Pressure vessel vales keep failing ?? at a loss.

[paulmapp8306]

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]

When was the last time the unvented cylinder was serviced ?

 

[paulmapp8306]

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]

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]

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

 

[paulmapp8306]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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