it shouldn't happen to a plumber........but it does!

[kay-jay]

Tip for no air locks.........do it live

Don'tbung the expansion or this may happen

View attachment 1338

This was actually from a job today where the cylinder had to be syphoned to remove it as there was no drain valve. The hose was tight in the draw off so created a vacuum which flattened the cylinder.

thanks tamz, can you explain how this happens by bunging the cwsc feed please? i would have thought that a bung simply stops the feed from flowing just like the the gatevalve would if it wern't siezed. i also thought that that a collapsed cylinder would only occur if the cylinder were to be emptied via a syphon effect, but surely unless the vent was bunged also the cylinder could 'breathe' and wouldn't collapse anyway.and if the vent and feed were bunged then the cylinder wouldn't drain?????
i'd really like to know where my thinking is flawed so i can avoid catastrophe on future jobs!!

thanks


KJ

 

[wheeto]

Hot water in the cylinder cools and contracts.

 

[easyt]

Doubt it. Water doesn't expand much unless frozen. Also hot water often cools.

Simply letting water out in a way that doen't allow air or water to replace it cause a vacuum. Atmospheric pressure does the rest. Fil an empty plastic coke bottle with water. Turn it upside down - vertical without a top. Water runs out and air cannot emter so bottle crushes. Plastic bottle regains shape. Copper cistern doesn't

 

[kay-jay]

yeah but as i said before unless you bung the vent as well surely the cylinder would take air in from there and not compress

 

[easyt]

It would but some folk drain the tank by pushing a tube in that way if there is no draindown and siphoning. This can block the air.

 

[kay-jay]

so bunging the feed isn;'t a problem unless i'm silly enough to bung the vent too?

 

[Twinhawk]

once you bing the tank the static head is then from the top of the cylinder which doesnt need drained, open a tap, .5 bar at most in a 3 story house isnt going to collapse a cylinder, maybe a direct one for heating with the weight of water

 

[kay-jay]

thats what i thought twinhawk, i was querying tamz post from above warning of cylinder collapse.
personally i can't see how it would collapse by using a bung but tamz has been at this game longer than me and i respect much of what he says in his posts.

 

[tamz]

99 times out of 100 you would be fine bunging it but there is the possibility that it can and does happen.

Things to watch for are if there is an essex or surrey flange on the cylinder where it can draw from inside the tank or if there is a pump on the draw off which kicks in.

Just be aware, .