Gledhill light unvented cylinder imploded

[Sharp Point]

is hazard a guess that bypassing the check valve has allowed it to siphon out.... probably through the hot draw off, that’s why Rakesh says water started coming out of the hot pipe. As we all know, you only need a litre or so to start a siphon off

Just an idea like

 

[Brambles]

Rakesh

For an accepted analysis of the failure will need a report prepared under Pressure Systems Safety Regulations (PSSR). The G3 qualification is to demonstrate competence to instal and commission. Whilst that would possibly help you, I think you would be better with a fault tree analysis report that covers both design and installation and thereafter, the operating situation that probably caused the failure.

As an aside, I don’t think that the information given to you by Heatrae Saudia (spelling ?) is correct. To resist a vacuum of delta 15bar against atmospheric pressure, the wall thickness of a 315 stainless 200 litre tank would need to over 10mm thick.

Andrew

 

[SimonG]

The drain down procedure is irrelevant. You didn't drain the cylinder.

Unless the cylinder syphoned down the cold because the combi valve was fitted wrong.

I would not expect something like this to happen when just cutting the flow to change a few taps.

 

[Chuck]

When I asked Gledhill technical how much pressure both negative or positive required to damage the cylinder, they were unable to comment. I also mentioned that I’d called Heatrae Sadia and went through the same scenario, however I was told by their technical team that there is no way their cylinder would implode under those circumstances and that there would have to be 15 bar of negative pressure to explode or implode their cylinders.

The Heatrae Sadia advice is either misquoted or wrong. The tank would need to be built like a submarine to withstand a pressure of 15 bar before collapsing. It would also need to be evacuated and then immersed in water to a depth of 150 m to achieve a 15 bar pressure difference.

The Gledhill warning notice states that emptying the cylinder while hot, sealing it, and then letting it cool will cause damage. Assuming that the cylinder starts at atmospheric pressure and 60°C at the time it is sealed then cools to 20°C. From this, we can deduce that a pressure difference of 0.12 bar or less may be sufficient to cause damage due to collapse.

 

[Rakesh]

The drain down procedure is irrelevant. You didn't drain the cylinder.

Unless the cylinder syphoned down the cold because the combi valve was fitted wrong.

I would not expect something like this to happen when just cutting the flow to change a few taps.

I’m thinking the same thing. I do t know how it would siphon from the hot water draw off. I’m thinking back siphon from cold inlet as the hot can only draw off from the very top of the cylinder.
Unless my understanding of the internals of an unvented cylinder is completely wrong.

 

[John stokes]

Gledhill lite is called lite for a reason... the wall thickness of the cylinder is half the industry standard and deep within the manufacturers instructions they recommend fitting an anti vac valve.
Do not use a pump when draining down, the bigger the cylinder the higher the risk

 

[Mastermixlee]

It's not fitted right no, but neither of the mistakes on Installation would cause the vacuum created that has imploded the cylinder.

There's integral non return valves on both hot and cold draw offs on the combination valves.

One could argue that the absence of an anti-vacuum valve could be a cause... But it's not compulsory - only recommended to avoid scenarios exactly like his where the specified drain down procedure isn't followed.

Best to have a Lever valve on outlet and another on balanced cold and utilise those for tap changes etc.

It's a tough one as your not g3 your technically not competent to have had knowledge of this, but on the flip side, it is in the User instructions regarding correct drain down procedure. So the responsibility would still fall down to yourself.

Boilermate thermal stores behave exactly the same way if any attempt to bung them is tried.

Installation should still be corrected regarding the pipe layout/expansion vessel location. But as it stands the 2 components would still be operating in the same way as if they are now and wouldn't have contributed to this scenario.

 

[chris watkins]

Long & short of it is the unvented cylinder is not installed correctly (as per manufactures instructions) therefore not your problem! (especially as it has happened before & still not corrected).
Don't get involved, don't try to help (I no it is hard), if they push it simply pass it over to your Public Liability insurance company.
You do have insurance cover don't you??

 

[Brambles]

Would not advise that “ostrich” attitude. The problem is a £1.5k fix. If you shun the Client and depend upon local work, word of mouth ( where you have no right of reply) may devastate your business.

A proposal to fix on a 50/50 basis may not be the bad deal it seems

Once you claim on a PLI, premiums rocket