes no difference.

Who ever this previous person was I would love to know if/how they resolved it.

I've even had Yorkshire water out to discuss if my pressure could be spiking at night.

John.g · Jul 23, 2022

Very strange, wonder is it actually leaking from the schrader needle valve itself or the valve body where it exits the EV, doesn't explain the reason but use a drop of soapy water to establish where (if) exactly its leaking from, if its the needle valve itself the you should surely be able to source a air tight cap for it.

Also why do you need to replace the EV, you can simply get it pumped up again?, also suggest getting a combined isol/drain service valve installed in the EV outlet pipe, there are purpose manufactured valves for this,

Pressure vessel vales keep failing ?? at a loss.

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